First segment of axon that connects distal axon segments to the neuronal CELL BODY at the axon hillock region. The axon initial segment is not protected by the MYELIN SHEATH and has properties critical for axonal growth. The axon initial segment and the axon hillock form an axonal trigger zone.

Synonyms: Axon Initial Segment, Axon Hillocks, Trigger Zone, Axonal, Axon Hillock, Axon Initial Segments, Axonal Trigger Zones, Axonal Trigger Zone