Wednesday, August 19, 2009

ANS innervation...

The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions.The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind.
It is classically divided into two subsystems: the parasympathetic nervous system and sympathetic nervous system.Relatively recently, a third subsystem of neurones that have been named 'non-adrenergic and non-cholinergic' neurones (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, particularly in the gut and the lungs.
With regard to function, the ANS is usually divided into sensory (afferent) and motor (efferent) subsystems. Within these systems, however, there are inhibitory and excitatory synapses between neurones.
The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

Function:Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest".
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to "fight" or "rest" situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second to second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below.

Sympathetic nervous system
Promotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits digestion.
Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
Blood flow to skeletal muscles and the lungs is not only maintained, but enhanced (by as much as 1200% in the case of skeletal muscles).
Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
Dilates pupils and relaxes the lens, allowing more light to enter the eye.
Provides vasodilation for the coronary vessels of the heart.
Inhibits peristalsis.

Parasympathetic nervous system
Promotes a "rest and digest" response, promotes calming of the nerves return to regular function, and enhances digestion.
Dilates blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut.
The parasympathetic nervous system can also constrict the bronchiolar diameter when the need for oxygen has diminished.
Dedicated cardiac branches of the Vagus and thoracic Spinal Accessory nerves impart Parasympathetic control of the Heart or Myocardium.
During accommodation, the parasympathetic nervous system causes constriction of the pupil and lens.
The parasympathetic nervous system stimulates salivary gland secretion, and accelerates peristalsis, so, in keeping with the rest and digest functions, appropriate PNS activity mediates digestion of food and indirectly, the absorption of nutrients.
Is also involved in erection of genitals, via the pelvic splanchnic nerves 2–4.

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