Physiological and morphological analysis of cat digastric motoneurons after intracellular staining with horseradish peroxidase.

Abstract

Horseradish peroxidase was injected into the somata of cat digastric motoneurons. Twelve motoneurons for which the spatial distribution of dendritic arborization and axon trajectory were iden-tified physiologically were studied with reference to the location of cell bodies in the trigeminal motor nucleus. It was revealed that, irrespective of the rostrocaudal level of the soma, dendrities of digastric motoneurons were present mainly in the medial half of the nucleus as well as in extranuclear regions. Interestingly very few dendritic branches were observed in the lateral half of the motor nucleus, this “unoccupied” region corresponding to the masseter subdivision of the nucleus. The axons ran a dorsomedial course and descended to the motor root without giving rise to collaterals. The average size of the soma was 44.2μm with a range of 36.9-50.1μm. A linear correlation was observed between the total stem dendrite diameter and the mean soma diameter, giving a coefficient of 0.534. No excitatory postsynaptic potential was recorded from the soma upon stimulation of the digastric nerve with a current below the threshold, indicating a lack of muscle spindles.