reflex

adj.

1. Bent, turned, or thrown back; reflected.
2. Physiology. Being an involuntary action or response, such as a sneeze, blink, or hiccup.
3. Produced as an automatic response or reaction: reflex opposition to change.

n.

1. 1. Something, such as light or heat, that is reflected.
   2. An image produced by reflection.
   3. A copy or reproduction.
2. Physiology. An involuntary response to a stimulus.
3. Psychology. An unlearned or instinctive response to a stimulus.
4. Linguistics. A form or feature that reflects or represents an earlier, often reconstructed, form or feature having undergone phonetic or other change.

tr.v., -flexed, -flex·ing, -flex·es. (rĭ-flĕksPRIMARY_STRESS)

1. To bend, turn back, or reflect.
2. To cause to undergo a reflex process.

[From Middle English reflexen, to refract light, bend back, from Latin reflexus, past participle of reflectere, to bend back. See reflect.]

A simple, unlearned, yet specific behavioral response to a specific stimulus. Reflexes are exhibited by virtually all animals from protozoa to primates. Along with other, more complex stimulus-bound responses such as fixed action patterns, they constitute much of the behavioral repertoire of invertebrates. In higher animals, such as primates, where learned behavior dominates, reflexes nevertheless persist as an important component of total behavior.

The simplest known reflexes require only one neuron or, in the strictest sense, none. For example, ciliated protozoa, which are single cells and have no neurons, nevertheless exhibit apparently reflexive behaviors. However, most reflexes require activity in a large sequence of neurons. The neurons involved in most reflexes are connected by specific synapses to form functional units in the nervous system. Such a sequence begins with sensory neurons and ends with effector cells such as skeletal muscles, smooth muscles, and glands, which are controlled by motor neurons. The central neurons which are often interposed between the sensory and motor neurons are called interneurons. The sensory side of the reflex arc conveys specificity as to which reflex will be activated. The remainder of the reflex response is governed by the specific synaptic connections that lead to the effector neurons. A familiar reflex is the knee-jerk or stretch reflex. It involves the patellar (kneecap) tendon and a group of upper leg muscles. Other muscle groups show similar reflexes.

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The term ‘reflex’ was first used to describe an automatic, almost immediate movement in response to a stimulus, involving a nerve circuit that traverses the spinal cord. It is now applied also to other types of automatic response to a stimulus, including those involving the brain. A reflex requires sensory receptors that detect the stimulus, sensory nerve fibres that conduct the information to the central nervous system (CNS), neurons in the CNS itself, nerve fibres conducting the command away from the CNS, and the effector. Sir Charles Sherrington (1857-1952) was the first to introduce the word ‘reflex’, taking the view that sensory information going into the cord was reflected out again along the motor nerve fibres, analogous to a beam of light being reflected by a mirror. Sherrington referred to the chain of structures — receptor, conductor, and effector — as a reflex arc.

The study by Sherrington and colleagues of the spinal reflex provided an understanding of the basis of the simplest neural circuits in the central nervous system, an understanding on which subsequent advances in neuroscience relied. Sherrington wished to remove the element of consciousness and consciously-guided movement, so that he could study the nature of the behaviour repertoire of the spinal cord. The experimental results prompted Sherrington to define the term ‘reflex’ and, with this, the implicit assumption that a reflex response is independent of consciousness. Elimination of the effects of consciousness could be achieved in experimental animals by surgically interrupting influences from higher centres. This afforded the means of unravelling features of the activity of the cord that had hitherto escaped analysis.

Spinal reflexes

Although simple manifestations of activity of the central nervous system, spinal reflexes are meaningful, in that each reflex subserves an obvious function. For example, the reflex withdrawal of the hand from a noxious object minimizes the damage inflicted on the organism by the noxious agent.

One of the best-known reflexes is the tendon jerk reflex. When a tendon is tapped, the muscle to which it is attached gives a twitch. An example is the ‘knee-jerk reflex’; a tap to the patellar tendon (just below the front of the knee) causes a reflex twitch in the quadriceps muscles (the muscle mass on the front of the thigh). This twitch may be sufficiently powerful to extend the lower leg at the knee. We now know that this reflex response is initiated from the class of sensory receptors called muscle spindle receptors. In animal experiments, Sherrington showed that the adequate stimulus for this reflex was a mere 0.01 mm elongation of the quadriceps muscle.

