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A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of
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 a ...
, and represents about 30% of ( )
fibers Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
going to the
muscle 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 mus ...
. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
. They receive input from the
reticular formation The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formatio ...
of the
pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of Va ...
in the
brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...
. Their
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
s are smaller than those of the alpha motor neurons, with a diameter of only 5 
μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles. The presence of
myelination Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be ...
in gamma motor neurons allows a conduction velocity of 4 to 24 meters per second, significantly faster than with non-myelinated axons but slower than in alpha motor neurons.


General background of muscles


Muscle spindles

Muscle spindles are the
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 ...
s located within muscles that allow communication to the spinal cord and brain with information of where the body is in space (proprioception) and how fast body limbs are moving with relation to space (
velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
). They are
mechanoreceptors A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, a ...
in that they respond to stretch and are able to signal changes in muscle length. The sensitivity of detecting changes in muscle length are adjusted by fusimotor neurons – gamma and beta motor neurons. Muscle spindles can be made up of three different types of muscle fibers: dynamic
nuclear bag fiber A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has many nuclei concentrated in bags and they cause excitation of the primary sensory fibers. There are two kinds of bag fibers based u ...
s (bag1 fibers), static nuclear bag fibers (bag2 fibers), and
nuclear chain fiber A nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers that, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle le ...
s.


Types of lower motor neurons

Muscle spindles are innervated by both sensory neurons and
motor neurons A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...
in order to provide
proprioception 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 ...
and make the appropriate movements via firing of motor neurons. There are three types of lower motor neurons involved in
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 a ...
: alpha motor neurons, gamma motor neurons, and beta motor neurons. Alpha motor neurons, the most abundant type, are used in the actual force for muscle contraction and therefore innervate
extrafusal muscle fiber Extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal striated muscle ti ...
s (muscle fibers outside of the muscle spindle). Gamma motor neurons, on the other hand, innervate only
intrafusal muscle fiber Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs ( proprioceptors). They detect the amount and rate of change in length of a muscle.Casagrand, Janet (2008) ''Action and Movement: Spinal Control o ...
s (within the muscle spindle), whereas beta motor neurons, which are present in very low amounts, innervate both intrafusal and extrafusal muscle cells. Beta motor neurons have a conduction velocity greater than that of both other types of lower motor neurons, but there is little currently known about beta motor neurons. Alpha motor neurons are highly abundant and larger in size than gamma motor neurons.


Alpha gamma co-activation

When the central nervous system sends out signals to alpha neurons to fire, signals are also sent to gamma motor neurons to do the same. This process maintains the tautness of muscle spindles and is called alpha gamma co-activation. The nuclei of spindle muscle cells are located in the middle of these spindles, but unlike extrafusal muscle fibers, the myofibril contractile apparati of spindle fibers are located only at both ends of spindle. Efferent stimulation of the spindle by gamma motor neurons contracts the myofibrils, tautening the central region of spindle—which maintains the muscle spindle's sensitivity to muscle's length change. Without gamma motor neurons, muscle spindles would be very loose as the muscle contracts more. This does not allow for muscle spindles to detect a precise amount of stretch since it is so limp. However, with alpha gamma co-activation and both alpha and gamma neurons firing, muscle fibers within the muscle spindles are pulled parallel to the extrafusal contraction causing the muscle movement. The firing of gamma motor neurons in sync with alpha motor neurons pulls muscle spindles from polar ends of the fibers as this is where gamma motor neurons
innervate A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...
the muscle. The spindle is innervated by
type Ia sensory fiber A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch cha ...
that go on to synapse with alpha motor neurons, completing the gamma-loop. The parallel pulling keeps muscle spindles taut and readily able to detect minute changes in stretch.


Fusimotor system

The central nervous system controls muscle spindle sensitivity via the fusimotor system that consists of muscle spindles along with gamma motor neurons also called fusimotor neurons. Beta motor neurons innervate extrafusal as well as intrafusal muscle fibers, and are more specifically named ''skeletofusimotor neurons''. Gamma motor neurons are the efferent (sending signals away from the central nervous system) part of the fusimotor system, whereas muscle spindles are the afferent part, as they send signals relaying information from muscles toward the spinal cord and brain.


Gamma bias

Gamma bias is gamma motor neurons' consistent level of activity. Smaller neurons require a smaller amount of excitatory input to reach its threshold compared to larger neurons. Therefore, gamma motor neurons (smaller in size than alpha motor neurons) are more likely to fire than the larger alpha motor neurons. This creates a situation with relatively few alpha motor neurons firing but some gamma motor neurons constantly firing in conditions where muscle stretch or force is not occurring. The sensitivity of sensory endings (primary and secondary endings - Ia, II) of the muscle spindle are based on the level of gamma bias (i.e. how much background level of gamma motor neuron discharge is taking place.)Butler, M.E. (1985) "Spasticity: A consideration in rehabilitation of the elderly" Rehabilitation Nursing 10(3): 14-19.


