Portion of the axon proximal to the neuronal cell body, at the level of the axon hillock. The action potentials that propagate along the axon are generated at the level of this initial segment. [ GOC:nln http://www.ncbi.nlm.nih.gov/pubmed/21551097 http://www.ncbi.nlm.nih.gov/pubmed/1754851 GOC:sl ]