40: The Vestibular System

Introduction

• The Vestibular Apparatus in the Inner Ear Contains Five Receptor Organs

  • Hair Cells Transduce Mechanical Stimuli into Receptor Potentials

  • The Semicircular Canals Sense Head Rotation

  • The Otolith Organs Sense Linear Accelerations

  • Most Movements Elicit Complex Patterns of Vestibular Stimulation

• Vestibulo-Ocular Reflexes Stabilize the Eyes and Body When the Head Moves

  • The Rotational Vestibulo-Ocular Reflex Compensates for Head Rotation

  • The Otolithic Reflexes Compensate for Linear Motion and Head Deviations

  • Vestibulo-Ocular Reflexes Are Supplemented by Optokinetic Responses

• Central Connections of the Vestibular Apparatus Integrate Vestibular, Visual, and Motor Signals

  • The Vestibular Nerve Carries Information on Head Velocity to the Vestibular Nuclei

  • A Brain Stem Network Connects the Vestibular System with the Oculomotor System

  • Two Visual Pathways Drive the Optokinetic Reflexes

  • The Cerebral Cortex Integrates Vestibular, Visual, and Somatosensory Inputs

  • The Cerebellum Adjusts the Vestibulo-Ocular Reflex

• Clinical Syndromes Elucidate Normal Vestibular Function

  • Unilateral Vestibular Hypofunction Causes Pathological Nystagmus

  • Bilateral Vestibular Hypofunction Interferes with Normal Vision

• An Overall View

Airplanes and submarines navigate in three dimensions using sophisticated guidance systems that register every acceleration and turn. Laser gyroscopes and computers make these navigational aids extremely precise. Yet the principles of inertial guidance are ancient: Vertebrates have used analogous systems for 500 million years and invertebrates for still longer.

In vertebrates the inertial guidance system is the vestibular system, comprising five sensory organs in the internal ear that measure linear and angular acceleration of the head. Acceleration of the head deflects hair bundles protruding from the hair cells in the inner ear; this distortion changes the cells' membrane potential, altering the synaptic transmission between the cells and the sensory neurons that innervate them. The signals from these vestibular neurons convey information on head velocity and acceleration to vestibular nuclei in the brain stem.

This information keeps the eyes still when the head moves, helps to maintain upright posture, and influences how we perceive our own movement and the space around us by providing a measure of the gravitational field in which we live. In this chapter we describe how the hair cells of the inner ear generate the signals for head acceleration and how these signals are integrated with other sensory information in the brain.

The Vestibular Apparatus in the Inner Ear Contains Five Receptor Organs

Vestibular signals originate in the labyrinths of the internal ear (Figure 40–1). The bony labyrinth is a hollow structure within the petrous portion of the temporal bone. Within it lies the membranous labyrinth, which contains sensors for both the vestibular and auditory systems.