Reflex arc

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. However, the brain will receive the sensory input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). However, autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord.[1]

During a somatic reflex, nerve signals travel along the following pathway:[1]

  1. Somatic receptors in the skin, muscles and tendons
  2. Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem
  3. An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse
  4. Efferent nerve fibers carry motor nerve signals from the anterior horn to the muscles
  5. Effector muscle innervated by the efferent nerve fiber carries out the response.

A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b) carry the response generated by the spinal cord to effector organ(s) during a reflex action.

Reflex Arc
In a reflex arc, an action potential never travels to the brain for processing and so results in a much quicker reaction. When a stimulus (A) is encountered, the signal from that stimulus will travel up the sensory neuron (B, in green) to the spinal column (C). There, it will likely pass through a short interneuron (D, in purple) before continuing down a motor neuron (E, in blue) to the origin of the signal. Then, a contraction of the muscles (F, in red) is triggered, moving the bone (G).

Monosynaptic vs. polysynaptic

Imgnotraçat arc reflex eng
Reflex arc demonstrated

When a reflex arc in an animal consists of only one sensory neuron and one motor neuron, it is defined as monosynaptic, referring to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain.

The patellar reflex (knee jerk)

Reflex arc of decapod including LG interneuron (1)
(A) Microscopic hairs etched along the tail of the decapod activate a somatic signal (2) in response to the presence of an environmental stimulus (1). (B) The action potential activated by the somatic interneuron (3) relays an impulse to the lateral giant (LG) interneuron (4). (C) The lateral giant interneuron executes a reflex by relaying impulses to various giant motor neurons (5) within the abdomen of the lobster. These muscular contractions result in the decapod being capable of successfully propelling itself through the water, away from the site of stimulus.

When the patellar tendon is tapped just below the knee, the tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps. This action potential travels to the L3 and L4 nerve roots of the spinal cord,[2] via a sensory axon which chemically communicates by releasing glutamate onto a motor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate CNS injury.[2]

The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons of antagonist muscles, blocking the sustained stimulation of these antagonistic (hamstring) muscles. The relaxation of the opposing muscle facilitates (by not opposing) the extension of the lower leg.

In lower animals reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.


  1. ^ a b Saladin, Kenneth (2015). Anatomy & Physiology: The Unity of Form and Function. New York: McGraw-Hill. pp. 496–497. ISBN 978-0073403717.
  2. ^ a b "Deep Tendon Reflexes". The Precise Neurological Exam. New York University School of Medicine. November 28, 2016. Retrieved November 28, 2016.

External links

Anal wink

The anal wink, anal reflex, perineal reflex, or anocutaneous reflex is the reflexive contraction of the external anal sphincter upon stroking of the skin around the anus.A noxious or tactile stimulus will cause a wink contraction of the anal sphincter muscles and also flexion. The stimulus is detected by the nociceptors in the perineal skin to the pudendal nerve, where a response is integrated by the spinal cord sacral segments S2-S4.

The absence of this reflex indicates that there is an interruption of the reflex arc, or damage to the spinal cord,which may be in the sensory afferent limb or the motor efferent limb. The synapse between the afferent and efferent limbs occurs in the lowest sacral segments of the spinal cord.

Axon reflex

The axon reflex (or the flare response) is the response stimulated by peripheral nerves of the body that travels away from the nerve cell body and branches to stimulate target organs. Reflexes are single reactions that respond to a stimulus making up the building blocks of the overall signaling in the body's nervous system. Neurons are the excitable cells that process and transmit these reflex signals through their axons, dendrites, and cell bodies. Axons directly facilitate intercellular communication projecting from the neuronal cell body to other neurons, local muscle tissue, glands and arterioles. In the axon reflex, signaling starts in the middle of the axon at the stimulation site and transmits signals directly to the effector organ skipping both an integration center and a chemical synapse present in the spinal cord reflex. The impulse is limited to a single bifurcated axon, or a neuron whose axon branches into two divisions and does not cause a general response to surrounding tissue.

