ABSTRACT
The concept of neurohumoral transmission between a nerve and either another nerve or the effector smooth muscle cell was first introduced by Elliott in 1905. In spite of the growing evidence to support the concept of neurohumoral transmission, there remained a few sceptics who were unwilling to accept the idea. Several criteria have been laid down for identification of a putative neurotransmitter. These have been proposed over the years by several authorities for the classical neurotransmitters. As the postganglionic sympathetic neurone enters the terminal organ it branches extensively and takes on a beaded appearance. There may be several thousand varicosities for each sympathetic neurone. Thus sympathetic neurones differ from somatic nerves, in which each branch ends in a discrete motor end plate. A secondary uptake mechanism is into extraneuronal or uptake sites. The two uptake compartments contain the two major metabolic enzymes for noradrenaline, monoamine oxidase and catechol-O-methyltransferase.
