Reflex

Abstract

Reflex, which MeSH classifies under psychophysiology, is an involuntary, stereotyped motor or autonomic response triggered by a specific stimulus and carried by a fixed neural pathway, the reflex arc. Sherrington gave the reflex its integrative account as the elementary unit of nervous action and coined the word synapse; Pavlov extended the idea to learning through the conditioned reflex. This article treats the reflex as a worked case: what defines it, the monosynaptic and polysynaptic circuits of the arc, how reflexes are measured — the tendon reflex, the H-reflex, the startle eyeblink, and prepulse inhibition — and the still-live question of how reflexes relate to voluntary action and whether even the simplest of them can be learned. The reflex matters because it exposes, in its most tractable form, the stimulus-response link that all of behaviour elaborates.

Keywords: reflex arc, stretch reflex, H-reflex, startle reflex, prepulse inhibition

A reflex is the nervous system's shortest answer to the world. Touch a hot surface and the hand is already withdrawing before the pain registers; a tap below the kneecap throws the lower leg forward regardless of intention. These responses are involuntary, stereotyped, and fast because they run over a fixed circuit that does not wait for the brain's deliberation. Charles Sherrington made the reflex the foundation of modern neurophysiology by treating this circuit — the reflex arc — as the elementary unit from which all coordinated action is built, and in doing so he named the synapse, the junction where its logic is decided (Sherrington, 1906). What makes the reflex a subject for cognitive psychology, and not only for physiology, is that it is the simplest complete case of the thing psychology studies: a stimulus reliably producing a response through a knowable mechanism, measurable to the millisecond, and — as Pavlov and later work showed — not nearly as fixed as it first appears.

What a Reflex Is

A reflex is an involuntary response to a stimulus that is mediated by the nervous system over a defined pathway. Three features define it. It is involuntary: it does not require, and often cannot be stopped by, a conscious decision. It is stereotyped: the same stimulus produces the same response, in form and roughly in magnitude, each time. And it is mediated by a fixed circuit, the reflex arc, so that the relationship between stimulus and response is built into the wiring rather than computed afresh. Sherrington's The Integrative Action of the Nervous System established this circuit as the elementary unit of nervous function and showed how reflexes are combined and coordinated into behaviour, introducing the principles of reciprocal innervation — excitation of a muscle accompanied by inhibition of its antagonist — and the final common path, the motor neuron on which many competing inputs converge (Sherrington, 1906).

The reflex arc has five elements in series: a receptor that transduces the stimulus into neural activity, an afferent neuron that carries that activity toward the central nervous system, an integration center where the signal is processed, an efferent neuron that carries the command outward, and an effector — a muscle or gland — that produces the response. The whole point of the arc is that it is short. Because the loop can close in the spinal cord without involving the brain, a reflex response can begin before the stimulus has even been consciously perceived, which is why reflex latencies are measured in tens of milliseconds while a deliberate reaction takes far longer. The demonstration below traces a signal around the arc and shows where a monosynaptic and a polysynaptic reflex diverge.

Figure 1

The Reflex Arc: From Stimulus to Response Over a Fixed Pathway

Types of Reflex

MeSH does not treat the reflex as a single undifferentiated category. It files psychophysiology as the parent kind and, beneath the Reflex descriptor, a set of narrower descriptors naming particular reflexes — from the somatic stretch and Babinski reflexes elicited in a neurologic examination to autonomic reflexes such as the baroreflex and the galvanic skin response. These subtypes are not a single clean taxonomy: they are cross-classified along several independent axes at once — by the effector (motor versus autonomic), by the receptor (stretch, acoustic, visual), by whether the response is normal or pathological, and by the circuit's synaptic depth (monosynaptic versus polysynaptic). A given reflex can therefore appear under more than one heading, and the placement below is a MeSH indexing classification rather than a claim that these are mutually exclusive kinds. Only subtypes that have their own article on this site are linked; the rest are glossed from their MeSH scope.

Note. These subtypes are cross-classified along independent axes (effector, receptor, normality, synaptic depth) and are not mutually exclusive kinds; the arrangement is a MeSH indexing classification. Only subtypes with a live route on this site are linked.