The tendon jerk reflex is the simplest reflex; within the central nervous system, the sensory nerve fibres form connections directly with the nerve cells that send out motor nerve fibres to innervate the effector muscle. The testing of these reflexes, together with a knowledge of the different levels of the spinal cord responsible for each of them, provides a clinical method of examining the integrity of a reflex arc involving particular peripheral nerves and segments of the spinal cord. Also, since tendon jerks are normally partly suppressed by nerve impulses descending from higher levels of the CNS, their exaggeration is a valuable sign of damage above the relevant spinal segments.

Transmission of information in the reflex arc

As with other cells in the body, each nerve cell is surrounded by its own thin lipid cell membrane. This membrane has a high electrical resistance. Conduction of nerve impulses along nerve fibres is subserved by an electrical mechanism. The nerve fibre acts as a cable with a conducting core (the cell sap) surrounded by its insulating membrane. Nerve impulses can propagate in either direction along the nerve fibre.

The study of spinal reflexes allowed early workers to deduce properties of transmission of information from the sensory nerve fibres to the motor nerve cells within the spinal cord. In consultation with Classics colleagues in the University of Liverpool, in 1897 Sherrington introduced into our language the noun synapse to describe those areas of functional contact, between nerve cells, that are specialized for transmission of nerve impulses. He deduced that it was synaptic transmission that conferred the reflex with the property of directionality. In the reflex arc, information entered the cord along sensory nerve fibres to elicit activity leaving the cord in motor nerve fibres but, because of the special properties of the synapse, information could not flow in the opposite direction. Synaptic transmission was subsequently shown to be subserved by a chemical mechanism. Action potentials in the sensory nerve fibres cause the release of a neurotransmitter chemical that diffuses to attach to specific recognition sites on the motor nerve cells. This attachment changes the electrical excitability of the nerve cells and may initiate nerve impulses in the motor nerve fibres. Synaptic transmission, the fundamental properties of which were initially revealed by the study of the spinal reflex, is the basis of the integrative activity of the nervous system. Modulation of synaptic transmission underlies the mechanism of action of most drugs, both therapeutic and drugs of abuse, that act on the brain.

Evolutionary aspects

In simple vertebrates the spinal cord and lower brain stem dominate, there being little or no developed forebrain. As higher centres have developed in the course of evolution, they have come to exert many of their effects by controlling and modifying the pre-existing spinal reflex mechanisms, not by replacing them. An example of modification of primitive cord activity by higher centres is afforded by another clinically useful test. When a firm stroke is applied to the sole of the foot, the primitive spinal reflex response, when influences from higher centres are absent, is withdrawal of the foot from the mildly noxious stimulus. The response of a normal human adult to this same stimulus, however, is a thrust, to push the stimulus away. This latter response is part of the complicated mechanism that allows us to stand; the pressure on the soles of our feet elicits a continuous muscular effort to keep the feet pushing against the ground to prevent us from falling. If a human adult suffers damage to the higher motor centres in the brain, the reflex reverts from its normal thrust to the more primitive withdrawal response. Doctors refer to the reflex as the Babinski response, named after the neurologist in Paris who first described its significance in 1896. It is a clinically useful indicator of the integrity of the higher motor centres together with the tracts projecting down from these centres to the motor nerve cells in the spinal cord. Normal new-born babies, in whom the higher central control of posture has yet to develop, show the primitive withdrawal response. This reverses to the normal adult response at the age of about 6 months. This is the time in development at which the tracts from higher motor centres become functional.

For different reflexes, the reflex responses range from simple to complex. The tendon jerk reflex is relatively simple and involves a relatively small region of the spinal cord. This contrasts with complicated, repeated movements, such as those occurring in a limb of a dog showing a scratch reflex to dislodge an insect biting its flank. For these more complicated reflexes, extensive regions of the cord are involved and the reflex circuits are correspondingly elaborate. Whereas the tendon jerk reflex is executed by a direct connection in the cord between the sensory nerve fibres and motor nerve cells, a scratch reflex depends on long pathways involving multiple synaptic relays, and the triggering into action of a rhythm generator responsible for the frequency and vigour of the scratching movements.

Reflexes interact with each other. The reflex response to a stimulus which is severely threatening to the well-being, or even to the life, of an animal, will, whilst commanding its own response, simultaneously switch off any other interfering reflexes that are less important in survival and that utilize the same muscles.

