The cord is a machine, not a cable
In the previous guide you met the motor system from the top down — the brain deciding, the upper motor neuron carrying the command down the cord, the lower motor neuron being the final wire to the muscle. It is tempting to picture the spinal cord as a bundle of telephone lines: orders go down, sensations come up, and nothing interesting happens in between. That picture is wrong, and unlearning it is the single most useful thing in this guide. The spinal cord is full of local circuits that compute. It can react, balance, and even rhythmically pattern movement, all on its own — sometimes faster than the brain could ever be told about it.
A useful analogy: the brain is a manager, but the spinal cord is a skilled floor worker who does not phone upstairs for every small decision. If your hand touches a hot stove, your arm is already pulling back before you consciously feel the pain — the cord handled it locally and told the brain afterward. This local autonomy is not a flaw or a shortcut; it is the design. And it has a direct consequence for rehabilitation: when injury cuts the brain off from the cord, those local circuits do not die. They keep running, now without their usual supervision — and that is where much of what we see after stroke and spinal cord injury comes from.
The stretch reflex: the cord's simplest loop
Start with the most famous trick in clinical medicine: the doctor taps just below your kneecap and your lower leg kicks out. This is the stretch reflex, and it is the cleanest example of the cord computing on its own. The tap stretches the tendon, which stretches the thigh muscle, which stretches a tiny sensor buried inside the muscle called a muscle spindle. That sensor fires a signal up a sensory nerve into the cord, where it connects — directly, with no other neuron in between — to the motor neuron that drives the same muscle. The muscle contracts and the leg kicks. The whole loop never reaches the brain. This is what the glossary means by the stretch reflex and deep tendon reflex.
Why would the body bother with this? Because the stretch reflex is a length-keeping servo. When you stand and your knee starts to buckle, the thigh muscle is suddenly stretched; the reflex catches it and stiffens the muscle before you can fall. It is a fast, automatic guard against being knocked out of position. There is a quieter partner here too: as the agonist is told to fire, the cord simultaneously inhibits the opposing muscle so it relaxes out of the way — a built-in courtesy called reciprocal inhibition. This is the same agonist-versus-antagonist teamwork you met in the kinesiology track, but now you can see where the timing actually comes from: it is wired into the cord itself, not micromanaged from above.
Muscle tone: the resistance you feel at rest
Now for a word that gets used loosely everywhere and precisely in rehab: tone. When a clinician takes a relaxed patient's arm and bends it back and forth, they feel a certain springy resistance. That resistance, in a muscle the person is not voluntarily using, is muscle tone — the topic of muscle tone and its neural control. It is emphatically not how "toned" a muscle looks in a mirror, and it is not muscle strength. It is the background tension a resting muscle has, and it comes from two ingredients mixed together: the passive stiffness of the muscle and its tissues, and a steady, low-level hum of the stretch-reflex circuit keeping a baseline of activity.
Normal tone is a quiet readiness — enough to hold posture without locking you up. The brain's descending pathways spend a lot of effort keeping this hum turned down to the right level; a great deal of what comes down from above is not "go" but "easy, not yet." That detail matters enormously, because it predicts what happens when the brake line is cut. Remove descending control and the stretch-reflex hum, no longer damped, can climb. Tone goes up. The limb, even a weak or paralyzed one, becomes stiff and hard to move. This is the heart of why a patient can be both profoundly weak and uncomfortably tight at the same time — a combination that baffles families who reasonably expect weakness to mean floppiness.
Central pattern generators: rhythm without thinking
The cord can do more than react — it can keep a beat. Built into the cord are networks of neurons that, once switched on, produce the rhythmic, alternating left-right pattern of stepping all by themselves, without needing the brain to command each muscle in turn. These are central pattern generators, and the glossary term is central pattern generators. Think of them as a built-in drum machine for walking: the brain presses "start" and sets the tempo, but the cord plays the actual rhythm. It is why a cat with its cord experimentally separated from its brain can still produce stepping movements on a treadmill — the leg rhythm was never really in the brain to begin with.
When injury unmasks the machinery
Put the pieces together and the after-effects of damage start to make sense. When an upper motor neuron pathway is cut — by a stroke, by spinal cord injury — the result is the upper motor neuron syndrome, and it has two faces. The negative features are losses: weakness, slowness, loss of dexterity, because the command to move is no longer getting through. The positive features are additions — things that appear because the cord's own circuits are now running unsupervised: brisk reflexes, spasticity (a velocity-dependent over-response of the stretch reflex, where moving a limb faster makes it resist harder), and signs like a sustained rhythmic beating called clonus or an up-going big toe. Nothing new was created; the brake came off old machinery.
There is a twist of timing here that surprises people. Immediately after a severe spinal cord injury, the limbs are often not stiff at all — they are flaccid, floppy, and the reflexes are gone. This is spinal shock, the term is spinal shock, and it is a temporary state in which the cord below the injury goes electrically silent for days to weeks. Then, as the cord "comes back online" without its brain connection, reflexes return and over the following weeks tone climbs and spasticity emerges. A family who saw a soft, limp leg in the first week and a stiff, resistant one a month later are not imagining things and the patient is not getting worse in the way they fear — they are watching the natural arc of an unmasked spinal cord.
Reading tone and reflexes as a map
Because reflexes and tone are wired segment by segment, they let an examiner localize where the trouble is — not just that something is wrong, but where along the wiring. The simple table below sketches the contrast that organizes the whole physiatric movement exam. You will meet the formal tools for measuring this later in the assessment track; for now, hold the pattern, because it is the skeleton everything else hangs on.
UPPER MOTOR NEURON LOWER MOTOR NEURON
(brain / cord pathway) (nerve to muscle)
Weakness yes, often a pattern yes, the muscles supplied
Muscle tone increased (spastic) decreased (flaccid)
Deep tendon reflexes brisk / overactive reduced / absent
Muscle wasting late, mild (from disuse) early, marked
Extra signs clonus, up-going toe muscle twitches (fasciculations)Step back and the big idea is this: the spinal cord is not where movement merely passes through — it is where a surprising amount of movement is actually made. Reflexes, reciprocal inhibition, tone regulation, and stepping rhythm all live partly or wholly in the cord. The brain's job is as much to shape, time, and restrain this machinery as to command it. When you grasp that, the after-effects of neurological injury stop looking like random damage and start looking like a machine running with one hand taken off the controls. That reframing is exactly what makes the next guides — on motor learning and neuroplasticity — make sense: therapy works by retraining a nervous system that still has a great deal of its own machinery intact.