Page 72 - 84 human physiolofy part-1

Chapter 4

In the SNS and other components of the peripheral nervous system, these synapses are made at
sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber
leaves the ganglion is called a postganglionic cell. As mentioned previously, the preganglionic cells of
the SNS are located between the first thoracic segment and the second or third lumbar segments of the
spinal cord. Postganglionic cells have their cell 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

72 | Human Physiology