The fine structure of neurons and synapses in the motor nuclei of the cat spinal cord

Abstract

Abstract The normal synaptic organization of the motor nuclei in the cat lumbosacral spinal cord was examined by light and electron microscopy. Neurons could be classified into three categories on the basis of their size in Nissl stained sections. In neurofibrillar stained sections, seven neurofibrillar rings were observed, on the average, to lie upon large neurons and proximal dendrites in the motor nuclei. Electron microscopic examination revealed no apparent differences in the fine structure of the three neuronal cell types. In contrast the synaptic covering, as determined by the actual number of synaptic contacts or “active” zones, varied greatly. Less than half of the perikaryal surface of large neurons was covered by “active” zones and very few or no “active” zones covered the perikaryal surface of smaller neurons. The synaptic knobs contacting neuronal cell bodies and dendritic processes were classified into seven morphological types on the basis of vesicle shape, the presence of neurofilaments, and the nature of the subsynaptic specializations. Synaptic contacts occurred more frequently on proximal dendrites and less frequently on perikarya and distal dendrites. Rounded vesicle knobs outnumbered flattened vesicle knobs on distal dendrites, while the reverse was true on cell bodies and proximal dendrites. Two quite large synaptic knobs were characteristically observed. One knob, occurring predominantly on cell bodies and proximal dendrites, exhibited an apposed subsynaptic cistern and elaborate subsynaptic granular endoplasmic reticulum. The other synaptic knob, the least commonly found of all synaptic types, appeared primarily on proximal dendrites and exhibited multiple synaptic contacts with specialized subsynaptic densities. No axo‐axonal synaptic contacts were observed in the motor nuclei.