aResponses are designated + to +++ to provide an approximate indication of the importance of sympathetic and parasympathetic nerve activity in the control of the various organs and functions listed. ^ Adrenergic receptors: ap a2, and subtypes thereof; br Cholinergic receptors: nicotinic (N); muscarinic (M), with subtypes 1-4. The receptor subtypes are described more fully in Chapters 7
and 10 and in Tables 6-2, 6-3 and 6-6. When a designation of subtype is not provided, the nature of the subtype has not been determined unequivocally. Only the principal receptor subtypes are shown. Transmitters other than acetylcholine and norepinephrine contribute to many of the responses.
cIn the human heart, the ratio of bi to ^2 is about 3:2 in atria and 4:1 in ventricles. While M2 receptors predominate, M3 receptors are also present.
dThe predominant a receptor subtype in most blood vessels (both arteries and veins) is a^ (see Table 6-8), although other a subtypes are present in specific vessels. The am is the predominant subtype in the aorta.
^Dilation predominates in situ owing to metabolic autoregulatory mechanisms.
^Over the usual concentration range of physiologically released circulating epinephrine, the b-receptor response (vasodilation) predominates in blood vessels of skeletal muscle and liver; the a-receptor response (vasoconstriction) in other abdominal viscera. The renal and mesenteric vessels also contain specific dopaminergic receptors whose activation causes dilation. ^Sympathetic cholinergic neurons cause vasodilation in skeletal muscle beds, but this is not involved in most physiological responses.
^The endothelium of most blood vessels releases NO, which causes vasodilation in response to muscarinic stimuli. However, unlike the receptors innervated by sympathetic cholinergic fibers in skeletal muscle blood vessels, these muscarinic receptors are not innervated and respond only to exogenously added muscarinic agonists in the circulation.
iWhile adrenergic fibers terminate at inhibitory b receptors on smooth muscle fibers and at inhibitory a receptors on parasympathetic (cholinergic) excitatory ganglion cells of the myenteric plexus, the primary inhibitory response is mediated via enteric neurons through NO, P2Y receptors, and peptide receptors.
Responses of Effector Organs to Autonomic Nerve Impulses (Continued)
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