[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) › Spinal cord reflexes [Physiology](https://durgical.com/en/physiology/) · [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) # 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](https://durgical.com/en/anatomy/spinal-cord/) · [Grey matter of the cord](https://durgical.com/en/anatomy/grey-matter-of-spinal-cord/) · [C5 nerve root](https://durgical.com/en/anatomy/c5-spine-and-pelvis/) · [C6 nerve root](https://durgical.com/en/anatomy/c6-spine-and-pelvis/) · [C7 nerve root](https://durgical.com/en/anatomy/c7/) · [L4 nerve root](https://durgical.com/en/anatomy/l4/) · [S1 nerve root](https://durgical.com/en/anatomy/s1/) ## Key points - Stretch reflex: Ia afferent → α motor neuron of the same muscle (monosynaptic), plus reciprocal inhibition of the antagonist through an inhibitory interneuron. - Reflex roots: biceps C5–C6, brachioradialis C6, triceps C7, patellar L3–L4, Achilles S1. - Gamma motor neurons set spindle sensitivity; descending input and the Jendrassik maneuver raise reflex gain. - The withdrawal (flexor) reflex is polysynaptic and comes with crossed extension of the opposite limb to keep the body upright. - Spinal shock after an acute cord transection: areflexia and flaccidity below the lesion for days to weeks, then hyperreflexia. - An extensor plantar response (Babinski sign) marks an upper motor neuron lesion; it is normal in infants until about 1–2 years of age. ## 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). ## Memory aid Reflex roots climb with the body: ankle S1, knee L4, then the arm, biceps 5–6 and triceps 7. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/spinal-cord-reflexes/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) - [Grey matter of the cord](https://durgical.com/en/anatomy/grey-matter-of-spinal-cord/) - [C5 nerve root](https://durgical.com/en/anatomy/c5-spine-and-pelvis/) - [C6 nerve root](https://durgical.com/en/anatomy/c6-spine-and-pelvis/) - [C7 nerve root](https://durgical.com/en/anatomy/c7/) - [L4 nerve root](https://durgical.com/en/anatomy/l4/) - [S1 nerve root](https://durgical.com/en/anatomy/s1/) ## Related topics - [Muscle spindle and Golgi tendon organ](https://durgical.com/en/physiology/muscle-spindle-and-golgi-tendon-organ/) - [Upper versus lower motor neuron lesions](https://durgical.com/en/physiology/upper-versus-lower-motor-neuron-lesions/) - [Motor cortex and the corticospinal tract](https://durgical.com/en/physiology/motor-cortex-and-the-corticospinal-tract/) - [Micturition and the bladder reflex](https://durgical.com/en/physiology/micturition-and-the-bladder-reflex/) - [Motor units, recruitment and summation](https://durgical.com/en/physiology/motor-units-recruitment-and-summation/) ## Related conditions - [Spinal cord injury](https://durgical.com/en/conditions/spinal-cord-injury/) - [Lumbar Disc Herniation](https://durgical.com/en/conditions/lumbar-disc-herniation/) - [Cervical radiculopathy](https://durgical.com/en/conditions/cervical-radiculopathy/) - [Cauda equina syndrome](https://durgical.com/en/conditions/cauda-equina-syndrome/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses) (OpenStax (Rice University), 2022); [16.4 The Sensory and Motor Exams](https://openstax.org/books/anatomy-and-physiology-2e/pages/16-4-the-sensory-and-motor-exams) (OpenStax (Rice University), 2022); [Spinal Cord Injuries](https://www.ncbi.nlm.nih.gov/books/NBK560721/) (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). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/spinal-cord-reflexes/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/spinal-cord-reflexes/