Vestibular System

The peripheral vestibular system consists of a set of three-dimensional angular velocity transducers, the semicircular canals, and a set of three-dimensional linear acceleration transducers, the otoliths (utricle and saccule) (see Fig. 12-1 (Figure Not Available) ). y These transducers are suspended by an assembly of membranes and connective tissue, the membranous labyrinth, within channels in the temporal bone, the bony labyrinth. Three important spatial arrangements characterize the semicircular canals. First, each canal within each

Figure 12-3 (Figure Not Available) Cortical auditory areas. Portions of the frontal and parietal lobes have been cut away above the sylvian fissure to expose the insula and the superior temporal lobe. The number of Heschl's gyrus is variable. In this illustration, primary auditory cortex (41) is shown on the first transverse gyrus of Heschl, and area 42 is shown on the second transverse gyrus. PT, planum temp<(Modified from Webster DB, Popper AN, Fay RF: The mammalian auditory pathway: neuroanatomy. New York, Springer-Verlag, 1992, p 17.)

labyrinth is perpendicular to the other canals, which is analogous to the spatial relationship between two walls and the floor of a rectangular room. Second, the planes of the semicircular canals, between the labyrinths, conform very closely to each other. The six individual semicircular canals become three co-planar pairs: (1) right and left lateral; (2) left anterior and right posterior; and (3) left posterior and right anterior. Third, the planes of the canals are close to the planes of the extraocular muscles. This relationship allows relatively simple connections between sensory neurons related to individual canals and motor output neurons, which are related to individual ocular muscles.

The otoliths register linear acceleration, and they respond both to linear head motion and to static tilt with respect to the gravitational axis. The otolithic membranes contain calcium carbonate crystals called otoconia and, therefore, they have substantially more mass than the surrounding structures. The increased mass of the otolithic membrane causes the maculae to be sensitive to gravity. In contrast, the cupulae of the semicircular canals have the same density as the surrounding endolymphatic fluid and are insensitive to gravity. Like the canals, the otoliths are arranged in a manner to enable them to respond to motion in all three dimensions. In an upright person, the saccule is vertical (parasagittal), whereas the utricle is horizontally oriented (near the plane of the lateral semicircular canals). In this posture, the saccule can sense linear acceleration in its plane, which includes the acceleration oriented along the occipitocaudal axis and linear motion along the anteroposterior axis. The utricle senses acceleration in its plane, which includes lateral accelerations along the interaural axis, as well as anteroposterior motion.

VESTIBULAR GANGLION AND NERVE

Vestibular nerve fibers are the afferent projections from the bipolar neurons of Scarpa's ganglion. The vestibular nerve transmits afferent signals from the labyrinths through the internal auditory canal (IAC). In addition to the vestibular nerve, the IAC also contains the cochlear nerve (hearing), the facial nerve, the nervus intermedius (a branch of the facial nerve), and the labyrinthine artery. The IAC travels through the petrous portion of the temporal bone to open into the posterior fossa at the level of the pons. The vestibular nerve enters the brain stem at the pontomedullary junction and contains two divisions, the superior and inferior vestibular nerves. The superior vestibular nerve innervates the utricle, as well as the superior and lateral canals. The inferior vestibular nerve innervates the posterior canal and the saccule.

VESTIBULAR NUCLEI

The primary vestibular afferents transmit information mainly to the vestibular nuclear complex and the cerebellum. The vestibular nuclear complex is the primary processor of vestibular input, and it implements direct, fast connections between incoming afferent information and motor output neurons. The cerebellum monitors vestibular performance and keeps it "calibrated." In both locations, vestibular sensory input is processed in association with somatosensory and visual sensory input.

The vestibular nuclear complex consists of four "major" nuclei (superior, medial, lateral, and descending) and at least seven "minor" nuclei. It is a large structure, located primarily within the pons but also extending caudally into the medulla. The superior and medial vestibular nuclei are relays for the vestibulo-ocular reflex (VOR). The medial vestibular nucleus is also involved in vestibulospinal reflexes (VSR), and it coordinates head and eye movements that occur together. The lateral vestibular nucleus is the principal nucleus for the vestibulospinal reflex. The descending nucleus is connected to all of the other nuclei and the cerebellum, but it has no primary outflow of its own. The vestibular nuclei are linked together via a system of commissures, which for the most part are mutually inhibitory. The commissures allow information to be shared between the two sides of the brain stem, and they implement the pairing of canals (discussed earlier).

In the vestibular nuclear complex, processing of the vestibular sensory input occurs concurrently with the processing of extravestibular sensory information (proprioceptive, visual, tactile, and auditory). Extensive connections between the vestibular nuclear complex, cerebellum, ocular motor nuclei, and brain stem reticular activating systems are required to formulate appropriate efferent signals to the VOR and VSR effector organs, the extraocular and skeletal muscles. The vestibular nucleus sends outflow to the oculomotor nuclei, spinal cord, cerebellum, and vestibular cortex.

Three major white matter pathways connect the vestibular nucleus to the anterior horn cells of the spinal cord. The lateral vestibulospinal tract originates from the ipsilateral lateral vestibular nucleus, which receives the majority of its input from the otoliths and the cerebellum. This pathway generates antigravity postural motor activity, primarily in the lower extremities, in response to the head position changes that occur with respect to gravity. The medial vestibulospinal tract originates from the contralateral medial, superior, and descending vestibular nuclei; it mediates ongoing postural changes in response to semicircular canal sensory input (angular head motion). The medial vestibulospinal tract descends only through the cervical spinal cord in the medial longitudinal fasciculus; it activates cervical axial musculature.

The reticulospinal tract receives sensory input from all of the vestibular nuclei, as well as all of the other sensory and motor systems involved with the maintaining balance. This projection has both crossed and uncrossed components, and it is very highly collateralized. As a result, its pathway through the entire extent of the spinal cord is poorly defined. The reticulospinal tract is probably involved in most balance reflex motor actions, including postural adjustments made to extravestibular sensory input (auditory, visual, and tactile stimuli).

The cerebellum is a major recipient of outflow from the vestibular nucleus complex, and it is also a major source of input to the vestibular nucleus itself. Most of the signal traffic is routed through the inferior cerebellar peduncle. The cerebellum is not required for vestibular reflexes, but when removed, vestibular reflexes become uncalibrated and ineffective. Originally, the vestibulocerebellum was defined as the portions of the cerebellum that received direct input from the primary vestibular afferents. It is now appreciated that most parts of the cerebellar vermis (midline) respond to vestibular stimulation. The cerebellar projections to the vestibular nuclear complex have an inhibitory influence on the vestibular nuclear complex. The cerebellar flocculus adjusts and maintains the gain of the VOR.y Lesions of the flocculus reduce the ability of experimental animals to adapt to disorders that reduce or increase the gain of the VOR. The cerebellar nodulus adjusts the duration of VOR responses; it is also involved with processing of otolith input.

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