Physiology→Neurophysiology→Spindles, GTOs and reflexes

MMed Phase I · Neurophysiology · Lesson 15

One receptor lies alongside the muscle.
The other lies in line with it.

Orientation

Rapid review

Prerequisites

What you should already have

Lesson 14 — motor control; Lesson 12 — sensory receptors.

Estimated study time

About 55 minutes

Plus the time it takes to redraw this lesson’s figures from memory, which is the fastest way to find out what you have not understood.

Why it matters

Where this shows up

Reflex testing is how you localise a lesion, monitor a neuraxial block and recognise magnesium toxicity. The gamma loop explains why muscle tone persists under anaesthesia until you either deepen it or paralyse the patient.

Learning outcomes

By the end of this lesson you should be able to:

Describe muscle spindle structure, its intrafusal fibre types and its sensory and motor innervation.
Explain alpha-gamma coactivation and why the spindle would otherwise fall silent during contraction.
Contrast the muscle spindle with the Golgi tendon organ by arrangement, stimulus, afferent and reflex effect.
Draw the monosynaptic stretch reflex arc and explain reciprocal innervation.
Describe the inverse stretch reflex and the clasp-knife response.
Draw the polysynaptic withdrawal reflex with its crossed extensor component, and identify the ascending limb that produces the sensation.
Relate spinal reflexes to clinical examination, spinal shock and the effects of anaesthesia.

Together these settle one syllabus objective: Spinal reflexes, muscle spindle and Golgi tendon organ. Tick it on the Physiology objective list once you can do all of the above without notes.

The minimum high-yield framework

Five components of a reflex arc — receptor, afferent neuron, integrating centre, efferent neuron, effector.
Spindle: in parallel, measures length. Ia afferent, monosynaptic excitatory to its own muscle — the only monosynaptic reflex in the body.
Golgi tendon organ: in series, measures tension. Ib afferent, disynaptic inhibitory to its own muscle.
The gamma loop keeps the spindle sensitive during contraction. Alpha-gamma co-activation means the spindle never falls silent as the muscle shortens.
Reciprocal innervation — the agonist is excited while the antagonist is inhibited through an interneuron.
Muscle tone is the resistance to passive stretch, and it is a low-grade continuous stretch reflex, maintained by gamma drive and modulated by descending tracts.
Withdrawal reflex — polysynaptic, multisegmental, ipsilateral flexion; with crossed extension contralaterally at higher stimulus intensity.

The unit of spinal function

The reflex arc

Five components, and two properties that follow from how many synapses lie between them.

Component	In the stretch reflex	In the withdrawal reflex
Receptor	Muscle spindle	Nociceptor — free nerve ending
Afferent neuron	Ia fibre, cell body in the dorsal root ganglion	Aδ and C fibres, cell body in the dorsal root ganglion
Integrating centre	The synapse itself — one, in the ventral horn	Interneuron pools across several segments
Efferent neuron	α motor neuron to the same muscle	α motor neurons to flexors across several segments
Effector	The stretched muscle	The flexor muscles of the limb

	Monosynaptic	Polysynaptic
Synapses in the arc	One	Two or more
Example	Stretch reflex — the only one	Withdrawal, crossed extensor, inverse stretch, most autonomic reflexes
Latency	Short and fixed	Longer and more variable, increasing with the number of interneurons
Response to stimulus strength	Little variation	Graded — a stronger stimulus recruits more interneurons and spreads to more segments (irradiation)
Susceptible to central modulation	Relatively little	Considerably — interneurons are the main target of descending inhibition and of many drugs

Length

The muscle spindle and the gamma loop

A receptor with its own motor supply, which is the only way it can stay useful while the muscle it monitors is shortening.

