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Cholinomimetics: Direct agonists
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The nervous system is divided into the central nervous system, that is the brain and spinal cord, and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs.
The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles, and the autonomic nervous system, which controls the involuntary activity of the smooth muscles and glands of our organs, and is further divided into the sympathetic and the parasympathetic nervous systems.
Parasympathetic neurons in the central nervous system project preganglionic fibers towards parasympathetic ganglia, which are collections of neurons near the organ they are supposed to affect.From there, postganglionic fibers project towards the target cell.
Both the preganglionic and postganglionic neurons release the neurotransmitter acetylcholine.
Acetylcholine released from preganglionic fibers acts on nicotinic receptors on the postganglionic neurons.
And acetylcholine released from postganglionic neurons acts on muscarinic and nicotinic receptors on target organs.
Nicotinic receptors are coupled to ion channels that let sodium in and potassium out, causing depolarization.
Muscarinic receptors are G-protein coupled receptors, which means they trigger secondary messenger proteins that activate a cascade of enzymes inside the cell.
The physiologic effects of the muscarinic and nicotinic stimulation can be remembered with the mnemonic: DUMB HAVES, so defecation; urination; muscle excitation; bronchospasm; heart bradycardia; autonomic ganglia stimulation; vasodilation; eye miosis, which is constriction of the pupil, and eye accommodation, which is contraction of the ciliary muscles of the iris to facilitate looking at near objects; and secretions from the lacrimal, salivary and sweat glands as well as glands in the GI tract.
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