The Parasympathetic Division

The parasympathetic division is also called the cran-iosacral division because it arises from the brain and sacral region of the spinal cord; its fibers travel in certain cranial and sacral nerves. The preganglionic neurons are located in the pons, medulla oblongata, and segments S2 to S4 of the spinal cord (fig. 15.7). They issue long pre-ganglionic fibers which end in terminal ganglia in or near the target organ (see fig. 15.1). (If a terminal ganglion is embedded within the wall of a target organ, it is also called an intramural8 ganglion.) Thus, the parasympathetic divi

8 intra = within + mur = wall sion has long preganglionic fibers, reaching almost all the way to the target cells, and short postganglionic fibers that cover the rest of the distance.

There is some neuronal divergence in the parasym-pathetic division, but much less than in the sympathetic. The parasympathetic division has a ratio of about two postganglionic fibers to every preganglionic. Furthermore, the preganglionic fiber reaches the target organ before even this slight divergence occurs. The parasympathetic division is therefore relatively selective in its stimulation of target organs.

Parasympathetic fibers leave the brainstem by way of the following four cranial nerves. The first three supply all parasympathetic innervation to the head and the last one supplies viscera of the thoracic and abdominal cavities.

1. Oculomotor nerve (III). The oculomotor nerve carries parasympathetic fibers that control the lens and pupil of the eye. The preganglionic fibers enter the orbit and terminate in the ciliary ganglion. Postganglionic fibers enter the eyeball and innervate the ciliary muscle, which thickens the lens, and the pupillary constrictor, which narrows the pupil.

2. Facial nerve (VII). The facial nerve carries parasympathetic fibers that regulate the tear glands, salivary glands, and nasal glands. Soon after the facial nerve emerges from the pons, its parasympathetic fibers split away and form two smaller branches. The upper branch ends at the sphenopalatine ganglion near the junction of the maxilla and palatine bones. Postganglionic fibers then continue to the tear glands and glands of the nasal cavity, palate, and other areas of the oral cavity. The lower branch crosses the middle-ear cavity and ends at the submandibular ganglion near the angle of the mandible. Postganglionic fibers from here supply salivary glands in the floor of the mouth.

3. Glossopharyngeal nerve (IX). The glossopharyngeal nerve carries parasympathetic fibers concerned with salivation. The preganglionic fibers leave this nerve soon after its origin and form the tympanic

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Spinal cord

Pelvic splanchnic nerves-

Regions of spinal cord: ]] Cervical | Thoracic Lumbar ] Sacral

Heart

Heart

Pelvic splanchnic nerves-

Ciliary Ganglion

Ganglia of C.N. III, VII, + IX: Q Sphenopalatine ganglion (?) Ciliary ganglion (3 Submandibular ganglion @ Otic ganglion

Lacrimal gland

Salivary glands

Stomach

Liver and gallbladder

Spleen Pancreas

^-Kidney and ureter

Colon

Small intestine

Descending colon

Rectum

Spinal cord

Regions of spinal cord: ]] Cervical | Thoracic Lumbar ] Sacral

Ovary

Uterus

Figure 15.7 Parasympathetic Pathways.

Which nerve carries the most parasympathetic nerve fibers?

Ganglia of C.N. III, VII, + IX: Q Sphenopalatine ganglion (?) Ciliary ganglion (3 Submandibular ganglion @ Otic ganglion

Lacrimal gland

Salivary glands

Stomach

Liver and gallbladder

Spleen Pancreas

^-Kidney and ureter

Colon

Small intestine

Descending colon

Rectum

Ovary

Uterus

Figure 15.7 Parasympathetic Pathways.

Which nerve carries the most parasympathetic nerve fibers?

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nerve, which crosses the eardrum and ends in the otic9 ganglion near the foramen ovale. The postganglionic fibers then follow the trigeminal nerve to the parotid salivary gland just in front of the earlobe.

4 Vagus nerve (X). The vagus nerve carries about 90% of all parasympathetic preganglionic fibers. It travels down the neck and forms three networks in the mediastinum—the cardiac plexus, which supplies fibers to the heart; the pulmonary plexus, whose fibers accompany the bronchi and blood vessels into the lungs; and the esophageal plexus, whose fibers regulate swallowing.

At the lower end of the esophagus, these plexuses give off anterior and posterior vagal trunks, each of which contains fibers from both the right and left vagus. These penetrate the diaphragm, enter the abdominal cavity, and contribute to the extensive abdominal aortic plexus mentioned earlier. As we have seen, sympathetic fibers synapse here. The parasympathetic fibers, however, pass through the plexus without synapsing and lead to the liver, pancreas, stomach, small intestine, kidney, ureter, and proximal half of the colon.

The remaining parasympathetic fibers arise from levels S2 to S4 of the spinal cord. They travel a short distance in the ventral rami of the spinal nerves and then form pelvic splanchnic nerves that lead to the inferior hypogastric (pelvic) plexus. Some parasympathetic fibers synapse here, but most pass through this plexus and travel by way of pelvic nerves to the terminal ganglia in their target organs: the distal half of the large intestine, the rectum, urinary bladder, and reproductive organs. The parasympathetic system does not innervate body wall structures (sweat glands, piloerector muscles, or cutaneous blood vessels).

The sympathetic and parasympathetic divisions of the ANS are compared in table 15.3.

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Responses

  • paul
    Which NT carries the post ganglion of parasympathic erve fibres?
    7 years ago
  • Filibert
    Which of the following is NOT a function of the parasympathetic division of the ANS?
    4 years ago

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