Ganglia are aggregations of neuronal somata and are of varying form and size. They occur in the dorsal roots of spinal nerves, in the sensory roots of the trigeminal, facial, glossopharyngeal, vagal and vestibulocochlear cranial nerves as well as in autonomic nerves and in the enteric nervous system. Each ganglion is enclosed within a capsule of fibrous connective tissue and contains neuronal somata and neuronal processes. Some ganglia, particularly in the autonomic nervous system (ANS), contain fibres from cell bodies that lie elsewhere in the nervous system and that either pass through, or terminate within, the ganglia.
The parasympathetic ganglia or craniosacral ganglia are a division of efferent nerve fibres and ganglia of the ANS. The second division is the sympathetic or thoracolumbar ganglia.
The parasympathetic ganglia are involuntary, and act with the sympathetic system to maintain body homeostasis among other functions. One of the important actions of the parasympathetic nervous system is associated with the ‘rest and digest’ response. Presynaptic parasympathetic neuron cell bodies are located in two sites within the central nervous system (CNS), their fibres exiting by two routes:
- In the gray matter of the brainstem, the fibres exit the CNS within cranial nerves III, VII, IX and X; constituting the cranial parasympathetic outflow.
- In the gray matter of the sacral segment of the spinal cord (S2 through S4), the fibres exit the CNS through the ventral roots of spinal nerves S2 through S4, and the pelvic splanchnic nerves that arise from their ventral rami; these fibres constitute the sacral parasympathetic outflow.
Not surprisingly, the cranial outflow provides parasympathetic innervation of the head, and the sacral outflow provides the parasympathetic outflow of the pelvic viscera. However, in terms of the innervation of the thoracic and the abdominal viscera, the cranial outflow through the vagus nerve (CN X) is dominant. It provides innervation to all the thoracic viscera and most of the gastrointestinal tract from the esophagus through most of the large bowel (to its left colic flexure). The sacral outflow supplies only the descending and sigmoid colon and rectum.
Regardless of the extensive influence of its cranial outflow, the parasympathetic system is much more restricted than the sympathetic system in its distribution. The parasympathetic system distribute only to the head, viscera cavities of the trunk, and erectile tissues of the external genitalia. With the exception of the latter, it does not reach the body wall or limbs, and except for the initial parts of the ventral rami of spinal nerves S2 – S4, its fibres are not components of spinal nerves or their branches.
Four discrete pairs of parasympathetic ganglia occur in the head. Elsewhere, presynaptic parasympathetic fibres synapse with postsynaptic cell bodies that occur singly in or on the wall of the target organ (intrinsic or enteric ganglia).
The parasympathetic ganglia is primarily a homeostatic or anabolic (energy-conserving) system promoting the quiet and orderly processes of the body, such as those that allow the body to feed and assimilate. Some specific functions of the parasympathetic ganglia includes the following, and are also illustrated with the diagram below:
- Constricts pupil (protects pupil from excessive bright light).
- Contracts ciliary muscle, allowing the lens to thicken for near vision (accommodation).
- Promotes secretion of the lacrimal glands.
- Promotes abundant watery secretions of salivary glands.
- Decreases the rate and strength of contraction (conserving energy); constrict coronary vessels in relation to reduced demand.
- Constricts bronchi (conserving energy) and promotes bronchi secretion of the lungs.
- Stimulates peristalsis and secretion of digestive juices. Contracts rectum, inhibits internal anal sphincter to cause defecation.
- Promotes building/conservation of glycogen; increases secretion of bile.
- Inhibits contraction of internal sphincter of urinary bladder, contracts detrusor muscle of the urinary bladder wall causing urination.
- Produces engorgement (erection) of erectile tissues of the external genitals.