Somatic Organs

'Somatic' means the body wall as opposed to the internal viscera. Inspection of the diagram indicates that these organs are structures in the dermis (arrector pili muscle of hair follicles, eccrine sweat glands and dermal arteries) and arteries in skeletal muscles. The diagram at the right shows a very short preganglionic axon (orange line) leaving the spinal cord to synapse with a postganglionic cell body (yellow) in the ganglionic chain. The postganglionic axon (dashed orange line) appears to curve back to contact the spinal cord before heading to its target is in the somatic region of the body. p>

The diagram at the left shows these neurons within an outline of the nerve itself. When the preganglionic axon (orange line) leaves the cord it travels a short distance then breaks out of the main nerve to enter a ganglion in the chain. This branch is called the white ramus. The axon of the postganglionic neuron (dashed orange line) leaves the ganglion via another short branch called the grey ramus. This axon re-enters the main nerve to travel to the somatic region of the body. This seemingly unusual arrangement is characteristic of sympathetic neurons heading to the somatic region of the body. Note the lack of parasympathetic neurons in this region.

Blood Vessels

The vast majority of somatic vessels are within skeletal muscles. The encircling smooth muscle has beta 2 receptors -- remember that B2 receptors give a strong response to epinephrine. Activation of beta 2 receptors causes a decrease in tone of these muscles and vasodilation that increases blood flow. Under high sympathetic activity epinephrine diffusing from the blood enhances this response.

Dermal Structures

Arrector Pili Muscle

These small smooth muscles originate on the lower surface of the dermis and insert on the side of hair follicles. Follicles are not part of the actual hair but are tubular depressions that house much of the hair. Norepinephrine activated alpha 1 receptors increases the tone of these cells. The response is that the follicle is pulled such that the enclosed hair sticks up straighter (hair standing on end) and some of the skin is pushed up into a bump (Goose bump).

Sweat Glands

Eccrine

The most abundant and widespread type of sweat gland is the eccrine gland. Their ducts lead directly to a pore at the surface of the skin. These produce a watery secretion primarily useful in thermoregulation -- evaporation from the skin surface decrease body heat. Their innervation is quite unusual because the postganglionic neurons secrete acetylcholine instead of the expected norepinephrine -- cholinergic sympathetic nerves. Some texts state some vessels also have sympathetic cholinergic innervation but this is not the case in humans. Activation of muscarinic 3 receptors on the secretory cells of these glands causes increased sweat production.

The secretory cells of eccrine sweat glands also have beta 1 receptors. However, there is no sympathetic adrenergic (i.e., norepinephrine) innervation and epinephrine is responsible for their activation; sweating is increased -- 'nervous' sweat'.

Apocrine

Apocrine sweat glands are predominantly located in the armpits and groin area. Their ducts do not lead to pores at the surface but rather into the side of hair follicles. From there the sweat is moved to the surface. The secretion is thick and contains many organic compounds that bacterial break down producing a foul odor. There is no innervation to these glands. Epinephrine stimulated beta 1 receptors induce production of this type sweat.

Last update: 10/6/2013