# Reflex Physiology

> FMGE Physiology notes on reflexes: stretch reflex arc, muscle spindle, gamma loop, root values of tendon jerks and UMN signs tested by NBE screening.

- Canonical URL: https://prepelephant.com/topics/fmge/physiology/reflexes-physiology-fmge
- Exam / course: FMGE · Subject: Physiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Reflex Physiology", PrepElephant, https://prepelephant.com/topics/fmge/physiology/reflexes-physiology-fmge

## Direct answer

Striking the patellar tendon stretches the quadriceps, deforming muscle spindles whose Ia afferents fire monosynaptically on the alpha motor neuron — the stretch (myotatic) reflex, the only monosynaptic reflex in the body, with a total delay of roughly 19-24 milliseconds. The spindle is the muscle's length sensor: nuclear bag fibres signal dynamic length change through Ia endings, nuclear chain fibres signal static length through Ia and II endings, and gamma motor neurons keep the spindle taut during shortening through alpha-gamma coactivation. The Golgi tendon organ, an Ib-fibre tension sensor, provides the inverse myotatic reflex — autogenic inhibition — through a disynaptic arc. Root values anchor the exam: biceps C5-C6, triceps C7, knee L2-L4, ankle S1-S2, with superficial abdominal T8-T12, cremasteric L1-L2 and plantar S1-S2. Upper motor neuron lesions convert the plantar response to extensor (Babinski sign) and release clonus and hyperreflexia; lower motor neuron lesions abolish jerks with wasting and fasciculations.

## What you must remember

- **Arc timings:** the knee jerk runs about 19-24 ms total — conduction in, one synapse, conduction out — and its central delay of under 0.5 ms is the classical proof of monosynaptic wiring.
- **Spindle wiring:** Ia (primary, annulospiral) endings report dynamic and static length; II (secondary, flower-spray) endings report static length; gamma-static and gamma-dynamic motor neurons set sensitivity; alpha-gamma coactivation keeps the spindle sensitive throughout contraction.
- **Inverse stretch reflex:** Golgi tendon organs (Ib) sense tension, cause autogenic inhibition through an inhibitory interneuron — the disynaptic protective circuit that prevents excessive force; with the stretch reflex it forms the servo loop around every movement.
- **Root values:** biceps jerk C5-C6, supinator C5-C6, triceps C7, knee jerk L2-L4, ankle jerk S1-S2; abdominal T8-T12, cremasteric L1-L2, plantar S1-S2; the Jendrassik manoeuvre reinforces absent jerks by increasing gamma drive.
- **Withdrawal circuitry:** the flexor (withdrawal) reflex is polysynaptic, accompanied by the crossed extensor reflex supporting posture, and persists after cord transection because it is wholly spinal.
- **UMN versus LMN signature:** upper lesions give hypertonia (clasp-knife after initial resistance), hyperreflexia, clonus, extensor plantar response, no significant wasting; lower lesions give flaccidity, wasting, fasciculations and abolished reflexes.
- **Renshaw cells:** recurrent collateral inhibition of the parent motor neuron (glycinergic) — the model presynaptic circuit targeted indirectly by tetanus toxin disinhibition.

## The knee jerk, dissected

Tap the tendon and the quadriceps is momentarily stretched. Within milliseconds the equatorial regions of its spindles deform; Ia fibres at 70-120 m/s bombard the alpha motor neurons of L3-L4, the quadriceps contracts, the leg swings, and the hamstrings relax through reciprocal innervation — all before the cortex knows, the movement already organised. Now make the movement voluntary: before the alpha neuron fires, gamma neurons co-activate and take up the slack inside the spindle, so the sensor stays on-scale as the muscle shortens. This servo loop explains clinical phenomena directly. Without gamma drive jerks are pendular or absent; with excess gamma bias the loop exaggerates into clonus; and the Jendrassik grip works by raising gamma discharge, sensitising the spindle to the same tap. Cerebellar disease lets the leg swing in a pendular, decayed series — the pendular knee jerk of screening fame.

## How the exam frames it

Two question shapes recur. The first quotes a reflex and asks its root value — ankle jerk S1-S2 (lost in prolapsed L5-S1 disc with sciatica), biceps C5-C6, triceps C7 — and the trick of pairing the absent reflex with the dermatome a level below, because the lesion's sensory root and reflex root may differ by one segment. The second gives a foot that dorsiflexes to plantar stroke: the Babinski sign, normal up to about one year of age before myelination of the corticospinal tract completes, and thereafter an upper motor neuron sign. Expect questions on the monosynaptic status of the stretch reflex, on reciprocal innervation, and on the Golgi tendon organ measuring tension while the spindle measures length.

## Frequently asked questions

### Why is the stretch reflex called monosynaptic?

The Ia afferent synapses directly on the alpha motor neuron with no interneuron, giving a central delay under 0.5 ms — the only monosynaptic reflex in the human body.

### What do nuclear bag and nuclear chain fibres detect?

Nuclear bag fibres (dynamic) signal rate of length change through Ia endings; nuclear chain fibres (static) signal steady length through Ia and II endings, together covering change and position.

### What is the root value of the ankle jerk?

S1-S2, assessed with the Achilles tendon; its loss with sciatica and a sensory deficit over the lateral foot localises an L5-S1 disc prolapse.

### What is alpha-gamma coactivation?

Gamma motor neurons fire with alpha neurons during voluntary movement, keeping the intrafusal fibres taut and the spindle sensitive as the extrafusal muscle shortens.

### What does a positive Babinski sign indicate after infancy?

Dorsiflexion of the great toe on plantar stroking reflects an upper motor neuron lesion (or delayed corticospinal myelination under one year of age), paired with spasticity and hyperreflexia.