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The parasympathetic nervous system is one of three divisions of the autonomic nervous system. Sometimes called the rest and digest system, the parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.
Relationship to sympathetic nervous system Sympathetic and parasympathetic divisions typically function against one another. But we cant say it as antagonistic, better to term it as complementary in nature. 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. Think of sympathetic as "fight or flight" and parasympathetic as "rest and digest". Physical Location The parasympathetic division is a division of the the visceral (autonomic) branch of the PNS (peripheral nervous system). The regions of the body associated with this division are the cranial and sacral regions of the spinal cord. Specifically, cranial nerves III, VII, IX, X (vagus n.) and in the sacral region (spinal nerves exiting from the sacrum) the spinal nerves S2 to S4. Similar to the sympathetic division, the parasympathetic division also has pre- and post- ganglionic neurons. Typically, in the parasympathetic division the ganglion will be closer to the area of innervation - unlike the sympathetic ganglion which form immediately lateral and inferior to the spinal nerve - making up the so called "chain ganglion". Receptors The parasympathetic nervous system uses only acetylcholine (ACh) as its neurotransmitter. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. Most transmissions occur in two stages: When stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on nicotinic receptors of the postganglionic nerve. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ. Types of muscarinic receptors The three main types of muscarinic receptors that are well characterised are: See also | ||||||||
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