8. The Vestibular System
Revised January 25, 2021
The learning objectives for this chapter are to:
Describe the organization of the vestibular system. In particular, be able to describe the
- Vestibular receptors
- Vestibular nerve and ganglion
- Vestibular nuclei
- Central connections of the vestibular nuclei
The vestibular system conveys sensory information about head orientation and motion from receptors in the inner ear. Its three main functions are
- Coordination of head and eye movement
- Helping to maintain upright posture and balance (equilibrium)
- Conscious perception of spatial orientation and motion
Lesions of the vestibular system cause
I. VESTIBULAR RECEPTORS
The inner (internal) ear contains the receptors for both the auditory system, discussed in Chapter 9, and the vestibular system.
Bony labyrinth. Within the petrous part of the temporal bone (#7270, #7271, #7272) lies the inner ear (#7348). It contains the bony labyrinth (fig 8a), a series of communicating bony cavities: the cochlea (part of the auditory system), the vestibule and the semicircular canals. The three semicircular canals are perpendicular to one another and correspond to the three planes of space (#7451). They are the lateral (horizontal) canal, and the superior (anterior) and posterior canals in the vertical plane.
Membranous labyrinth. The bony labyrinth encloses the membranous labyrinth, a closed system of sacs and ducts filled with endolymph and bathed in perilymph. The membranous labyrinth includes two sacs, the utricle (#7354) and saccule (#7353, #7366) within the vestibule; and the semicircular ducts within the semicircular canals. The semicircular ducts, utricle and saccule contain the receptors of the vestibular system.
Semicircular ducts. Each semicircular duct has a dilated end, the ampulla, within a corresponding dilation of the semicircular canal (#7368). Each ampulla contains an ampullary crest (crista ampullaris) (#7444) with epithelium that contains mechanoreceptive hair cells. The hair cells are receptors whose stereocilia and kinocilium are embedded in a gelatinous mass called the cupula. Angular acceleration (#7452) sets up currents in the endolymph that cause the cupula 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.
Utricle and saccule. 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 the macula (#7446) of the utricle and the macula of the saccule. The hair cells have a gelatinous covering that contains rounded crystals of calcium carbonate called otoliths. Gravitational pull on the otoliths causes deflection (#7447) of the stereocilia and kinocilium of the hair cells in the maculae. The utricular macula lies horizontally when the head is upright and detects horizontal linear acceleration. The saccular macula is oriented vertically and is the major gravitational sensor.
What is the result of vigorous and excessive stimulation of these receptors? Carnival rides work their wonder through vestibular hair cells.
II. VESTIBULAR NERVE AND GANGLION
The primary sensory axons of the vestibular system comprise the vestibular nerve (#7448), a division of the vestibulocochlear nerve (VIII). The cell bodies of these bipolar neurons are located in the vestibular (Scarpa's) ganglion (#7449) in the lateral end of the internal auditory canal (internal acoustic meatus). The peripheral axonal processes of these cell bodies innervate the hair cells, and the central axonal processes enter the brain stem at the cerebellopontine angle. How do these axons enter the cranial cavity (#5440)? These axons enter which cranial fossa?
Vestibular schwannoma. Benign tumors of the Schwann cells in cranial nerve VIII generally arise from the vestibular part of VIII and are called vestibular schwannomas (old term, still used: acoustic neuromas) (#7966). This is one of the top three benign intracranial tumors (meningiomas, vestibular schwannomas, and pituitary adenomas) and by far the commonest tumor of the cerebellopontine angle.
The cochlear (auditory) nerve is generally affected first, causing ipsilateral hearing loss often with tinnitus (ringing in the ear). Vestibular symptoms, imbalance or infrequently vertigo, are next most common. The adjacent facial nerve (VII) is resistant to gradual compression and facial weakness is unusual. Instead, as the tumor gets quite large (#10866), the trigeminal nerve (V) is usually the next cranial nerve to be affected, with symptoms such as facial numbness and tingling.
III. VESTIBULAR NUCLEI
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 in the lateral floor of ventricle IV (#4608). Note their relationships with the inferior cerebellar peduncles (fig 8b), the solitary nucleus and tract, and the somatic and visceral motor nuclei.
IV. CENTRAL CONNECTIONS OF THE VESTIBULAR NUCLEI
The vestibular nuclei have pathways to the
A. Extraocular muscles - to coordinate eye movements with head movements
B. Spinal cord - to maintain upright posture and balance
C. Cerebral cortex - to perceive spatial orientation and motion
(There are also connections with the cerebellum, especially cerebellar efferents from the vestibulocerebellum to the vestibular nuclei, discussed in Chapter 11: The Cerebellum.)
A. Vestibuloocular pathways: mlf
Medial longitudinal fasciculus (mlf). The central connections of the vestibular system are important in understanding conjugate eye movements. The medial longitudinal fasciculus (mlf) 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 (#6168, #6325, #6328, #6329, #6335, #6337, #4315). Secondary vestibular neurons project rostrally via the mlf to the motor neurons in the abducens nucleus (#6696), trochlear nucleus (#6702) and oculomotor nucleus (#6217).
Vestibuloocular reflex (VOR). The mlf is a vestibuloocular pathway important in mediating conjugate eye movements, especially the vestibuloocular reflex (VOR), which normally functions to keep the eyes fixed on a target and hold images of the seen world steady on the retina during brief head rotations. The integrity of the VOR is routinely tested in the comatose patient to help evaluate brain stem function. The two tests of the VOR in the comatose patient are the oculocephalic reflex (doll's eyes phenomenon) and caloric (thermal) testing.
B. Vestibulospinal tracts
The vestibular nuclei give rise to two vestibulospinal tracts.
The medial vestibulospinal tract projects bilaterally within the descending part of the mlf (#6339) to end in the ventral horns mainly in the upper cervical cord. It promotes stabilization of head position.
The lateral vestibulospinal tract projects ipsilaterally in the anterolateral white matter (#5505) to end in the ventral horns throughout the cord. It functions in reflex postural mechanisms to activate motor neurons of extensor (antigravity) muscles and promote upright posture and balance.
C. Vestibulothalamocortical pathway
Thalamus: VLp (Vim)
The vestibular nuclei project bilaterally to the thalamus, particularly its cerebellar territory: the posterior part of the ventral lateral nucleus (VLp) (#6592) (fig 11h), also called the ventral intermediate nucleus (Vim) in humans.
Lesions of this region cause transient vestibular signs and symptoms such as tilted perception of visual vertical and corresponding deviations of stance and gait.
Vestibular cortex: the posterior insula
Thalamic vestibular neurons project to cerebral cortex. Unilateral stimulation of the vestibular labyrinth results in mainly ipsilateral activation of vestibular cortex. In contrast to the other sensory systems, there is no unimodal primary vestibular cortex. That is, there is no cortical area that receives only vestibular input. Instead, there are multiple multisensory vestibular cortical areas: their neurons also receive somatosensory and/or visual motion input.
The dominant vestibular area in humans is in the posterior insula (corresponding to the monkey parieto-insular vestibular cortex (PIVC), considered to be the core vestibular region in monkeys). Infarctions of this region cause subjective visual vertical tilt.
The vestibular system works in silence and does not usually intrude on consciousness except under extreme circumstances. We are, for example, conscious of vertigo. Primarily, we come to "experience" the vestibular system when it malfunctions.
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 irritative lesions evoke?
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