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The sense organ for the myotactic reflex is the muscle spindle. Stretch of the muscle spindle stimulates primary Ia afferents that enter the cord through the sensory dorsal roots.
These Ia fibers synapse directly on the alpha motor neurons that fire and cause the stretched muscle to contract. The resistance of a muscle to stretch is called tone. Transection of the motor nerves to muscle produces flaccidity.
Hyperactive stretch reflexes result in a hypertonic or spastic muscle. This slide illustrates the site of action of various drugs (discussed in the following slides) used in the treatment of spasticity.