Stretched muscle spindles initiate a stretch
reﬂex, causing contraction of the stretched muscle and
inhibition of its antagonist.
When stretch activates muscle spindles, the
associated sensory neurons (blue) transmit afferent
impulses at higher frequency to the spinal cord.
The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal ﬁbers of the
stretched muscle. Sensory ﬁbers also synapse with
interneurons (green) that inhibit motor neurons (purple)
controlling antagonistic muscles.
Efferent impulses of alpha motor neurons
cause the stretched muscle to contract, which
resists or reverses the stretch.
Efferent impulses of alpha motor neurons to
antagonist muscles are reduced (reciprocal inhibition).
Tapping the patellar ligament stretches the
quadriceps and excites its muscle spindles.
The motor neurons (red) send activating
impulses to the quadriceps causing it to
contract, extending the knee.
Afferent impulses (blue) travel to the
spinal cord, where synapses occur with motor
neurons and interneurons.
The interneurons (green) make inhibitory
synapses with ventral horn neurons (purple) that
prevent the antagonist muscles (hamstrings)
from resisting the contraction of the quadriceps.
The events by which muscle stretch is damped
The patellar (knee-jerk) reflex—an example of a stretch reflex
Cell body of