spinal shock
Right after a serious spinal cord injury, the cord below the damage does not just lose its connection to the brain — for a while it seems to switch off entirely, like a circuit board that has tripped. Even the simple automatic reflexes that normally live in the cord (the knee-jerk, for example) go silent, the muscles below go completely floppy, and tone disappears. This temporary state of a 'stunned' cord is called spinal shock, and understanding it prevents a serious early misjudgement.
In spinal shock the cord below the lesion is hyporeflexic and flaccid: no reflexes, no spasticity, paralysed limbs that feel limp, and often no bladder or bowel activity. It usually begins immediately and lasts hours to days to a few weeks, then gradually resolves as the cord's reflex machinery comes back online. One classic early sign that the spinal-shock phase is ending is the return of the bulbocavernosus reflex (a small reflex contraction of the anal sphincter when the head of the penis or the clitoris is squeezed). As the weeks pass, reflexes reappear and often become exaggerated, and flaccid limbs may turn spastic — this is not the cord getting worse but its lower-motor-neuron circuits waking up below a now-disconnected brain.
Spinal shock matters enormously for prognosis, because during it you cannot tell how bad the injury truly is. A completely flaccid, areflexic body looks like the worst possible (complete) injury, but some of that is just the temporary shutdown. That is exactly why the definitive complete-versus-incomplete classification is not locked in until spinal shock has resolved — judging too early can hand someone a falsely hopeless prognosis. It is also a different thing entirely from neurogenic shock, the dangerous drop in blood pressure and heart rate that can accompany high injuries; sharing the word 'shock' causes endless confusion.
On the day of injury a young woman's legs are utterly limp with no reflexes at all. Three weeks later her knee-jerks return and grow brisk, and her legs begin to stiffen. The team explains this is spinal shock resolving — the cord's reflexes waking up below the injury — not a sign of further damage.
Returning, brisk reflexes after spinal shock are the cord 'waking up', not the injury worsening.
Do not confuse spinal shock (a temporary loss of cord reflexes) with neurogenic shock (a circulatory collapse from lost sympathetic tone in high injuries). They share a word but are unrelated; and because spinal shock masks the true severity, the definitive AIS grade is deferred until it resolves.