Vestibular nerve axons synapse on sensory hair cells located in a macula (saccule or utricle) or a crista ampularis (semicircular duct) of the vestibular apparatus sense organ. Hair cell excitation (depolarization) triggers action potentials (APs) in vestibular nerve axons. The frequency of action potentials per vestibular nerve axon is proportional to the excitation of the hair cells on which the axon synapses.
Hair cells are excited (depolarized) by head acceleration toward the ipsilateral side; simultaneously, contralateral hair cells are hyperpolarized. Thus, when the frequency of APs increases on one side, APs decrease on the other side. (The right vestibular apparatus is a mirror image of the left and, during acceleration, cilia are displaced in opposite directions on the two sides.)
Normal head posture reflects bilateral balance in vestibular apparatus excitation, vestibular nerve AP frequency, and vestibular reflexes. (Vestibular reflexes shift eyes, tilt the head & extend limbs.) Head acceleration upsets the excitation balance, triggering vestibular reflex activity that restores normal posture and balances vestibular input.
If vestibular imbalance is caused not by head acceleration but by a destructive lesion. The imbalance triggers vestibular reflexes which alter posture, but the reflexes cannot restore vestibular balance and so the altered posture persists until the damage is reversed and/or the nervous system undergoes some degree of accommodation. The typical vestibular syndrome involves falling, downward head tilt and nystagmus slow phase directed toward the lesion (impaired) side.
(Note: It is possible, although unlikely, for a lesion to produce vestibular excitation or to damage inhibitory cerebellar cortex. This imbalance, which results in a head tilt toward the normal side, produces a paradoxical vestibular syndrome and it is rare.)
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