Skeletal muscles (commonly referred to as muscles) are
organs
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a fu ...
of the
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
muscular system
The muscular system is an organ (anatomy), organ system consisting of skeletal muscle, skeletal, smooth muscle, smooth, and cardiac muscle, cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the bo ...
and typically are attached by
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
s to
bone
A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
s of a
skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
.
The
muscle cell
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...
s of skeletal muscles are much longer than in the other types of
muscle tissue
Muscle tissue (or muscular tissue) is soft tissue that makes up the different types of muscles in most animals, and give the ability of muscles to contract. Muscle tissue is formed during embryonic development, in a process known as myogenesis. Mu ...
, and are often known as
muscle fibers
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...
.
The muscle tissue of a skeletal muscle is
striated – having a striped appearance due to the arrangement of the
sarcomere
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s.
Skeletal muscles are voluntary muscles under the control of the
somatic nervous system
The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.
The somatic nervous system consists of sensory nerves carryi ...
. The other types of muscle are
cardiac muscle
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...
which is also striated and
smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
which is non-striated; both of these types of muscle tissue are classified as involuntary, or, under the control of the
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
.
A skeletal muscle contains multiple
fascicles – bundles of muscle fibers. Each individual fiber, and each muscle is surrounded by a type of
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
layer of
fascia
A fascia (; plural fasciae or fascias; adjective fascial; from Latin: "band") is a band or sheet of connective tissue, primarily collagen, beneath the skin that attaches to, stabilizes, encloses, and separates muscles and other internal organs. ...
. Muscle fibers are formed from the
fusion
Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole.
Fusion may also refer to:
Science and technology Physics
*Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...
of developmental
myoblasts
Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development.
Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called ''myotubes''. In the early development of ...
in a process known as
myogenesis
Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development.
Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called ''myotubes''. In the early development o ...
resulting in long
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinat ...
d cells. In these cells the
nuclei termed ''myonuclei'' are located along the inside of the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
. Muscle fibers also have multiple
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
to meet energy needs.
Muscle fibers are in turn composed of
myofibrils
A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. Each myofibr ...
. The myofibrils are composed of
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
and
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
filaments called
myofilament
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togethe ...
s, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
.
Muscles are predominantly powered by the
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
of
fat
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
s and
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s, but
anaerobic
Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to:
* Anaerobic adhesive, a bonding a ...
chemical reactions are also used, particularly by
fast twitch fibers. These chemical reactions produce
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) molecules that are used to power the movement of the
myosin head
The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a he ...
s.
Structure
Gross anatomy
There are more than 600 skeletal muscles in the human body, making up around 40% to 50% of body weight.
Most muscles occur in bilaterally-placed pairs to serve both sides of the body. Muscles are often classed as groups of muscles that work together to carry out an action. In the
torso
The torso or trunk is an anatomical term for the central part, or the core, of the body of many animals (including humans), from which the head, neck, limbs, tail and other appendages extend. The tetrapod torso — including that of a human ...
there are several
major muscle groups including the
pectoral
Pectoral may refer to:
* The chest region and anything relating to it.
* Pectoral cross, a cross worn on the chest
* a decorative, usually jeweled version of a gorget
* Pectoral (Ancient Egypt), a type of jewelry worn in ancient Egypt
* Pectoralis ...
, and
abdominal muscles
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
;
intrinsic and extrinsic muscles are subdivisions of muscle groups in the
hand
A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each "h ...
,
foot
The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
,
tongue
The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...
, and
extraocular muscles
The extraocular muscles (extrinsic ocular muscles), are the seven extrinsic muscles of the human eye. Six of the extraocular muscles, the four recti muscles, and the superior and inferior oblique muscles, control movement of the eye and the ot ...
of the
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
. Muscles are also grouped into
compartments including
four groups in the arm,
and the
four groups in the leg.
Apart from the contractile part of a muscle consisting of its fibers, a muscle contains a non-contractile part of dense fibrous connective tissue that makes up the
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
at each end. The tendons attach the muscles to bones to give skeletal movement. The length of a muscle includes the tendons.
Connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
is present in all muscles as
deep fascia
Deep fascia (or investing fascia) is a fascia, a layer of dense connective tissue that can surround individual muscles and groups of muscles to separate into fascial compartments.
This fibrous connective tissue interpenetrates and surrounds the m ...
. Deep fascia specialises within muscles to enclose each muscle fiber as
endomysium
The endomysium, meaning ''within the muscle'', is a wispy layer of areolar connective tissue that ensheaths each individual Skeletal muscle#Skeletal muscle fibers, muscle fiber, or muscle cell. It also contains capillaries and nerves. It overlies ...
; each muscle fascicle as
perimysium
Perimysium is a sheath of connective tissue that groups muscle fibers into bundles (anywhere between 10 and 100 or more) or fascicles.
Studies of muscle physiology suggest that the perimysium plays a role in transmitting lateral contractile ...
, and each individual muscle as
epimysium
Epimysium (plural ''epimysia'') (Greek ''epi-'' for on, upon, or above + Greek ''mys'' for muscle) is the fibrous tissue envelope that surrounds skeletal muscle. It is a layer of dense irregular connective tissue which ensheaths the entire muscle ...
. Together these layers are called ''mysia''. Deep fascia also separates the groups of muscles into muscle compartments.
Two types of
sensory receptor
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
s found in muscles are
muscle spindle
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...
s, and
Golgi tendon organ
The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a musc ...
s. Muscle spindles are
stretch receptor
Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles, and are neurologically linked to the medulla in the brain stem via afferent nerve fibers. Examples include stretch receptors in the arm and leg muscles ...
s located in the muscle belly. Golgi tendon organs are
proprioceptor
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
s located at the
myotendinous junction
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
that inform of a
muscle's tension.
Skeletal muscle cells
![Blausen 0801 SkeletalMuscle](https://upload.wikimedia.org/wikipedia/commons/6/6f/Blausen_0801_SkeletalMuscle.png)
Skeletal muscle cells are the individual contractile cells within a muscle, and are often termed as muscle fibers.
