Anatomy and Neuronal Localization
The efferent pathway of the parasympathetic nervous system consists of two neurons. Unlike the sympathetic division, the parasympathetic system does not have systemic effects, and its innervation zone is restricted.
- First (preganglionic) neurons — located in the CNS. They form the craniosacral outflow:
- Cranial division: nuclei of cranial nerves III, VII, IX, and X (midbrain, pons, medulla oblongata).
- Sacral division: lateral horns of the spinal cord at segments S2–S4.
- Processes of these neurons (preganglionic fibers) are long, type B fibers.
- Second (postganglionic) neurons — their cell bodies lie in peripheral autonomic ganglia. A hallmark of the parasympathetic system is the location of ganglia directly within the walls of innervated organs (intramural) or very close to them. Postganglionic fibers are short, type C fibers.
Mechanism of Impulse Transmission
The parasympathetic nervous system is also termed cholinergic, because nerve impulse transmission at both stages is mediated by a single neurotransmitter — acetylcholine (ACh).
- In the autonomic ganglion, the first neuron releases acetylcholine, which binds to nicotinic acetylcholine receptors (nAChRs) on the membrane of the second neuron.
- In the target organ, the second neuron also releases acetylcholine, but here it acts on muscarinic acetylcholine receptors (mAChRs).
System Effects on Organs
The biological role of the system is to promote rest and decrease energy production. Fibers travel within cranial and pelvic nerves to specific organs:
- Eye (n. oculomotorius, ciliary ganglion): pupillary constriction (miosis) and contraction of the ciliary muscle (accommodation).
- Head glands (n. intermedius, n. glossopharyngeus): vasodilation, lacrimation, and watery saliva secretion.
- Cardiovascular system (n. vagus): decreased vascular tone, diminished cardiac output, bradycardia, constriction of coronary arteries.
- Respiratory system (n. vagus): airway constriction (bronchoconstriction), decreased respiratory rate, increased secretion.
- Digestive system (n. vagus, pelvic splanchnic nerves): increased GI motility and secretion.
- Excretory system (pelvic nerves): stimulation of bladder emptying (contraction of the detrusor muscle and relaxation of the internal urethral sphincter).
- Uterus (during labor): contraction of circular uterine body muscles with relaxation of longitudinal muscles.
Exceptions: The parasympathetic system does not innervate the adrenal glands or the walls of most blood vessels.
Regulation and Clinical Significance
The highest regulatory center of the autonomic nervous system is the hypothalamus. Its rostral (anterior) region is responsible for increasing parasympathetic (trophotropic) activity. Stimulation leads to hypotension, bradycardia, enhanced peristalsis, and decreased epinephrine secretion.
In clinical practice and pharmacology, the effects of the system are modulated via receptors:
- Blockade of muscarinic receptors (e.g., by atropine, neuroleptics, or tricyclic antidepressants) causes dry mouth, tachycardia, urinary retention, constipation, mydriasis, and cycloplegia. Overdose of antimuscarinics can cause delirium with altered mental status.
- Chronic misuse of anticholinergics leads to cognitive and memory impairment.