Sympathetic Nervous System

Original Editor - lucinda hampton

Top Contributors - Lucinda hampton, Kim Jackson and Ahmed M Diab  

Introduction

The sympathetic nervous system (SNS) is one of the two divisions of the autonomic nervous system (ANS), along with the parasympathetic nervous system (PNS), These systems primarily work unconsciously in opposite ways to regulate many functions and parts of the body.

• The SNS governs the "fight or flight" response while the PNS controls the "rest and digest" response.
• Fibers from the SNS innervate tissues in almost every organ system and provide physiological regulation over diverse body processes including pupil diameter, gut motility (movement), and urinary output.
• The main overall end effect of the SNS is to prepare the body for physical activity, a whole-body reaction affecting many organ systems throughout the body to redirect oxygen-rich blood to areas of the body needed during intense physical demand

Actions

The actions of the sympathetic nervous system occur in concert with other neural or hormonal responses to stress eg increases in corticotropin and cortisol secretion.

The effects in which SNS acts in direct contrast to the PNS function include the following:

• Eye, sympathetic activation causes the radial muscle of the iris (alpha-1) to contract, which leads to mydriasis, allowing more light to enter. The ciliary muscle (beta-2) relaxes, allowing for far vision to improve.
• Heart (beta-1, beta-2), sympathetic activation causes an increased heart rate, the force of contraction, and rate of conduction, allowing for increased cardiac output to supply the body with oxygenated blood.
• Lungs, bronchodilation (beta-2) and decreased pulmonary secretions (alpha-1, beta-2) occur to allow more airflow through the lungs.
• Stomach and intestines, decreased motility (alpha-1, beta-2) and sphincter contraction (alpha-1), as well as contraction of the gallbladder (beta-2), occur to slow down digestion to divert energy to other parts of the body.
• Exocrine and endocrine pancreas (alpha-1, alpha-2) decreases both enzyme and insulin secretion.
• Urinary bladder, there is relaxation of the detrusor muscle and contraction of the urethral sphincter to help stop urine output during sympathetic activation.
• Kidney (beta-1) increases renin secretion to increase intravascular volume.
• Salivary glands (alpha-1, beta-2) work through small volume potassium and water secretion.

In humans, chronic stress results in long-term stimulation of the fight-or-flight response, which leads to constant production and secretion of catecholamines (e.g., epinephrine) and hormones such as cortisol.

Long-term stress-induced secretion of these substances is associated with a variety of physiological consequences eg hyperglycemia (high blood glucose levels), which can lead to type 2 diabetes mellitus, and hypertension (high blood pressure), which can lead to cardiovascular disease.

Structure

The sympathetic nerves run parallel to the spinal cord on both sides of the vertebral column. Sympathetic nerves branch off from these main nerve chains and smaller branches travel into the spinal cord and beyond into major organs, glands, and other groupings of nerves (sometimes called ganglia).

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, activate goose bumps, start sweating and raise blood pressure.

Cells of the SNS originate toward the middle of the spinal column, intermediolateral cell column, near the thoracic vertebrae, numbers 1 thru 12 (T1-T12). This is where the parts of the nerves that have not yet run through a ganglion (pre-ganglionic cell bodies) are located.

Axons (the largest "tentacle" of a nerve cell) leave the spinal cord and synapse (connect, with a space in between) onto the sympathetic chain ganglion; some axons leaving the sympathetic chain then synapse on their destination organs.

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. The one exception is with sweat glands which receive sympathetic innervation but have muscarinic acetylcholine receptors which are normally characteristic of the PNS.

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