Human Physiology/The Nervous System

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bodies in the ganglia and send their axons to target organs or glands.

The ganglia include not just the sympathetic trunks but also the superior cervical ganglion (which sends sympathetic

nerve fibers to the head), and the celiac and mesenteric ganglia (which send sympathetic fibers to the gut).

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; increase peristalsis in the

esophagus; 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

noradrenaline or adrenaline. An exception is with sweat glands which receive sympathetic innervation but have

muscarinic acetylcholine receptors which are normally characteristic of 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. The sympathetic system cell bodies are located on the spinal cord excluding the cranial and sacral

regions. The preganglonic neurons exit from the vertebral column and synapse with the postganglonic nerouns in the

sympathetic trunk.

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

While an oversimplification, it is said that the parasympathetic system acts in a reciprocal manner to the effects of

the sympathetic nervous system; in fact, in some tissues innervated by both systems, the effects are synergistic.

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.

The three main types of muscarinic receptors that are well characterised are:

• The M1 muscarinic receptors are located in the neural system.

• The M2 muscarinic receptors are located in the heart, and act to bring the heart back to normal after the actions of

the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial cardiac

muscle, and reducing conduction velocity of the atrioventricular node (AV node). Note, they have no effect on the

contractile forces of the ventricular muscle.

• The M3 muscarinic receptors are located at many places in the body, such as the smooth muscles of the blood

vessels, as well as the lungs, which means that they cause vasoconstriction and bronchoconstriction. They are also

in the smooth muscles of the gastrointestinal tract (GIT), which help in increasing intestinal motility and dilating

sphincters. The M3 receptors are also located in many glands that help to stimulate secretion in salivary glands

and other glands of the body.