Distribution of synapses on fast and slow pyramidal tract neurons in the cat. An electron microscopic study.

Distributions of synapses on various portions of fast and slow pyramidal tract neurons (PTNs) in cat motor cortex were studied with electron microscopy. PTNs were identified by their antidromic invasion following stimulation of the medullary pyramid and were classified into fast and slow PTNs according to conduction velocities of their axons. Two fast and two slow PTNs were intracellularly labeled and, by systematic sampling, electron micrographs from various portions of these neurons were examined to compare the distributions of different types of synapses. It was found that most synapses formed on apical and basal dendrites of fast PTNs were with the dendritic shafts. In slow PTNs, while synapses on apical dendrites were mostly axospinous, about 70% of the sampled synapses on basal dendrites of slow PTNs were established with the dendritic shafts. Virtually all synapses on apical dendrites of slow PTNs belonged to asymmetrical type and most of the synapses sampled from basal dendrites of fast PTNs were also asymmetrical. On the other hand, about 29% of the synapses found on apical dendrites of fast PTNs were symmetrical and a trend was observed for this type of synapses to increase their number with increasing proximity to the cell body. Over 28% of the synapses on basal dendrites of slow PTNs were also symmetrical and seemed to be mainly distributed in layer VI. All synapses formed on the soma were symmetrical both for the fast and slow PTNs.