In the diagram a neuron's cell body, axon and terminal are drawn out but keep in mind that there are actually hundreds of these grouped together that make up nerves.
The system has two divisions based on the where the nerves exit the central nervous system. Nerves exiting the brain and sacral spinal cord (blue) are collectively know as the "carnio-sacral outflow" or the parasympathetic division (PSNS). Nerves exiting other sections of the spinal cord (orange)-- with the exception of the cervical region -- are known as the "thoraco-lumbar outflow" or the sympathetic division (SNS).
This is the involuntary portion of the nervous system. Its nerves begin within the central nervous system and connect with target organs via two-neuron pathways. The system is totally efferent. The cells within innervated organs have membrane receptors to which neurotransmitters attach. These cells are smooth muscle, cardiac pacemaker cells, cardiac muscle cells or glandular cells. Smooth muscle cells respond by either increasing or decreasing their muscle tone, the cardiac pacemaker increases or decreases the heart rate, the cardiac muscle contracts more or less strongly with each beat and glands increase or decrease their secretions.
The major point to always keep in mind is that many -- but not all -- organs are responding to neurotransmitters of both divisions simultaneously.
The circle with a long hollow tail running the length of the diagram represents the brain and spinal cord.
Nerves of the autonomic nervous system contain numerous, parallel, two-neuron chains. A neuron consists of a cell body ( colored circle) and its axon represented by a line extending from it. The axon of the first neuron is solid while that of the second neuron is dashed.
Nerves consists of hundreds of individual neurons and when wrapped in connective tissue have an obvious bulge where the cell bodies of the second neurons are located. Such a bulge is called a "ganglion". It is for this reason that the first neurons in a nerve are described as "preganglionic" and the second neurons as "postganglionic." In the diagram ganglia can be thought of as cell bodies of the postganglionic neurons. These are the light blue and yellow circles outside the central nervous system. Similar circles drawn inside the brain and spinal cord (dark blue and orange) represent the cell bodies of preganglionic neurons. In the brain these regions are called "nuclei" and in the spinal cord they are "lateral grey horns".
The thorax houses the heart shown to the right of the upper spinal cord. The circle inside the heart is the pacemaker. The tubular structure inside the heart represents small coronary arterioles.
The branching structure to the right of the heart represents the bronchial tree. The donut-shaped structures around the branches are the bronchial muscles that regulate the diameter of the airways. The tubular structure above the lungs represents the bronchial arteries.
The abdominopelvic region contains the gastrointestinal tract; the diagram shows the stomach, pyloric sphincter, small and large intestines and internal anal sphincter. These organs are illustrated at the lower right portion of the diagram. The tubular structure to the left of the small intestine represents splanchnic arteries that supply these organs. The arrows leaving it represent epinephrine (E) and norepinephrine (NE) diffusing to these organs.
The urinary bladder and its internal urethral sphincter are shown below the splanchnic artery and to the left of the large intestine.
An adrenal gland is shown just beneath the heart. Its central portion is the medulla which releases the hormones epinephrine (E) and norepinephrine (NE) into the capillaries (shown to the right) that pass through the gland.
Arteries are shown throughout the diagram as curved tubes encircled by a donut. The donut represents vascular smooth muscle that encircles resistance arteries. When these muscles contract the diameter of the vessel narrows and reduces blood flow; when they are relaxed the pressure of the blood causes the vessel diameter to enlarge and flow to increase.
A facial profile containing a blood vessel that supplies the linings (mucosae) of the nose (nasal) and mouth (oral) is shown in the upper-left corner. At the top-center of the diagram are two illustrations of the iris of the eye showing the pupil constricted and dilated. Just below and to the right of the dilated pupil is a representation of the functional unit of salivary glands.
Illustrations at the lower left of the diagram depict structures found in the skin. The oval structure to the left is an apocrine sweat gland entering the hair follicle. These are most abundant in the armpits and groin area. The structure running from the follicle to the lower surface of the skin is the arrector pili muscle. The smaller oval structure to the right of the follicle is an eccrine sweat gland. This is the 'normal sweat' that evaporates to cool the body. Below these structures is a dermal artery.
The spindle-shaped structure at center left represents the skeletal muscles. The tubular structure within it represents arteries.
Last update: 10/3/2013