stretch reflex and deep tendon reflex

When a doctor taps just below your kneecap and your lower leg kicks out on its own, you are watching the body's fastest safety loop in action. You did not decide to kick; a tiny stretch of the muscle was sensed and answered before your brain even knew. This is the stretch reflex, and the knee tap is the most famous example of testing it — the deep tendon reflex.

Inside muscles are tiny stretch sensors called muscle spindles. A sudden stretch — like a tap on the tendon — fires these sensors, which send a signal straight into the spinal cord. There it connects to a lower motor neuron that immediately tells the same muscle to contract, pulling back against the stretch. The whole loop runs through the spinal cord without waiting for the brain, which is why it is so fast and why we call it a reflex. The deep tendon reflex test (knee, ankle, biceps, and so on) is simply the clinician triggering this loop on purpose to check that the sensing nerve, the spinal segment, and the motor nerve are all working.

These taps are gold in the clinic because the answer is graded. A normal brisk response means the loop is intact. A weak or absent response points to a lower motor neuron or nerve problem (the relay is broken). An exaggerated, jumpy response — sometimes with repeating beats called clonus — points to an upper motor neuron problem, because the brain's normal damping of the reflex has been lost. The same reflex also underlies muscle tone and, when it runs out of control, much of spasticity, so understanding it explains a great deal of what therapists feel in a stiff limb.