Original teaching diagram

Two proprioceptors, two reflexes, opposite signs

The muscle spindle lies in parallel with the extrafusal fibres, so it is stretched when the muscle lengthens and unloaded when it shortens. Its Ia afferent makes a monosynaptic excitatory connection onto the α motor neuron of its own muscle — the only monosynaptic reflex in the body — and a disynaptic inhibitory connection, through an interneuron, onto the antagonist. That second limb is reciprocal innervation, and it is why a stretch reflex produces movement rather than co-contraction.

The Golgi tendon organ lies in series at the musculotendinous junction, so it is loaded by tension however that tension arises — including by active contraction, which unloads the spindle. Its Ib afferent acts through an inhibitory interneuron, so the reflex is disynaptic and inhibitory to its own muscle. Series against parallel is the whole distinction: it determines what each organ measures and therefore the sign of its reflex.

The spindle is an encapsulated fusiform bundle of intrafusal fibres lying in parallel with the force-generating extrafusal fibres. It contains two kinds of intrafusal fibre and receives two kinds of afferent:

Element	Description	Signals
Nuclear bag fibres	Nuclei clustered in a central bag; typically 2 per spindle	Predominantly dynamic — the rate of change of length
Nuclear chain fibres	Nuclei in a row; typically 4-5 per spindle, shorter and thinner	Predominantly static — absolute length
Primary (annulospiral) ending	Ia afferent, large myelinated, wraps the central region of both fibre types	Both static length and, strongly, rate of change — so it fires a burst on rapid stretch
Secondary (flower-spray) ending	II afferent, smaller, mainly on chain fibres	Static length only
γ (fusimotor) efferent	Small motor neuron from the anterior horn, supplying the contractile poles of the intrafusal fibres	Not an output — it sets the sensitivity of the spindle

The stretch reflex itself is the Ia afferent making a monosynaptic excitatory connection onto the α motor neurons of its own muscle, and a disynaptic inhibitory connection onto those of the antagonist. Tapping the patellar tendon stretches quadriceps, the spindles fire, quadriceps contracts and the hamstrings relax. Its clinical value is that it is the shortest possible pathway: testing a tendon reflex tests one specific segment, and only that segment.

Reflex	Root	Peripheral nerve
Biceps	C5, C6	Musculocutaneous
Supinator (brachioradialis)	C5, C6	Radial
Triceps	C7, C8	Radial
Knee (patellar)	L3, L4	Femoral
Ankle (Achilles)	S1, S2	Tibial

Tension

The Golgi tendon organ

In series, so it is loaded by tension however that tension is produced — including by the contraction that unloads the spindle.

The Golgi tendon organ is an encapsulated ending among the collagen bundles of the musculotendinous junction, arranged in series with the muscle fibres. Its Ib afferent acts through an inhibitory interneuron, so the reflex is disynaptic and inhibitory to its own muscle and excitatory to the antagonist — the inverse stretch reflex, or autogenic inhibition.

	Muscle spindle	Golgi tendon organ
Location	Within the belly, in parallel with extrafusal fibres	At the musculotendinous junction, in series
Stimulus	Muscle length and its rate of change	Muscle tension
Response to passive stretch	Increases firing	Increases firing, but less sensitively
Response to active contraction	Decreases firing (unloaded), unless γ co-activation intervenes	Increases firing — this is the discriminating answer
Afferent	Ia (primary) and II (secondary)	Ib
Efferent supply to the receptor	γ motor neurons	None
Synapses in the reflex	One — monosynaptic	Two — disynaptic, via an inhibitory interneuron
Effect on its own muscle	Excitatory	Inhibitory
Function	Maintains length; the servo mechanism of tone and posture	Regulates tension; contributes to smooth force control and, at extremes, protects against damaging load

The resting state

Muscle tone

Not a separate phenomenon, but the stretch reflex operating continuously at a low level.

Tone therefore depends on three things, and abnormalities of tone map onto them:

Spindle sensitivity, set by γ drive from the anterior horn.
The integrity of the reflex arc — afferent, synapse and α motor neuron.
Descending modulation, principally inhibitory from cortex through the reticulospinal system, and facilitatory from the vestibulospinal system.

