Afferent and efferent innervation of the cristae ampullares of the toadfish (opsanus tau)

The vestibular system is responsible for the sense of balance and equilibrium of vertebrates. The end-organ for sensing angular acceleration in this system are the semicircular canals whose sensory hair cells are located on the cristae ampullares of the canals. These hair cells are innervated by both afferent and efferent fibers. Based on their physiology, the afferent fibers of the toadfish (Opsanus tau) horizontal semicircular canal crista(HSCC) have been divided into three categories: low-gain, high-gain and acceleration units (Boyle et al., J. Neurophysiol.1991, 66:1505-21). Low-gain afferent supply hair cells in the periphery of the crista; the high-gain and acceleration units are located in the center of the crista. Stimulation of the efferent fibers modifies the electrical activity of the afferent fibers, with the excitatory effect being greatest in the center of the crista. We have studied the morphology of the afferent and efferent fibers that innervate the hair cells of the HSCC using electron microscopic thin section reconstructions. Our preliminary data suggest that the proportion of afferents to efferents is reduced in the center of the crista. The observed regional morphological differences might be the morphological basis of the physiological effects of efferent stimulation.