The primary control of systemic peripheral resistance is at the level of arterioles. Smooth muscle cells surround arterioles at the entrance to capillary beds. Neurotransmitters released from varicosities in sympathetic motor neurons bathe the tissue. These varicosities are not distinct synapses like the neuromuscular junction, but are sites along the axon terminals that release neurotransmitter.
Gap junctions between the smooth muscle cells allow signals to pass from one cell to another, allowing them to contract as a unit, causing vasoconstriction. Arteriole smooth muscle cells are not innervated by the parasympathetic nervous system, except for the clitoris and penis where its activation causes engorgement and erection. Rather, vasodilation is due to the reduction in sympathetic tone and relaxation of the smooth muscle cells.
A sympathetic response increases blood pressure. It diverts blood from most tissues to the heart, liver, and skeletal muscles. This is accomplished by a single system acting on different receptors. Alpha adrenergic receptors in most of the body (such as the GI tract) cause vasoconstriction in response to norepinephrine released from sympathetic neurons. On the other hand liver and muscle tissues have beta adrenergic receptors that respond to circulating epinephrine with vasodilation. Actively metabolizing tissues also produce metabolites (paracrines) that cause vasodilation that increases local blood flow.
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