parasympatisk

Parasympatisk, or the parasympathetic division, is one of the two main components of the autonomic nervous system. It promotes rest-and-digest activities that conserve energy and support maintenance functions such as digestion, urination, and defecation. In contrast to the sympathetic division, it generally slows activity and promotes recovery processes.

Anatomy and transmission: the parasympathetic system has a craniosacral outflow, with preganglionic neurons arising from brainstem nuclei (oculomotor III, facial VII, glossopharyngeal IX, and vagus X) and from sacral spinal segments S2–S4. Preganglionic fibers are myelinated and synapse on terminal (intramural) ganglia near or within target organs; postganglionic fibers are unmyelinated and release acetylcholine onto muscarinic receptors. The vagus nerve supplies a large portion of parasympathetic innervation to thoracic and abdominal organs.

Physiological effects: parasympathetic activity slows heart rate and decreases atrioventricular conduction; constricts pupils; stimulates salivation, lacrimation, digestion, and glandular secretions; promotes bronchoconstriction and bronchial secretions; and enhances peristalsis, glandular activity, and bladder contraction. Overall, it supports energy storage, nutrient absorption, and elimination.

Clinical and pharmacological relevance: acetylcholine is the primary neurotransmitter at parasympathetic synapses. Drugs that mimic or block its action have therapeutic uses and toxicology implications. Muscarinic agonists (for example, bethanechol) can stimulate bladder or GI activity, while antimuscarinic drugs (such as atropine) reduce secretions and slow certain reflexes. Organophosphates inhibit acetylcholinesterase, causing cholinergic crisis with excessive parasympathetic effects.