Autonomic neuroeffector mechanisms: recent developments.

Peripheral autonomic control mechanisms are more sophisticated than formerly recognized. The autonomic neuroeffector junction is defined, with emphasis on 'en passage' release of transmitter from varicosities in extensive terminal branching nerve fibres. In addition to the classical autonomic transmitters acetylcholine and noradrenaline, a multiplicity of other neurotransmitter substances are also present, including peptides, purines, indoleamines and amino acids. There is coexistence of different combinations of these substances which, upon release, act either as cotransmitters or neuromodulators. Examples are described, including cotransmission and neuromodulation at sympathetic, parasympathetic and sensory-motor neuroeffector junctions. The involvement of locally released agents in peripheral control is also described, including substances produced secondarily as a result of release of neurotransmitters, as well as the release of substances from vascular endothelial cells leading to vasodilatation.