Stretch Reflex

The stretch reflex, also called the myotatic reflex, is an automatic contraction of a skeletal muscle in response to its being stretched. The reflex is initiated by muscle spindles, sensory receptors located within the muscle that detect changes in muscle length and, particularly through their primary sensory endings, the rate of stretch .^1,2

When a muscle is stretched, activity increases in group Ia sensory afferents from its muscle spindles. These sensory fibers enter the spinal cord and make direct excitatory connections with alpha motor neurons that supply the same muscle. The motor neurons then activate the muscle fibers, producing contraction that opposes the imposed change in muscle length. Because this direct sensory-to-motor pathway contains a single synapse, it is called the monosynaptic component of the stretch reflex .^1,2

The same Ia sensory input also contributes to reciprocal inhibition. Through inhibitory interneurons in the spinal cord, activation of the stretched muscle can be accompanied by reduced activation of motor neurons supplying its antagonist muscles. This arrangement helps coordinate opposing muscle groups during reflex responses and movement .¹

The familiar knee-jerk reflex is an example of a stretch reflex. Tapping the patellar tendon briefly stretches the quadriceps and its muscle spindles, producing reflex contraction of the quadriceps. The stretch reflex should not be confused with force-related sensory feedback from Golgi tendon organs, which use group Ib afferents and participate in different spinal circuits .¹