Vestibular System
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Introduction
Maintaining your balance requires integration from multiple sensory inputs, including joint receptors, muscle spindles, and the vestibular apparatus
Vestibular Apparatus
The vestibular apparatus lies within the bony labyrinth, along with the cochlea. The membranous labyrinth is filled with endolymph and surrounded by perilymph.
Linear acceleration of the head is determined by the utricle (horizontal) and the saccule (vertical). Small calcium carbonate crystals called otoliths are located in a gel above hair cells in the macular epithelium, and horizontal and vertical movements cause hair cell depolarization. The paired structures, and therange of hair cells within them, mean all directions of head tilt are unambiguously interpreted.
Rotational acceleration is measured by the semicircular (horizontal, anterior, and posterior) canals. Canals are paired with each other - horizontal-horizontal and anterior-posterior. Hair cells are clustered in the crista ampularis, located in the bulges along the canal called ampullae. Hair bundles project into the gelatinous cupula. Movement of the head causes bending of the hair cells as endolymph within the canals causes the cupula to bend.
Hair cells
Signals contonuously, with graded signals being important
Mechanical deformation occurs at the tips of cilia. A protein link at the tips tugs cation channels open.
Endolymph is high in K, which depolarizes the cell
A ribbon synapse is present basally, which guides glutamate-filled vesicles to the synapse.
cilia cover a 10 um long receptor cell, which itself generates a graded potential that passively spreads to its basal side. Depolarization of this cell is the gate, with a synapse of a myelinated interneuron transducing the signal to the CNS.
Vestibular Processing and Output
Upon synapsing with hair cells, axons of bipolar neurons leave through the vestibular nerve, with cell bodies in the vestibular (Scarpa's) ganglia. Dendrites extend along the vestibulocochlear nerve (CN VIII), travelling alongside the facial nerve (CN VII) through the internal acoustic meatus, and enter the brainstem at the pontomedullary junction.
Some proceed directly to the cerebellum via the juxtarestiform body in the inferior cerebral peduncle. Most, however, synapse in the four vestibular nuclei of the inferior pons.
Vestibular nuclei
There are four vestibular nuclei - inferior, medial, lateral, and superior.
Inputs to the vestibular nuclei, in addition to those from the vestibular system, include projections from:
- flocconodular lobe of cerebellum by way of juxtarestiform body
- spinal cord, some directly from spinovestibular fibres, but most via cerebellar or reticular relays
- contralateral vestibular nuclei. extensively.
Ascending pathways control oculomotor movements, while descending pathways control posture and gait.
Targets of vestibular outputs include:
CN III, IV, VI
- via ascending medial longitudinal fasciculus
- from crossed and uncrossed inputs of all four vestibular nuclei
- coordinates head and eye movements
spinal cord
- the lateral vestibulospinal tract (uncrossed) innervates extensor muscle motor neurons in the anterior horn, causing quick movements during postural reflexes
- the medial longitudinal fasciculi (crossed and uncrossed) coordinates head and eye movements by innervating upper motor nerves in the cervical gray
cerebellum
- input is via juxtarestaform body of the inferior peduncle
- reciprocal projection; vestibular nuclei process information, which can then be sent out to lower motor neurons to coordinate head and eye movements
reticular formation
- gives rise to secondary projections which project to visceral brainstem nuclei, ie the dorsal motor nucleus of CN X and others
- connections with visceral centre thought to be related to motion sickness
thalamus
- from here, projects to cerebral cortex, possibly somatosensory, to provide conscious awareness of space
Vestibular Reflexes
Vestibuloocular reflex (VOR) -
stabilizes vision while head is moving. This can be suppressed, however, as occurs during gaze shift.
Information about velocity from the vestibular nuclei travels bilaterally to abducens nuclei (CN VI) and subsequently to the oculomotor nuclei (CN III) via the medial longitudinal fasciculus to move the eyes in compensation.
- fastest reflex of the body
- three neuron arc
- push-pull setup as different muscles are working in the two eyes
- eyes move in same plane but opposite to head
Vestibulospinal reflexes
collaborates with the cervicospinal reflex to maintain postural stability
Vestibular Dysfunction
Resources and References
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