Reflex Activity

This chapter discusses the importance of reflex activity. The reflex responses of skeletal muscle to stretch are best understood because they are most easily investigated. The main stretch reflexes are negative feedback responses of two types—extension of the receptor opposes extension in one and tension of the receptor opposes tension in the other. The tendon jerk or pluck reflex is a brief short latency contraction of an abruptly stretched muscle most easily elicited from the antigravity muscles but not confined to them. The excitability of the stretch reflex arcs is modified by the balance of facilitatory and inhibitory impulses acting at the somadendritic region of their motoneurons. Proprioceptive signals from receptors in the ligaments and joints of the first three cervical vertebrae ascend and cross in the medulla oblongata to the contralateral vestibular nucleus and the medial longitudinal bundle. These signals act in combination with signals from the labyrinth to maintain ocular stability during simultaneous head and neck movements and to adjust the postural muscles to maintain the upright posture during axial rotation.