The axon reflex arc is distinct from the spinal cord reflex arc. In the spinal cord reflex pathway the afferent neuron transmits information to spinal cord interneurons. These interneurons act collectively, process and make sense of inbound stimuli, and stimulate effector neurons acting as an integration center. The effector neurons leaving the integration center transmit a response to the original tissue the reflex arose resulting in a response. The axon reflex results in a localized response to only the locally innervated cells of the single neuron where the signal originated. The axon reflex pathway does not include an integration center or synapse that relays communication between neurons in the spinal cord reflex. The stimulus, therefore, is diverted to the effector organ without entering the neuronal cell body and therefore indicates that the axon reflex is not a true reflex where afferent impulses pass through the central nervous system before stimulating efferent neurons.

The axon reflex was discovered and was described as "a new type of peripheral reflex" that bypasses the integration center and synapse in the central nervous system. The discovery of the axonal reflex found that the axon reflex activates local arterioles causing vasodilation and muscle contraction. This muscle contraction was observed in people with asthma where the released neuropeptides caused the smooth muscle in the airway to contract. Similarly the release of cholinergic agents at sudomotor nerve terminals evokes an axon reflex that stimulates sweat glands inducing the body to sweat in response to heat. The axon reflex is possible through the transmission of signals from the cutaneous receptors on the skin.

Balloonist theory

Balloonist theory was a theory in early neuroscience that attempted to explain muscle movement by asserting that muscles contract by inflating with air or fluid. The Greek physician Galen believed that muscles contracted due to a fluid flowing into them, and for 1500 years afterward, it was believed that nerves were hollow and that they carried fluid. René Descartes, who was interested in hydraulics and used fluid pressure to explain various aspects of physiology such as the reflex arc, proposed that "animal spirits" flowed into muscle and were responsible for their contraction. In the model, which Descartes used to explain reflexes, the spirits would flow from the ventricles of the brain, through the nerves, and to the muscles to animate the latter.In 1667, Thomas Willis proposed that muscles may expand by the reaction of animal spirits with vital spirits. He hypothesized that this reaction would produce air in a manner similar to the reaction that causes an explosion, causing muscles to swell and produce movement.

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 to quickly depress the biceps brachii tendon as it passes through the cubital fossa. Specifically, the test activates the stretch receptors inside the biceps brachii muscle which communicates mainly with the C5 spinal nerve and partially with the C6 spinal nerve to induce a reflex contraction of the biceps muscle and jerk of the forearm.

A strong contraction indicates a 'brisk' reflex, and a weak or absent reflex is known as 'diminished'. Brisk or absent reflexes are used as clues to the location of neurological disease. Typically brisk reflexes are found in lesions of upper motor neurons, and absent or reduced reflexes are found in lower motor neuron lesions.

A change to the biceps reflex indicates pathology at the level of musculocutaneous nerve, segment C5/6 or at some point above it in the spinal cord or brain.

Charles Scott Sherrington

Sir Charles Scott Sherrington (27 November 1857 – 4 March 1952) was an English neurophysiologist, histologist, bacteriologist, and a pathologist, Nobel laureate and president of the Royal Society in the early 1920s. He received the Nobel Prize in Physiology or Medicine with Edgar Adrian, 1st Baron Adrian, in 1932 for their work on the functions of neurons. Prior to the work of Sherrington and Adrian, it was widely accepted that reflexes occurred as isolated activity within a reflex arc. Sherrington received the prize for showing that reflexes require integrated activation and demonstrated reciprocal innervation of muscles (Sherrington's law). Through his seminal 1906 publication, The Integrative Action of the Nervous System, he had effectively laid to rest the theory that the nervous system, including the brain, can be understood as a single interlinking network. His alternative explanation of synaptic communication between neurons helped shape our understanding of the central nervous system.

Escape reflex

Escape reflex, a kind of escape response, is a simple reflectory reaction in response to stimuli indicative of danger, that initiates an escape motion of an animal.

Escape reflexes control the seemingly chaotic motion of a cockroach running from under the foot when one tries to squash it.

In higher animals examples of escape reflex include the withdrawal reflex, e.g., the withdrawal of a hand in response to a pain stimulus. Sensory receptors in the stimulated body part send signals to the spinal cord along a sensory neuron. Within the spine a reflex arc switches the signals straight back to the muscles of the arm (effectors) via an intermediate neuron (interneuron) and then a motor neuron; the muscle contracts. There often is an opposite response of the opposite limb. Because this occurs automatically and independently in the spinal cord, not the brain, the brain only becomes aware of the response after it has taken place.

Escape reflex arcs have a high survival value, enabling organisms to take rapid action to avoid potential danger.

Various animals may have specialized escape reflex circuits.

