Vascular Innervations – Constriction

Peripheral Distribution of Post-Ganglionic Fibres

The blood vessels of the skin, that is, the small arteries and arterioles leading to the capillary bed are composed chiefly of involuntary muscle fibres arranged in a circular fashion. Like cardiac muscle these muscle fibres are supplied by two types of nerve fibres – classified as inhibitory and/or excitatory. Those which cause constriction of the arteriolar musculature are called vasoconstrictor, those which inhibit, and in consequence cause relaxation of the muscular rings, are termed vasodilator.

The vasoconstrictor fibres belong to the thoraco-lumbar (sympathetic) division of the involuntary nervous system. These constrictor fibres arise from groups of nerve cells situated in the lateral horns of the spinal cord, extending in man from the first thoracic to the second or third lumbar segment inclusive. All the arterioles of the body wherever situated are supplied with filaments whose ultimate source is this relatively limited region of the central nervous system.

The fibres of the sympathetic system emerge from the cord through the anterior root of the spinal segment in which their cell bodies are placed. In a cross section of the anterior root they appear as fine medullated fibres. They separate almost immediately, however, from the voluntary motor fibres of the anterior root and enter the corresponding ganglion of the sympathetic chain. Thus, the spinal nerves from the 1st thoracic to the 2nd or 3rd lumbar, but not others, are connected each to a vertebral ganglion by a delicate white strand composed of preganglionic fibres and known as the white ramus communicans.

A preganglionic sympathetic fibre, after entering the ganglion, may pursue one of three courses:

I. Form synapses with cells in the ganglion which it first enters.

2. Pass up or down the sympathetic trunk for some distance to terminate in a ganglion at a level higher or lower than that of the segment from which it originated.

3. Traverse the gangliated cord without interruption to find a cell station in either a prevertebral or a terminal ganglion.

The ganglia of the sympathetic chain are connected with the spinal nerves by delicate filaments called gray rami communicantes. They are composed of the axons of the ganglion cells and are therefore called post-ganglionic. The gray rami in the thoracic and upper lumber regions join the spinal nerves close to the points at which the white rami arise. Their constituent fibres are continued to the periphery for the supply of the blood vessels, sweat glands and smooth muscle of the skin. Whereas, as already mentioned, only a limited number of the spinal nerves possess white rami, every spinal nerve receives a gray ramus.

The peripheral distribution of the post-ganglionic fibres follow the course of the larger arteries, supplying short sections of these vessels with single axons which terminate in the muscular layer of the vessel.

Experimental research has shown the pattern of distribution of post ganglionic fibres has no correlation to the segmental position of the pre-ganglionic cell body in the lateral horn of the cord. Although it is known that there are sympathetic dermatomes in man, these do not correspond to the well-known sensory dermatomes.