8. The Vestibular System

The objectives for this chapter are to:

  1. Visualize the vestibular apparatus of the inner ear.
  2. Study the central organization of the vestibular system.

The vestibular system is important in maintaining equilibrium and orientation in 3-dimensional space. In this regard the geometry of the labyrinth is important. The semicircular canals, utricle (#7354), and saccule (#7353, #7366) contain the receptors of the vestibular system (fig 8a). These structures are part of the membranous labyrinth located deep in the temporal bone (#7270, #7271, #7272). The membranous labyrinth is a fluid-filled (endolymph) channel that is located, like a hand in a glove, in the bony labyrinth (#7348). Both labyrinths have three semicircular canals that are perpendicular to one another and that correspond to the three planes of space (#7451). A dilatation called the ampulla occurs at the base of each semicircular canal. Ampullae (#7368) contain specialized regions of hair cells called the cristae ampullaris (#7444). The hair cells are receptors whose stereocilia and kinocilium are embedded in a gelatinous mass called the cupola. Angular acceleration (#7452) sets up currents in the endolymph that cause the cupola to sway. This deflects the cilia and excites the hair cells (#7445). Consequently, the semicircular canals are sensitive to angular acceleration of the head and function in dynamic (or kinetic) equilibrium.

Gravitational pull and linear acceleration, which represent static equilibrium, are detected by the utricle (#7367) and saccule (#7366). These structures also have collections of hair cells in a specialized area called the macula. The hair cells have a gelatinous covering that contains particles called otoliths. Gravitational pull on the otoliths causes deflection (#7447) of the stereocilia and kinocilium of the hair cells in the maculae (#7446). Are the receptors of the utricle, saccule, and semicircular canals chemoreceptor or mechanoreceptors? What is the result of vigorous and excessive stimulation of these receptors? Carnival rides work their wonder through vestibular hair cells.

The primary sensory axons of the vestibular system comprise the vestibular nerve (#7448), a division of cranial nerve VIII. The cell bodies of these axons are located in the vestibular ganglion (#7449), which is comparable to a dorsal root ganglion. The peripheral axonal processes of these cell bodies innervate the hair cells, and the central axonal processes enter the brain stem. How do these axons enter the cranial cavity (#5440)? These axons enter which cranial fossa?

The central connections of the vestibular system (#4481) are important in understanding conjugate eye movements. Instead of delineating each vestibular nucleus (there are four), understand where they are located in the brain stem. They extend from caudal medulla to mid-pons beneath the floor of ventricle IV (#4608). Note their relationships with the inferior cerebellar peduncles (fig 8b), solitary complex, and the somatic and visceral motor nuclei. What natural stimuli excite the vestibular system? The vestibular system can also be unnaturally stimulated, i.e., irritated, by inner ear infections, infections in the mastoid process (#5451) or by a tumor pressing on the vestibular nerve or nuclei (#15179). What sensations will these irritative lesions evoke?

Two tracts deserve attention. The first tract is the medial longitudinal fasciculus (mlf). This is a fairly complex fascicle that contains axons from the vestibular nuclei of both sides. The mlf runs longitudinally near the midline beneath ventricle IV and the periaqueductal gray matter of the midbrain (#4315, #6168, #6325, #6328, #6335, #6337). Its axons terminate in nuclei VI (#6696), IV (#6702), and III (#6217).

The mlf is a vestibulo-ocular pathway important in mediating conjugate eye movements. In addition, some mlf axons descend into the cervical spinal cord (#6339) and terminate in the ventral horn of cervical segments. Some anatomists refer to these axons as the medial vestibulospinal tract. Thus, the mlf is involved in the coordination of head and eye movements. As might be expected, this fasciculus is one of the first tracts in the brain stem to become myelinated (for example, its myelination occurs at the time an infant begins to lift and turn its head). What is the significance of coordinated eye and head movements?

The "dolls eyes" or oculocephalic reflex is a test of brain stem function. It utilizes the medial longitudinal fasciculus. If you would like to see an animation of this reflex click on the underlined text at the end of this sentence: oculocephalic.

The second fiber tract to study briefly is the lateral vestibulospinal tract. It extends from its source in the lateral vestibular nucleus (#6167, #6237) as far caudally as the sacral spinal cord. Throughout most of its course, it intermingles with anterolateral system fibers in the ventral lateral white matter and cannot be identified as a separate tract (#5505). The lateral vestibulospinal tract functions in reflex postural mechanisms. Its axons selectively excite motor neurons of extensor (or antigravity) muscles.

The vestibular nuclei project bilaterally to the thalamus, particularly to the rostral part of VPL (#4005). However, these projections do not form a discrete pathway that can be followed in the usual stained sections. The cortical area for the vestibular system is not well defined but is thought to be in the postcentral gyrus (#4208) near the representation for the face. The vestibular system works in silence and does not usually intrude on consciousness except under extreme circumstances. We are, for example, conscious of dizziness. Primarily, we come to "experience" the vestibular system when it malfunctions.

Click for the Syllabus Quiz.
Click for the Vestibular Pathway Quiz
Click for the Vestibulospinal Pathway Quiz
Click for the Movie From: "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charlston, South Carolina.

Movies

Chapter 7 Chapter 9