Reflexes mediated by cranial nerves

The cranial nerves (the nerves that arise from the brain rather than the spinal nerves that arise from the cord) provide the pathways to and from the central nervous system for reflexes utilizing the muscles of the head, such as those controlling movements of the eyeball, face, and tongue. The nerve cells giving rise to the cranial motor nerve fibres lie in clusters (nuclei) in the brain stem; they represent an upward extension of the homologous groups of nerve cells in the spinal cord. Examples of reflexes involving the cranial nerves are the closure of the eyelids when the cornea is stimulated, or gagging when the back of the throat is irritated.

Autonomic reflexes

These reflexes produce effects such as: changing the rate or force of contraction of the heart; contraction or relaxation of smooth muscle; glandular secretion. The reflexes are mediated by the sympathetic or parasympathetic nerves of the autonomic nervous system, in response to information reaching the central nervous system from a variety of receptors in the organs and tissues. For example, when a light shines in the eye, there is constriction of the pupil produced by contraction of the circular smooth muscle of the iris; when a person rises rapidly from bed or bath, the heart rate promptly increases in response to a fall in blood pressure; in response to the taste of a lemon, there is an outpouring of saliva.

In conclusion, the study of reflexes alone cannot solve the problems of higher neural function, of emotion, or of psychology. However, by providing a basis from which a study of higher functions could develop, the unravelling of the properties of reflexes was an historic, essential early step in the development of neuroscience.

— Oliver Holmes

See also central nervous system; motor neuron; reaction time; synapse.

adjective

Acting or happening without apparent forethought, prompting, or planning: automatic, impulsive, instinctive, involuntary, spontaneous, unpremeditated. See planned/unplanned.

noun

Imitative reproduction, as of the style of another: echo, imitation, reflection, repetition. See same/different/compare.

(rē′fleks)
n

A reflected action or movement; the sum total of any specific involuntary activity

For more information on reflex, visit Britannica.com.

A reflex is an involuntary response formulated in the spinal cord to a specific stimulus.

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An action or movement not controlled by conscious thought. A reflex may be anything from a hiccup to the involuntary response of a body part, such as the action that occurs in the knee-jerk reflex.

A reflected involuntary action or movement; the sum total of any particular automatic response mediated by the nervous system.
A reflex is built into the nervous system and does not need the intervention of conscious thought to take effect.
For reflexes used in clinical examination of a patient see under individual titles including anal, blink, corneal, conjunctival, crossed extensor, extensor thrust, eyelid, gastrocnemius, hopping, palpebral, panniculus, patellar, perineal, placing, pupillary, withdrawal.

• accommodation r. — the coordinated changes that occur when the eye adapts itself for near vision; they are constriction of the pupil, convergence of the eyes and increased convexity of the lens.
• r. action — an involuntary response to a stimulus conveyed to the nervous system and reflected to the periphery (see also reflex).
• r. bradycardia — bradycardia occurring as a reflex initiated by severe atrial or ventricular stretch.
• chain r. — a series of reflexes, each serving as a stimulus to the next, making a complete activity.
• complete r. — one requiring no feedback from the forebrain.
• conditioned r. — see conditioned response.
• cutaneous trunci r. — see panniculus reflex.
• dazzle r. — a test of the retina, optic nerve, and central retinal pathways. Shining a bright light into the eye should cause squinting.
• r. dyssynergia — see detrusor-urethral dyssynergia.
• gastroileal r. — increase in ileal motility and opening of the ileocecal valve when food enters the empty stomach.
• light r. — see pupillary light reflex.
• nociceptive r. — reflexes initiated by painful stimuli.
• palatal r. — see swallowing reflex.
• rectosphincteric r. — relaxation of the anal sphincter which occurs with distention of the rectum and during defecation.
• stepping r. — movements of progression elicited when the animal is held upright and inclined forward with the feet touching a flat surface.
• stretch r. — reflex contraction of a muscle in response to passive longitudinal stretching.
• superficial r. — any withdrawal reflex elicited by noxious or tactile stimulation of the skin, cornea or mucous membrane, including the corneal and pharyngeal reflexes.
• tapetal r. — tapetal reflection of light.
• tendon r. — a method of testing the patency of reflex arc. The tendon is stretched sharply by tapping it close to its insertion. A positive reaction is a sharp contraction of the muscle of which the tendon is part. The patellar reflex is the best known of these reflexes. Absence indicates a defect in the reflex arc, an exaggerated response suggests an upper motor neuron lesion. Called also myotactic reflex.
• tonic r. — see tonic neck response.
• triceps surae r. — Achilles reflex.
• trigemino-abducens r. — a test of the ophthalmic and mandibular divisions of the trigeminal nerve, and the abducens nerve. Light touching of the cornea should result in retraction of the globe and protrusion of the nictitating membrane.
• r. walking — spinal reflexes can provide support and uncoordinated use of the hindlegs that resembles walking, in dogs with transection of the spinal cord between T13 and L4.
• whisker r. — pinching the pinna elicits a twitch of the whiskers in cats; used in assessment of depth of anesthesia.

