Corpus: Parasympathetic nervous system

The parasympathetic nervous system is a part of the autonomic nervous system and acts as the antagonist of the sympathetic nervous system. Its primary role is to regulate bodily functions that promote recovery and energy storage (trophotropic functions). It also helps maintain the body's internal balance, known as homeostasis.

The core areas of the parasympathetic nervous system are located in the brainstem and the sacral spinal cord. Parasympathetic fibers are carried by the oculomotor nerve (cranial nerve III), facial nerve (VII), glossopharyngeal nerve (IX), and especially the vagus nerve (X). These fibers control the muscles and glands in the head and neck region. Additionally, the trigeminal nerve (cranial nerve V) also partly carries parasympathetic fibers originating from the facial nerve. The vagus nerve extends to the internal organs of the chest (thorax) and parts of the abdomen.

From the sacral spinal cord segments S1 to S3, parasympathetic pelvic splanchnic nerves emerge. These innervate the lower part of the large intestine (approximately from the left colonic flexure, Cannon's point) and structures of the pelvis, including the urinary bladder.

The parasympathetic nervous system uses acetylcholine (ACh) as its primary neurotransmitter at both the first synapse in the ganglion and the second synapse at the target organ. This differs from the sympathetic nervous system, which uses noradrenaline as the neurotransmitter at the second synapse.

In both systems, acetylcholine is the neurotransmitter at the first synapse, where it acts on nicotinic receptors (nAChRs) to transmit signals between the first and second neurons. At the second synapse, the parasympathetic system uses acetylcholine acting on muscarinic receptors (mAChRs), while the sympathetic system uses noradrenaline acting on adrenergic receptors.

Additionally, the parasympathetic nervous system also uses co-transmitters such as nitric oxide (NO) and vasoactive intestinal peptide (VIP).

3.1. Organotropic effects of the parasympathetic nervous system

• Heart: Slows heart rate (negative chronotropic effect) and decreases conduction speed (negative dromotropic effect).
• Blood vessels: Direct effects are uncertain, but it has vasodilatory effects in the genital region. Indirect vasodilation can occur through inhibition of the tonic sympathetic system, though this varies by organ.
• Bronchi: Causes constriction and increases mucus secretion.
• Gastrointestinal tract: Stimulates glandular secretion and increases peristalsis.
• Urinary bladder: Contracts the detrusor muscle and relaxes the sphincter.
• Uterus: Causes contraction.
• Eye: Contracts the sphincter pupillae muscle, leading to pupil constriction (miosis), and contracts the ciliary muscle, improving near vision (accommodation).
• Salivary glands: Increases saliva production.

Drugs that enhance the parasympathetic nervous system are called parasympathomimetics. These are categorized into:

• Direct mimetics: Act directly on acetylcholine receptors (e.g., acetylcholine, pilocarpine, muscarine, arecoline).
• Indirect mimetics: Inhibit acetylcholinesterase, the enzyme that breaks down acetylcholine, thereby increasing its levels (e.g., physostigmine, neostigmine, pyridostigmine).

Drugs that inhibit the parasympathetic nervous system are called parasympatholytics. Examples include atropine, scopolamine, tropicamide, and their derivatives.