Answer:
Muscarinic receptors 1 and 3 are coupled to Gq protein, excitatory subnunit, which increase intracellular calcium, promoting: contraction of the sphincter muscle of the iris, increased intestinal motility, bronchoconstriction, increase secretion of histamine in stomach.
Explanation:
In the parasympathetic nervous system (PNS), the acting neurotransmitter is acetylcholine (Ach). Ach acts on two types of receptors: nicotinic receptors and muscarinic receptors. When Ach is released into the synaptic cleft, it then binds to a particular receptor and then performs its actions.
Muscarinic receptors (M) are coupled to G protein. We have several muscarinic receptors, from 1 to 7. M 1, 3 and 5, the odd ones, are coupled to the Gq protein, an excitatory subunit, which activates a cascade, leading to increased intracellular calcium. M 2 and 4, the pairs receptors, are coupled to the Gi protein, an inhibitory subunit, as it increases the potassium permeability.
These receptors have different locations: M1 is located in the central nervous system, gastric parietal cells and autonomic ganglia; M2 is located in the heart, and does presynaptic modulation of neurotransmitter release, as I said earlier; M3 located in smooth muscle and glands; M4 located in the central nervous system; and M5 also in the central nervous system.
A visible effect of Ach's action on an M3 receptor occurs in the eyeball, especially in the sphincter muscle of the iris. M3 is an excitatory receptor and, when activated by Ach, promotes contraction of the sphincter muscle of the iris, leading to shrinkage of the pupil, a process known as miosis. Another effect also on the eyeball, and by Ach's action on M3, is increased tear secretion, as the parasympathetic stimulates secretion of our glands, and the tear gland is not left out.
Acetylcholine is released, binds to the gastrointestinal system M3 receptor, and promotes increased intestinal motility as well as increased intestinal secretions, which is of paramount importance for digestion.
The bronchi, by parasympathetic action in M3, contract, thus promoting bronchoconstriction.
By acting in M1, Ach increase the secretion of histamine in stomach.
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