The Reflex Arc: Monosynaptic and Polysynaptic

The single most important structural distinction among reflexes is the number of synapses their arc contains. The stretch reflex — the response measured by the tendon tap — is the only reflex in the human body known to be genuinely monosynaptic: a muscle spindle, the stretch receptor embedded in the muscle, sends a fast afferent fibre into the spinal cord that makes a single excitatory synapse directly onto the alpha motor neuron supplying the same muscle, which then contracts. One receptor, one afferent neuron, one synapse, one efferent neuron, one muscle. Its speed and simplicity are exactly why the tendon reflex is the physician's first probe of an intact spinal segment (Hultborn, 2006).

Most reflexes are polysynaptic: one or more interneurons are interposed between the afferent input and the motor output, so the arc contains several central synapses. The flexor-withdrawal reflex that pulls a limb from a painful stimulus is polysynaptic, and it recruits reciprocal innervation — the same afferent volley that excites the flexors inhibits the extensors, and, through crossed pathways, does the opposite in the other limb so it can bear the shifted weight. Hultborn's synthesis traces how the interneuronal networks of the spinal cord, worked out from Eccles and Lundberg onward, implement this coordination, and how descending pathways from the brain continually adjust the gain of even the simplest spinal reflex (Hultborn, 2006). The circuitry of the human spinal cord, and the methods used to dissect it in intact people, are set out in detail by Pierrot-Deseilligny and Burke (Pierrot-Deseilligny & Burke, 2012). The synapse count matters because every synapse adds a delay of roughly a millisecond: a reflex's latency is a readout of how many synapses its signal crossed, as the worked example below makes explicit.

Measuring Reflexes

Because a reflex is a fixed input-output relationship, it can be measured with unusual precision, and different measurements open windows onto different levels of the nervous system. The oldest is the clinical tendon reflex: a calibrated tap stretches the muscle spindle and the resulting twitch is graded by eye. Its electrophysiological refinement is the H-reflex, or Hoffmann reflex, which bypasses the spindle altogether by stimulating the afferent nerve electrically and recording the reflex muscle response. Because it drives the same monosynaptic arc without the receptor, the H-reflex isolates the excitability of the spinal reflex pathway itself, and its size under different conditions is a standard measure of how descending and segmental inputs bias that pathway (Schieppati, 1987).

At the other end of the nervous system, the startle reflex offers a different probe. A sudden intense stimulus — most conveniently a loud noise — evokes a fast whole-body flinch whose most reliable human component is the eyeblink, recorded as the electromyographic response of the muscle that closes the eye. Standardizing this measurement across laboratories required an agreed methodology for eliciting, recording, and quantifying the eyeblink, which Blumenthal and colleagues codified in the committee guidelines that remain the reference procedure (Blumenthal et al., 2005). The neural circuit of startle is short and well mapped, which is what makes it useful (Koch, 1999).

Startle becomes a tool for studying the brain, and not just the brainstem, through prepulse inhibition: when a weak, non-startling prepulse precedes the startling stimulus by 30–500 milliseconds, the startle response is reduced. This reduction is an operational measure of sensorimotor gating — the nervous system's automatic filtering of a flood of sensory input so that a leading stimulus dampens the response to what follows. Prepulse inhibition is heritable, is disrupted in schizophrenia and related disorders, and translates almost unchanged between rodents and humans, which is why it became a central assay in psychiatry and pharmacology (Geyer et al., 2001). Its brainstem mechanisms and its status as a hallmark of gating have been reviewed in detail (Gómez-Nieto et al., 2020). The demonstration below shows how the prepulse's lead time governs the size of the inhibition.

Reflex, Learning, and Voluntary Action

The reflex looks like the opposite of learning — a response wired in advance — and yet the reflex is where the experimental study of learning began. Pavlov showed that a reflex could be captured by a new stimulus: pair a neutral signal with one that already evokes salivation, and the signal alone comes to evoke it. This conditioned reflex, described in his Conditioned Reflexes, turned the reflex from a fixed reaction into a unit that could be recombined by experience, and it founded the reflexological tradition that shaped classical conditioning and behaviourism (Pavlov, 1927). The reflex, in Pavlov's hands, became the atom of learned as well as innate behaviour.