Golgi tendon reflex

The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous system. In a Golgi tendon reflex, skeletal muscle contraction causes the antagonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex. Though muscle tension is increasing during the contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited. However, antagonistic muscles are activated.


The 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-reflex test is performed using an electric stimulator, which gives usually a square-wave current of short duration and small amplitude (higher stimulations might involve alpha fibers, causing an F-wave, compromising the results), and an EMG set, to record the muscle response. That response is usually a clear wave, called H-wave, 28-35 ms after the stimulus, not to be confused with an F-wave. An M-wave, an early response, occurs 3-6 ms after the onset of stimulation. The H and F-waves are later responses. As the stimulus increases, the amplitude of the F-wave increases only slightly, and the H-wave decreases, and at supramaximal stimulus, the H-wave will disappear. The M-wave does the opposite of the H-wave. As the stimulus increases the M-wave increases. There is a point of minimal stimulus where the M-wave is absent and the H-wave is maximal.

H-reflex is analogous to the mechanically induced spinal stretch reflex (for example, knee jerk reflex). "The primary difference between the H-reflex and the spinal stretch reflex is that the H-reflex bypasses the muscle spindle, and, therefore, is a valuable tool in assessing modulation of monosynaptic reflex activity in the spinal cord." Although stretch reflex gives just qualitative information about muscle spindles and reflex arc activity; if the purpose of the test to compare performances from different subjects, H-reflex should be used. In that case, in fact, latencies (ms) and amplitudes (mV) of H-wave can be compared.

H-reflex amplitudes measured by EMG are shown to decrease significantly with applied pressure such as massage and tapping to the cited muscle. The amount of decrease seems to be dependent on the force of the pressure, with higher pressures resulting in lower H-reflex amplitudes. H-reflex levels return to baseline immediately after pressure is released except in high pressure cases which had baseline levels returned within the first 10 seconds.After about 5 days in zero gravity, for instance in orbit around Earth, the h-reflex diminishes significantly. It is generally assumed that this is due to a marked reduction in the excitability of the spinal cord in zero gravity. Once back on Earth, a marked recovery occurs during the first day, but it can take up to 10 days to return to normal. The H-reflex was the first medical experiment completed on the International Space Station.


A hiccup (also spelled hiccough) is an involuntary contraction (myoclonic jerk) of the diaphragm that may repeat several times per minute. The hiccup is an involuntary action involving a reflex arc. Once triggered, the reflex causes a strong contraction of the diaphragm followed about 0.25 second later by closure of the vocal cords, which results in the classic "hic" sound.

Hiccups may occur individually, or they may occur in bouts. The rhythm of the hiccup, or the time between hiccups, tends to be relatively constant.

A bout of hiccups, in general, resolves itself without intervention, although many home remedies are often used to attempt to shorten the duration. Medical treatment is occasionally necessary in cases of chronic hiccups.

Historical fallacy

The historical fallacy is a logical fallacy originally described by philosopher John Dewey in The Psychological Review in 1896. Most simply put, the fallacy occurs when a person reads into a process the results that occur only because of that process. Dewey writes:

"A set of considerations which hold good only because of a completed process, is read into the content of the process which conditions this completed result. A state of things characterizing an outcome is regarded as a true description of the events which led up to this outcome; when, as a matter of fact, if this outcome had already been in existence, there would have been no necessity for the process."


Hyporeflexia refers to below normal or absent reflexes (areflexia). It can be detected through the use of a reflex hammer. It is the opposite of hyperreflexia.

Hyporeflexia is generally associated with a lower motor neuron deficit (at the alpha motor neurons from spinal cord to muscle), whereas hyperreflexia is often attributed to upper motor neuron lesions (along the long, motor tracts from the brain). The upper motor neurons are thought to inhibit the reflex arc, which is formed by sensory neurons from intrafusal fibers of muscles, lower motor neurons (including alpha and gamma motor fibers) and appurtenant interneurons. Therefore, damage to lower motor neurons will subsequently result in hyporeflexia and/or areflexia.

Note that, in spinal shock, which is commonly seen in the transection of the spinal cord (Spinal cord injury), areflexia can transiently occur below the level of the lesion and can, after some time, become hyperreflexic. Furthermore, cases of severe muscle atrophy or destruction could render the muscle too weak to show any reflex and should not be confused with a neuronal cause.