Types


Static

Static gamma motor neurons innervate static nuclear bag fibers (bag2 fibers), a type of
nuclear bag fiber A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has many nuclei concentrated in bags and they cause excitation of the primary sensory fibers. There are two kinds of bag fibers based u ...
and
nuclear chain fiber A nuclear chain fiber is a specialized sensory organ contained within a muscle. Nuclear chain fibers are intrafusal fibers that, along with nuclear bag fibers, make up the muscle spindle responsible for the detection of changes in muscle le ...
s. Both of these fiber types are part of the intrafusal muscle spindle fibers, where the static gamma motor neurons innervate onto. Nuclear chain fibers' nuclei are organized in longitudinal columns, which is where it gets its name from, whereas the nuclear bag fibers' nuclei are clumped in the midsection of the muscle spindle. There is approximately a 2:1 ratio of nuclear chain fibers to nuclear bag fibers. The static gamma motor neurons increase their firing, in response to an increase in magnitude of change in length and controls the static sensitivity of the stretch reflex. For this reason, this type of gamma motor neuron is mostly used in the maintenance of postures and slower movements such as lifting a box, rather than activities requiring rapid changes due to rapid change in muscle length.


Dynamic

Dynamic gamma motor neurons innervate the dynamic nuclear bag fibers (bag1 fibers), another type of nuclear bag fiber smaller than the static nuclear bag fibers. This type of gamma motor neuron can enhance the sensitivities of Ia sensory neurons. It is done so because the dynamic nuclear bag fibers, which are innervated by the dynamic gamma motor neurons, receive Ia sensory innervation. Furthermore, the firing of dynamic gamma motor neurons removes the slack in dynamic nuclear bags, bringing Ia fibers closer to the firing threshold. Dynamic gamma motor neurons alter muscle spindle sensitivity and increases its discharge in response to
velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
, the rate of change, of muscle length rather than simply the magnitude as it is with static gamma motor neurons. Therefore, this type of gamma motor neuron can be used for activities requiring quick changes in muscle length to adjust such as balancing on a rail.


Effects of nuclear chain fibers

The effect of nuclear chain fibers on primary endings is to drive the discharge up to a
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of around 60 Hz in a linear fashion, above which the discharge can become irregular. The activities of bag2 fibers show an initial sharp peak in discharge, which gets less as the receptor adapts. Bag2 fibers also reduce the dynamic sensitivity of the
Ia afferent A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch chang ...
and sometimes also reduce the length sensitivity. Activation of bag1 fibers has the effect of increasing both the length sensitivity and the dynamic sensitivity of the primary ending.Boyd I (1980) The action of the three types of intrafusal fiber in isolated cat muscle spindles on the dynamic and length sensitivities of primary and secondary sensory endings. In: Muscle Receptors and Movement (Taylor A, Prochazka A, eds), pp 17 - 32. London: MacMillan. It is believed that the secondary sensory endings serve to measure length and
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 a ...
s of nuclear chain fibers at the pole via the static γ-motoneurons both excite the ending and increase its length sensitivity. Bag1 and bag2 fibers receive very little innervation from secondary endings, and activation of these fibers has a minimal effect on the discharge of the secondary ending.


Development

Gamma motor neurons develop similarly to alpha motor neurons at the beginning. They originate in the basal plate, which is the ventral portion of the
neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, ...
in the developing
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
.
Sonic hedgehog Sonic hedgehog protein (SHH) is encoded for by the ''SHH'' gene. The protein is named after the character ''Sonic the Hedgehog''. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organogenesis a ...
genes (Shh) are an important part of the development process that is secreted by the
notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...
creating gradients of concentrations. After the hedgehog genes, various other molecular markers and transcription factors play a role in differentiating motor neurons into the specific gamma motor neurons. Gamma motor neurons, like all cells, express specific genetic markers at birth. Muscle spindle derived
GDNF Glial cell line-derived neurotrophic factor (GDNF) is a protein that, in humans, is encoded by the ''GDNF'' gene. GDNF is a small protein that potently promotes the survival of many types of neurons. It signals through GFRα receptors, particul ...
neurotrophic factors Neurotrophic factors (NTFs) are a family of biomolecules – nearly all of which are peptides or small proteins – that support the growth, survival, and differentiation of both developing and mature neurons. Most NTFs exert their trop ...
must also be present for postnatal survival. Wnt7A is a secreted signaling molecule selectively in gamma motor neurons by embryonic day 17.5 of mice. This is the earliest molecule present in gamma motor neurons that differentiates them from alpha motor neurons, illustrating the divergence of these two types of lower motor neurons. In addition, serotonin receptor 1d (5-ht 1d) has been concluded to be a novel marker for gamma motor neurons enabling researchers to distinguish between the various types of lower
motor neurons A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...
. Mice lacking this serotonin receptor 1d, displayed lower monosynaptic reflex (a reflex arc involving only a sensory and motor neuron), which may be caused by a reduced response to sensory stimulation in motor neurons. In addition,
knockout A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, muay thai, mixed martial arts, karate, some forms of taekwondo and other sports involving strikin ...
mice without this serotonin receptor exhibited more coordination on a balance beam task, suggesting that less activation of motor neurons by Ia afferents during movement could reduce the unnecessary excess of muscle output. Another distinguishing molecular marker of gamma motor neurons is transcription factor Err3. It is expressed at high levels in gamma motor neurons, but very little in alpha motor neurons. On the other hand, neuronal DNA binding protein NeuN, are present in significantly greater quantities in alpha motor neurons. Osteopontin, a protein also expressed in bones, hence the "osteo-" prefix, is a marker for alpha motor neurons. This in turn can provide scientists a way of eliminating gamma motor neurons if alpha motor neurons are of interest. One study in particular made this conclusion based on the fact that osteopontin was present in larger cell bodies, indicating the alpha motor neurons as they have larger cell bodies than gamma motor neurons.