A single muscle such as the
biceps
The biceps or biceps brachii ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join ...
in a young adult male contains around 253,000 muscle fibers.
Skeletal muscle fibers are the only muscle cells that are
multinucleate
Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinat ...
d with the
nuclei often referred to as myonuclei. This occurs during
myogenesis
Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development.
Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called ''myotubes''. In the early development o ...
with the
fusion
Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole.
Fusion may also refer to:
Science and technology Physics
*Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...
of myoblasts each contributing a nucleus.
Fusion depends on muscle-specific proteins known as
fusogens called ''myomaker'' and ''myomerger''.
Many nuclei are needed by the skeletal muscle cell for the large amounts of proteins and enzymes needed to be produced for the cell's normal functioning.
A single muscle fiber can contain from hundreds to thousands of nuclei.
A muscle fiber for example in the human biceps with a length of 10 cm can have as many as 3000 nuclei.
Unlike in a non-muscle
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
where the nucleus is centrally positioned, the myonucleus is elongated and located close to the
sarcolemma
The sarcolemma (''sarco'' (from ''sarx'') from Greek; flesh, and ''lemma'' from Greek; sheath) also called the myolemma, is the cell membrane surrounding a skeletal muscle fiber or a cardiomyocyte.
It consists of a lipid bilayer and a thin oute ...
. The myonuclei are quite uniformly arranged along the fiber with each nucleus having its own ''myonuclear domain'' where it is responsible for supporting the volume of cytoplasm in that particular section of the myofiber.
A group of muscle
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s known as
myosatellite cell
Myosatellite cells, also known as satellite cells, muscle stem cells or MuSCs, are small multipotent cells with very little cytoplasm found in mature muscle. Satellite cells are precursors to skeletal muscle cells, able to give rise to satellite ...
s, also ''satellite cells'' are found between the
basement membrane
The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...
and the sarcolemma of muscle fibers. These cells are normally quiescent but can be activated by exercise or pathology to provide additional myonuclei for muscle growth or repair.
Attachment to tendons
Muscles attach to
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
s in a complex interface region known as the musculotendinous junction also known as the myotendinous junction, an area specialised for the primary transmission of force.
At the muscle-tendon interface, force is transmitted from the sarcomeres in the muscle cells to the tendon.
Muscles and tendons develop in close association, and after their joining at the myotendinous junction they constitute a dynamic unit for the transmission of force from muscle contraction to the skeletal system.
Arrangement of muscle fibers
Muscle architecture
Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pr ...
refers to the arrangement of muscle fibers relative to the axis of
force generation, which runs from a
muscle's origin to its
insertion
Insertion may refer to:
*Insertion (anatomy), the point of a tendon or ligament onto the skeleton or other part of the body
*Insertion (genetics), the addition of DNA into a genetic sequence
*Insertion, several meanings in medicine, see ICD-10-PCS
...
.
The usual arrangements are types of
parallel
Parallel is a geometric term of location which may refer to:
Computing
* Parallel algorithm
* Parallel computing
* Parallel metaheuristic
* Parallel (software), a UNIX utility for running programs in parallel
* Parallel Sysplex, a cluster of IBM ...
, and types of
pennate muscle
A pennate or pinnate muscle (also called a penniform muscle) is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. This type of muscle generally allows higher force production but a smaller ra ...
. In parallel muscles the fascicles run parallel to the axis of force generation, but the fascicles can vary in their relationship to one another, and to their tendons.
These variations are seen in
fusiform
Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a b ...
,
strap
A strap, sometimes also called strop, is an elongated wikt:flap, flap or ribbon, usually of leather or other flexible materials.
Thin straps are used as part of clothing or baggage, or bedding such as a sleeping bag. See for example spaghetti s ...
, and
convergent muscles.
A convergent muscle has a triangular or fan-shape as the fibers converge at its insertion and are fanned out broadly at the origin.
A less common example of a parallel muscle is a circular muscle such as the
orbicularis oculi
The orbicularis oculi is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short ...
, in which the fibers are longitudinally arranged, but create a circle from origin to insertion.
[Lieber, Richard L. (2002) ''Skeletal muscle structure, function, and plasticity''. Wolters Kluwer Health.] These different architectures, can cause variations in the tension that a muscle can create between its tendons.
The fibers in
pennate muscle
A pennate or pinnate muscle (also called a penniform muscle) is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. This type of muscle generally allows higher force production but a smaller ra ...
s run at an angle to the axis of force generation.
This
pennation angle reduces the effective force of any individual fiber, as it is effectively pulling off-axis. However, because of this angle, more fibers can be packed into the same muscle volume, increasing the
physiological cross-sectional area
In muscle physiology, physiological cross-sectional area (PCSA) is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point. It is typically used to describe the contraction properties of pennate muscl ...
(PCSA). This effect is known as fiber packing, and in terms of force generation, it more than overcomes the efficiency-loss of the off-axis orientation. The trade-off comes in overall speed of muscle shortening and in the total excursion. Overall muscle shortening speed is reduced compared to fiber shortening speed, as is the total distance of shortening.
All of these effects scale with pennation angle; greater angles lead to greater force due to increased fiber packing and PCSA, but with greater losses in shortening speed and excursion. Types of pennate muscle are
unipennate
Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pr ...
,
bipennate
Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pr ...
, and
multipennate
Muscle architecture is the physical arrangement of muscle fibers at the macroscopic level that determines a muscle’s mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force pr ...
. A unipennate muscle has similarly angled fibers that are on one side of a tendon. A bipennate muscle has fibers on two sides of a tendon. Multipennate muscles have fibers that are oriented at multiple angles along the force-generating axis, and this is the most general and most common architecture.
Muscle fiber growth
Muscle fibers grow when exercised and shrink when not in use. This is due to the fact that exercise stimulates the increase in
myofibrils
A myofibril (also known as a muscle fibril or sarcostyle) is a basic rod-like organelle of a muscle cell. Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. Each myofibr ...
which increase the overall size of muscle cells. Well exercised muscles can not only add more size but can also develop more
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
,
myoglobin
Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglobi ...
