Chapter 7 - Mechanisms of Neuroeffector Transmission

Summary

Axons of postganglionic neurons branch many times close to their effector cells and have hundreds to thousands of varicosities, which contain transmitter(s) packed in vesicles. Excitation of the postganglionic neurons spreads over all its branches and invades all varicosities. Signal transmission from postganglionic neurons to most effector cells occurs through specific neuroeffector synapses. In the heart, acetylcholine released by parasympathetic cardiomotor axons reacts with junctional muscarinic receptors that are coupled via a intracellular second-messenger pathway to the cellular effectors. Arterioles and small arteries are influenced by neural release of noradrenaline and ATP from the varicosities of the vasoconstrictor axons. The ATP reacts with junctional purinoceptors and opens ligand-gated cation channels, which cause activation of voltage-sensitive calcium channels and resultant action potentials. Noradrenaline released from the postganglionic vasoconstrictor terminals reacts with junctionally and extrajunctionally located a-adrenoceptors leading to slow depolarization in some blood vessels. The influence exerted by autonomic neurons on their effector tissues may be modulated by local and remote non-neural signals. The mechanisms of neuroeffector transmission in different autonomic targets are diverse. As no exceptions have yet been found, it is not far-fetched to assume that neuroeffector transmission is specific for all target cells innervated by postganglionic neurons.