Sympathetic nervous system: Encyclopedia - Sympathetic nervous system

Sympathetic nervous system

The sympathetic nervous system (SNS) is one half of the autonomic nervous system; the parasympathetic nervous system (PNS) is the other.

Sympathetic nervous system - Function

The sympathetic nervous system activates what is often termed the "fight or flight response". This response is also known as sympathico-adrenal response of the body, as the pre-ganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the secretion of epinephrine and to a lesser extent norepinephrine from it. Therefore this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.

Western science typically looks at the SNS as an automatic regulation system, that is, one that operates without the intervention of conscious thought. Some evolutionary theorists suggest that the sympathetic nervous system operated in early man to maintain human survival (Origins of Consciousness, Robert Ornstein; et al.), as the sympathetic nervous system is responsible for priming the body for action.

Sympathetic nervous system - Organization

The sympathetic nerves emerge near the spinal cord, inside the vertebral column, travelling along the path of the nerve roots. After a few twists and turns, sympathetic nerves branch off and travel into major organs, glands, and other groupings of nerves (sometimes called ganglia). [1]

Cells of the SNS originate toward the middle of the spinal column, intermediolateral cell column, starting with the thoracic vertebrae numbers 1 through 12 (T1-T12) and ending at lumbar vertebrae numbers 1 and 2 (L1-L2, the top two).

Axons (the largest "tentacle" of a nerve cell) leave the spinal cord and synapse (connect, with a space in between) to several major ganglion hubs which then distribute the nerves to various parts of the body.

These hubs include: the superior cervical ganglion (which sends sympathetic nerve fibers to the head), the celiac and mesenteric ganglion (which sends sympathetic fibers to the gut), and two chain ganglia – these run parallel to the spine on both sides of the spinal cord (bilaterally), and extend out to the thorax and peripheral parts of the body.

In order to reach the target organs and glands, the axons must travel long distances in the body, and to accomplish this many axons link up with the axon of a second cell. The ends of the axons do not make direct contact, but rather link across a space, the synapse.

As the location where many axons (and dendrites)meet and criss-cross is called a ganglion, in tracing the path of two-neuron linkages in the sympathetic nervous system, the terms “preganglionic” and “postganglionic” are used. Preganglionic nerves are the axons that are located before the intersection with the ganglion. Postganglionics are those axons of the second neuron that travel from the ganglia to the target organ or gland.

Sympathetic nervous system - Information Transmission

Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in different parts of the body simultaneously. For example, the sympathetic nervous system can accelerate heart rate, widen bronchial passages, decrease motility (movement) of the large intestine, constrict blood vessels, cause pupil dilation, piloerection (goose bumps) and perspiration (sweating), and raise blood pressure. Afferent messages carry sensations such as heat, cold, or pain.

The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically activated by acetylcholine, and the target synapse is mediated by adrenergic receptors physiologically activated by either norepinephrine or epinephrine. An exception is with sweat glands which receive sympathetic innervation but have muscarinic acetylcholine receptors which are normally characteristic of the PNS. Another exception is with certain deep muscle blood vessels, which have acetylcholine receptors and which dilate (rather than constrict) with an increase in sympathetic tone.

See also

• Autonomic nervous system
• Parasympathetic nervous system
• Epinephrine

Brain - Spinal cord - Central nervous system - Peripheral nervous system - Somatic nervous system - Autonomic nervous system - Sympathetic nervous system - Parasympathetic nervous system

Category: Sympathetic nervous system

Adapted from the Wikipedia article "Sympathetic nervous system", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki