Figure 1. Hair cells of the inner ear transduce mechanical into electrical signals. (a) At rest, hair cells release glutamate as a transmitter and afferent nerve fibres discharge at a rate of about 90 impulses per second. (b) At the onset of a head rotation to the right, the endolymph in the semicircular canals follows the head less rapidly (dashed arrows) and bends the cupula with the hair bundles. Displacement of the hair bundle towards its longest process leads to a depolarizing receptor potential, displacement in the opposite direction leads to a hyperpolarizing receptor potential. The rate of transmitter release of hair cells is coupled to the receptor potential and changes correspondingly. As a result, the discharge rate in afferent fibres of the right side increases by the same amount as the discharge rate on the left side decreases. The amount of this difference in discharge rates represents the degree of head acceleration.

Figure 2. Semicircular canal-related neurons in the vestibular nuclei are connected across the midline (dashed vertical line) by commissural fibres that mediate a differential amplification of signals that originate in functional canal pairs. At the onset of a head turn to the left, the discharge rate in afferent fibres increases on the left side (N. VIII_L) and decreases on the right side (N. VIII_R). Central vestibular neurons on the left side (VN_L) receive an increased excitation from vestibular afferent fibres. This excitation is amplified because of the simultaneous reduction of commissural inhibition in these neurons. Central vestibular neurons on the right side (VN_R) receive a reduced excitation from vestibular afferent fibres but an increased inhibition from commissural fibres because of the increased activation on the left side. As a result, the difference in the discharge rates present in bilateral vestibular nerve afferent fibres is centrally amplified by a commissure that functions as a differential amplifier.

Figure 3. Neural organization of the horizontal vestibulo-ocular reflex. Afferent fibres from the horizontal semicircular canal (HC) activate central vestibular neurons (VN). The axons of part of these neurons (open circle) cross the midline (dashed vertical line) to excite abducens (AB-) motor neurons (MOT) and internuclear neurons (INT) on the contralateral side. Inhibitory vestibular neurons (closed circle) project to the corresponding abducens neurons on the ipsilateral side. This push–pull organization activates agonistic and inhibits antagonistic pairs of extraocular muscles. The crossing axons of abducens internuclear neurons with their excitatory input to medial rectus oculomotor neurons (OC) provide a basis for conjugate horizontal eye movements. Green, activated pathways.