Visceral Efferent Endings

Autonomic neuromuscular endings control such diverse functions as heart rate, intestinal and urogenital activity, pupillary size, and blood pressure. The mor phologic features of this type of ending are shown in A, which illustrates a three-dimensional reconstruction of the smooth muscle lining the colon. Bundles of the unmyelinated postganglionic fibers that innervate intestinal muscle are enveloped by individual Schwann cells. As these bundles run between smooth muscle cells, each axon exhibits beadlike swellings filled with synaptic vesicles at various points along its length. At these varicosities (“boutons en passant”), the surrounding Schwann cell membranes are drawn back so that the released transmitter substance can diffuse into the interstitial space and act on nearby smooth muscle cells. After forming numerous varicosities, an individual axon loses its Schwann cell sheath; after a short distance, it forms a final terminal ending similar in structure to the earlier varicosities.

Autonomic nerve endings in exocrine glands are structurally similar to autonomic neuromuscular endings. In the case of the mandibular gland (B), bundles of unmyelinated postganglionic fibers in Schwann cell sheaths form varicosities and terminal endings in the spaces between secretory cells. In this gland, as in many structures innervated by autonomic fibers, two types of endings are seen. Sympathetic endings, which in this gland excite mucous cells to produce mucous saliva, are filled with densely staining vesicles indicating the presence of the transmitter norepinephrine. Parasympathetic endings, which act on serous cells to produce watery saliva, are filled with clear vesicles that contain acetylcholine.

The neurosecretory endings of the posterior pituitary gland (C) and adrenal medulla are adapted to allow the transmitter substance released by the arrival of an action potential in the nerve terminal to enter the bloodstream and be carried to target cells in other parts of the body. In the posterior pituitary, axons of neurons in the supraoptic and paraventricular nuclei of the hypothalamus run between supporting cells called pituicytes, to terminate directly on the basement membrane that delimits the collagen space around a capillary. Vesicles within the terminals contain one of the two posterior pituitary hormones, oxytocin and vasopressin (antidiuretic hormone). The morphology of the endings suggests that a hormone released by the arrival of action potentials in the terminals is able to diffuse through the collagen space and enter the capillary via pores between the endothelial cells. This diffusion process may be aided by mast cells, which are known to play a role in capillary permeability.

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