A knee tap can bypass the brain and still move your leg
A reflex is an almost instant, unplanned response to a stimulus traveling a reflex arc—sometimes through a single synapse, sometimes many. Signals need not reach awareness; the spinal cord can finish the job. René Descartes sketched the idea in 1664; Marshall Hall named and explained “reflex” in 1833.
Sensory input hits a synapse, then a motor neuron fires the target muscle or gland. Short-latency loops use one junction; long-latency paths cross several. Survival flavors the catalog: startle to surprise, a cat’s righting twist in free fall, withdrawal from pain, escape circuits spanning multiple nuclei. Stretch (myotatic) reflexes contract a lengthened muscle and, with EMG, hint whether trouble is peripheral (diminished jerks) or central (brisk ones). The patellar jerk maps to spinal segments L2–L4; the Achilles jerk to S1–S2. H-reflexes are the electrical cousins; vibration elicits tonic vibration reflexes. Golgi tendon organs invert the stretch story when tendon tension rises.
Human newborns carry primitive automatic reactions such as the Moro startle that fade with maturity. Clinicians grade tendon jerks from 0 to 4, calling 2+ typical—though healthy people may live at 1+ or 3+. Another scale runs from –4 (absent) to +4 (clonus) with 0 as normal. Breathing, digestion, and heartbeat can count as reflex under broad definitions, yet breath-holding shows voluntary override via intercostals.
Reflexes are not fixed robots. At rest, stretch reflexes stabilize posture by resisting lengthening; during voluntary motion their gain drops or reverses so resistance does not fight the intended move. Presynaptic inhibition, spinal interneurons, and descending brain signals reshape sensory weight mid-behavior. Descartes imagined a mechanical statue triggered by external pokes; Hall’s 1833 Royal Society paper rooted involuntary motion in medulla and cord, separate from conscious brain will—the tap on the tendon as a window into wiring.
Source: Reflex
