Spinal cord reflexes !
- Regulation
- Exam priority: high
A reflex is a fast, automatic loop through the spinal cord: tap the tendon and the muscle jerks, with no decision from the brain.
Updated 2026-11-01
Also called: Stretch reflex, Deep tendon reflexes, Withdrawal reflex
See it in 3D: Spinal cord · Grey matter of the cord · C5 nerve root · C6 nerve root · C7 nerve root · L4 nerve root · S1 nerve root
What it is
Spinal reflexes are circuits that turn a sensory input into a motor output inside the cord. The stretch reflex is monosynaptic: a muscle spindle afferent excites the alpha motor neuron of the same muscle. Others are polysynaptic: the withdrawal reflex with its crossed extension, and the tendon organ reflex that relaxes a muscle under high tension. Testing them tells the examiner whether a lesion sits in a root, in the cord or above it.
Deep tendon reflexes and their roots
| Parameter | Value | Note |
|---|---|---|
| Biceps | C5–C6 | |
| Brachioradialis | C6 | |
| Triceps | C7 | C8 contributes |
| Patellar (knee jerk) | L3–L4 | mainly L4 |
| Achilles (ankle jerk) | S1 |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Gamma motor neuron activity: Keeps spindles taut and raises reflex sensitivity
- +Loss of descending inhibition (upper motor neuron lesion): Hyperreflexia, clonus and spasticity
- −Ib input from Golgi tendon organs: Inhibits the motor neurons of the same muscle under high tension
- −Renshaw cells (recurrent inhibition, glycine): Limit and stabilize motor neuron firing
- −Damage to root, nerve or anterior horn (lower motor neuron): Reduced or absent reflexes
Main spinal reflexes
| Reflex | Receptor | Synapses | Effect |
|---|---|---|---|
| Stretch (myotatic) | Muscle spindle (Ia) | Monosynaptic | Contracts the stretched muscle |
| Reciprocal inhibition | Muscle spindle (Ia) → inhibitory interneuron | Disynaptic | Relaxes the antagonist |
| Tendon organ (autogenic inhibition) | Golgi tendon organ (Ib) | Disynaptic | Relaxes a muscle under high tension |
| Withdrawal (flexor) | Nociceptors | Polysynaptic | Flexes the limb away from the stimulus |
| Crossed extensor | Nociceptors of the opposite limb | Polysynaptic | Extends the other limb to bear weight |
Clinical notes
- An L5–S1 disc herniation compressing S1: reduced ankle jerk, weak plantar flexion, numb lateral foot.
- An L3–L4 disc compressing L4: reduced knee jerk and weak knee extension.
- Cauda equina syndrome: lost ankle reflexes, saddle anesthesia and bladder dysfunction; a surgical emergency.
- Clonus and brisk reflexes point to a lesion above the reflex arc (brain or cord).
Test yourself
Think of the answer, then tap the question to see it.
Why is the knee jerk absent in the first days after a thoracic cord injury but brisk months later?
At first, spinal shock removes tonic descending facilitation and the cord below the lesion falls silent. Later, loss of descending inhibition, receptor supersensitivity and sprouting make the reflex arc hyperexcitable.
Tapping the patellar tendon contracts the quadriceps and relaxes the hamstrings. What circuit relaxes them?
Reciprocal inhibition: the same Ia afferent excites an inhibitory interneuron in the cord that inhibits the hamstring motor neurons.
A patient with sciatica has an absent ankle reflex and numbness of the lateral foot. Which root and which disc?
The S1 root, most often compressed by an L5–S1 disc herniation.
What is the clasp-knife phenomenon and which receptor was classically blamed?
In spasticity, resistance to passive stretch gives way suddenly. It was attributed to Golgi tendon organ (Ib) inhibition, although other spinal mechanisms also contribute.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Muscle spindle and Golgi tendon organ
- Upper versus lower motor neuron lesions
- Motor cortex and the corticospinal tract
- Micturition and the bladder reflex
- Motor units, recruitment and summation
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 14.3 Motor Responses (OpenStax (Rice University), 2022); 16.4 The Sensory and Motor Exams (OpenStax (Rice University), 2022); Spinal Cord Injuries (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 55). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.