The Vestibular System

Thickened endings of SCC = ampullae (ampullae=plural, ampulla=singular) Two major subsystems: (1) Semicircular canals, (2) Otolith system (utricle and saccule). (from Perkins & Kent, 1986)

(from Deutsch & Richards, 1979) Another view showing the ampullae of the SCCs and the utricle and saccule of the otolith system

receptors in ampullae = cristae receptors in otolith system (utricle & saccule) = maculae (from Perkins & Kent, 1986)

Cristae in ampullae of SCC responsible for sensing angular acceleration (pitch, roll, & yaw) Maculae in the otolith system are responsible for sensing linear acceleration (speeding up or slowing down) (from Perkins & Kent, 1986)

The motion-detecting receptors in the vestibular system are hair cells that operate on general principles that are identical to those we discussed for HCs in the cochlea: Motion in one direction (short hairs tipping in the direction of tall hairs) produces an electrical disturbance in the HC that stimulates the firing of 8 th N fibers; Motion in the other direction (tall hairs tipping in the direction of short hairs) inhibits the firing of 8th N fibers

Vestibular and cochlear branches of the 8 th nerve (from Deutsch & Richards, 1979)

What the Auditory and Vestibular Systems Have in Common (and why some disease processes can affect both hearing & balance) 1.Common Fluid System. The membranous labyrinth is one continuous fluid system serving both hearing and balance. If there’s something wrong with this fluid system, both hearing and balance will be affected. Example: Meniere’s Disease (more later). 2.Hair Cell Motion Detectors. Hearing and balance both involve the detection of motion: slowly varying (i.e., low frequency) in the case of balance, higher frequency vibratory motion in the case of hearing. In both cases, the motion detectors are hair cells that operate on nearly identical principles. We’ll see later that Meniere’s Disease affects the HCs in both the cochlea and the vestibular system – hearing and balance are both affected.

3.Innervation. Hearing and balance are innervated by separate branches of the same cranial nerve. A tumor that presses on the 8 th N after the vestibular and cochlear branches have joined can affect both hearing and balance. There are many case of “acoustic neuromas” (8 th N tumors) in which the earliest symptoms are problems with balance rather than hearing. (More later.)