Acetylcholine parasympathetic effector sites

Neurotransmission in autonomic ganglia is more complex than depolarization mediated by a single transmitter 190 Muscarinic receptors are widely distributed at postsynaptic parasympathetic effector sites 190 Stimulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber 191 Competitive blocking agents cause muscle paralysis by preventing access of acetylcholine to its binding site on the receptor 191... 

Hyoscine competitively inhibits the actions of acetylcholine at the muscarinic receptors of autonomic effector sites innervated by parasympathetic nerves. It has a central as well as a peripheral action, as it is lipid-soluble and crosses the blood-brain barrier. 

In normal circumstances, acetylcholine is hydrolyzed almost immediately by acetylcholinesterase close to receptors in the synaptic cleft at sites of action (see Bowman, 1993). The normal function of acetylcholinesterase is to hydrolyze acetylcholine in the synaptic cleft, parasympathetic effector organ or neuromuscular junction, in order to terminate... 

Actions of acetylcholine (ACh) are referred to as muscarinic based on the observation that muscarine acts selectively at certain sites and, quahtatively, produces the same effects as ACh. Peripheral muscarinic acetylcholine receptors are found primarily on autonomic effector cells innervated by postganghonic parasympathetic nerves and on some cells that receive little or no cholinergic innervation but express muscarinic receptors e.g., vascular endothelial cells). There are also muscarinic receptors in ganglia and the adrenal meduUa, where muscarinic stimulation seems to modulate the effects of nicotinic stimulation. Within the central nervous system (CNS), the hippocampus, cortex, and thalamus have high densities of muscarinic receptors. 

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