,
glycogen
Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body.
Glycogen functions as one o ...
and a higher density of
capillaries
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
. However muscle cells cannot divide to produce new cells, and as a result there are fewer muscle cells in an adult than in a newborn.
Muscle naming
There are a number of terms used in the naming of muscles including those relating to size, shape, action, location, their orientation, and their number of heads.
;By size: ''brevis'' means short; ''longus'' means long; ''longissimus'' means longest; ''magnus'' means large; ''major'' means larger; ''maximus'' means largest; ''minor'' means smaller, and ''minimus'' smallest; ''latissimus'' means widest, and ''vastus'' means huge.
These terms are often used after the particular muscle such as
gluteus maximus
The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. It is the single largest muscle in the human ...
, and
gluteus minimus
The gluteus minimus, or glutæus minimus, the smallest of the three gluteal muscles, is situated immediately beneath the gluteus medius.
Structure
It is fan-shaped, arising from the outer surface of the ilium, between the anterior and infer ...
.
;By relative shape: ''deltoid'' means triangular; ''quadratus'' means having four sides; ''rhomboideus'' means having a
rhomboid
Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled.
A parallelogram with sides of equal length (equilateral) is a rhombus but not a rhomboid.
...
shape; ''teres'' means round or cylindrical, and ''trapezius'' means having a
trapezoid
A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium ().
A trapezoid is necessarily a Convex polygon, convex quadri ...
shape;
''serratus'' means saw-toothed; ''orbicularis'' means circular; ''pectinate'' means comblike; ''piriformis'' means pear-shaped; ''platys'' means flat and ''gracilis'' means slender.
Examples are the
pronator teres
The pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two attachments, to t ...
, and the
pronator quadratus
Pronator quadratus is a square-shaped muscle on the distal forearm that acts to pronate (turn so the palm faces downwards) the hand.
Structure
Its fibres run perpendicular to the direction of the arm, running from the most distal quarter of the ...
.
;By action: ''
abductor'' moving away from the midline; ''
adductor'' moving towards the midline; ''
depressor'' moving downwards; ''
elevator
An elevator or lift is a wire rope, cable-assisted, hydraulic cylinder-assisted, or roller-track assisted machine that vertically transports people or freight between floors, levels, or deck (building), decks of a building, watercraft, ...
'' moving upwards; ''
flexor
A flexor is a muscle that flexes a joint. In anatomy, flexion (from the Latin verb ''flectere'', to bend) is a joint movement that decreases the angle between the bones that converge at the joint. For example, one’s elbow joint flexes when one ...
'' moving that decreases an angle; ''
extensor
In anatomy, extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extendin ...
'' moving that increase an angle or straightens; ''
pronator'' moving
to face down; ''
supinator
In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm.
Structure
Supinator consists of two planes of fibers, between whi ...
'' moving
to face upwards;
''
internal rotator
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
''
rotating
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
towards the body; ''
external rotator
Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...
'' rotating away from the body; ''
sphincter
A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the hum ...
'' decreases the size, and ''tensor'' gives tension to; ''
fixator muscle
Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location.
Types
There are three types of muscle tissue in the body: skeletal, smooth ...
s'' serve to fix a joint in a given position by stabilizing the prime mover whilst other joints are moving.
;By number of heads:''biceps'' two; ''triceps'' three and ''quadriceps'' four.
;By location: named after the near main structure such as the
temporal muscle
In anatomy, the temporalis muscle, also known as the temporal muscle, is one of the muscles of mastication (chewing). It is a broad, fan-shaped convergent muscle on each side of the head that fills the temporal fossa, superior to the zygomatic a ...
(temporalis) near to the
temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral cortex.
The temporal bones are overlaid by the sides of the head known as the temples, and house the structures of the ears. Th ...
.
Also ''supra-'' above; ''infra-'' below, and ''sub-'' under.
;By fascicle orientation: Relative to the midline, ''rectus'' means parallel to the midline; ''transverse'' means perpendicular to the midline, and ''oblique'' means diagonal to the midline.
Relative to the axis of the generation of force – types of ''parallel'', and types of ''pennate'' muscles.
Fiber types
Broadly there are two types of muscle fiber: Type I, which is slow, and Type II which are fast. Type II has two divisions of type IIA (oxidative), and type IIX (glycolytic), giving three main fiber types.
These fibers have relatively distinct metabolic, contractile, and motor unit properties. The table below differentiates these types of properties. These types of properties—while they are partly dependent on the properties of individual fibers—tend to be relevant and measured at the level of the motor unit, rather than individual fiber.
Fiber color
Traditionally, fibers were categorized depending on their varying color, which is a reflection of
myoglobin
Myoglobin (symbol Mb or MB) is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglobi ...
content. Type I fibers appear red due to the high levels of myoglobin. Red muscle fibers tend to have more mitochondria and greater local capillary density. These fibers are more suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP (
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
). Less oxidative Type II fibers are white due to relatively low myoglobin and a reliance on glycolytic enzymes.
Twitch speed
Fibers can also be classified on their twitch capabilities, into fast and slow twitch. These traits largely, but not completely, overlap the classifications based on color, ATPase, or MHC.
Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly. These mainly include the ATPase type II and MHC type II fibers. However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well-developed, Bioenergetic systems#Aerobic and anaerobic metabolism, anaerobic, short term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow twitch fibers. Fast twitch muscles are much better at generating short bursts of strength or speed than slow muscles, and so fatigue more quickly.
The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of Bioenergetic systems#Aerobic and anaerobic metabolism, aerobic energy transfer. These mainly include the ATPase type I and MHC type I fibers. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity.
Fibers that become slow-twitch develop greater numbers of mitochondria and capillaries making them better for prolonged work.
;Type distribution:
Individual muscles tend to be a mixture of various fiber types, but their proportions vary depending on the actions of that muscle. For instance, in humans, the quadriceps muscles contain ~52% type I fibers, while the Soleus muscle, soleus is ~80% type I.
The orbicularis oculi muscle of the eye is only ~15% type I.
