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(par″ă-stĕr′ năl) [para- + sternal] Adjacent to the sternum.


(par″ă-soo′ĭ-sīd″) [para- + suicide] An intentional act of injuring or harming oneself without the intention of committing suicide.


(par″ă-soo″ĭ-sīd′ ăl), adj.


(par″ă-sim″pă-thet’ik) [para- + sympathetic] Pert. to the craniosacral division of the autonomic nervous system.

parasympathetic nervous system

In the autonomic nervous system, those efferent (motor) circuits in which the preganglionic neurons are located in the brainstem or the most caudal segments of the spinal cord and the main postganglionic neurotransmitter is acetylcholine.

 ANATOMY: The primary motor cells of the parasympathetic nervous system are found in the brainstem visceral motor nuclei (including, the Edinger-Westphal nucleus, superior and inferior salivatory nuclei, dorsal motor nucleus of the vagus, and nearby reticular nuclei) and in the lateral horns of the sacral segments of the spinal cord. The axons, the “preganglionic axons,” of these neurons exit through cranial nerves (specifically, CN III, CN VII, CN IX, and CN X) or through sacral ventral roots (specifically, S2, S3, and S4) and synapse on neurons in small peripheral parasympathetic ganglia, all of which are located near the effector cells. The vagus nerve (CN X) is the major conduit for those preganglionic parasympathetic axons that innervate the autonomic ganglia of the viscera, including the gastrointestinal tract, lungs, heart, pancreas, liver, gallbladder, kidney, and ureters. In general, preganglionic parasympathetic axons tend to be long, while postganglionic parasympathetic axons are usually short.

 PHYSIOLOGY AND PHARMACOLOGY: Activation of the parasympathetic nervous system relaxes an organism and allows it to focus inward, to digest food, and to rebuild. Parasympathetic activity constricts pupils, decreases heart rate, narrows airways, increases motility of the gastrointestinal tract, and increases storage of glucose.

 Most of these effects are produced by acetylcholine interacting with cholinergic receptors on effector cells (smooth muscle, cardiac muscle, and secretory cells, although parasympathetic postganglionic axons also affect target cells by secreting other active chemicals, such as nitric oxide and a number of peptides.) There are two main classes of cholinergic receptors—muscarinic and nicotinic; autonomic effector cells typically have muscarinic receptors. Drugs other than acetylcholine can act on cholinergic receptors, and receptor subtypes differ in their sensitivity to specific drugs. For example, bethanechol, carbachol, muscarine, and pilocarpine selectively activate muscarinic cholinergic receptors and atropine, ipratropium, and scopolamine selectively block muscarinic cholinergic receptors. The availability of selective drugs allows some autonomic medical symptoms to be targeted selectively. SEE: autonomic nervous system for illus. and table; sympathetic nervous system.


(par″ă-sim″pă-thō-lit′ ik) [para- + sympatholytic] Anticholinergic.


(păr″ă-sĭm″pă-thō-mĭm-ĕt′ĭk) [″ + ″ + mimetikos, imitative] Producing ...

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