Detailed scientific illustration showing the human nervous system reflex arc in a leg, spinal cord, and muscle spindle.

“Here’s how it works: every reflex has a little pathway called a reflex arc. It starts with a receptor — in this case, a muscle spindle inside your thigh muscle. When the doctor taps your tendon, it stretches the muscle and the spindle senses that stretch. That signal travels up a sensory nerve fiber — we call it a group Ia afferent fiber — straight into your spinal cord Reflexes Physiology CAPS 205 . Inside the spinal cord, that sensory nerve connects directly with a motor neuron — the nerve that controls your muscle. The motor neuron instantly sends a signal back down to the same muscle, making it contract. That’s the quick kick you see.” See more