Part 5
A movement of the head in space can be quantified as the sum of a rotation
(around a given axis) and a translation (along a given straight line)
Rotational vestibulo-ocular reflex (RVOR): semicircular canals
Translational (linear) vestibulo-ocular reflex (LVOR or TVOR): otoliths
Bony labyrinth is filled with perilymph (high Na and low K) and the membranous
labyrinth is filled with endolymph (low Na and high K)
Semicircular canals: 3 pairs are roughly orthogonal to detect head rotations in all
three directions
Horizontal is often called lateral
Filled with endolymph and contain elaborations called ampullae, which convert
fluid motion within the canals into an electrical signal
Cupula: gelatinous membrane that lies on top of the crista into which the hairs of
the hair cells protrude
Movement of the endolymph caused by the head rotation presses the cupula
causing the hair bundles to bend
Hair cells hyperpolarize when moving away from the kinocilium and depolarize when
moving toward the kinocilium
Vestibular bipolar cells (primary vestibular neurons) contact the hair cells and
receive the depolarization or hyperpolarization resulting from the hair movements
High resting rate (tonic rate) around 90 spikes/sec (bidirectional detectors)
Davis mechanoelectric theory
Hair faces the endolymph and the basolateral membrane surrounded by
perilymph, leading to an ionic imbalance that generates a polarization with a high
tonic firing rate
Mechanical bending of the hairs causes a modulation of the electrical resistance
that modulates the firing rate of the vestibular nerve connection
Depolarization effects are much larger than hyperpolarizing effects for the same
absolute amount of hair displacement
Ewald’s second law: Excitation is more effective than inhibition in changing
labyrinthine activity
Otoliths detect gravity/linear acceleration
Saccule and utricle act as bidimensional linear acceleration and gravity detectors
Rotational VOR: purpose is to avoid slippage of the image of the world on the
retina during head rotations
Achieved by having the eyes rotating in opposite direction with respect to the
rotation of the head and of the same amount
No need of visual input, only vestibular inputs
Yaw (around z axis) rotations will generate conjugate horizontal eye rotations
Pitch (around y axis) rotations will generate conjugate vertical eye rotations
Roll (around x axis) rotations will generate conjugate counterrolling (torsion) eye
rotations
The SCCs and otoliths from the two inner ears are matched in functional pairs
For horizontal rotations: excitationipsilateral SCC & inhibition: contralateral SCC
An increase in activity in one anterior canal will cause a decrease in activity in the
contralateral posterior canal and vice versa
During vertical and counterrolling rotations all four A/P SCCs are active
Anterior canal: excited by downward head motion
Excites: ipsilateral SR and contralateral IO
Inhibits: ipsilateral IR and contralateral SO
Posterior canal: excited by upward head motion
Excites: contralateral IR and ipsilateral SO
Inhibits: contralateral SR and ipsilateral IO
For the otoliths, the saccule and utricle outputs are also organized in a ipsi/contra
push-pull circuitry
Allows the almost full recovery from the complete loss of one inner ear after a
period of adaptation (rebalance) of the signals during the acute phase, when
strong nystagmus occurs
Three-neurons arc: the direct path from the hair cells to the EOM is a short 3neurons arc, which maximizes the response speed
Primary vestibular neurons (Scarpa’s ganglions)-have dendrites connected
with the hair cells and synapse on the secondary vestibular neurons
Secondary vestibular neurons in the vestibular nuclei
Motorneurons in the oculomotor complex, directly or indirectly projecting to
the EOMs