biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology

, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous movement in response to a

stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation Stimulation is the encouragement of development or the cause of activity generally. For example, "The press provides stimulation of political discourse." ...

. A reflex is made possible by

neural pathway A neural pathway is the connection formed by axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term dialect (from Lat ...

s called

reflex arc A reflex arc is a neural pathway A neural pathway is the connection formed by axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling di ...

s which can act on an impulse before that impulse reaches the brain. The reflex is then an automatic response to a stimulus that does not receive or need conscious thought.

Types of human reflexes

Myotatic reflexes

The myotatic or muscle

stretch reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction Muscle contraction is the activation of tension Tension may refer to: Science * Psychological stress * Tension (physics), a force related to ...

es (sometimes known as ''deep tendon reflexes'') provide information on the integrity of the

central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecu ...

and

peripheral nervous system The peripheral nervous system (PNS) is one of two components that make up the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, ...

. Generally, decreased reflexes indicate a peripheral problem, and lively or exaggerated reflexes a central one. A stretch reflex is the contraction of a muscle in response to its lengthwise stretch. *

Biceps reflex Biceps reflex is a reflex test that examines the function of the C5 reflex arc and the C6 reflex arc. The test is performed by using a tendon hammer A tendon or sinew is a tough band of fibrous connective tissue Connective tissue is one of the ...

( C5, C6) *

Brachioradialis reflex The brachioradialis reflex (also known as supinator reflex'' is observed during a neurological exam by striking the brachioradialis tendon (at its insertion at the base of the wrist into the radial styloid process (radial side of wrist around 4 inc ...

(C5, C6, C7) *

Extensor digitorum reflexThe Extensor digitorum reflex is tested as part of the neurological examination A neurological examination is the assessment of sensory neuron and motor neuron, motor responses, especially reflexes, to determine whether the nervous system is impaired ...

(C6, C7) *

Triceps reflexThe triceps reflex, a deep tendon reflex, is a reflex as it elicits involuntary contraction of the triceps brachii muscle The triceps, also triceps brachii (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic ...

(C6, C7, C8) *

Patellar reflex The patellar reflex or knee-jerk (in American English knee reflex) is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from t ...

or knee-jerk reflex ( L2, L3, L4) *

Ankle jerk reflexThe ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsiflexion, dorsiflexed. It is a type of stretch reflex that tests the function of the gastrocnemius muscle and the nerve that su ...

(Achilles reflex) ( S1, S2) While the reflexes above are stimulated mechanically, the term

H-reflexThe H-reflex (or Hoffmann's reflex) is a reflectory reaction of muscles after electrical stimulation of sensory fibers (Ia afferents stemming from muscle spindles) in their innervating nerves (for example, those located behind the knee). The H-refl ...

refers to the analogous reflex stimulated electrically, and

tonic vibration reflexTonic vibration reflex is a sustained contraction of a muscle subjected to vibration. This reflex is caused by vibratory activation of muscle spindles — muscle receptors sensitive to stretch. Tonic vibration reflex is evoked by placing a Vibrato ...

for those stimulated to vibration.

Tendon reflex

tendon reflex Tendon reflex (or T-reflex) may refer to: *The stretch reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction Muscle contraction is the activation of tension Tension may refer to: Scienc ...

is the contraction of a muscle in response to striking its tendon. The

Golgi tendon reflex Golgi may refer to: *Camillo Golgi Camillo Golgi (; 7 July 184321 January 1926) was an Italians, Italian biologist and pathologist known for his works on the central nervous system. He studied medicine at the University of Pavia (where he later s ...

is the inverse of a stretch reflex.

Reflexes involving cranial nerves

Reflexes usually only observed in human infants

Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes. These automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include: *

Asymmetrical tonic neck reflex The asymmetrical tonic neck reflex (ATNR) is a primitive reflexPrimitive reflexes are reflex action In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry ...

(ATNR) *

Palmomental reflexThe palmomental reflex (PMR) is a primitive reflex consisting of a twitch of the chin muscle elicited by stroking a specific part of the palm. It is present in infancy and disappears as the brain matures during childhood but may reappear due to proc ...

Moro reflex Moro reflex while sleeping The Moro reflex is an infantile reflex that develops between 28 and 32 weeks of gestation and disappears at 3–6 months of age. It is a response to a sudden loss of support and involves three distinct components: # spr ...

, also known as the startle reflex *

Palmar grasp reflex Palmar grasp reflex (or grasp reflex) is a Primitive reflexes, primitive and involuntary reflex found in infants of humans and most primates. When an object, such as an adult finger, is placed in an Infant, infant's palm, the infant's fingers refl ...

