Terminal inflated portion of the axon, containing the specialized apparatus necessary to release neurotransmitters. The axon terminus is considered to be the whole region of thickening and the terminal bouton is a specialized region of it. [ GOC:mc GOC:dph http://www.ncbi.nlm.nih.gov/pubmed/10218156 http://www.ncbi.nlm.nih.gov/pubmed/8409967 GOC:nln ]