Motor units within the muscle, however, have minimal variation between the fibers of that unit. It is this fact that makes the size principal of motor unit recruitment viable.
The total number of skeletal muscle fibers has traditionally been thought not to change.
It is believed there are no sex or age differences in fiber distribution; however, proportions of fiber types vary considerably from muscle to muscle and person to person. Among different species there is much variation in the proportions of muscle fiber types.
Sedentary men and women (as well as young children) have 45% type II and 55% type I fibers.
People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type I fibers.
Sprint athletes, on the other hand, require large numbers of type IIX fibers.
Middle-distance event athletes show approximately equal distribution of the two types. This is also often the case for power athletes such as throwers and jumpers.
It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle.
It is thought that if you perform endurance type events for a sustained period of time, some of the type IIX fibers transform into type IIA fibers. However, there is no consensus on the subject.
It may well be that the type IIX fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes, not a change in fiber type.
Fiber typing methods
![Denervation atrophy - atp94 - intermed mag](https://upload.wikimedia.org/wikipedia/commons/b/b7/Denervation_atrophy_-_atp94_-_intermed_mag.jpg)
There are numerous methods employed for fiber-typing, and confusion between the methods is common among non-experts. Two commonly confused methods are histochemical staining for myosin ATPase activity and immunohistochemical staining for myosin heavy chain (MHC) type. Myosin ATPase activity is commonly—and correctly—referred to as simply "fiber type", and results from the direct assaying of ATPase activity under various conditions (e.g. pH).
Myosin heavy chain staining is most accurately referred to as "MHC fiber type", e.g. "MHC IIa fibers", and results from determination of different MHC isoforms.
These methods are closely related physiologically, as the MHC type is the primary determinant of ATPase activity. However, neither of these typing methods is directly metabolic in nature; they do not directly address oxidative or glycolytic capacity of the fiber.
When "type I" or "type II" fibers are referred to generically, this most accurately refers to the sum of numerical fiber types (I vs. II) as assessed by myosin ATPase activity staining (e.g. "type II" fibers refers to type IIA + type IIAX + type IIXA ... etc.).
Below is a table showing the relationship between these two methods, limited to fiber types found in humans. Subtype capitalization is used in fiber typing vs. MHC typing, and some ATPase types actually contain multiple MHC types. Also, a subtype B or b is not expressed in humans by either method.
Early researchers believed humans to express a MHC IIb, which led to the ATPase classification of IIB. However, later research showed that the human MHC IIb was in fact IIx,
indicating that the IIB is better named IIX. IIb is expressed in other mammals, so is still accurately seen (along with IIB) in the literature. Non human fiber types include true IIb fibers, IIc, IId, etc.
Further fiber typing methods are less formally delineated, and exist on more of a spectrum. They tend to be focused more on metabolic and functional capacities (i.e., oxidative vs. glycolytic, fast vs. slow contraction time). As noted above, fiber typing by ATPase or MHC does not directly measure or dictate these parameters. However, many of the various methods are mechanistically linked, while others are correlated ''in vivo''.
For instance, ATPase fiber type is related to contraction speed, because high ATPase activity allows faster Muscle contraction#Crossbridge cycling, crossbridge cycling.
While ATPase activity is only one component of contraction speed, Type I fibers are "slow", in part, because they have low speeds of ATPase activity in comparison to Type II fibers. However, measuring contraction speed is not the same as ATPase fiber typing.
Muscle fiber type evolution
Almost all multicellular animals depend on muscles to move. Generally, muscular systems of most multicellular animals comprise both slow-twitch and fast-twitch muscle fibers, though the proportions of each fiber type can vary across organisms and environments. The ability to shift their phenotypic fiber type proportions through training and responding to the environment has served organisms well when placed in changing environments either requiring short explosive movements (higher fast twitch proportion) or long duration of movement (higher slow twitch proportion) to survive.
Bodybuilding has shown that changes in muscle mass and force production can change in a matter of months. Some examples of this variation are described below.
Examples of muscle fiber variation in different animals
Invertebrates
American lobster, ''Homarus americanus'', has three fiber types including fast twitch fibers, slow-twitch and slow-tonic fibers.
Slow-tonic is a slow twitch-fiber that can sustain longer contractions (Tetanic contraction, tonic).
In lobsters, muscles in different body parts vary in the muscle fiber type proportions based on the purpose of the muscle group.
Vertebrates
In the early embryonic development, development of vertebrate embryos, growth and formation of muscle happens in successive waves or phases of
myogenesis
Myogenesis is the formation of skeletal muscular tissue, particularly during embryonic development.
Muscle fibers generally form through the fusion of precursor myoblasts into multinucleated fibers called ''myotubes''. In the early development o ...
. The myosin heavy chain Isotype (immunology), isotype is a major determinant of the specific fiber type. In zebrafish embryos, the first muscle fibers to form are the slow twitch fibers. These cells will undergo migration from their original location to form a monolayer of slow twitch muscle fibers. These muscle fibers undergo further Cellular differentiation, differentiation as the embryo matures.
Reptiles
In larger animals, different muscle groups will increasingly require different fiber type proportions within muscle for different purposes. Turtles, such as ''Red-eared slider, Trachemys scripta elegans'', have complimentary muscles within the neck that show a potential inverse trend of fiber type percentages (one muscle has high percentage of fast twitch, while the complementary muscle will have a higher percentage of slow twitch fibers). The complementary muscles of turtles had similar percentages of fiber types.
Mammals
Chimpanzee muscles are composed of 67% fast-twitch fibers and have a maximum dynamic force and power output 1.35 times higher than human muscles of similar size. Among mammals, there is a predominance of type II fibers utilizing glycolytic metabolism. Because of the discrepancy in fast twitch fibers compared to humans, chimpanzees outperform humans in power related tests. Humans, however, will do better at exercise in aerobic range requiring large metabolic costs such as walking (bipedalism).
