Spinal shock and the isolated cord

Mechanism
Exam priority: medium

When the spinal cord is cut, everything below goes silent for a while; later the cord's own reflexes return, sometimes too strongly.

Updated Nov 13, 2026

Also called: Mass reflex, Autonomic dysreflexia

See it in 3D: Spinal cord · Grey matter of the cord · Sympathetic trunk

Key points

Spinal shock = loss of reflexes and tone below the lesion (a neurological state); neurogenic shock = hypotension with bradycardia from lost sympathetic outflow (a hemodynamic state).
Cause: sudden loss of tonic descending facilitation (reticulospinal, vestibulospinal, corticospinal), not damage to the cord below the lesion.
Reflexes return over days to weeks; the bulbocavernosus reflex is often one of the first to reappear; later hyperreflexia and spasticity develop.
Mass reflex: in a chronic isolated cord, one strong stimulus below the lesion triggers widespread flexor spasm, bladder and bowel emptying, sweating and a rise in blood pressure.
Autonomic dysreflexia (lesions at or above about T6): a stimulus below the lesion causes a sympathetic surge with severe hypertension; reflex bradycardia and flushing appear above the lesion.

What it is

Spinal shock is the temporary loss of all cord function below an acute transection: reflexes, muscle tone and autonomic reflexes. It happens because cord neurons lose the constant excitatory drive they get from the brain. Over days to weeks the isolated cord regains its own reflexes, which may then become exaggerated and can trigger dangerous autonomic surges.

How it works, step by step

Cord neurons below the lesion normally sit close to threshold thanks to steady excitatory input from brainstem and cortex.
Transection removes that input at once, so motor and autonomic neurons are hyperpolarized and reflexes fail (areflexia, flaccid limbs).
Sympathetic preganglionic neurons (T1–L2) lose their drive; vascular tone and, with high lesions, cardiac sympathetic drive fall.
Blood pressure falls sharply; with lesions above the cardiac outflow, unopposed vagal tone slows the heart.
Over days to weeks, cord neurons become more excitable (receptor up-regulation, sprouting of remaining synapses), and reflexes return.
Without descending inhibition, reflexes overshoot: brisk tendon reflexes, clonus, spasms and a Babinski sign.
Autonomic reflexes inside the cord also return, so bladder distension can drive a large, unchecked sympathetic discharge below the lesion.

Spinal shock vs neurogenic shock

Feature	Spinal shock	Neurogenic shock
What it is	Loss of reflexes and tone below the lesion	Distributive shock from loss of sympathetic tone
Key signs	Flaccid paralysis, areflexia, atonic bladder	Hypotension, bradycardia, warm dry skin
Level	Any cord level	Usually cervical or high thoracic (above about T6)
Course	Reflexes return over days to weeks	Improves as vascular tone recovers

Clinical notes

A cord-injured patient who is hypotensive and bradycardic with warm skin fits neurogenic shock; a tachycardic, cold patient suggests bleeding instead.
Autonomic dysreflexia is often triggered by a full bladder, a blocked catheter or constipation; removing the stimulus is the first step.
During spinal shock the bladder is atonic and retains urine; later a reflex (spastic) bladder empties without voluntary control.

Test yourself

Think of the answer, then tap the question to see it.

What is the main reason reflexes disappear below an acute spinal cord transection?

Cord neurons suddenly lose the tonic excitatory input from brainstem and cortex, so they fall far below threshold; the reflex circuits themselves are intact.

A patient with a C6 cord injury has BP 80/50 mmHg, heart rate 48/min and warm skin. What is the likely cause?

Neurogenic shock: loss of sympathetic vascular tone causes vasodilation, and loss of cardiac sympathetic drive leaves vagal tone unopposed, causing bradycardia.

Why does a patient with a T4 lesion and a blocked catheter develop severe hypertension with bradycardia?

Bladder distension triggers a cord sympathetic reflex below the lesion that the brain cannot inhibit; baroreceptors sense the hypertension and slow the heart through the intact vagus.

What is the mass reflex?

In a chronic isolated cord, a strong stimulus below the lesion spreads through cord circuits to cause widespread flexor spasm, bladder and bowel emptying, sweating and raised blood pressure.

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Connections

See it in 3D3

The structures of Durgical's 3D anatomy where it happens.

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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: Spinal Cord Injuries (StatPearls (NCBI Bookshelf), 2025); Neuroanatomy, Spinal Cord (StatPearls (NCBI Bookshelf), 2025); Spinal Cord Injury | MedlinePlus (MedlinePlus, US National Library of Medicine, 2025).

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