The majority of our glands and smooth muscle tissue is located in the visceral (i.e., internal) region of our bodies. However, in our somatic (i.e., not internal) region, smooth muscle regulates blood flow to our skeletal muscle and our skin; smooth muscle is also responsible for producing "goose bumps." And, there are two types of sweat glands in our skin.
The insert to the left is not part of the model. It is an outline of a spinal nerve showing the rami (i.e., branches) where the sympathetic neurons are located. The preganglionic neuron (orange) starts in the spinal cord and heads out the nearest spinal nerve. However, when barely into the nerve, the axon (orange line) deviates out via the white ramus communicans that leads to the nearby sympathetic (chain) ganglion. There it synapses with the postganglionic cell body (gold circle) whose axon (gold line) immediately returns to the same spinal nerve via the gray ramus communicans. These axons travel within spinal nerves to their destinations in the somatic regions to synapse with the smooth muscle and glands there.
The insert to the right --from the model-- shows these rami. The short preganglionic axon(orange) leaves the spinal cord to synapse with the nearby sympathetic (chain) ganglion (gold circle); it is labeled "white ramus" because this is the route it travels to get to the somatic regions. The postganglionic axon (dashed orange line) is drawn as curving backward toward the cord to emphasize that this segment travels in the "gray ramus". After joining the spinal nerve it travels to the target organ.
After penetrating the skeletal muscle the postganglionic axon separates from the nerve and enters a blood vessel where it synapses with the circular smooth muscle (donut) in the vessel's wall. There it releases norepinephrine (not shown) which binds with the stimulatory (color-code green) alpha 1 receptors. However, these receptors are scarce in this location and the resulting vasoconstriction is limited. This effect is further minimized because the neuron terminal (orange bar) has inhibitory (color-code red) alpha 2 receptors to which norepinephrine also binds decreasing the subsequent release of norepinephrine from the neuron. This neuronal activity is establishing a base line tone (i.e., resting-state vasoconstriction) that insures a somewhat greater blood flow to skeletal muscle than to organs whose blood vessels have more alpha 1 receptors and no alpha 2 receptors.
Another unique feature of skeletal muscle blood vessels is that their smooth muscle also has inhibitory (color-code red) beta 2 receptors. Epinephrine (E) is thirty times more potent than norepinephrine in stimulating (arrow) beta 2 receptors although they are equally potent in stimuating (arrows) alpha 1 receptors. Because of the abundance of beta 2 receptors on blood vessels in this location, and because of the potency of the more plentiful epinephrine, vasodilation is ensured when sympathetic stimulation is greater than base line.
DISCLAIMER: Earlier literature states that skeletal muscle vessels are innervated by cholinergic sympathetic neurons. However, recent investigations find that muscarinic receptors reside on the endothelium of these vessels and binding of acetylcholine agonists cause nitric oxide release that causes the observed vasodilation. Muscarinic sites are not located on the vascular smooth muscle.
The insert to the left illustrates a hair and the arrector pili muscle attached near the base of its follicle (i.e., capsule surrounding the base of the hair); the other end of this small muscle is attached to the undersurface of the skin. Hair follicles are set at an angle in the skin and the muscle is attached "in the shadow of the hair". Accordingly, when the muscle contracts the base of the follicle is pulled toward the skin where the muscle is inserted. This causes the shaft of the hair to press against the skin on the side opposite the muscle causing a "goosebump."
The arrector pili contains stimulatory (color-code green) alpha 1 receptors. These can be activated by norepinephrine (not shown) released from symapthetic postganglionics (dashed orange line) or by blood-borne norepinephrine and epinephrine.
This type of sweat gland in concentrated in the genital, anal and axillary regions. Its duct opens into a hair follicle instead of directly onto the surface of the skin. The insert at the right indicates that this coiled gland has stimulatory (color-code green) alpha 1 receptors activated by blood-borne norepinephrine (NE) & epinephrine (E) (arrow).
It is also activated neurologically in the same manner as other organs lying in the somatic regions; a white and gray ramus are involved in the pathway. As is typical of postganglionic SNS neurons (dashed orange line), the transmitter secreted by the terminal (orange bar) is norepinephrine (not shown). Because norepinephrine (previously called noradrenalin) is related to adrenalin(now called epinephrine) neurons that secrete it are referred to as "adrenergic." With one exception, all SNS postganglionics are adrenergic.
This is the exception. Eccrine sweat glands, found all over the body, helps regulate body temperature by covering the skin with sweat that cools by evaporation. These are illustrated in the right portion of the insert. These coiled glands secrete sweat directly onto the surface of the skin. They too are innervated by the SNS division of the ANS but, at this target, the postganglionics secrete acetylcholine (not shown) instead of norepinephrine. The stimulatory (color-code green) muscarinic 3 (M3) receptors bind this neurotransmitter and cause secretion. Secretion can be enhanced by binding of blood-borne epinephrine (E) & norepinephrine (NE) (arrow) to stimulatory (color-code green) beta 1 receptors.
Last update: 11/12/2005