autonomic nervous system

Sympathetic and parasympathetic ganglia are innervated by preganglionic neurons in the spinal cord. Sympathetic preganglionic axons arise from neurons in the thoracic and upper lumbar spinal cord. The preganglionic neurons that innervate the head and thoracic organs are in the upper and middle thoracic segments, and those that innerate the abdominal and pelvic organs are in the lower thoracic and upper lumbar segments. The parasympathetic preganglionic axons arise from neurons in the brainstem and sacral spinal cord. Many organs--including the salivary glands, heart, bladder, and sex organs--receive inputs from both the sympathetic and parasympathetic systems. Other targets receive only sympathetic innervation, including the sweat glands, the adrenal medulla, the piloerector muscles of the skin, and most blood vessels. The neurons innervated by the preganglionic sympathetic axons are mostly found in the sympathetic chain ganglia, whereas the parasympathetic motor neurons are located in ganglia within the organs they control.

The enteric nervous system, although it receives sympathetic and parasympathetic innervation, acts to some degree independently of the rest of the autonomic system. A rich intrinsic circuitry of sensory neurons, interneurons, and motor neurons interconnects different levels of the gut and coordinates activity along its length. The enteric system governs gut motility, secretion, and the transfer of substances across the gut epithelium.

Sensory inputs from the viscera modulate autonomic activity. Like other primary sensory neurons, the relevant cell bodies lie in dorsal root and cranial nerve ganglia; the visceral sensory axons that enter the spinal cord terminate mainly in the intermediate gray matter, near the preganglionic neurons of the thoracolumnar and sacral cord. Those that enter the brainstem (in cranial nerves VII, IX, and X) terminate in the nucleus of the solitary tract. Sensory fibers that travel in the sympathetic nerves convey visceral sensations, usually pain. Other fibers, including most of those that travel in the parasympathetic nerves to the nucleus of the solitary tract, convey information that does not reach consciousness, but which is important for reflex integration (for example, arterial baroreceptors and chemoreceptors).

Neuroscience, Sinaur Associates (QP355.2.N487 1997)

Dictionary of Sexology Project: Main Index

The Autonomic Nervous System: The Official Line

"The autonomic nervous system is predominantly an efferent system, transmitting impulses to the viscera. It is primarily involved in reflex arcs, involving an autonomic or somatic afferent limb. There may be a reflex response to this causing contraction of smooth muscle in certain organs and influencing the function of the heart and glands." - The Autonomic Nervous System, Dr S Bakewell

And in English...

So what does that mean? We're talking about the viscera - the internal organs - which includes the heart, stomach and intestines, all of which are controlled by our nervous system. That part is pretty much independent from our conscious awareness of who we are, and because of this autonomy it's known as the autonomic nervous system. In most situations we're happily unaware of its function; we don't consciously need to make our heart beat faster, nor do we need to change the size of our blood vessels - the body adjusts these automatically. It is possible to alter some functions of the autonomic nervous system; individuals can be trained to lower their heart rate or even to reduce blood pressure, but the majority of us are pleased to let our bodies just get on with the job.

The autonomic nervous system is particularly important in two situations; emergencies or other stressful situations, and the opposite - restful situations. The body's response in such cases parallels the nature of these situations; in a stressful situation the body is faced with the choice of coping with the situation or running away - the so-called fight or flight response. A non-emergency situation gives the body the opportunity to relax and make efficient use of the digestive system. To go with the nicely-rhyming fight or flight, this is occasionally termed rest and digest.

To achieve these responses, the autonomic nervous system controls muscles. Muscles in the skin, around blood vessels, in the eye, in the stomach, intestines and bladder, and in the heart itself; the autonomic nervous system manipulates all these to put the body into the optimum state for the current situation. It also controls the various glands within the body.

The autonomic system is divided into three parts; the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.

The Sympathetic Nervous System

The sympathetic nervous system is that which is concerned with the fight or flight response. A situation in which a threat is suddenly perceived will trigger the sympathetic nervous system and thus a cascade of sudden internal changes. Consider returning home late at night to find a slavering Yog-Sothoth waiting in the hallway: The body prudently directs energy to increase blood pressure, make the heart beat more rapidly and slow unnecessary activities, such as digestion. The body is then ready for an appropriate response - an all out bloody battle for survival, or a potentially more prudent rapid vacation of the area.

The Parasympathetic Nervous System

The parasympathetic nervous system is more concerned with the conservation of energy. In a relaxed situation with no immediate threat, perhaps whilst enjoying an appropriate beverage in a sunny corner outside The Slug And Lettuce, the blood pressure is decreased, the heart beats slower, and leisurely digestion of your recent pub lunch can begin.

The Enteric Nervous System

The enteric nervous system is, meanwhile, occupied with the workings of the gastrointestinal tract, pancreas and gall bladder, and is an essential part of maintaining normal gastric functions. It should be noted, of course, that all three divisions of the autonomic nervous system work constantly to maintain normal internal functions, not merely when a more extreme response is needed.

It will also be noted, in general, that the effect of the sympathetic and parasympathetic responses are almost direct opposites. The iris is dilated by the sympathetic response, and constricted by a parasympathetic response. The same is true of the salivary glands, heart, lungs and stomach. In some cases, such as the adrenal gland, there is no parasympathetic response, although the sympathetic nervous system will stimulate the secretion of norepinephrine and epinephrine.

Sources

Neuroscience Education
Online Medical Dictionary
Hutchinson's Encyclopedia
The Enteric Nervous System: A Second Brain