Genetic conservation versus functional conservation
Across species, certain gene sequences have been preserved, but do not always have the same functional purpose. Within the zebrafish embryo, the ''PRDM1, Prdm1'' gene down-regulates the formation of new slow twitch fibers through direct and indirect mechanisms such as ''SOX6, Sox6'' (indirect). In mice, the PRDM1, ''Prdm1'' gene is present but does not control slow muscle genes in mice through ''SOX6, Sox6''.
Plasticity
In addition to having a genetic basis, the composition of muscle fiber types is flexible and can vary with a number of different environmental factors. This plasticity can, arguably, be the strongest evolutionary advantage among organisms with muscle.
In fish, different fiber types are expressed at different water temperatures.
Cold temperatures require more efficient metabolism within muscle and fatigue resistance is important. While in more tropical environments, fast powerful movements (from higher fast-twitch proportions) may prove more beneficial in the long run.
In rodents such as rats, the transitory nature of their muscle is highly prevalent. They have high percentage of hybrid muscle fibers and have up to 60% in fast-to-slow transforming muscle.
Environmental influences such as diet, exercise and lifestyle types have a pivotal role in proportions of fiber type in humans. Aerobic exercise will shift the proportions towards slow twitch fibers, while explosive powerlifting and sprinting will transition fibers towards fast twitch.
In animals, "exercise training" will look more like the need for long durations of movement or short explosive movements to escape predators or catch prey.
Microanatomy
![1023_T-tubule](https://upload.wikimedia.org/wikipedia/commons/9/94/1023_T-tubule.jpg)
Skeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of two contractile proteins
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
, and
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
– that are two of the
myofilament
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togethe ...
s in the myofibrils. The myosin forms the thick filaments, and actin forms the thin filaments, and are arranged in repeating units called
sarcomere
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s. The interaction of both proteins results in muscle contraction.
The sarcomere is attached to other organelles such as the mitochondria by intermediate filaments in the cytoskeleton. The costamere attaches the sarcomere to the sarcolemma.
Every single organelle and macromolecule of a muscle fiber is arranged to ensure that it meets desired functions. The cell membrane is called the sarcolemma with the cytoplasm known as the sarcoplasm. In the sarcoplasm are the myofibrils. The myofibrils are long protein bundles about one micrometer in diameter. Pressed against the inside of the sarcolemma are the unusual flattened myonuclei. Between the myofibrils are the
mitochondria
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
.
While the muscle fiber does not have smooth endoplasmic cisternae, it contains sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the Calcium in biology, calcium ions needed to cause a muscle contraction. Periodically, it has dilated end sacs known as terminal cisternae. These cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infolding called a transverse tubule (T tubule). T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction. Together, two terminal cisternae and a transverse tubule form a triad (anatomy), triad.
Development
![Gray20](https://upload.wikimedia.org/wikipedia/commons/c/c0/Gray20.png)
All muscles are derived from paraxial mesoderm. During embryonic development in the process of somitogenesis the paraxial mesoderm is divided along the embryo's length to form somites, corresponding to the Segmentation (biology), segmentation of the body most obviously seen in the vertebral column.
Each somite has three divisions, sclerotome (which forms vertebra (anatomy), vertebrae), Dermatome (embryology), dermatome (which forms skin), and myotome (embryology), myotome (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form epaxial and hypaxial muscles, respectively. The only epaxial muscles in humans are the erector spinae muscles, erector spinae and small vertebral muscles, and are innervated by the dorsal rami of the spinal nerves. All other muscles, including those of the limbs are hypaxial, and innervated by the ventral rami of the spinal nerves.
During development, myoblasts (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation of
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
frameworks, usually formed from the somatic lateral plate mesoderm. Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongated multinucleated skeletal muscle cells.
Between the tenth and the eighteenth weeks of gestation, all muscle cells have fast myosin heavy chains; two myotube types become distinguished in the developing fetus – both expressing fast chains but one expressing fast and slow chains. Between 10 and 40 per cent of the fibers express the slow myosin chain.
Fiber types are established during embryonic development and are remodelled later in the adult by neural and hormonal influences.
The population of satellite cells present underneath the basal lamina is necessary for the postnatal development of muscle cells.
Function
The primary function of muscle is muscle contraction, contraction.
Following contraction, skeletal muscle functions as an endocrine organ by secreting myokines – a wide range of cytokines and other peptides that act as signalling molecules.
Myokines in turn are believed to mediate the health benefits of exercise. Myokines are secreted into the bloodstream after muscle contraction. Interleukin 6 (IL-6) is the most studied myokine, other muscle contraction-induced myokines include BDNF, FGF21, and Secreted Protein, Acidic and Rich in Cysteine, SPARC.
Muscle also functions to produce body heat. Muscle contraction is responsible for producing 85% of the body's heat.
This heat produced is as a by-product of muscular activity, and is mostly wasted. As a homeostatic response to extreme cold, muscles are signaled to trigger contractions of shivering in order to generate heat.
Contraction
![1008_Skeletal_Muscle_Contraction](https://upload.wikimedia.org/wikipedia/commons/2/24/1008_Skeletal_Muscle_Contraction.jpg)
Contraction is achieved by the muscle's structural unit, the muscle fiber, and by its functional unit, the motor unit.
Muscle fibers are excitable cells stimulated by motor neurons. The motor unit consists of a motor neuron and the many fibers that it makes contact with. A single muscle is stimulated by many motor units. Muscle fibers are
subject to depolarization by the neurotransmitter acetylcholine, released by the motor neurons at the neuromuscular junctions.
In addition to the
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
and
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
myofilament
Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togethe ...
s in the myofibrils that make up the contractile
sarcomere
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s, there are two other important regulatory proteins – troponin and tropomyosin, that make muscle contraction possible. These proteins are associated with actin and cooperate to prevent its interaction with myosin. Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum Calcium signaling, releases ionic calcium (Ca
2+), which then interacts with the regulatory protein troponin. Calcium-bound troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the myosin-binding sites on actin. This allows for myosin and actin ATP-dependent cross-bridge cycle, cross-bridge cycling and shortening of the muscle.
