Neuroscience '03 at RWJMS

Course Director, Richard S. Nowakowski, Ph.D. (e-mail: neuro@umdnj.edu)
Department of Neuroscience and Cell Biology
Robert Wood Johnson Medical School

Vestibular System

by David P. Crockett (Room R-306, 235-4522, crockett@umdnj.edu)

I. Functions of the Vestibular System.

  1. The vestibular system is designed to detect the position and motion (or acceleration) of the head in space. As a sensory system we are hardly aware of it. It does not enter consciousness as does vision, audition, somesthesis, olfaction and taste. Even though we are not usually aware of the sensations produced by the vestibular apparatus, the normal functioning of which is essential for motor coordination, eye movements and posture. Disease processes or malfunctions that affect the vestibular system can lead to dizziness, and nausea. Thus, it is only through disease that we are aware of vestibular sensations.
    1. The major symptom of vestibular disease is vertigo (dizziness) that is accompanied by (except in its most mild forms) some degree of nausea, vomiting pallor, perspiration, and some difficulty of walking. There may be sensations of "spinning" or rhythmic movements resulting from nystagmus. There may be a sensation of leaning or being pulled to the ground (as if by a magnet). Oscillopsia (=illusory movements of the environment, especially induced by the movement of the head is another sign of vestibular malfunctioning.
    2. Falling is a major health problem with the elderly; part of the problem may be decrements in vestibular functioning with age.
  2. The labyrinth or inner ear is composed of two parts: the bony and membranous labyrinth.
  3. The vestibular labyrinth consist of two functional components:
  4. Vestibular sensation is carried by the vestibular portion of eighth nerve.

II. The vestibular labyrinth is part of the membranous labyrinth.

  1. Endolymph fills the vestibular labyrinth and perilymph surrounds it.
    1. Disruption of the flow of either the perilymph or the endolymph has serious consequences of labyrinthine functioning.
      • Ménière's syndrome
  2. Specialized regions of the vestibular labyrinth contain receptors:
    1. Ampullae (associated with semicircular canals) a. ampullary crest) b. Cupula (=small cup):
    2. Utricle and Saccule (otolithic organs)
      • Macula

III. The Arrangement of the vestibular hair cells is an integral part of their functioning:

  1. The hair cells are polarized structurally and functionally.
    1. Free surface of each hair cell contains 40-70 stereocilia and a single kinocilium: Stereocilia are organized in size places with the tallest adjacent to the kinocilium: a morphological axis of polarity; and they are embedded in the overlying cupula of the ampullary crest.
    2. Bending of stereocilia towards the kinocilium leads to depolarization and as a consequence an increase in bipolar cell (primary afferent) firing; while bending away from the kinocilium leads to hyperpolarization and therefore a decrease in primary afferent firing. The cells contain mechanically sensitive ion channels..
    3. The cells of the ampullae are arranged in an orderly pattern in reference to the morphological axis of polarity: the kinocilium.
      1. In the vertical ducts, the kinocilium faces away from the utricle.
      2. In the horizontal ducts, the kinocilium faces the utricle.
  2. Semicircular ducts work in pairs:
  3. Hair Cells in the utricle are polarized toward the striola.

IV. The Central Connections of the vestibular system reflect the vestibular systems static and dynamic functions:

  1. The central axons of the vestibular system run with the eight cranial nerve.
    1. Scarpa's ganglion
    2. The vestibular ganglion is divided into two divisions:
  2. Each Nucleus of the vestibular system has distinctive connections:
    1. Lateral Vestibular Nucleus (Deiter's nucleus):
    2. Medial and Superior Vestibular nuclei:
    3. Inferior vestibular nucleus
    4. Cerebellum:
    5. Vestibular thalamus
    6. Cerebral Cortex.
  3. Vestibular Systems: Clinical Aspects:
    1. Nystagmus: Involuntary rhythmic movements of the eye consisting of a rapid movement in one direction (= saccade) followed by a slow movement in the opposite direction. They are related to an imbalance of synchronized impulses from vestibular sources. Nystagmus can be elicited in clinical examinations, but when it occurs spontaneously, it is a sign of a disease process (e.g., a lesion). Nystagmus is named by the direction of the fast saccade component. Horizontal nystagmus is the most common, although vertical and rotary nystagmus can occur.
    2. Rotation Test.
      • The subject is seated on a rotating (Bárány) chair. The head is tilted forward 30 degrees, bringing the horizontal canals parallel to the ground. The chair is rotated quickly 10 -12 times and then stopped. The subject feels a spinning sensation (vertigo). In the healthy person: During the spin the fast component (saccade) is in the direction of the spin; post-rotary nystagmus is in the opposite direction.
    3. Caloric or thermal test.
      • Irrigating the external auditory meatus with warm or cold water sets up convection currents in the endolymph, which displaces the crista ampullaris. Normally nystagmus is towards the side irrigated with the warm water and is away from the side irrigated with cold water. Each external auditory meatus is irrigated 30 s with first water at 30 degrees C and then at 44 degrees C (body temperature ±7 degrees C), with a pause of at least 5 min between irrigations. With cold water irrigation, first there is a slight tonic deviation of the eyes to the side being irrigated. This is followed by nystagmus to the opposite side. To test horizontal canals: have the patient tilt the head back 60 degrees, bringing horizontal canals into a vertical position.

Study Aids: Some additional websites with interesting information on the vestibular system: