Tendon reflexes and pathologic reflexes

The tendon reflex (deep tendon reflex, muscle stretch reflex, myotatic reflex) is a monosynaptic proprioceptive reflex. The afferent arc of the reflex starts from the stretch receptor in the muscle called the muscle spindle. The receptor is activated by sudden stretch, and an electric impulse generated there is conducted through Ia fibers to the anterior horn cells of the same segment, where the α‎ motor neurons are monosynaptically activated. The efferent impulse is conducted through the α‎ motor fibers to contract the muscle fibers from which the afferent impulse originated. The tendon reflex is decreased or lost by a lesion of either the afferent arc or the efferent arc of the reflex. In addition, as the tone of muscle spindle is modulated by the γ‎ motor fibers, the tendon reflex can be decreased also by impairment of the γ‎ motor fibers. Therefore, patients who are considered to have impairment of the peripheral nervous system with loss of tendon reflexes and yet no abnormality in the muscle strength or somatosensory function may have involvement of the γ‎ motor fibers. There is some evidence for this situation with acute cerebellar lesions. As the lower motor neuron is inhibited by a component of the supraspinal drive presynaptically via spinal interneurons, impairment of the supraspinal drive is expected to cause an increase in the excitability of the anterior horn cells and consequently an enhancement of the tendon reflex (see Chapter 16-3D, p. XX).