The Parasympathetic Nervous System
The autonomic nervous system contains three subsystems: the sympathetic nervous system and the parasympathetic nervous system, which are usually in opposition, and the enteric nervous system.
The parasympathetic nervous system, also known as the craniosacral division, is a branch of the autonomic nervous system (ANS). Its presynaptic neuron cell bodies located in two regions within the central nervous system (CNS), and their fibres exciting by two routes. The two regions where their neuron cell bodies are located are the medulla (a brainstem part) which constitutes the cranial half of the system, as well as the sacral segment of the spinal cord (sacral half of the system).
In the gray matter of the medulla, the fibres exit the CNS within the following cranial nerves (CN):
The associated cranial nerves form the cranial preganglionic parasympathetic fibres or what is known as the cranial parasympathetic outflow; 75% of all parasympathetic fibres are in the vagus nerve.
The preganglionic fibres from the medulla and spinal cord project to ganglia very close, or attached, to the target organ and makes a synapse.
The presynaptic parasympathetic neuron cell bodies of the sacral half of the system lie within the gray matter of the sacral segments (S2 – S4) of the spinal cord. These fibres leave the spinal cord through the anterior roots of the sacral spinal nerves S2 – S4 and the pelvic splanchnic nerves that arise from their anterior branches. They join together, after exiting the spinal cord, to form the pelvic nerves and mainly innervate the viscera of the pelvic cavity. These fibres constitute the sacral parasympathetic outflow (or sacral part of the preganglionic parasympathetic fibres). Parasympathetic synapses use the neurotransmitter acetylcholine, and are called cholinergic pathways.
The ANS controls and regulates the internal viscera without any conscious recognition or effort by the individual, and is thus referred to as involuntary. It plays a crucial role in the maintenance of homeostasis through the opposing functions of its two anatomically and functionally distinct divisions, the parasympathetic and sympathetic nervous systems. Both systems provide some degree of nervous input to a given tissue at all times and either increase/enhances or decrease/inhibits the activity of the innervated structure. Parasympathetic fibres are sent to various viscera to ensure different involuntary functions, such as:
- constriction of the pupil (protecting the pupil from excessive bright light)
- contraction of the ciliary muscle, (allowing the lens to thicken for near vision e.g., accommodation)
- promotion of the secretion of the lacrimal glands
- promotion of abundant watery secretions of salivary glands, decreasing the rate and strength of contraction (conserving energy)
- constriction of coronary vessels in relation to reduced demand
- constriction of the bronchi (conserving energy)
- promotion of bronchi mucus secretion of the lungs
- sending impulses to induce peristalsis and secretion of digestive juices
- contraction of the rectum during defecation
- inhibition of the internal anal sphincter to cause defecation
- promoting the building/conservation of glycogen
- increases secretion of bile
- inhibition of the contraction of internal sphincter of urinary bladder
- contraction of the detrusor muscle of the urinary bladder wall causing urination
- stimulation of engorgement (erection) of erectile tissues of the external genitals
However, to ensure homeostasis in the activities of these organs, the sympathetic nervous system also innervates those structures in an antagonistic way. Furthermore, it can clearly be seen that the cranial outflow provides parasympathetic innervation of the head, and the sacral outflow provides the parasympathetic innervations of the pelvic viscera. However, in terms of nerve contribution to the thoracic and abdominal viscera, the cranial outflow, through the vagus nerve is dominant.
Apart from the opposing actions of the sympathetic and parasympathetic nervous system, a striking anatomical difference between both systems relates to the axons. Those of the preganglionic neurons of the parasympathetic nervous system are longer than those of the sympathetic system. They also synapse with postganglionic neurons within terminal ganglia which are close to or embedded within the effector tissues.
However, the axons of the parasympathetic postganglionic neurons are very short, and usually unmyelinated, compared with the sympathetic’s. This is due to their proximity to the cells of the effector tissue. In addition, the parasympathetic fibres are more restricted than the sympathetic. They do not reach the body walls or the limbs.