The facial motor nucleus of the opossum: cytology and axosomatic synapses.

The normal cytology and synaptic organization of the opossum facial motor nucleus was determined by use of Nissl preparations, Golgi impregnations, one-micron plastic sections and electron microscopy. Neurons from all regions of the nucleus can be classified into three categories based on the size and shape of their perikarya, the distribution of Nissl substance and extent of their dendritic arbors. Large neurons (30--50 mu) exhibit numerous Nissl granules and their dendrites often span more than one-half the width of the nucleus in the transverse plane. Their proximal dendrites measure 4--7 mu in diameter, taper to 0.5--2 mu and display few spines. Medium neurons (20--30 mu) can be distinguished from the large nerve cells by their size and their less extensive dendritic arbor. Proximal dendrites measure 4--7 mu in diameter, tapes to 0.5--2 mu and are relativel aspinous. Small neurons (10--20 mu) display a thin rim of cytoplasm which is either uniformly basophilic or achromatic. The dendrites of these neurons are relatively unbranched and taper from 4--5 mu proximally to 0.5--2 mu distally. Their axons give off one or two collaterals within the nuclear borders. More than one-half the perikaryal membrane of large and medium neurons is covered by presynaptic profiles which were grouped into three primary categories based upon differences in their vesicle shape (spherical, pleomorphic, or ellipsoidal). Only a few axosomatic contacts are present on small neurons. Further differentiation of axosomatic synaptic endings is suggested by differences in vesicle size, in pre- and postsynaptic membrane densities and in the size of the synaptic terminals. Vesicle size was determined by a cybergraphic tablet and a PDP-12 computer system and is expressed as a mean area in nm2.