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Dansk (Danish)
n. - refleks, afspejling, genskin
adj. - refleks-, reflekteret
v. tr. - reflektere

idioms:

• reflex action    reflekshandling
• reflex arc    refleksbue
• reflex camera    spejlreflekskamera

Nederlands (Dutch)
reflex, (mv) reactievermogen, weerkaatsing, spiegelbeeld, manifestatie van verschijnsel, woord afgeleid van eerdere vorm, groter dan 180 graden (hoek), teruggebogen, weerkaatst, op zichzelf betrekking hebbend

Français (French)
n. - (gén, Physiol) réflexe
adj. - (Math) rentrant (angle), (Phys) réfléchi
v. tr. - réfléchir (la lumière), rabattre, plier

idioms:

• reflex action    réflexe
• reflex arc    (Math) arc rentrant
• reflex camera    reflex

Deutsch (German)
n. - Reflex
adj. - Reflex-, (Winkel) überstumpf
v. - zurückbiegen

idioms:

• reflex action    Reflexhandlung
• reflex arc    Reflexbogen
• reflex camera    Spiegelreflexkamera

Ελληνική (Greek)
n. - (φυσιολ.) (εξαρτημένο) ανακλαστικό, ρεφλέξ
adj. - (αντ)ανακλαστικός, άθελος
v. - κάμπτομαι

idioms:

• reflex action    αντανακλαστική αντίδραση, αντανακλαστικό, ρεφλέξ
• reflex arc    (φυσιολ.) αντανακλαστό (νευρικό) τόξο
• reflex camera    φωτογραφική μηχανή ρεφλέξ

Italiano (Italian)
riflessione, riflesso

idioms:

• reflex action    azione riflessa
• reflex arc    arco riflesso
• reflex camera    macchina fotografica reflex

Português (Portuguese)
n. - reflexo (m), resultado (m)
adj. - reflexo, introspectivo
v. - recurvar

idioms:

• reflex action    contração involuntária
• reflex arc    arco reflexo
• reflex camera    câmara de reflexo

Русский (Russian)
рефлекс

idioms:

• reflex action    рефлекс
• reflex arc    порядок нервов при рефлексе
• reflex camera    зеркальный фотоаппарат

Español (Spanish)
n. - reflejo, imagen reflejada, acción refleja
adj. - reflejo, imagen reflejada
v. tr. - doblar, encorvar o dirigir hacia atrás

idioms:

• reflex action    movimiento reflejo, acción refleja
• reflex arc    arco reflejo
• reflex camera    cámara reflex

Svenska (Swedish)
n. - reflex
adj. - reflekterad
v. - återkasta

中文（简体）(Chinese (Simplified))
反射, 映象, 反映, 反射的, 反作用的, 反省的, 使经受反射, 使折回

idioms:

• reflex action    反射作用
• reflex arc    反射弧
• reflex camera    反射式照相机

中文（繁體）(Chinese (Traditional))
n. - 反射, 映象, 反映
adj. - 反射的, 反作用的, 反省的
v. tr. - 使經受反射, 使折回

idioms:

• reflex action    反射作用
• reflex arc    反射弧
• reflex camera    反射式照相機

한국어 (Korean)
n. - 영상 그림자, 반영, 반사 작용
adj. - 반사 작용의, 반사된, 휘어진
v. tr. - 반전시키다, 휘게 하다, 반사 시키다

日本語 (Japanese)
adj. - 反射の, 反射的な, 反射された, 反動の
n. - 反射的な動作, 反射神経, 反映, 反射されたもの

idioms:

• conditioned reflex    条件反射
• reflex action    反射
• reflex arc    反射弓
• reflex camera    レフレックスカメラ

العربيه (Arabic)
‏(الاسم) رد فعل, ضو أو حرارة منعكسه (صفه) ملتو (فعل) يحني, يلوي‏

עברית (Hebrew)
n. - ‮רפלקס, תגובה, החזר‬
adj. - ‮מוחזר, חוזר, משתקף, בא כתגובה‬
v. tr. - ‮כפף, החזיר, שיקף‬

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twin reflex	merrell reflex
honda reflex	norelco reflex