NOTICE: Balanced input from right & left Vestibular Nerves
results in
balanced reflexes, leading to Normal Head Posture (square to the horizon).
Vestibular reflex activity involves more than neck muscles. The eyes, trunk and
limbs are also engaged to keep the head square and preclude falling.
NOTICE: Persistent Head Tilt occurs because unilateral nerve damage generates
a persistent bilateral
vestibular nerve AP rate asymmetry that drives
persistent asymmetric reflex activity. [AP = Action Potential]
NOTICE: External forces that disturb head position, trigger differential
vestibular apparatus excitation. This results in asymmetric vestibular nerve
AP rates. The reflex differential works to restore normal head position.
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 become excited (depolarized) by head acceleration toward one side. Simultaneously, hair cells of the contralateral side become hyperpolarized. Thus, when the frequency of APs increases on one side, AP frequency decreases 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 balanced excitation in bilateral vestibular apparatuses, leading to balanced bilateral AP frequency in vestibular nerves, and comparable opposing vestibular reflexes. (Vestibular reflexes shift eyes, tilt the head & extend limbs.) Head acceleration (generated by external force) upsets the excitation balance, triggering differential vestibular reflex activity that restores normal posture and balances vestibular excitation.
If vestibular imbalance is caused not by head acceleration but by a destructive lesion. Then, imbalanced vestibular nerve activity triggers vestibular reflexes that alter posture, but the reflex activity can not restore vestibular nerve balance and so altered posture persists indefinitely, 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 all 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.)
The cat cartoon was created by Kurtis Scaletta.