Pattern	Character	Lesion
Spasticity	Velocity-dependent — resistance increases the faster you move the limb — and gives way abruptly (clasp-knife). Pyramidal distribution	Upper motor neuron. Loss of descending inhibition releases the stretch reflex
Rigidity	Not velocity-dependent; present equally throughout the range (lead-pipe), or interrupted by tremor (cogwheel)	Extrapyramidal — basal ganglia, as in Parkinsonism
Hypotonia	Reduced or absent resistance; flaccid	Lower motor neuron lesion, spinal shock, cerebellar disease, or deep anaesthesia

Spinal shock

Immediately after complete cord transection, all reflex activity below the lesion is abolished: flaccid paralysis, areflexia, loss of tone, urinary retention and loss of sympathetic vasomotor tone (neurogenic shock, if the lesion is above T6). It lasts days to weeks, and is attributed to the sudden withdrawal of tonic descending facilitation from the reticulospinal and vestibulospinal tracts.

Recovery proceeds in a recognised order — reflexes return first, then become exaggerated as denervation supersensitivity and axonal sprouting develop, giving the chronic spastic picture. The anaesthetic significance is direct: suxamethonium is safe within about the first 24 hours, but from roughly 24-48 hours until at least 6-12 months the upregulation of extrajunctional nicotinic receptors makes it dangerous, and after the acute phase these patients are at risk of autonomic dysreflexia.

Protective reflexes

Withdrawal and crossed extension

The prototype polysynaptic reflex, and the one you see under anaesthesia.

A noxious stimulus activates Aδ and C nociceptors. Their afferents enter the dorsal horn and excite interneuron pools that spread over several segments — necessarily, since flexing a limb requires muscles from several myotomes. Those interneurons excite ipsilateral flexor motor neurons and, through inhibitory interneurons, inhibit the extensors. The limb withdraws.

With a stronger stimulus, commissural interneurons cross the cord and produce the crossed extensor reflex: the contralateral limb extends to take the weight. The response also shows irradiation — increasing stimulus strength recruits more segments and a larger response — and after-discharge, in which the response outlasts the stimulus because of reverberating interneuron circuits.

	Stretch reflex	Withdrawal reflex
Stimulus	Muscle stretch	Noxious cutaneous stimulus
Receptor	Muscle spindle	Nociceptor
Afferent	Ia	Aδ and C
Synapses	One	Many
Segments involved	One	Several — multisegmental
Effect	Contraction of the stretched muscle	Flexion of the whole limb, and contralateral extension
Graded with stimulus strength	Little	Markedly
Present under general anaesthesia	Depressed but present; abolished by deep anaesthesia or neuromuscular blockade	Suppressed by adequate depth; its presence at incision is a sign of inadequate anaesthesia or analgesia

Previously examinedOctober 2024 — the pathway from receptor to effector muscle when a patient moves in response to a surgical stimulus. Worked answers in the library

Viva prompt

Compare the muscle spindle with the Golgi tendon organ.

Answer it out loud, then reveal the model answer

Structure it on the geometry, because every other difference follows from it.

Position. The spindle lies in parallel with the extrafusal fibres, within the muscle belly. The Golgi tendon organ lies in series with them, at the musculotendinous junction.

Adequate stimulus. A parallel element is stretched when the muscle lengthens, so the spindle measures length and its rate of change. A series element bears the force transmitted through it, so the Golgi tendon organ measures tension.

Behaviour during active contraction — the discriminating question. Contraction shortens the muscle, so the parallel spindle is unloaded and its firing falls, unless γ co-activation takes up the slack. The same contraction raises tension, so the series Golgi tendon organ increases its firing. Passive stretch increases both.

Reflex connections. Ia from the spindle is monosynaptic and excitatory to its own muscle. Ib from the Golgi tendon organ acts through an inhibitory interneuron, so it is disynaptic and inhibitory to its own muscle.

