Sympathetic vasodilator fibers are intermingled with other sympathetic fibers. They have acetylcholine and not norepinephrine as their neurotransmitter. The sympathetic vasodilator nerves are activated by cortico-hypothalamic-reticulo-spinal pathways, which are quite separate from the vasomotor center-thoracolumbar spinal paths.
Yet, they are called sympathetic because they discharge only in emergencies. They are not influenced by the baroreceptor and chemoreceptor afferents and do not participate in the usual cardiovascular reflexes.
Sympathetic vasodilator fibers are not important in providing improved nutritive flow to the muscles, either during or before exercise. The sympathetic vasodilator nerves provide a safety valve that prevents dangerous increase in blood pressure before the onset of exercise. Its mechanism is as follows.
Before exercise, even the thought of exercise brings about considerable sympathetic discharge, which causes widespread vasoconstriction in the muscle beds. As a result, the total peripheral resistance and blood pressure tends to rise to dangerous levels.
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At this time, the discharge of the sympathetic vasodilator nerves lowers the blood pressure to safe levels. It does so by dilating the A-V anastomotic channels in the muscle. Opening of these channels does not improve nutritive flow to the muscles. Once exercise commences, the precapillary sphincters are dilated by local metabolites and blood is diverted from the A-V channels to the muscle capillaries which now offer lesser resistance.