Parasympaticus

Parasympaticus refers to the parasympathetic division of the autonomic nervous system, historically described in some texts as the parasympathicus. It operates in contrast to the sympathetic division and together they regulate the body's unconscious functions. The parasympathetic nervous system is known for its craniosacral outflow, with preganglionic neurons located in the brainstem (cranial nerves III, VII, IX, and X) and in the sacral spinal cord (S2–S4). Its ganglia are typically located near or within the target organs, resulting in long preganglionic and short postganglionic fibers.

Neurotransmission in the parasympathetic pathway is primarily cholinergic. Preganglionic neurons release acetylcholine (ACh) onto nicotinic receptors in the ganglia, and postganglionic neurons release ACh onto muscarinic receptors on the target organs. This organization contrasts with the sympathetic division, where postganglionic signaling often uses norepinephrine. The effects of parasympathetic activation are therefore focused on maintaining quiet, restorative functions.

The main physiological actions of the parasympathetic system include slowing the heart rate, increasing glandular secretions (such as salivary, lacrimal, and bronchial secretions), stimulating digestion through enhanced gastric and pancreatic activity and intestinal motility, constricting the pupils (miosis) and accommodating the lens, and promoting contraction of the urinary bladder and rectum. It also contributes to sexual arousal responses.

Clinical relevance includes the use of anticholinergic drugs, which inhibit parasympathetic activity and can cause dry mouth, tachycardia, and blurred vision, and cholinergic drugs or acetylcholinesterase inhibitors, which enhance parasympathetic effects. Abnormal parasympathetic function can be involved in dysautonomia, glaucoma treatment (e.g., pilocarpine), and various toxin exposures such as organophosphates.