Muscle tone

Although muscles can be in a relaxed state, muscles have a general resting level of tension. This is termed
muscle tone In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state.O’Sullivan, S. B. (2007). ...
and is maintained by the motor neurons innervating the muscle. Its purpose is to maintain posture and assist in quicker movements, since if muscles were completely loose, then more neuronal firing would need to take place. The amount of tension in the muscles depends primarily on the resting level discharge of alpha motor neurons and Ia spindle afferents. Gamma motor neurons are also involved through their action on intrafusal muscle fibers. The intrafusal muscle fibers control the resting level of the Ia afferent pathway, which in turn creates a steady level of alpha neuron activity. Muscle tone can also be due to tonic discharge of gamma motor neurons. The activation to these neurons are mostly from the descending fibers of the facilitatory
reticular formation The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formatio ...
.Khurana, Indu. (2006) Textbook of Medical Physiology. Elsevier. Chapter 10. pg 1076. This leads to the stretching of muscle spindle, activation of alpha motor neurons and finally a partially contracted muscle. The cerebellum is the alpha-gamma motor neuron linkage . Therefore, with the
cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebe ...
the muscle tension is maintained via alpha motor neurons as well as the gamma motor neurons.


Abnormal activity

Hypotonia Hypotonia is a state of low muscle tone (the amount of tension or resistance to stretch in a muscle), often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases ...
can be due to damage to alpha neurons or Ia afferents carrying sensory information to the alpha neurons. This creates a decrease in muscle tone. Opposite to this, hypertonia is caused by damage to descending pathways that terminate in the spinal cord. It increases muscle tone by increasing the total responsiveness of alpha motor neurons from its Ia sensory input. Spasms can be caused by a disparity between how much alpha and gamma motor neurons are firing, i.e. too much gain of one or the other. The imbalance causes an inaccurate reading from muscle receptors in the muscle spindle. Therefore, the sensory neurons feeding back to the brain and spinal cord are misleading. For example, if a patient has over active gamma motor neurons, there will be a resistance to passive movement causing stiffness, also called spasticity. This is often found in individuals with damage to higher centers affecting the descending pathways. This can sometimes cause a gamma-bias (constant discharge of some gamma motor neurons) to be greater or less than usual. In the case for patients with excess gamma bias, the sensory endings within muscle spindles are discharging too frequently causing there to be more muscle activity than appropriate. Furthermore, this hyperactivity in the gamma spindle loop can cause spasticity. Gamma motor neurons assist in keeping the muscle spindle taut, thus adjusting sensitivity. Therefore, if proper gamma motor neuronal firing does not occur, muscle movement can be adversely affected.
Fine motor skills Fine motor skill (or dexterity) is the coordination of small muscles in movement with the eyes, hands and fingers. The complex levels of manual dexterity that humans exhibit can be related to the nervous system. Fine motor skills aid in the grow ...
such as movements with the fingers and eyes are affected most, since any lack of tautness within the muscle spindle hinders its ability to detect the amount of stretch through the sensory endings. This means that the muscle will not be able to precisely move accordingly.
Lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classif ...
s controlling descending pathways in lower motor neurons to the upper limbs, can cause a loss in patient's ability to have fine movement control. In clinical settings, it is possible to test whether someone has an abnormally low or high gamma gain simply by moving the patients arm. Gamma gain is the process where acceleration, velocity, and length of muscle changes are scaled up equally, enabling more accurate movements to take place in the appropriate situation. If it is more difficult to bend a patients arm at the elbow back and forth, then he/she has higher gamma gain while someone whose arm moves very easily will have lower gamma gain. Oscilloscopes can be used to measure action potentials of an axon from a motor neuron in order to assess general muscle activity. Though it cannot distinguish alpha motor neurons from gamma motor neurons, it is useful in understanding whether one has abnormal motor neuron activity. With low rates of activity of the descending pathway, fewer and smaller motor neurons are activated, leading to a small amount of muscle force. This will appear on the oscilloscope as lower peaks on the y-axis.


References


External links

*
NIF Search - Gamma Motor Neuron
via the Neuroscience Information Framework {{Nervous tissue Efferent neurons Somatic motor system