If reflexes can be attached to new stimuli, can the reflex arc itself be trained? Wolpaw's work answered yes: even the spinal stretch reflex, about as low-level a response as exists, can be operantly conditioned — rewarded up or down over weeks until its size shifts and the change is stored as plasticity within the spinal cord itself. This makes the simplest reflex a tractable model of motor learning and a route to rehabilitation after spinal injury (Thompson & Wolpaw, 2014). Findings like these blur the tidy line between reflex and voluntary action. Prochazka and colleagues argued that the everyday distinction — reflexes as automatic, voluntary acts as willed — does not survive contact with the physiology, since voluntary movements lean on reflex circuitry and reflexes are continuously modulated by descending, goal-related control; the two are better seen as ends of a continuum than as separate categories (Prochazka et al., 2000). The reflex, in other words, is not the rigid mechanism it appears to be, and that is precisely why it remains scientifically interesting.

Worked Example

Consider what determines a reflex's latency — the time from stimulus to response — because latency is what clinicians and researchers actually measure. Model it as the sum of three delays: peripheral conduction, central synaptic delay, and neuromuscular transmission. Let the total peripheral path, afferent plus efferent, be a length L in metres, travelled at a conduction velocity v in metres per second, so the peripheral time is L divided by v, converted to milliseconds by multiplying by one thousand. Add a central delay of about one millisecond for each synapse the signal must cross, n synapses in all, and a fixed one millisecond for transmission across the neuromuscular junction. So total latency T, in milliseconds, is one thousand times L over v, plus n, plus one.

Take a monosynaptic stretch reflex with a one-metre peripheral path — roughly a tall person's leg-and-back loop — carried by fast fibres at fifty metres per second, crossing one central synapse. The peripheral time is one thousand times one over fifty, which is twenty milliseconds; add one for the single synapse and one for the neuromuscular junction, and the latency is twenty-two milliseconds. Now suppose disease demyelinates those fibres and halves the conduction velocity to twenty-five metres per second, changing nothing else. The peripheral time doubles to forty milliseconds, and the latency rises to forty-two milliseconds — nearly double. The single fact this makes vivid is that peripheral conduction dominates the latency: in the healthy reflex it is over ninety percent of the total, which is why a slowed reflex points the clinician straight at the nerve rather than the spinal cord. Finally, keep the fast fibres but make the reflex polysynaptic with four central synapses instead of one: the latency becomes twenty plus four plus one, or twenty-five milliseconds. Three extra synapses cost only three milliseconds — a small, fixed price that nonetheless reliably separates a monosynaptic from a polysynaptic reflex when the peripheral path is held constant. The demonstration above varies L, v, and n and shows the three delays adding up.

Discussion

The reflex earns its foundational place in the nervous-system sciences by being the simplest thing the nervous system does that is still recognisably a complete act: a stimulus, a pathway, a response, all measurable and all mechanistically transparent. Sherrington's insight was that this simplicity is not a curiosity at the margin of behaviour but its building block — that coordinated action is assembled from reflexes competing for the final common path and shaped by reciprocal innervation. A century of work has honoured that insight while complicating it. The reflex arc is real and its latency really does read out the number of synapses crossed; but the arc is also continuously tuned by descending control, its gain set by the state of the whole organism, and even its lowest links are plastic.

The deepest lesson of the reflex for cognitive psychology is where its boundary lies. Pavlov showed the reflex reaching upward into learning; Wolpaw showed learning reaching down into the spinal reflex; Prochazka showed that the reflex-voluntary distinction, so intuitive from the outside, dissolves under the physiology. The reflex is therefore not the trivial base case it is often taught as, a mere warm-up before the interesting cognition. It is the clearest available view of the stimulus-response relationship that every more elaborate cognitive process is built on top of and out of — which is why measuring it precisely, and understanding exactly how it can be modified, continues to matter.

