Anatomy and neuron 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 limited.
- First (preganglionic) neurons are located in the CNS. They form the craniosacral system:
- Cranial division: nuclei of cranial nerve pairs III, VII, IX, and X (midbrain, pons, medulla oblongata).
- Sacral division: lateral horns of the spinal cord at the S2–S4 segments.
- The processes of these neurons (preganglionic fibers) are long and belong to type B.
- Second (postganglionic) neurons — their cell bodies lie in peripheral autonomic ganglia. The parasympathetic system is characterized by the location of ganglia directly in the walls of innervated organs (intramurally) or close to them. Postganglionic fibers are short and belong to type C.
Mechanism of impulse transmission
The parasympathetic nervous system is also called 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 (N-AChR) 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 (M-AChR).
How does the system affect organs?
The biological significance of the system is to ensure rest and slow down energy production. Fibers run within cranial and pelvic nerves to specific organs:
- Eye (n. oculomotorius, ciliary ganglion): pupil constriction (miosis) and contraction of the ciliary muscle (accommodation).
- Head glands (n. intermedius, n. glossopharyngeus): vasodilation, lacrimation, and secretion of watery saliva.
- Cardiovascular system (n. vagus): decrease in vascular tone, weakening of heart activity, bradycardia, constriction of coronary arteries.
- Respiratory system (n. vagus): bronchial constriction (bronchoconstriction), weakening of respiration, increased secretion.
- Digestion (n. vagus, pelvic nerves): enhancement of GI motility and secretion.
- Excretory system (pelvic nerves): stimulation of urinary bladder emptying (contraction of the detrusor muscle and relaxation of the internal sphincter).
- Uterus (during labor): contraction of circular muscles of the uterine body with relaxation of longitudinal muscles.
Exceptions: the parasympathetic system does not innervate the adrenal glands and the walls of most blood vessels.
Regulation and clinical significance
The highest regulatory center of the autonomic nervous system is the hypothalamus. Its rostral division is responsible for increasing parasympathetic (trophotropic) activity. Its stimulation leads to hypotension, pulse slowing, increased peristalsis, and decreased adrenaline 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 antimuscarinic drugs can cause psychosis with altered mental status (delirium).
- Long-term abuse of anticholinergics leads to memory impairment.
Class and exam questions
What neurotransmitter do postganglionic parasympathetic fibers release?
Acetylcholine. It acts on muscarinic acetylcholine receptors (M-AChRs) of the target organ cells.
Where are the first neurons of the parasympathetic nervous system located?
In the craniosacral division: in the nuclei of cranial nerves III, VII, IX, and X (brainstem) and in the lateral horns of the sacral spinal segments (S2–S4).
Which organs lack parasympathetic innervation?
The adrenal glands and the walls of most blood vessels.
How does the parasympathetic system affect the urinary bladder?
It ensures the act of micturition: the released acetylcholine causes contraction of the muscular wall (detrusor) and relaxation of the internal sphincter.
What else to study on this topic
- Sympathetic nervous system
- Hypothalamus
- Autonomic ganglia
- Cholinergic Receptors
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