CHAPTER 10. THE VESTIBULAR SYSTEM

OBJECTIVES:
  1. BE ABLE TO IDENTIFY THE VESTIBULAR COMPONENTS OF THE INTERNAL EAR;
  2. BE ABLE TO IDENTIFY IN HISTOLOGICAL PREPARATIONS THE VESTIBULAR RECEPTORS AND IN GROSS SPECIMENS AND BRAINSTEM SECTIONS THE VESTIBULAR PATHS.

Vestibular Receptors

The vestibular system receptors are located in the utricle, saccule, and semicircular ducts (Fig. 9-1).  Those in the utricle and saccule are called maculae while those in the semicircular ducts are called ampullary crests or cristae ampullares.

The Maculae

The maculae in the utricle and saccule are static vestibular receptor organs that respond to changes in the position of the head and linear acceleration and play a major role in the postural adjustments needed to keep the body on an even keel with the head. The utricle and saccule are oriented at right angles to each other with that of the utricle nearly in the horizontal plane and that of the saccule almost in the sagittal plane.

Observe that the maculae consist of neuroepithelial hair cells and supporting cells (Sl. 86). Overlying the hair cells is the gelatinous otolithic membrane which contains calcium carbonate crystals, the otoliths or otoconia. When the position of the head changes, the force of gravity shifts the otolithic membrane, which bends the neuroepithelial hairs embedded in it, and transduces this mechanical stimulus into a receptor potential. The hair cells are in synaptic contact with terminals of the dendrites of the vestibular ganglion cells.

Ampullary Crests

The ampullary crests in the semicircular ducts are dynamic vestibular receptor organs. At one end of each duct is an enlargement, the ampulla. In each ampulla a part of the wall is thickened and projects into the cavity of the duct as the ampullary crest. This vestibular receptor organ is composed of neuroepithelial hair cells and supporting cells (Sl. 87). Overlying each crest is a gelatinous substance, the cupula, in which is embedded the free ends of the stereocilia of the hair cells of the crest. One of the hairs in each hair cell is larger and is called the kinocilium. The hair cells are in synaptic contact with terminals of the dendrites of the vestibular ganglion cells.

The semicircular ducts are arranged and named according to their corresponding canals. The anterior and posterior ducts are oriented vertically but at right angles to each other, whereas the lateral duct is oriented horizontally. Thus, rotation of the head in any direction will stimulate the receptors in functional pairs of semicircular ducts: the right and left horizontal, the right anterior and left posterior, or, the right posterior and left anterior.

When the head begins to rotate the endolymph in the semicircular duct lags behind and prevents the cupula from moving. As a result the stereocilia embedded in the cupula are bent in the direction opposite to the rotation. The hair cells are polarized so that when the stereocilia are bent toward the kinocilium they are depolarized, whereas when they are bent away from the kinocilium they are hyperpolarized. In this way the receptors in the right and left semicircular ducts work in pairs: when one side is excited, the other is inhibited.

Vestibular Nerve

The vestibular part of CN VIII is formed by the central processes of the bipolar cells of the vestibular ganglion. Identify the vestibulocochlear nerve entering the brainstem at the pontomedullary junction in the cerebellar angle (Pl. 8).  The vestibular division is medial to the cochlear (Sls. 19, 20). The vestibular fibers then pass dorsally between the inferior cerebellar peduncle (restiform body) and the spinal trigeminal tract and, upon reaching the vestibular nuclear complex, most fibers terminate. Others, however, pass directly into the cerebellum.

Vestibular Nuclei

Review the vestibular nuclear complex beneath the vestibular area in the floor and wall of the 4th ventricle (Sls. 14, 15, 17, 18, 19, 20, 21, 22).

The inferior vestibular nucleus is in the rostral medulla and is the easiest to identify because it has a speckled appearance due to the presence of large bundles of myelinated fibers. The medial vestibular nucleus is the largest and is located in the lateral part of the floor of the 4th ventricle in the rostral medulla and caudal pons. The lateral vestibular nucleus is limited to the region of the pontomedullary junction. It intermingles with the inferior vestibular nucleus caudally and the superior vestibular nucleus rostrally. It contains several populations of neurons, the most conspicuous of which is a magnocellular group frequently referred to as Deiter's neurons or nucleus. The superior vestibular nucleus is limited to the caudal pons where it is located in the wall of the 4th ventricle.

Vestibulospinal Reflex

Axons of vestibular ganglion cells carrying impulses from the maculae synapse in the medial, lateral, and inferior vestibular nuclei. The lateral vestibular nucleus gives rise to the lateral vestibulospinal tract, while the medial and inferior vestibular nuclei give rise to the medial vestibulospinal tracts.

The significance of the vestibulospinal connections is that when the head is tilted the maculae of the utricle and saccule influence the lateral, medial, and inferior vestibular nuclei which, in turn, project via the lateral and medial vestibulospinal tracts to the motor nuclei supplying the neck, trunk, and limb muscles needed to keep the body on an even keel with the head.

Vestibulo-ocular Reflex

The axons of vestibular ganglion cells carrying impulses from the ampullary crests in the semicircular ducts synapse in the superior and medial vestibular nuclei. Vestibulo-ocular fibers then pass via the pontine and midbrain parts of the MLF and the adjacent reticular formation (Sls. 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33) to the nuclei of CN VI (Sls. 22), CN IV (Sls. 29, 30), and CN III (Sls. 31, 32, 33).  Note the close relationship of the MLF to these nuclei.

Rotation of the head in any direction results in reflex turning of the eyes in the direction opposite to the rotation. This vestibulo-ocular reflex includes three neurons: 1) bipolar vestibular ganglion cells carrying afferent impulses from receptors in the semicircular ducts, 2) interneurons in the vestibular nuclei, and 3) efferent neurons in the oculomotor, trochlear, and abducens nuclei.

Note that integrity of the vestibulo-ocular reflex requires that the central parts of the pons and midbrain be intact.