Current Directions

Three lines of work are active. The first is the modern re-mapping of the startle circuit: reviews of the acoustic startle response in mammals are integrating decades of lesion and stimulation work with newer optogenetic and circuit-tracing methods, refining exactly which brainstem neurons generate startle and which forebrain inputs gate it (Zheng & Schmid, 2023). The second is the continued use of prepulse inhibition as a translational assay of sensorimotor gating, where the current effort is to tie the behavioural measure to specific brainstem and midbrain mechanisms so that a gating deficit can be read as a marker of a definable circuit fault rather than a diffuse abnormality (Gómez-Nieto et al., 2020). The third is the clinical and methodological maturation of the H-reflex: recent work revisits the Hoffmann reflex as a quantitative window on spinal excitability in health, injury, and rehabilitation, standardizing how it is elicited and interpreted (Arias-Carrión & Ortega-Robles, 2026). Across all three, the theme is the same — turning a classic reflex measurement into a precise index of an identified neural circuit.

Common Misconceptions

A reflex is just any fast reaction.

A reflex is a specific involuntary response carried by a fixed reflex arc, not merely a quick one. A fast but deliberate reaction — hitting a brake — is a voluntary act routed through the brain, and it is far slower than a spinal reflex precisely because it is not one (Prochazka et al., 2000).

Reflexes bypass the brain entirely.

Many spinal reflexes can close without the brain, but the brain continuously sets their gain through descending pathways, so a reflex is modulated by the state of the whole nervous system rather than sealed off from it (Hultborn, 2006).

Reflexes are fixed and cannot be changed.

Pavlov attached reflexes to new stimuli by conditioning, and even the spinal stretch reflex can be operantly trained up or down, with the change stored as plasticity in the cord itself (Thompson & Wolpaw, 2014).

Glossary

Afferent neuron.

A sensory neuron that carries signals from a receptor toward the central nervous system; the input limb of the reflex arc.

Conditioned reflex.

A reflex response transferred, through Pavlovian conditioning, to a stimulus that did not originally elicit it; Pavlov's extension of the reflex to learning.

Efferent neuron.

A motor neuron that carries commands from the central nervous system to an effector; the output limb of the reflex arc.

Final common path.

Sherrington's term for the motor neuron on which many converging inputs act, the single route by which the nervous system reaches a muscle.

Galvanic skin response.

A reflexive change in the skin's electrical conductance driven by sympathetic sweat-gland activity; an autonomic reflex widely used as an index of arousal.

H-reflex.

The Hoffmann reflex, an electrically evoked analogue of the stretch reflex that bypasses the muscle spindle, used to probe spinal reflex excitability in humans.

Monosynaptic reflex.

A reflex whose arc contains a single central synapse between the afferent and efferent neurons; the stretch reflex is the canonical example.

Muscle spindle.

A stretch receptor embedded in skeletal muscle whose afferent fibres initiate the stretch reflex.

Polysynaptic reflex.

A reflex whose arc includes one or more interneurons, and therefore several central synapses, between input and output; withdrawal reflexes are polysynaptic.

Prepulse inhibition.

The reduction of a startle response when the startling stimulus is preceded by a weak prepulse; a standard operational measure of sensorimotor gating.

Psychophysiology.

The study of the relationship between psychological states and physiological activity, the field under which MeSH files the reflex.

Receptor.

A specialized structure that transduces a stimulus into neural activity; the starting point of a reflex arc.

Reciprocal innervation.

Sherrington's principle that excitation of one muscle group is accompanied by inhibition of its antagonist, coordinating reflex movement.

Reflex arc.

The fixed neural pathway of a reflex, running from receptor through afferent neuron, integration center, and efferent neuron to an effector.

Sensorimotor gating.

The nervous system's automatic filtering of sensory input so that a weak lead stimulus suppresses the response to a following one, indexed by prepulse inhibition.

Startle reflex.

A fast, whole-body protective response to a sudden intense stimulus, measured in humans through its eyeblink component.

Stretch reflex.

The monosynaptic contraction of a muscle in response to its own stretch, the basis of the tendon-tap reflex.

Synapse.