Innervation of the receptor. Only the spindle has a motor supply — the γ efferents to the contractile poles of the intrafusal fibres. It is not an output pathway; it sets the gain of the receptor, and through alpha-gamma co-activation keeps the spindle sensitive during shortening.

Function. The spindle is the sensor of a length servo — the basis of muscle tone, posture and the tendon reflexes. The Golgi tendon organ provides tension feedback for smooth force control, and at extremes protects against damaging load.

Likely follow-ups

What happens to the discharge of each during active contraction of the muscle?
What is the function of the gamma efferent supply?

Retrieval check

Why does the muscle spindle need its own motor innervation when no other sensory receptor does?
Say it out loud, then check
Because it lies in parallel with the muscle it monitors, so contraction unloads it and it would fall silent exactly when length information is most needed. The gamma efferents shorten the contractile poles alongside the extrafusal fibres, keeping the sensory region taut so the spindle stays sensitive throughout the contraction.
Give the physiological explanation for spasticity after a stroke.
Say it out loud, then check
The reflex arc is intact but descending inhibition, mainly reticulospinal, has been lost. The stretch reflex is released and operates at abnormally high gain, so resistance to passive stretch is increased and is velocity-dependent, with hyperreflexia and clonus. Because the muscle is still innervated it does not waste or fasciculate.

Anaesthetic application

Reflexes in anaesthetic practice

Reflexes as monitors

Tendon reflexes are the standard bedside monitor of magnesium toxicity: loss of the patellar reflex is the recognised early warning sign, appearing before respiratory depression, and is why it is checked serially during magnesium infusion in pre-eclampsia. Reflex level is also used to assess the upper limit of a neuraxial block and the resolution of regional anaesthesia.

Anaesthesia and reflex activity

Polysynaptic reflexes are far more susceptible to anaesthetic depression than monosynaptic ones, because every additional synapse is another site of action. Volatile agents depress spinal reflex transmission directly — a substantial part of the immobility they produce is spinal, not cortical. Movement in response to incision is therefore a spinal reflex, and its presence indicates inadequate depth or analgesia rather than awareness.

Reflexes and the airway

Laryngospasm is a protective polysynaptic reflex: afferents in the internal branch of the superior laryngeal nerve, integration in the brainstem, and efferents in the recurrent laryngeal nerve producing sustained adduction. Like other polysynaptic reflexes it is exaggerated at light planes of anaesthesia and abolished at deep ones, which is why the extremes of anaesthetic depth are safe and the middle is not.

Consolidation

The lesson in one paragraph

A reflex is a stereotyped involuntary response mediated by an arc of five components: receptor, afferent neuron, integrating centre, efferent neuron and effector. The muscle spindle lies in parallel with the extrafusal fibres and therefore measures length and its rate of change, signalling through Ia and II afferents; its Ia connection is monosynaptic and excitatory to its own muscle — the only monosynaptic reflex in the body — and disynaptically inhibitory to the antagonist, which is reciprocal innervation. Because contraction unloads a parallel receptor, the spindle alone among sensory receptors has a motor supply: the γ efferents, co-activated with α motor neurons, shorten the intrafusal poles and keep the spindle sensitive throughout contraction. The Golgi tendon organ lies in series at the musculotendinous junction and therefore measures tension, including the tension of active contraction that unloads the spindle; its Ib afferent acts through an inhibitory interneuron, so the inverse stretch reflex is disynaptic and inhibitory. Muscle tone is the resistance to passive stretch and is itself a continuous low-grade stretch reflex, so it depends on γ drive, on an intact arc, and on descending modulation — loss of descending inhibition gives velocity-dependent spasticity, basal ganglia disease gives non-velocity-dependent rigidity, and an interrupted arc gives flaccidity. The flexor withdrawal reflex is polysynaptic and multisegmental with contralateral crossed extension, shows irradiation and after-discharge, and is depressed by anaesthesia far more readily than the monosynaptic stretch reflex.