General characteristics
The parasympathetic nervous system acts locally, regulating the functions of individual organs. The main neurotransmitter transmitting signals across synapses is acetylcholine. Its effect is short-lived because the enzyme cholinesterase rapidly degrades it.
A distinctive anatomical feature of the system is long preganglionic fibers and short postganglionic fibers. This is because the autonomic ganglia, where the impulse is relayed from the second neuron, are located either very close to the innervated organ or directly within its wall (intramurally).
Cranial division
The cell bodies of the first (preganglionic) neurons of this division lie in the brainstem nuclei. Their axons run within four pairs of cranial nerves: oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X).
Relay to short postganglionic fibers occurs in the cranial ganglia:
- Ciliary;
- Pterygopalatine;
- Submandibular (source error states mandibular, but transcribed as is: mandibular);
- Otic.
These nerves innervate the smooth muscles of the eye (causing pupil constriction — miosis and accommodation) and stimulate secretion by the lacrimal and salivary glands.
Thoracic and abdominal organs receive parasympathetic innervation from the vagus nerve. The fibers originate from its dorsal nucleus and reach the heart, lungs, and digestive tract down to the distal third of the transverse colon. The ganglia are located near the organs or intramurally (Auerbach's and Meissner's plexuses).
Sacral division
The centers of the sacral division are located in the spinal cord segments S2–S4. From here, signals travel via the pelvic splanchnic nerves and the pelvic plexus to the pelvic organs and lower bowel.
This division innervates:
- Colon (below the distal third of the transverse colon);
- Rectum;
- Urinary bladder;
- Genitals.
Parasympathetic activity promotes emptying of the urinary bladder (contraction of the detrusor muscle and relaxation of the sphincter) and rectum, and mediates erection.
Effects on organs
| Organ | Effect of the parasympathetic system |
|---|---|
| Eye | Pupil constriction (miosis), accommodation |
| Salivary glands | Secretion of abundant watery saliva |
| Heart | Decreased heart rate (bradycardia), constriction of coronary arteries |
| GI tract | Increased peristalsis and secretion, relaxation of sphincters |
| Urinary bladder | Contraction of the detrusor, relaxation of the sphincter (micturition) |
| Adrenal glands | No innervation |
Briefly: answers to topical questions
Which specific parasympathetic nuclei are located in the brainstem?
Four parasympathetic cranial nerve nuclei are located in the brainstem. These include:
- Edinger-Westphal nucleus (accessory oculomotor nucleus) (cranial nerve III) — innervates the intrinsic muscles of the eye.
- Superior salivatory nucleus (cranial nerve VII) — provides secretory fibers to the lacrimal and salivary glands.
- Inferior salivatory nucleus (cranial nerve IX) — associated with the parotid gland.
- Dorsal nucleus of the vagus nerve (cranial nerve X) — innervates thoracic and abdominal organs.
Which types of cholinergic receptors are found in the synapses of parasympathetic target organs?
Muscarinic cholinergic receptors are located in the synapses of the parasympathetic nervous system target organs. Specifically, they are present in the following structures:
- Muscular coat of the urinary bladder — contains muscarinic detrusor receptors, stimulation of which by acetylcholine causes muscle contraction.
- Bronchial smooth muscle — contains muscarinic cholinergic receptors involved in autonomic regulation.
Which specific smooth muscles of the eye are innervated by parasympathetic fibers?
Two intrinsic smooth muscles of the eye are innervated by parasympathetic fibers. These include:
- Sphincter pupillae muscle (m. sphincter pupillae) — a circular muscle whose contraction causes miosis and increases depth of focus.
- Ciliary muscle (m. ciliaris) — provides accommodation by changing lens curvature.
Which organs and tissues of the body lack parasympathetic innervation?
Parasympathetic innervation is absent in the smooth muscle of blood vessel walls and in the adrenal glands. Sweat glands, arrector pili muscles, and the spleen have predominantly sympathetic innervation, though this is not framed in the cited sources as a complete absence of parasympathetic innervation.
Class and exam questions
What is the main anatomical difference between parasympathetic and sympathetic pathways?
In the parasympathetic system, preganglionic fibers are long and postganglionic fibers are short because the ganglia lie directly at or within the target organs.
Do blood vessels have parasympathetic innervation?
No, smooth muscles of blood vessel walls lack parasympathetic innervation (an important clinical exception).
Which part of the nervous system is responsible for urination?
The primary motor innervation of the urinary bladder is provided by the parasympathetic system (segments S2–S4). It contracts the detrusor and relaxes the sphincter.
What else to study on this topic
- Features of sympathetic fibers and the adrenal medulla
- Horner's syndrome: classical triad and causes
- Carotid artery dissection and pathogenesis of innervation disturbance
- Pathological vasodilation in splanchnic nerve lesions
- Sympathetic innervation of pelvic organs
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