The junction at which one neuron communicates with another; Sherrington coined the term, and the number of synapses in an arc distinguishes monosynaptic from polysynaptic reflexes.

Tendon reflex.

The clinically elicited stretch reflex produced by tapping a muscle's tendon, such as the knee-jerk.

Key Researchers

Terry D. Blumenthal (contemporary). Psychophysiologist at Wake Forest University and lead author of the committee guidelines that standardized how the human startle eyeblink is elicited, recorded, and quantified. Google Scholar - Faculty Page

Mark A. Geyer (contemporary). Distinguished Professor of Psychiatry Emeritus at UC San Diego whose work established prepulse inhibition of the startle reflex as a translational assay of sensorimotor gating. ORCID - Wikipedia - Wikidata - Google Scholar

Hans Hultborn (contemporary). Professor Emeritus of Neuroscience at the University of Copenhagen whose synthesis of spinal reflex circuitry traces the mechanisms of the stretch reflex and its interneuronal control. ORCID - Google Scholar - Faculty Page

Ivan Pavlov (1849-1936). Russian physiologist and 1904 Nobel laureate whose study of the conditioned reflex extended the reflex from fixed innate responses to acquired stimulus-response associations, founding the reflexological tradition. Wikipedia - Wikidata

Charles Scott Sherrington (1857-1952). British neurophysiologist and 1932 Nobel laureate who coined synapse and, in The Integrative Action of the Nervous System, established the reflex arc as the elementary unit of nervous integration. Wikipedia - Wikidata

Jonathan R. Wolpaw (contemporary). Neuroscientist at the National Center for Adaptive Neurotechnologies who showed that even the simplest spinal reflexes are plastic and can be operantly conditioned, providing a model of motor learning and a route to rehabilitation. Faculty Page

Frequently Asked Questions

What is a reflex?
A reflex is an involuntary, stereotyped response to a stimulus, carried by a fixed neural pathway called the reflex arc. Because the pathway is built into the wiring, the response is fast and does not require a conscious decision (Sherrington, 1906).

What is a reflex arc?
The reflex arc is the circuit that produces a reflex: a receptor transduces the stimulus, an afferent neuron carries the signal to an integration center, an efferent neuron carries the command outward, and an effector (a muscle or gland) produces the response (Sherrington, 1906).

What is the difference between a monosynaptic and a polysynaptic reflex?
A monosynaptic reflex has a single central synapse between the afferent and efferent neurons, as in the stretch reflex; a polysynaptic reflex interposes one or more interneurons and so crosses several synapses, as in a withdrawal reflex. Each extra synapse adds about a millisecond of delay (Hultborn, 2006).

Are reflexes learned or innate?
Most basic reflexes are innate, but Pavlov showed a reflex can be attached to a new stimulus through conditioning, and later work showed that even the spinal stretch reflex can be modified by operant training (Pavlov, 1927; Thompson & Wolpaw, 2014).

What is the H-reflex?
The H-reflex, or Hoffmann reflex, is an electrically evoked version of the stretch reflex: stimulating the afferent nerve directly drives the same monosynaptic spinal pathway while bypassing the muscle spindle, which makes it a precise measure of spinal reflex excitability in humans (Schieppati, 1987).

What is prepulse inhibition?
Prepulse inhibition is the reduction of a startle response when the startling stimulus is preceded by a weak prepulse. It is a standard operational measure of sensorimotor gating and is disrupted in schizophrenia and related disorders (Geyer et al., 2001).

Can reflexes be modified by training?
Yes. Beyond Pavlovian conditioning of the response to new stimuli, the size of a spinal reflex itself can be operantly conditioned up or down over weeks, with the change stored as plasticity within the spinal cord, giving both a model of motor learning and a rehabilitation strategy (Thompson & Wolpaw, 2014).

Is a reflex the same as a reaction?
No. A reflex runs over a fixed arc and is involuntary; a reaction such as pressing a brake is a voluntary act routed through the brain and is much slower. The boundary is not perfectly sharp, since voluntary movements use reflex circuitry and reflexes are tuned by descending control (Prochazka et al., 2000).

References