Stretch reflex

A stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.

Muscle spindles are sensory apparatus sensitive to stretch of the muscle in which they lie.

Examples
A person standing in the upright position begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the side will stretch. Because of this, stretch receptors in those muscles contract to correct posture.

The patellar (knee-jerk) reflex is an example.

Another example is the group la fibres in the calf muscle, which synapse with motor neurons supplying muscle fibres in the same muscle. A sudden stretch, such as tapping the Achilles' tendon, causes a reflex contraction in the muscle as the spindles sense the stretch and send an action potential to the motor neurons which then cause the muscle to contract; this particular reflex causes a contraction in the soleus-gastrocnemius group of muscles. This reflex can be enhanced by the Jendrassik maneuver.

Supraspinal control
Inhibitory signals on gamma neurons through lateral reticulospinal tract from


 * Motor Area 6
 * Pleo cerebellum
 * Red nucleus

Facilitatory signals on gamma neurons through ventral reticulospinal tract from


 * Motor Area 4
 * Neocerebellum
 * Vestibular nucleus

Spinal control

 * Withdrawer reflex
 * Damping & Loading reflex
 * Jendrassik manoeuvre

Pathology
The clasp-knife response is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. It is one of the characteristic responses of a upper motor neurone lesion.