Final answer:
The vasomotor center has preganglionic sympathetic fibers known as vasomotor nerves, which extend to blood vessels to regulate their constriction, primarily by releasing norepinephrine. Some neurons also induce vasodilation using acetylcholine and nitric oxide.
Step-by-step explanation:
The vasomotor center has preganglionic sympathetic fibers that extend from it to the blood vessels. These vasomotor nerves are crucial in regulating the constriction of blood vessels in response to various signals, primarily those from the cardiovascular center. The primary neurotransmitter released by these sympathetic neurons is norepinephrine, which induces vasoconstriction affecting peripheral resistance, blood pressure, flow, and therefore cardiac output.
Not all the neurons in the vasomotor center induce vasoconstriction, however. There is a subset of neurons, often cholinergic, which promote vasodilation in specific vessels like those in the brain and skeletal muscles. Acetylcholine released by these neurons stimulates the production of nitric oxide (NO) from endothelial cells, leading to the relaxation of smooth muscle fibers and vessel dilation. Some neurons may also bind to ß2 receptors or release NO directly as a neurotransmitter.
Furthermore, the vasomotor centers are not anatomically distinct but function interdependently, coordinating with the cardiovascular center to maintain hemodynamic balance in response to stimuli such as changes in blood pressure or stress.