Hyporeflexia may have other causes, including hypothyroidism, electrolyte imbalance (e.g. excess magnesium), drug induced (e.g. the symptoms of benzodiazepine intoxication include confusion, slurred speech, ataxia, drowsiness, dyspnea, and hyporeflexia).

Diseases associated with hyporeflexia include

Centronuclear myopathy

Guillain–Barré syndrome

Lambert-Eaton myasthenic syndrome

Polyneuropathy (Achilles and plantar reflexes)


Mephenoxalone (trade names Dorsiflex, Moderamin, Control-OM) is a muscle relaxant and mild anxiolytic. It inhibits neuron transmission, relaxing skeletal muscles by inhibiting the reflex arc. As the effect of muscle relaxation, mephenoxalone affects mental condition, and is also a treatment for nervousness and anxiety.

Pharyngeal reflex

The pharyngeal reflex, gag reflex, or laryngeal spasm, is a reflex contraction of the back of the throat, evoked by touching the roof of the mouth, the back of the tongue, the area around the tonsils, the uvula, and the back of the throat. It, along with other aerodigestive reflexes such as reflexive pharyngeal swallowing, prevents objects in the oral cavity from entering the throat except as part of normal swallowing and helps prevent choking.

Rectoanal inhibitory reflex

The rectoanal inhibitory reflex (RAIR) (also known as the anal sampling mechanism, anal sampling reflex, or anorectal sampling reflex) is a reflex characterized by a transient involuntary relaxation of the internal anal sphincter in response to distention of the rectum. The RAIR provides the upper anal canal with the ability to discriminate between flatus and fecal material.

The ability of the rectum to discriminate between gaseous, liquid and solid contents is essential to the ability to voluntarily control defecation. The RAIR allows for voluntary flatulation to occur without also eliminating solid waste, irrespective of the presence of fecal material in the anal canal.

Richard Dugard Grainger

Richard Dugard Grainger FRCS FRS (1801 – 1 February 1865) was an English surgeon, anatomist and physiologist.

Grainger was born in Birmingham, the son of a surgeon, and educated at a grammar school. He was the brother of Edward Grainger, whose anatomical school he carried forward. He ran the private Webb Street anatomy school for twenty years before joining St Thomas's Hospital as a lecturer from 1842 to 1860.

He was elected a Fellow of the Royal College of Surgeons of England and delivered their Hunterian oration in 1848. He was an inspector for the Children's Employment Commission (1841), the Board of Health (1849), author of a report on cholera (1850) and inspector under the Burials Act 1853. Grainger refused money from a testimonial, which was then used to found the Grainger prize. He was the author of Elements of general anatomy (1829) and Observations on... the spinal cord (1837).

He was elected a Fellow of the Royal Society in January 1846 for his work on the spinal cord, which supported Marshall Hall's work on the reflex arc. His application citation read: The Discoverer of the structure of the Fallopian Tube in Mammalia. The Author of Observations on the Structure & Functions of the Spinal Cord, &c. Distinguished for his acquaintance with the science of physiology. Eminent as a Physiologist.

A tall, stooping man, he was a medical and social reformer, and was active in the Christian Medical Association.

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 afferent nerves or sensory nerves, and efferent nerves or motor nerves. Afferent nerves are responsible for relaying sensation from the body to the central nervous system; efferent nerves are responsible for sending out commands from the CNS to the body, stimulating muscle contraction; they include all the non-sensory neurons connected with skeletal muscles and skin. The a- of afferent and the e- of efferent correspond to the prefixes ad- (to, toward) and ex- (out of).

Tensor network theory

For the tensor network theory used in quantum physics, see matrix product stateTensor network theory is a theory of brain function (particularly that of the cerebellum) that provides a mathematical model of the transformation of sensory space-time coordinates into motor coordinates and vice versa by cerebellar neuronal networks. The theory was developed by Andras Pellionisz and Rodolfo Llinas in the 1980s as a geometrization of brain function (especially of the central nervous system) using tensors.

Triceps reflex

The triceps reflex, a deep tendon reflex, is a reflex as it elicits involuntary contraction of the triceps brachii muscle. It is initiated by the Cervical (of the neck region) spinal nerve 7 nerve root (the small segment of the nerve that emerges from the spinal cord). The reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C7 and C8 spinal nerves.

Withdrawal reflex

The withdrawal reflex (nociceptive flexion reflex or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.