Excitation-contraction coupling
Excitation contraction coupling is the process by which a Action potential#Muscular action potentials, muscular action potential in the muscle fiber causes the myofibrils to contract. This process relies on a direct coupling between the sarcoplasmic reticulum calcium release channel RYR1 (ryanodine receptor 1), and Voltage-dependent calcium channel, voltage-gated L-type calcium channels (identified as dihydropyridine receptors, DHPRs). DHPRs are located on the sarcolemma (which includes the surface sarcolemma and the transverse tubules), while the RyRs reside across the SR membrane. The close apposition of a transverse tubule and two SR regions containing RyRs is described as a triad and is predominantly where excitation–contraction coupling takes place. Excitation–contraction coupling occurs when depolarization of skeletal muscle cell results in a muscle action potential, which spreads across the cell surface and into the muscle fiber's network of T-tubules, thereby depolarizing the inner portion of the muscle fiber. Depolarization of the inner portions activates dihydropyridine receptors in the terminal cisternae, which are close to ryanodine receptors in the adjacent Endoplasmic reticulum#Sarcoplasmic reticulum, sarcoplasmic reticulum. The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes (involving conformational changes that allosterically activates the ryanodine receptors). As the ryanodine receptors open, is released from the sarcoplasmic reticulum into the local junctional space and diffuses into the bulk cytoplasm to cause a calcium spark. Note that the sarcoplasmic reticulum has a large calcium buffering capacity partially due to a calcium-binding protein called calsequestrin. The near synchronous activation of thousands of calcium sparks by the action potential causes a cell-wide increase in calcium giving rise to the upstroke of the calcium transient. The released into the cytosol binds to Troponin C by the actin filaments, to allow crossbridge cycling, producing force and, in some situations, motion. The SERCA, sarco/endoplasmic reticulum calcium-ATPase (SERCA) actively pumps back into the sarcoplasmic reticulum. As declines back to resting levels, the force declines and relaxation occurs.
Muscle movement
The Efferent nerve fiber, efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement. Nerves move muscles in response to somatic nervous system, voluntary and autonomic nervous system, autonomic (involuntary) signals from the brain. Deep muscles, superficial muscles, Template:Muscles of head, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the human brain, brain, directly anterior to the central sulcus that divides the frontal and parietal lobes.
In addition, muscles react to reflex action, reflexive nerve stimuli that do not always send signals all the way to the brain. In this case, the signal from the afferent fiber does not reach the brain, but produces the reflexive movement by direct connections with the efferent nerves in the Spinal cord, spine. However, the majority of muscle activity is volitional, and the result of complex interactions between various areas of the brain.
Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor cortex of the brain's cerebral cortex. Commands are routed through the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal system contribute signals to influence muscle tone and response.
Deeper muscles such as those involved in Human position, posture often are controlled from nuclei in the brain stem and basal ganglia.
Proprioception
In skeletal muscles,
muscle spindle
Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be ...
s convey information about the degree of muscle length and stretch to the central nervous system to assist in maintaining posture and joint position. The sense of where our bodies are in space is called proprioception, the perception of body awareness, the "unconscious" awareness of where the various regions of the body are located at any one time. Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.
Energy consumption
![1016 Muscle Metabolism](https://upload.wikimedia.org/wikipedia/commons/f/f6/1016_Muscle_Metabolism.jpg)
Muscular activity accounts for much of the body's energy consumption. All muscle cells produce
adenosine triphosphate
Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP) molecules which are used to power the movement of the
myosin head
The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a he ...
s. Muscles have a short-term store of energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of
glycogen
Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body.
Glycogen functions as one o ...
. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume lactic acid produced and excreted by skeletal muscles during exercise.
Skeletal muscle uses more calories than other organs.
At rest it consumes 54.4 kJ/kg (13.0 kcal/kg) per day. This is larger than adipose tissue (fat) at 18.8 kJ/kg (4.5 kcal/kg), and bone at 9.6 kJ/kg (2.3 kcal/kg).
Efficiency
The mechanical efficiency, efficiency of human muscle has been measured (in the context of Watercraft rowing, rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolism, metabolic cost, as can be calculated from oxygen consumption. This low efficiency is the result of about 40% efficiency of generating Adenosine triphosphate, ATP from food energy, losses in converting energy from ATP into mechanical work inside the muscle, and mechanical losses inside the body. The latter two losses are dependent on the type of exercise and the type of muscle fibers being used (fast-twitch or slow-twitch). For an overall efficiency of 20 percent, one watt of mechanical power is equivalent to 4.3 kcal per hour. For example, one manufacturer of rowing equipment calibrates its Indoor rower, rowing ergometer to count burned calories as equal to four times the actual mechanical work, plus 300 kcal per hour, this amounts to about 20 percent efficiency at 250 watts of mechanical output. The mechanical energy output of a cyclic contraction can depend upon many factors, including activation timing, muscle strain trajectory, and rates of force rise & decay. These can be synthesized experimentally using Work Loop, work loop analysis.
Muscle strength
Muscle strength is a result of three overlapping factors: ''physiological strength'' (muscle size, cross sectional area, available crossbridging, responses to training), ''neurological strength'' (how strong or weak is the signal that tells the muscle to contract), and ''mechanical strength'' (muscle's force angle on the lever, moment arm length, joint capabilities).
Vertebrate muscle typically produces approximately of force per square centimeter of muscle cross-sectional area when isometric and at optimal length. Some invertebrate muscles, such as in crab claws, have much longer
sarcomere
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
s than vertebrates, resulting in many more sites for actin and myosin to bind and thus much greater force per square centimeter at the cost of much slower speed. The force generated by a contraction can be measured non-invasively using either mechanomyography or phonomyography, be measured in vivo using tendon strain (if a prominent tendon is present), or be measured directly using more invasive methods.
The strength of any given muscle, in terms of force exerted on the skeleton, depends upon Muscle contraction#Force-length and force-velocity relationships, length, shortening speed, cross sectional area, Pennate muscle, pennation,
sarcomere
A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called musc ...
length,
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
isoforms, and neural activation of motor units. Significant reductions in muscle strength can indicate underlying pathology, with the chart at right used as a guide.
