Parasympatiska

Parasympatiska, or the parasympathetic nervous system, is one of the two main divisions of the autonomic nervous system. It typically promotes rest and digest activities and functions to conserve energy and maintain routine bodily processes, in contrast to the sympathetic division.

Anatomy and neurochemistry: The parasympathetic system has a craniosacral outflow, with preganglionic neurons in brainstem nuclei (oculomotor III, facial VII, glossopharyngeal IX, vagus X) and in the sacral spinal cord (S2–S4). Preganglionic fibers synapse in terminal or intramural ganglia near or within target organs, where postganglionic neurons release acetylcholine. Effector tissues predominantly respond to acetylcholine via muscarinic receptors; nicotinic receptors are found on postganglionic neuron cell bodies. The vagus nerve provides substantial parasympathetic input to thoracic and much of the abdominal viscera.

Major actions: It slows heart rate, reduces conduction velocity, and lowers blood pressure; constricts bronchial airways and stimulates bronchial secretions; increases gastrointestinal motility and secretions; promotes urination and defecation; causes pupil constriction (miosis) and lens rounding for near vision; stimulates lacrimal and certain salivary glands.

Clinical relevance: Alterations in parasympathetic activity influence many organ systems. Anticholinergic drugs inhibit muscarinic receptors and cause dry mouth, constipation, urinary retention, and tachycardia. Cholinergic agonists enhance parasympathetic actions. Poisoning with organophosphates or nerve agents can produce excessive parasympathetic stimulation, with salivation, lacrimation, urination, defecation, GI distress, miosis, and bradycardia. Assessment of parasympathetic function is used in autonomic testing, alongside heart rate variability and pupil response.