Transmission of the Nerve Impulse

TRANSMISSION OF THE NERVE IMPULSE

Microscopic studies of the structure of the terminal axons of the autonomic nerves have shown that the ax-ons branch many times on entering the effector tissue, forming a plexus among the innervated cells. “Swollen” areas found at intervals along the terminal axons are re-ferred to as varicosities (Figs. 9.2 and 9.3). Within each varicosity are mitochondria and numerous vesicles con-taining neurotransmitters.

The vesicles are intimately involved in the release of the transmitter into the synaptic or neuroeffector cleft in response to an action potential. Following release, the transmitter must diffuse to the effector cells, where it in-teracts with receptors on these cells to produce a re-sponse. The distance between the varicosities and the effector cells varies considerably from tissue to tissue. Smooth muscle, cardiac muscle, and exocrine gland cells do not contain morphologically specialized regions comparable to the end plate of skeletal muscle.

In the autonomic ganglia, the varicosities in the terminal branches of the preganglionic axons come into close contact primarily with the dendrites of the ganglionic cells and make synaptic connection with them.