The ''maximum holding time'' for a contracted muscle depends on its supply of energy and is stated by Rohmert's law to Exponential decay, exponentially decay from the beginning of exertion.
The "strongest" human muscle
Since three factors affect muscular strength simultaneously and muscles never work individually, it is misleading to compare strength in individual muscles, and state that one is the "strongest". But below are several muscles whose strength is noteworthy for different reasons.
* In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of for 2 seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.
* If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. This is because the tension exerted by an individual skeletal muscle fiber does not vary much. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the Quadriceps, quadriceps femoris or the
gluteus maximus
The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. It is the single largest muscle in the human ...
.
* Because muscle strength is determined by cross-sectional area, a shorter muscle will be stronger "pound for pound" (i.e., by mass, weight) than a longer muscle of the same cross-sectional area. The myometrial layer of the uterus may be the strongest muscle by weight in the female body. At the time when an infant is delivered, the entire uterus weighs about 1.1 kg (40 oz). During childbirth, the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force with each contraction.
* The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the human eyeball, eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." However, eye movements (particularly saccades used on facial scanning and reading) do require high speed movements, and eye muscles are exercised nightly during rapid eye movement sleep.
* The statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that the tongue consists of eight muscles, not one.
Force generation
Muscle architecture#Force generation, Muscle force is proportional to
physiological cross-sectional area
In muscle physiology, physiological cross-sectional area (PCSA) is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point. It is typically used to describe the contraction properties of pennate muscl ...
(PCSA), and muscle velocity is proportional to muscle fiber length. The torque around a joint, however, is determined by a number of biomechanical parameters, including the distance between muscle insertions and pivot points, muscle size and architectural gear ratio. Muscles are normally arranged in opposition so that when one group of muscles contracts, another group relaxes or lengthens.
Antagonism in the transmission of nerve impulses to the muscles means that it is impossible to fully stimulate the contraction of two Antagonist (muscle), antagonistic muscles at any one time. During ballistic motions such as throwing, the antagonist muscles act to 'brake' the Agonist (muscle), agonist muscles throughout the contraction, particularly at the end of the motion. In the example of throwing, the chest and front of the shoulder (anterior deltoid) contract to pull the arm forward, while the muscles in the back and rear of the shoulder (posterior deltoid) also contract and undergo eccentric contraction to slow the motion down to avoid injury. Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder.
Contracting muscles produce vibration and sound. Slow twitch fibers produce 10 to 30 contractions per second (10 to 30 Hz). Fast twitch fibers produce 30 to 70 contractions per second (30 to 70 Hz).
Peak Performance – Endurance training: understanding your slow twitch muscle fibers will boost performance The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist. The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example. The sound is usually described as a rumbling sound. Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear. The rumbling sound can also be heard when the neck or jaw muscles are highly tensed.
Signal transduction pathways
Skeletal muscle fiber-type phenotype in adult animals is regulated by several independent signaling pathways. These include pathways involved with the Ras subfamily, Ras/mitogen-activated protein kinase (MAPK) pathway, calcineurin, calcium/calmodulin-dependent protein kinase IV, and the peroxisome proliferator γ coactivator 1 (PGC-1). The MAPK/ERK pathway, Ras/MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve-dependent induction of the slow program in regenerating muscle. Calcineurin, a Ca2+/calmodulin-activated phosphatase implicated in nerve activity-dependent fiber-type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow-type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins. Ca2+/calmodulin-dependent protein kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow-type calcineurin-generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis.
Contraction-induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium/calmodulin-dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle.
PGC1-α (PPARGC1A), a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective slow twitch (ST) muscle genes and also serves as a target for calcineurin signaling. A peroxisome proliferator-activated receptor δ (PPARδ)-mediated transcriptional pathway is involved in the regulation of the skeletal muscle fiber phenotype. Mice that harbor an activated form of PPARδ display an "endurance" phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers. Thus—through functional genomics—calcineurin, calmodulin-dependent kinase, PGC-1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against insulin resistance and obesity.
The transition from aerobic to anaerobic metabolism during intense work requires that several systems are rapidly activated to ensure a constant supply of ATP for the working muscles. These include a switch from fat-based to carbohydrate-based fuels, a redistribution of blood flow from nonworking to exercising muscles, and the removal of several of the by-products of anaerobic metabolism, such as carbon dioxide and lactic acid. Some of these responses are governed by transcriptional control of the fast twitch (FT) glycolytic phenotype. For example, skeletal muscle reprogramming from an ST glycolytic phenotype to an FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family. Moreover, the hypoxia-inducible factor 1-α (HIF1A) has been identified as a master regulator for the expression of genes involved in essential hypoxic responses that maintain ATP levels in cells. Ablation of HIF-1α in skeletal muscle was associated with an increase in the activity of rate-limiting enzymes of the mitochondria, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway in these animals. However, hypoxia-mediated HIF-1α responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria.
Other pathways also influence adult muscle character. For example, physical force inside a muscle fiber may release the transcription factor serum response factor from the structural protein titin, leading to altered muscle growth.
Exercise
Physical exercise is often recommended as a means of improving motor skills, physical fitness, fitness, muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
, bone, and the nerves that stimulate the muscles. One such effect is muscle hypertrophy, an increase in size of muscle due to an increase in the number of muscle fibers or cross-sectional area of myofibrils. Muscle changes depend on the type of exercise used.
Generally, there are two types of exercise regimes, aerobic and anaerobic. Aerobic exercise (e.g. marathons) involves activities of low intensity but long duration, during which the muscles used are below their maximal contraction strength. Aerobic activities rely on aerobic respiration (i.e. citric acid cycle and electron transport chain) for metabolic energy by consuming fat, protein, carbohydrates, and oxygen. Muscles involved in aerobic exercises contain a higher percentage of Type I (or slow-twitch) muscle fibers, which primarily contain mitochondrial and oxidation enzymes associated with aerobic respiration. On the contrary, anaerobic exercise is associated with activities of high intensity but short duration, such as sprinting or weight training, weight lifting. The anaerobic activities predominately use Type II, fast-twitch, muscle fibers. Type II muscle fibers rely on glucogenesis for energy during anaerobic exercise. During anaerobic exercise, type II fibers consume little oxygen, protein and fat, produce large amounts of lactic acid and are fatigable. Many exercises are partially aerobic and anaerobic; for example, soccer and rock climbing.
