Box 105

Functional Considerations: Complexity of Smooth Muscle Innervation

The description of autonomic innervation of smooth muscle according to the sympathetic and parasympathetic classification and their respective adrenergic and cholinergic transmitters may be more complex than traditionally thought. Viewed with the TEM, nerve terminals with synaptic vesicles that appear to be empty are considered cholinergic (i.e., secreting the neurotransmitter acetylcholine); nerve terminals with synaptic vesicles filled with dense granular material are considered adrenergic (i.e., secreting the neurotransmitter norepinephrine). Other neurotransmitters have been identified and are collectively called purinergic. They are likely to be contained in large vesicles with an opaque content.

Nerve endings in smooth muscle tissue that contain primarily mitochondria and no vesicles are considered sensory. Both sympathetic and parasympathetic fibers innervate smooth muscle, and it is difficult to distinguish between the fibers. In some smooth muscle, the adrenergic neurotransmitters stimulate and the cholinergic transmitters inhibit contraction; in other smooth muscle, the reverse is true.

cytoplasmic morphology is difficult to distinguish from that of the endothelial cell. In postcapillary venules and pericytic venules, they may form a nearly complete investment of the vessel with cells that resemble smooth muscle cells (see Chapter 12).

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