Morphology of physiologically identified second‐order vestibular neurons in cat, with intracellularly injected HRP

Abstract

The morphology of horizontal canal second-order type I neurons was investigated by intracellular staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of the cell bodies and axons. Axons penetrated in and around the abducens nucleus were identified as originating from type I neurons by their characteristic firing pattern to horizontal rotation and by their monosynaptic response to stimulation of the ipsilateral vestibular nerve. A total of 47 type I neurons were stained. The cell bodies were located in the rostral portion of the medial vestibular nucleus (MVN) and were large or medium sized and had rather elongated shapes and rich dendritic arborizations. The neurons were divided into two groups: those which projected to the contralateral side of the brain stem (type Ic neurons) and those which projected to the ipsilateral side of the brainstem (type Ii neurons). All stem axons of type Ic neurons crossed the midline and bifurcated into rostral and caudal branches in the contralateral medial longitudinal fasciculus (MLF). Two or three collaterals arising close to this bifurcation distributed terminals in a relatively wide area in the contralateral abducens nucleus. Some of these collaterals projected further to the contralateral MVN and thus are vestibular commissural axons. Some of the rostral and caudal stem axons had collaterals which projected to the contralateral nucleus prepositus hypoglossi (PH), nucleus raphe pontis, or medullary reticular formation. There were at least six classes of type Ii neurons, most of which distributed to a relatively limited region in the ipsilateral abducens nucleus and they were categorized according to their future projections into the following categories: A) no further collaterals beyond the abducens nucleus; B) collaterals in the abducens nucleus and a branch descending and terminating in ipsilateral PH; C) projected to the abducens nucleus, PH, and an area rostral to the abducens nucleus; D) projected to the abducens nucleus and to ipsilateral reticular formation rostral and caudal to the abducens nucleus; E) collaterals in the abducens nucleus and a thick caudal stem axon entering and descending in ipsilateral MLF; F) a thick caudal stem axon entering and descending in ipsilateral MLF and no collaterals to the abducens nucleus. Some type Ii neurons also had recurrent collaterals which projected back to the ipsilateral MVN; these may inhibit type II neurons during ipsilateral rotation.

Neuronal loss in human medial vestibular nucleus

“…The caudal projections of the MVN end in the cervical spinal cord (Peterson et al, 1978). The MVN is thus an important center for coordinating eye, head, and neck movements (Ohgaki et al, 1988;Baloh and Honrubia, 1990).…”

“…Based on its connections, the MVN has been implicated in the coordination of eye, head, and neck movements (Ohgaki et al, 1988;Baloh and Honrubia, 1990) and in vestibular compensation (Maeda, 1988;Baloh and Honrubia, 1990). The rostral part is thought to be one of the centers for the vestibulo-ocular reflex (Highstein, 1973a,b).…”

Detection of rotating gravity signals

“…Cells with relatively large dendritic trees, potentially composed of spines and regions with large input impedances, have been described in the vestibular nuclei (Mitsacos et al 1983;Ohgaki et al 1988;Highstein and McCrea 1988). In addition, the presence of GABA and glycine receptors that are associated with C1-channels and are possible candidates for shunting inhibition has been documented (cf., Precht 1979;Walberg et al 1990).…”

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

“…Several studies have used retrograde tracing or electrophysiological techniques to locate the cells in the VMN of the cat that project the nuclei of the third, fourth, and sixth cranial nerves (Graybiel and Hartwieg 1974;Gacek 1977;Uchino et al 1982;Isu and Yokota 1983;Langer et al 1986;Ohgaki et al 1988;Spencer et al 1989;Hoddevik et al 1991). In all reports, these cells are shown ventral to the calretinin area.…”