The Small Pyramidal Neuron of the Rat Cerebral Cortex the Axon Hillock and Initial Segment

The axon of the pyramidal neuron in the cerebral cortex arises either directly from the peri-karyon or as a branch from a basal dendrite. When it arises from the perikaryon, an axon hillock is present. The hillock is a region in which there is a transition between the cytologi-cal features of the perikaryon and those of the initial segment of the axon. Thus, in the hillock there is a diminution in the number of ribosomes and a beginning of the fasciculation of microtubules that characterize the initial segment. Not all of the microtubules entering the hillock from the perikaryon continue into the initial segment. Distally, the axon hillock ends where the dense undercoating of the plasma membrane of the initial segment commences. Dense material also appears in the extracellular space surrounding the initial segment. The initial segment of the pyramidal cell axon contains a cisternal organelle consisting of stacks of flattened cisternae alternating with plates of dense granular material. These cisternal organelles resemble the spine apparatuses that occur in the dendritic spines of this same neuron. Axo-axonal synapses are formed between the initial segment and surrounding axon terminals. The axon terminals contain clear synaptic vesicles and, at the synaptic junctions, both synaptic complexes and puncta adhaerentia are present. The axon originates from the neuronal peri-karyon at the axon hillock. In light microscope preparations stained with basic dyes, the axon hillock is characterized by an absence of basophilic substance from its cytoplasm, and this is the only cytological feature that distinguishes it from the remainder of the perikaryon. In the same type of preparation, the axon emerging from the hillock is also devoid of Nissl substance, so that generally it is apparent only as a faintly outlined and unstained strand with smooth contours. These features distinguish the axon from the dendrites, which, in addition to containing Nissl substance, have irregular contours. When an emergent axon acquires a myelin sheath, the sheath does not commence immediately. Instead there is a length of axon that remains bare. This is the initial segment of the axon, and i~ is a region of the neuron that has special properties. It is here that the action potential probably originates, and the membrane of the initial segment is considered to have a lower threshold to excitation than the membrane of dendrites (Eccles, 1964). Although numerous electron microscope studies of central nervous tissue have been carried out in …