An electron microscope study of motoneurones and interneurones in the cat abducens nucleus identified by retrograde intraaxonal transport of horseradish peroxidase

Abstract

AbstractAbducens motoneurones innervating the lateral rectus muscle and abducens interneurones projecting to the oculomotor nucleus have been studied in the cat by light and electron microscopy following labelling with horseradish peroxidase. Motoneurones ranged from 15–60 μm in diameter and typically contained circular nuclei with smooth or slightly irregular nuclear membranes and extensive cisternae of granular endoplasmic reticulum. Interneurones ranged from 25–50 μm in diameter and comprised two cytologically distinct cell types. One type was fusiform in profile and contained nuclei with fluted or deeply invaginated nuclear membranes and poorly developed granular endoplasmic reticulum. The other interneurone type resembled the motoneurones cytologically, having multipolar or circular profiles, smooth or slightly irregular nuclear membranes, and well developed cisternae of granular endoplasmic reticulum.The densities with which boutons were distributed on the somata of motoneurones and interneurones were similar, as were the ratios of boutons containing spheroidal or flattened synaptic vesicles. Motoneurones varied significantly in the density with which boutons were distributed on their somata, and had greater densities on their dendrites than on their somata. Density was, furthermore, not correlated with motoneurone soma diameter. The proportion of spheroidal or flattened vesicle boutons on the somata of motoneurones was almost equal, with comparatively little variation between individual motoneurones. Motoneurone dendrites, however, with the exception of lateral dendrites, had a relatively higher proportion of boutons containing spheroidal synaptic vesicles. The lateral dendrites, which extend toward the medial vestibular nucleus, a known source of inhibitory afferents to the abducens motoneurones, had a higher proportion of boutons containing flattened vesicles.The results indicate that abducens motoneurones and interneurones cannot be distinguished by any simple morphological criteria. The somadendritic distribution of boutons on motoneurones appears to be governed by certain broad rules.