effective postural responses. We can quickly see why persons with unilateral loss of vestibular function have loss of balance or difficulty maintaining gaze stabilization when they turn their head quickly to the affected side. Otoliths The two otolith structures, the saccule and the utricle, are contained within the vestibule. The saccule has a vertical orientation, whereas the utricle is oriented horizontally (see Figure 1). The otoliths are the gravity-sensitive structures of the vestibular system, responding to linear motion and head position. The vertically oriented saccule senses motion predominantly in the sagittal plane, such as forward tilt (pitch) and vertical linear acceleration, while the horizontally oriented utricle senses motion predominantly in the horizontal plane, such lateral tilt (roll) or horizontal linear acceleration of the head. Together, they are able to detect linear acceleration in all possible vectors of the vertical, horizontal, and transverse planes of motion (Herdman & Clendaniel, 2014; Jacobson & Shepard, 2008). A classic example of otolith function is that of riding in an elevator. Although from a visual perspective it appears as though the environment is stationary, the otoliths are able to register the vertical linear acceleration that is taking place, thus making it possible to appreciate that vertical movement is occurring. Unlike the semicircular canals that rely on a hydrodynamic system to translate head movement into neural firing, calcium carbonate crystals (otoconia) embedded in the gelatinous macula of the otoliths detect linear acceleration and static head tilt with respect to gravity (see Figure 3). Figure 3: Anatomy of the Macula of the Otoliths
by a central strip of hair cells called a striola (see Figure 4). The kinocilia in the utricle are oriented toward their striola, whereas kinocilia in the saccule are oriented away from their striola, creating a push-pull arrangement in the otoliths in response to static head tilt and linear acceleration. This anatomical organization also results in excitatory and inhibitory neural firing within each macula, creating redundancy in the push-pull arrangement, which is thought to be the reason the otoliths may have less vulnerability to unilateral vestibular loss than the semicircular canals (Baloh & Halmagyi, 1996; Herdman & Clendaniel, 2014). Figure 4: Otolith Organization
Vestibular Nerve Neural projections carrying afferent information from the vestibular apparatus arise from the hair cells of the semicircular canals and otoliths and join with the cochlear nerve to become the CN VIII. The cell bodies of the primary vestibular afferents are located in the vestibular ganglion, also known as Scarpa’s ganglion . The vestibulocochlear nerve passes through the internal auditory canal, along with the facial nerve (CN VI) and labyrinthine artery, to synapse on the lateral, medial, inferior, and superior vestibular nuclei at the pontomedullary junction of the brainstem, located at the cerebellopontine angle. Projections are also sent to the cerebellum to monitor vestibular performance and make necessary adjustments for posture and coordination of limb movements. Inputs to the vestibular nuclei also include sensory information from spinal cord afferents, inputs from the cerebellum, and projections from contralateral vestibular nuclei. Outputs from the vestibular nuclei include projections to the cerebellum, the lateral and medial vestibulospinal tracts, reticulospinal tract, thalamus, motor nuclei of the extra-ocular muscles (ascending medial vestibulospinal tract), cerebral cortex, and the vestibular apparatus. The role of these connections in maintaining postural control will be discussed in later sections. information, which drives appropriate motor responses for effective maintenance of balance and postural control. The four vestibular nuclei (superior, inferior, medial, and lateral) are located in the brainstem at the level of the pons and medulla, and serve as the cranial nuclei of the vestibular nerve. The pathways for the vestibulo-ocular reflex (VOR) arise from the superior and medial nuclei. The lateral vestibular nucleus gives rise to the vestibulospinal pathways with some contribution from the medial vestibular nucleus. The inferior vestibular nucleus is connected to all other vestibular nuclei and the cerebellum but has no primary motor output.
As a result, neural firing is elicited from a shifting or shearing force of the otoconia on the macula, perpendicular to the hair cells, causing the hair cells to deflect in response to gravity- referenced head movement. In contrast to mechanoreceptors of the semicircular canals, which are organized in clusters, the hair cells of the otoliths are organized in curved sheets, bisected Central vestibular structures The CNS components of the vestibular system are the cerebellum and the vestibular nuclei. The cerebellum is the central adaptive processor of the vestibular complex, readjusting and recalibrating signals from the vestibular system to drive smooth coordinated movement and postural control. It does this through integrating inputs from the vestibular system and spinal cord afferents with motor output neurons, maintaining head- eye coordination, postural control during static and dynamic activities, and coordination of limb movements (Herdman & Clendaniel, 2014; Jacobson & Shepard, 2008). Contextual orientation to upright and the environment is achieved through cerebellar integration of somatosensory, visual, and vestibular
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