GROSS ANATOMY

Detailed explanation-1: -When a stretch reflex is activated, it not only causes contraction of the synergistic muscles, but also caused relaxation i.e. has an inhibitory effect on the antagonist muscles. Once a stretch reflex occurs, the impulse is sent from the stretched muscle spindle, to the alpha motor neuron.

Detailed explanation-2: -The stretch reflex is designed as a protective mechanism, to prevent strain and tear injuries to the muscles and tendons. When the muscle spindle is excited an impulse is immediately received to contract the muscle, thereby protecting it from being pulled forcefully or stretched beyond a normal range of motion.

Detailed explanation-3: -Thus, when a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle (Figure 2.2). This inhibition is accomplished by an inhibitory interneuron in the spinal cord.

Detailed explanation-4: -The classic stretch reflex involves the stretch receptor in the skeletal muscle spindle and its afferent Ia fiber in the posterior nerve root, which directly or indirectly excites the alpha motor neuron in the spinal cord.

Detailed explanation-5: -Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other. Reciprocal inhibition in stretch reflex.