DISCLAIMER: Receptors are currently a hot topic and there is conflicting information concerning the locations and concentrations of the various types. The information presented here reflects only the most widely accepted viewpoints as of this writing.
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 usually smooth muscle, cardiac muscle or glandular.
The system has two divisions based on the where the nerves exit the central nervous system. Nerves exiting the brain and sacral spinal cord (shown in blue) are collectively know as the "carnio-sacral outflow" or the parasympathetic division (PSNS). Nerves exiting other sections of the spinal cord (shown in gold) are known as the "thoraco-lumbar outflow" or the sympathetic division (SNS). Many organs have dual innervation and both divisions of this system control them.
The circle with a long hollow tail running the length of the model 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 (solid colored circle); its axon is represented by a line extending from it. The axon of a first neuron is solid while that of a second neuron is dashed.
Nerves consists of hundreds of individual neurons and when wrapped in their connective tissues they 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 are described as "postganglionic." In the model ganglia are the same as the cell bodies of the postganglionic neurons that are the blue and gold circles drawn outside the central nervous system. Similar circles drawn inside the brain and spinal cord are the cell bodies of preganglionic neurons; those in the brain are called "nuclei" while those in the cord are in a region called the "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 the coronary arteries.
The branching structure to the right of the heart represents the bronchial tree of the respiratory system. The donut-shaped structures around the branches are the bronchial muscles that regulate the diameter of the airways. The scalloped structure above it represents the bronchial glands. The tubular structure to the right of the lungs represents the bronchial arteries.
The abdominopelvic region contains the gastrointestinal tract; the model shows the stomach, pyloric sphincter, small and large intestines and internal anal sphincter. These organs are illustrated at the lower right portion of the model. The splanchnic arteries that supply this organ system are located to the left of the small intestine.
The urinary bladder and its internal urethral sphincter are shown below the splanchnic arteries and to the left of the large intestine.
The 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 model 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 and flow to increase.
A facial profile containing a blood vessel that supply the linings (mucosae) of the nose (nasal) and mouth (oral) is shown in the upper-left corner.
At the top-center of the model 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 scalloped structure that represents salivary glands; the structure directly beneath the gland symbolizes the arteries to the glands.
Illustrations at the lower left of the model depict structures found in the skin. The left-most illustration is a hair with the arrector pili muscle attached to its base of its follicle. The next two illustrations are sweat glands and a dermal artery is shown below these.
The arteries that supply skeletal muscle are the target organs under consideration. Specifically, it is the circular smooth muscle (donut) that is the tissue innervated by the ANS.
Last update: 8/14/2005