Rooting reflexPrimitive reflexes are reflex actions originating in the central nervous system that are exhibited by normal infants, but not neurologically intact adults, in response to particular stimuli. These reflexes are suppressed by the development of the ...

Sucking reflexPrimitive reflexes are reflex actions originating in the central nervous system that are exhibited by normal infants, but not neurologically intact adults, in response to particular Stimulus (physiology), stimuli. These reflexes are suppressed by the ...

* Symmetrical tonic neck reflex (STNR) * Tonic labyrinthine reflex (TLR)

Other kinds of reflexes

Other reflexes found in the central nervous system include: * Abdominal reflexes (T6-L1) * Gastrocolic reflex * Anocutaneous reflex (S2-S4) *

Baroreflex The baroreflex or baroreceptor reflex is one of the body's homeostasis, homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressur ...

Cough reflex The cough reflex has both sensory ( afferent) and motor ( efferent) components, the former mainly via the vagus nerve The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve Cranial nerves are the nerve A ...

Cremasteric reflex Cremasteric Reflex The cremasteric reflex is a superficial (i.e., close to the skin's surface) reflex observed in human males. This reflex is elicited by lightly stroking or poking the superior and medial (inner) part of the thigh—regardless of ...

(L1-L2) *

Diving reflex The diving reflex, also known as the diving response and mammalian diving reflex, is a set of physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology i ...

* Muscular defense *

Photic sneeze reflex The photic sneeze reflex (also known as Autosomal Dominant Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome or photosneezia, from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el ...

Scratch reflex The scratch reflex is a response to activation of sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an ...

Sneeze A sneeze (also known as sternutation) is a semi-autonomous, convulsive A convulsion is a medical condition where body muscles Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecule ...

Startle reflex In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative Affect (psychology), affect.Rammirez-Moreno, Da ...

Withdrawal reflexThe withdrawal reflex (nociceptive flexion reflex or flexor withdrawal reflex) is a spinal reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. ...

** Crossed extensor reflex Many of these reflexes are quite complex requiring a number of synapses in a number of different nuclei in the

CNS

(e.g., the

escape reflex Escape reflex, or escape behavior, is any kind of escape response In animal behaviour, escape response, escape reaction, or escape behaviour is a rapid series of movements performed by an animal in response to possible predation. Some types of escap ...

). Others of these involve just a couple of synapses to function (e.g., the

withdrawal reflexThe withdrawal reflex (nociceptive flexion reflex or flexor withdrawal reflex) is a spinal reflex The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching within the muscle. ...

). Processes such as

breath Breathing (or ventilation) is the process of moving air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of th ...

breath

ing,

digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such a ...

, and the maintenance of the

heartbeat Heartbeat or heartbeats may refer to: Physiology *Cardiac cycle, of the heart *Contraction of the cardiac muscle, muscles of the heart, or a perceived effect of it, such as: **Heart sounds, the noises generated by the beating heart and the resulta ...

can also be regarded as reflex actions, according to some definitions of the term.

Grading

medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge), skills (proced ...

, reflexes are often used to assess the health of the

nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

Doctors Doctor or The Doctor may refer to: Personal titles * Doctor (title), the holder of an accredited academic degree * A medical practitioner, including: ** Physician ** Surgeon *Other roles ** Doctor of the Church, a title given to those with g ...

Doctors

will typically grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+.

Reflex modulation

Naively, we might imagine that reflexes are immutable. In reality, however, most reflexes are flexible and can be substantially modified to match the requirements of the behavior in both vertebrates and invertebrates. A good example of reflex modulation is the

. When a muscle is stretched at rest, the stretch reflex leads to contraction of the muscle, thereby opposing stretch (resistance reflex). This helps to stabilize posture. During voluntary movements, however, the intensity (gain) of the reflex is reduced or its sign is even reversed. This prevents resistance reflexes from impeding movements. The underlying sites and mechanisms of reflex modulation are not fully understood. There is evidence that the output of sensory neurons is directly modulated during behavior—for example, through

presynaptic inhibitionPresynaptic inhibition is an inhibitory input to a neuron to make it less likely to fire an action potential and communicate with downstream neurons. Inhibition can be provided both at the postsynapse (IPSP) and the presynapse. Presynaptic inhibition ...

. The effect of sensory input upon motor neurons is also influenced by interneurons in 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. It encloses the central canal of the spinal cord, which contain ...

ventral nerve cord The ventral nerve cord (VNC) is a major structure of the invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all ani ...

and by descending signals from the brain.

References

{{Authority control Reflexes Animal physiology