The mechanism of transmission of excitation from sympathetic nerves to smooth muscle appears to be much like transmission at cholinergic junctions. 1) There is a spontaneous release of packets of NE from sympathetic nerves. 2) On nerve stimulation, many packets of NE reach the effector cells. 3) Stimulation of the nerves leads to increase of spontaneous release of NE. 4) NE released from the nerves depolarises the postjunctional smooth muscle membrane, and this leads to a spike and contraction. 5) The prejunctional terminations of the sympathetic nerves are packed with vesicles and mitochondria. The main difference from the skeletal neuromuscular junction is the slow time course of the junction potentials and the long delay before their appearance after stimulation.
In the vas deferens, where there are junctions with separations of only 200 Å between nerve and muscle, the minimum delay recorded is 6 msec and the duration of SEJPs may be less than 100 msec. It is possible that these delays are due to the reaction of NE with the postjunctional receptors.
In the mesenteric artery and gut, the delay is much longer, of the order of 150 msec. Few, if any, junctions with nerve-muscle separation of 200 Å have been observed in these preparations and the majority of varicose fibres run in bundles with wide separations from muscle membranes. These long delays might therefore be due to diffusion of NE over considerable distances. That NE can diffuse over long distances without being inactivated is another point of difference from cholinergic transmission.
The sensitivity of different smooth muscle cells, or even of different regions of the muscle membrane, to NE might be influenced by the nature of the sympathetic end apparatus in that it is likely to differ in those cells receiving transmitter primarily from localised close junctions compared with cells receiving transmitter by diffusion from many sources at variable distances.
The variations in the geometry of the end apparatus may account for the variation in functional organisation of different organs. For example, close junction on most muscle cells is correlated with fast, coordinated contraction (e.g., rat vas deferens), whereas wide separation of nerves from muscle, arranged so that different cells receive different amounts of transmitter, is correlated with graded and regional differentiation of contraction and relaxation (e.g., intestine).