MORPHOLOGICAL ANALYSIS OF CAT MASSETERIC MOTONEURONS AFTER INTRACELLULAR STAINING WITH HORSERADISH-PEROXIDASE

Intracellular injection of horseradish peroxidase (HRP) into 58 masset eric motoneurons identified by antidromic activation was performed in cats under pentobarbital anesthesia. Monosynaptic EPSPs were evoked by masseteric nerve stimuli in 52 cells, and were absent in the remainin g six cells. The antidromic nature of the evoked spikes was confirmed by IS-SD separation observed at high frequency (50 Hz) stimulation. Mo toneurons with monosynaptic excitation from masseter afferents showed IPSPs following stimulation of lingual and inferior alveolar nerves. M otoneurons which did not show monosynaptic excitation from masseter af ferents showed no IPSPs from the above nerves. There were no differenc es in cell size or the number of stem dendrites between motoneurons wi th and without monosynaptic EPSPs. No recurrent collaterals were obser ved in any motor axone. Motoneurons with monosynaptic EPSPs were locat ed at all rostrocaudal levels throughout the trigeminal motor nucleus, whereas motoneurons without such EPSPs were encountered only at the m iddle level. Dendtites of motoneurons with monosynaptic EPSPs did nor extend into the medial portion of the nucleus where motoneurons innerv ating the anterior belly of the digastric muscle were located. In cont rast, motoneurons without monosynaptic EPSPs had dendrite branches ext ending well into the medial part. The results show that there are two subpopulations of masseteric motoneurons that differ in peripheral inp uts as well as dendritic morphology. (C) 1998 Elsevier Science B.V.