The presence of lactic acid has an inhibitory effect on ATP generation within the muscle. It can even stop ATP production if the intracellular concentration becomes too high. However, endurance training mitigates the buildup of lactic acid through increased capillarization and myoglobin. This increases the ability to remove waste products, like lactic acid, out of the muscles in order to not impair muscle function. Once moved out of muscles, lactic acid can be used by other muscles or body tissues as a source of energy, or transported to the liver where it is converted back to pyruvate. In addition to increasing the level of lactic acid, strenuous exercise results in the loss of potassium ions in muscle. This may facilitate the recovery of muscle function by protecting against fatigue.
Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and generally subsides two to three days later. Once thought to be caused by lactic acid build-up, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.
Clinical significance
Muscle disease
Diseases of skeletal muscle are termed myopathies, while diseases of nerves are called neuropathies. Both can affect muscle function or cause muscle pain, and fall under the umbrella of neuromuscular disease. The cause of many myopathies is attributed to mutations in the various associated muscle proteins. Some inflammatory myopathies include polymyositis and inclusion body myositis
Neuromuscular diseases affect the muscles and their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A number of movement disorders are caused by neurological disorders such as Parkinson's disease and Huntington's disease where there is central nervous system dysfunction.
Symptoms of muscle diseases may include Muscle weakness, weakness, spasticity, myoclonus and myalgia. Diagnostic procedures that may reveal muscular disorders include testing Creatine kinase#Laboratory testing, creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). In some cases, muscle biopsy may be done to identify a myopathy, as well as genetic testing to identify DNA abnormalities associated with specific myopathies and Muscular dystrophy, dystrophies.
A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound produced by a muscle comes from the shortening of actomyosin Protein filament, filaments along the axis of the muscle. During Muscle contraction, contraction, the muscle shortens along its length and expands across its width, producing vibrations at the surface.
Hypertrophy
Independent of strength and performance measures, muscles can be induced to grow larger by a number of factors, including hormone signaling, developmental factors, strength training, and disease. Contrary to popular belief, the number of muscle fibres cannot be increased through Physical exercise, exercise. Instead, muscles grow larger through a combination of muscle cell growth as new protein filaments are added along with additional mass provided by undifferentiated satellite cells alongside the existing muscle cells.
Biological factors such as age and hormone levels can affect muscle hypertrophy. During puberty in males, hypertrophy occurs at an accelerated rate as the levels of growth-stimulating hormones produced by the body increase. Natural hypertrophy normally stops at full growth in the late teens. As testosterone is one of the body's major growth hormones, on average, men find hypertrophy much easier to achieve than women. Taking additional testosterone or other anabolic steroids will increase muscular hypertrophy.
Muscular, spinal and neural factors all affect muscle building. Sometimes a person may notice an increase in strength in a given muscle even though only its opposite has been subject to exercise, such as when a bodybuilder finds her left biceps stronger after completing a regimen focusing only on the right biceps. This phenomenon is called cross education.
Atrophy
Every day between one and two percent of muscle is broken down and rebuilt. Physical inactivity, Inactivity, malnutrition, disease, and aging can increase the breakdown leading to muscle atrophy or sarcopenia. Sarcopenia is commonly an age-related process that can cause frailty syndrome, frailty and its consequences. A decrease in muscle mass may be accompanied by a smaller number and size of the muscle cells as well as lower protein content.
Spaceflight, Human spaceflight, involving prolonged periods of immobilization and weightlessness is known to result in muscle weakening and atrophy resulting in a loss of as much as 30% of mass in some muscles. Such consequences are also noted in some mammals following hibernation.
Many diseases and conditions including cancer, AIDS, and heart failure can cause muscle loss known as cachexia.
Research
Myopathies have been modeled with cell culture systems of muscle from healthy or diseased tissue Biopsy, biopsies. Another source of skeletal muscle and progenitors is provided by the directed differentiation of Induced pluripotent stem cells, pluripotent stem cells.[ ]
Research on skeletal muscle properties uses many techniques. Electrical muscle stimulation is used to determine force and contraction speed at different frequencies related to fiber-type composition and mix within an individual muscle group. In vitro muscle testing is used for more complete characterization of muscle properties.
The electrical activity associated with muscle contraction is measured via electromyography (EMG). Skeletal muscle has two physiological responses: relaxation and contraction. The mechanisms for which these responses occur generate electrical activity measured by EMG. Specifically, EMG can measure the action potential of a skeletal muscle, which occurs from the Hyperpolarization (biology), hyperpolarization of the motor axons from nerve impulses sent to the muscle. EMG is used in research for determining if the skeletal muscle of interest is being activated, the amount of force generated, and an indicator of muscle fatigue. The two types of EMG are intra-muscular EMG and the most common, surface EMG. The EMG signals are much greater when a skeletal muscle is contracting verses relaxing. However, for smaller and deeper skeletal muscles the EMG signals are reduced and therefore are viewed as a less valued technique for measuring the activation. In research using EMG, a muscle contraction, maximal voluntary contraction (MVC) is commonly performed on the skeletal muscle of interest, to have reference data for the rest of the EMG recordings during the main experimental testing for that same skeletal muscle.
Research into the development of artificial muscles includes the use of electroactive polymers.
See also
* Facioscapulohumeral muscular dystrophy
* Hill's muscle model
* In vitro muscle testing
* Musculoskeletal injury
* Muscle relaxant
* Microtrauma
* Muscle memory
* Myomere
* Myotomy
* Preflexes
References
{{DEFAULTSORT:Skeletal Muscle
Skeletal muscle,
Muscular system
Somatic motor system
Muscle tissue