When Your Family Physician Taps Your Left Patellar Tendon with a Mallet

Question

When your family physician taps your left patellar tendon with a mallet,
A) the intrafusal muscle fibers in your left quadriceps contract.
B) many motor neurons innervating your left quadriceps are excited by 1a afferent fibers.
C) a single motor neuron innervating your left quadriceps is excited by a 1a afferent fiber.
D) a single motor neuron innervating your left quadriceps is inhibited by a 1a afferent fiber.

Accepted Answer

When the patellar tendon is tapped, it stretches the quadriceps muscle, activating the muscle spindle. This sends signals via 1a afferent fibers to the spinal cord, exciting many motor neurons that innervate the quadriceps, causing it to contract and produce the knee-jerk reflex.