ABSTRACT
The autonomic nervous system provides an important mechanism for regulating the cardiovascular system in vertebrates. Some features of the autonomic innervation are common to nearly all vertebrates. These include an inhibitory vagal innervation of the heart and a constrictor innervation of at least some blood vessels by spinal autonomic neurons that synthesize catecholamines. However, there seems to be an evolutionary trend for cardiac excitation to be achieved by catecholamines released from endogenous or nearby chromaffin tissue in cyclostomes, elasmobranchs and dipnoans, but to be mediated primarily by spinal autonomic neurons in teleosts and most tetrapods. All vertebrates have developed reflexes that modify the cardiovascular system in response to stimuli such as hypoxia and exercise. The neuroeffector mechanisms used to achieve autonomic effects seem to vary between parts of the cardiovascular system and between species. Cotransmission involving “classical transmitters”, peptides and purines is widespread, although there are no obvious evolutionary trends in the neurotransmitters used by autonomic neurons controlling homologous structures.
