Large vessels are controlled by the sympathetic division; note that all neurons to blood vessels are orange. These neurons release norepinephrine, NE -- not shown in the illustrations -- onto receptors bound to smooth muscle (donut) cells that encircle the vessels. Hormones also play a role in vasoconstriction and vasodilation.
Postganglionic sympathetic neurons (orange) are scattered among the smooth muscle cells (donut) that encircle arteries. Alpha 1 receptors are characteristic of vascular smooth muscle although they are sparce in some vessels. These receptors are stimulatory -- color-coded green -- and their contraction narrows the vessel diameter thus reducing blood flow.
The adrenal hormones epinephrine E and norepinephrine NE also bind to alpha 1 receptors reaching these targets via the blood. The potency of norepinephrine is the same regardless of the source; epinephrine is less potent with respect to its affect on these receptors.
The smooth muscle of blood vessels supplying the skeletal muscles is unique because, in addition to alpha 1 receptors, they also have beta 2 receptors. These receptors are inhibitory (color-code red) indicating that, when activated, they cause relaxation of the muscle. This causes the vessel diameter to increase thus increasing blood flow to skeletal muscles.
Epinephrine, E is the only naturally occurring compound that activates beta 2 receptors and, even then, its potency is low. Therefore, the degree of vasodilation is dependent on the density of these receptors on the vascular muscle cells in addition to the concentration of epinephrine in the blood.
Alpha 2 receptors -- color-coded red -- are inhibitory. Their location is unique; they are found mainly on the post-synaptic neuron terminals (orange bar) at the end of neurons innervating blood vessels. Veins have a higher density of these receptors than arteries.
The inset to the right illustrates the details of alpha 2 receptor activation. Norepinephrine released from the terminal (solid arrow pointing from orange bar to NE) will bind to both the alpha 1 receptors (solid arrow from NE to alpha 1) on the vascular muscle (donut) and to alpla 2 receptors (solid arrow from NE to alpha 2) on the terminal (orange bar). The dashed arrow pointing from the alpha 2 receptor to NE symbolizes an inverse relationship, i. e., activation of alpha 2 receptors decreases NE release. The overall result is a dampening of the neural signal to the vascular muscle to contract.
The pulmonary circuit has no autonomic innervation. It is a low-pressure circuit mainly because its arterial walls are thin. The main flow-control mechanism is due to local concentrations of dissolved respiratory gases.
The coronary circuit has no autonomic innervation. The main flow-control mechanism is due to local concentrations of metabolites and respiratory gases.
last update: 10/21/05