Sympathetic Vasodilator Outflow

Abstract

Sympathetic vasodilator neurons take their origin in the motor cortex and pass, in close proximity to motor fibers, to the hypothalamus, where they are probably relayed to lower neurons. Continuing caudally, they have a new relay station in the collicular region and reach the ventrolateral part of the medulla oblongata. At this site, the vasodilator tract forms a longitudinal bundle 1-2 mm above the ventral surface and runs to the lateral horns of the spinal cord. The sympathetic vasodilator fibers are cholinergic. They are probably confined to the skeletal muscles (and possibly to the coronaries of the heart). The arguments presented by earlier authors in support of the outflow of sympathetic vasodilator nerves to the intestines, and to the skin of the ear in the dog are reviewed and found to be inadequate. The existence of adrenergic vasodilator nerves is rejected. With regard to the functional significance of the sympathetic vasodilator nerves, it is suggested that they are capable of increasing muscular blood flow in situations of emergency and in other conditions requiring sudden muscular effort. It is also postulated that the primary vasodilatation stated to occur at the beginning of muscular exercise might be due to vasodilator activity. It is found that the sympathetic vasodilator nerves are not involved in depressor reflexes or in other vascular reactions associated with the regulation of the blood pressure. Such vasodilator reactions are considered to be produced by inhibition of vasoconstrictor tone.