Parasympathetic System

Introduction

The parasympathetic system is a craniosacral network characterized by long preganglionic fibers and ganglia located near target organs. Dominated by vagal activity, it promotes restorative functions, and an optimal vagal tone is essential for maintaining physiological stability and health.

• The parasympathetic system promotes energy conservation and restoration during resting conditions. It supports recovery by enhancing digestion, nutrient absorption, and tissue repair. This system maintains internal balance by reducing metabolic demand.
• Most parasympathetic functions are mediated through the vagus nerve, which regulates major visceral activities.

Structural Organization

• The parasympathetic system represents the craniosacral division of the autonomic nervous system. Its cranial component arises from the brainstem, while the sacral component originates from sacral spinal segments.
• Parasympathetic activity produces localized responses due to limited divergence, with an approximate ratio of 1:15.
• Unlike the sympathetic system, it lacks a circulating hormonal mechanism, resulting in targeted organ-specific effects. For example, vagal stimulation slows heart rate without significantly influencing other organs simultaneously.
• Parasympathetic ganglia are located near or within target organs. Consequently, preganglionic fibers are long and postganglionic fibers are short.

Cranial Component

• The cranial component consists of preganglionic neurons situated in brainstem nuclei. These neurons project through cranial nerves supplying head, neck, thoracic, and abdominal organs.
• Major cranial nerves involved include the oculomotor, facial, glossopharyngeal, and vagus nerves.
• Brainstem nuclei act as integration centers for autonomic reflexes controlling these organs. This structural organization ensures precise and efficient regulation of visceral functions.

Cranial Nerve III

• Cranial nerve three (oculomotor nerve) carries parasympathetic fibers originating from the Edinger–Westphal nucleus in the midbrain.
• Preganglionic fibers travel within this nerve and synapse in the ciliary ganglion located in the orbit.
• Postganglionic fibers enter the eye and supply the sphincter pupillae, causing pupillary constriction. They also innervate the ciliary muscle, which enables accommodation for near vision.
• A small proportion of fibers supply choroidal blood vessels, contributing to vascular regulation.
• Most fibers are directed toward the ciliary muscle, with fewer fibers supplying the iris. This pathway is essential for pupillary reflexes and visual accommodation.

Table 32.1: Differences between sympathetic and parasympathetic systems.

Feature	Sympathetic System	Parasympathetic System
Origin	Thoracolumbar (T1–L3 spinal segments)	Craniosacral (brainstem and S2–S4)
Preganglionic Fibers	Short, cholinergic	Long, cholinergic
Ganglia Location	Near spinal cord (paravertebral, prevertebral)	Near or within target organs (intramural)
Postganglionic Fibers	Long, mainly adrenergic	Short, cholinergic
Divergence Ratio	High (~1:100), produces widespread effects	Low (~1:1–1:15), produces localized effects
Functional Role	Energy expenditure, stress response	Energy conservation, restorative functions
Neurotransmitters	Noradrenaline (α, β receptors); acetylcholine in some fibers	Acetylcholine (muscarinic receptors)
Overall Response	Diffuse, coordinated activation	Targeted, organ-specific action

Table 32.2: Organ-wise Effects.

Organ/System	Sympathetic Effect	Parasympathetic Effect
Eye	Pupil dilation; eyelid elevation	Pupil constriction; accommodation
Glands	Thick or reduced secretion	Profuse watery secretion
Heart	Increased rate and contractility	Decreased rate and contractility
Lungs	Bronchodilation	Bronchoconstriction
Gastrointestinal Tract	Reduced motility; sphincter contraction	Increased motility; sphincter relaxation
Urinary Bladder	Detrusor relaxation; sphincter contraction	Detrusor contraction; sphincter relaxation
Reproductive Organs	Ejaculation	Erection
Skin	Sweating; piloerection	Minimal direct effect
Blood Vessels	Predominant vasoconstriction	Limited direct control

Cranial Nerve VII

• Cranial nerve seven carries parasympathetic fibers from the superior salivatory nucleus in the brainstem.
• Preganglionic fibers pass through the greater petrosal nerve and synapse in the pterygopalatine ganglion.
• Postganglionic fibers supply the lacrimal gland and glands of the nasal and palatal mucosa, increasing secretion.
• Another pathway travels via the chorda tympani to the submandibular ganglion.
• Postganglionic fibers from this ganglion innervate the submandibular and sublingual glands, enhancing saliva production.

Cranial Nerve IX

• Cranial nerve nine carries parasympathetic fibers from the inferior salivatory nucleus in the medulla.
• Preganglionic fibers pass through the lesser petrosal nerve and synapse in the otic ganglion.
• Postganglionic fibers reach the parotid gland and stimulate salivary secretion.
• This nerve also transmits chemoreceptor and baroreceptor signals from the carotid body and sinus to regulate cardiovascular activity.

Cranial Nerve X

• Cranial nerve ten (vagus nerve) arises from the nucleus ambiguus and the dorsal motor nucleus in the medulla. It provides nearly seventy-five percent of total parasympathetic outflow, supplying thoracic and abdominal organs.
• Preganglionic fibers travel to intramural ganglia located within target organs such as the heart, lungs, and gastrointestinal tract.
• In the heart, the right vagus mainly influences the sinoatrial node, while the left vagus affects the atrioventricular node, reducing heart rate.
• In the lungs, vagal activity produces bronchoconstriction and increases glandular secretion.
• The nerve innervates the esophagus, stomach, small intestine, and proximal large intestine. It enhances gastrointestinal motility and secretion, supporting digestion.
• Neurotransmitters released include acetylcholine and vasoactive intestinal peptide.
• Additional branches supply organs such as the liver, pancreas, kidneys, and spleen.
• This extensive distribution enables coordinated regulation of visceral functions.

Sacral Component

• The sacral component of the parasympathetic system supplies pelvic organs.
• Preganglionic neurons arise from sacral spinal segments S2 to S4 in the intermediolateral region.
• Fibers travel to ganglia located near or within target organs. They innervate the distal colon, rectum, urinary bladder, internal anal sphincter, and reproductive organs, regulating their function.

Parasympathetic Functions

• The parasympathetic system promotes energy conservation and restoration under resting conditions. It enhances gastrointestinal motility, secretion, digestion, and absorption, supporting nutrient utilization.
• Most effects are stimulatory, except for the cardiovascular system where activity is reduced. It also facilitates growth, repair, and reproductive functions, maintaining overall physiological balance.

Neurotransmitters and Receptors

• Most parasympathetic fibers are cholinergic and release acetylcholine at both preganglionic and postganglionic synapses.
• Acetylcholine acts on two receptor types: muscarinic and nicotinic receptors.

Muscarinic Receptors

• Muscarinic receptors are present in the heart, smooth muscle, and glandular tissues. Their activation decreases heart rate and conduction, while increasing smooth muscle contraction and glandular secretion. These effects are mediated by reduced cyclic adenosine monophosphate and increased intracellular calcium through inositol trisphosphate pathways.
• Muscarinic receptors also enhance potassium channel activity in cardiac nodal tissue. These receptors are blocked by atropine.

Nicotinic Receptors

• Nicotinic receptors are located in autonomic ganglia, neuromuscular junctions, and the adrenal medulla. Their activation produces rapid excitatory responses by opening sodium and potassium ion channels. These receptors function as ligand-gated ion channels, enabling fast synaptic transmission.
• Nicotinic receptors in autonomic ganglia mediate transmission between preganglionic and postganglionic neurons. Their action can be inhibited by ganglionic blockers and neuromuscular blocking agents.
• Together, these receptor systems ensure effective transmission and regulation of parasympathetic activity.