A study of the axon initial segment and proximal axon of neurons in the primate motor and somatic sensory cortices

Abstract

The axon initial segments of pyramidal cells and large and small stellate cells in the primate sensori-motor cortex have a typical membrane undercoating and bundles of neurotubules. Those of pyramidal cells are directed towards the white matter whereas those of large and small stellate cells often run obliquely or towards the cortical surface and may be curved. Cisternal organs in these initial segments are related to symmetrical axon terminals, frequently coming into close apposition to the non-synaptic part of these terminals adjacent to the synapse between the axon terminal and initial segment. The dense plates of cisternal organs and the membrane undercoating of the initial segment are specifically stained by ethanolic phosphotungstic acid (ethanolic PTA). Pyramidal initial segments have spines which receive only symmetrical synapses, as do the shafts of the initial segments of each cell type. The full length of the initial segment was studied for fourteen pyramidal and two large stellate cells. All gave rise to myelinated axons although two pyramidal cells had lengths of unmyelinated axon between the initial segment and myelinated axon. One of these lengths of unmyelinated axon made an asymmetric synapse on to a dendrite just after losing its initial segment features. Quantitative analysis of these complete initial segments showed that whereas the diameter of the initial segment and the axon it gave rise to were approximately proportional to the size of the parent cell soma over a considerable range of cell diameters, the length of the initial segment appeared to be unrelated to either its diameter or the size of its parent soma but varied between 30 and 55 μm apparently at random. Synapses were evenly distributed along the full length of the complete pyramidal initial segments, but the density of synapses on the initial segments of supragranular pyramids was about three times that on those of infragranular pyramids and cisternal organs were similarly more frequent in the initial segments of supragranular pyramids.