Initial Segment and Axon Hillock
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The cone-shaped region of the cell body where the axon originates is termed the
axon hillock. This area is free of
ribosomes and most other cell organelles, with the exception of cytoskeletal elements and organelles that are being transported down the axon. The
neurofilaments in the axon hillock become clustered together as fascicles. The region between the axon hillock and the beginning of the myelin sheath is known as the
initial segment. In many cases, this region is the anatomical location for the initiation of the action potential. The area under the
axolemma in this region has material that stains darkly when viewed by EM. This region is shown in Figure 8.4. At the distal-most end of the axon and its collaterales are small branches whose tips are button-shaped cytoplasmic enlargements called
terminal boutons or nerve endings.
