Specialized Regions of the Vestibular System Contain Receptors
Each vestibular structure of the membranous labyrinth has a region of epithelial lining that has become specialized into a set of secondary receptor cells (see Chapter 7) called hair cells (Figure 11-2).
These hair cells form the basis of a sensory receptor organ within each vestibular structure. Each hair cell has several hairlike cilia at its apex that are arranged in order according to size. At its base the hair cell synapses with a sensory neuron that carries action potentials to the brainstem. The cell bodies of these sensory neurons are located in Scarpa’s ganglia, and their axons collectively form the vestibular portion of the vestibulocochlear nerve (cranial nerve VlII). The hairlike cilia from all the hair cells within any one vestibular structure project into a gelatinous mass; displacement of this gelatinous mass in a given direction causes all the hair cell cilia to bend in that direction.At rest, when the cilia arc not deflected, the sensory neurons that synapse with the vestibular hair cells transmit action potentials spontaneously at about IOO per second (Figure 11-3). When the hair cell cilia are bent in a direction toward the largest cilium, the hair cells depolarize, the release of transmitter from the hair cell onto the sensory neuron increases, and the neuron’s action potential frequency increases. When the cilia are bent in the opposite direction, toward the smaller cilia, hair cell membranes hyperpolarize, transmitter release decreases, and the action potential frequency of sensory neurons decreases. Therefore, displacement of the hair cell cilia in either of these directions can be detected by the brain as an increase or a decrease from the resting action potential frequency'. Deflections in other directions are much less effective. How the brain uses this information to detect the direction of head movement is described later.