Mechanoreceptor - Muscle Spindles and The Stretch Reflex

Muscle Spindles and The Stretch Reflex

The knee jerk is the popularly known stretch reflex (involuntary kick of the lower leg) induced by a physician tapping the knee with a rubber-headed hammer. The hammer strikes a tendon that inserts an extensor muscle in the front of the thigh into the lower leg. Tapping the tendon stretches the thigh muscle, which activates stretch receptors within the muscle called muscle spindles. Each muscle spindle consists of sensory nerve endings wrapped around special muscle fibers called spindle fibers (also called intrafusal fibers). Stretching a spindle fiber initiates a volley of impulses in the sensory neuron (a I-a neuron) attached to it. The impulses travel along the sensory axon to the spinal cord where they form several kinds of synapses:

1. Some of the branches of the I-a axons synapse directly with alpha motor neurons.These carry impulses back to the same muscle causing it to contract. The leg straightens.
2. Some of the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord. These, in turn, synapse with motor neurons leading back to the antagonistic muscle, a flexor in the back of the thigh. By inhibiting the flexor, these interneurons aid contraction of the extensor.
3. Still other branches of the I-a axons synapse with interneurons leading to brain centers, e.g., the cerebellum, that coordinate body movements.