Acetylcholine

Approximately 5% of brain neurons have receptors for acetylcholine (ACh). Most CNS responses to ACh are mediated by a large family of G protein-coupled muscarinic M1 receptors that lead to slow excitation when activated. The ionic mechanism of slow excitation involves a decrease in membrane permeability to potassium. Of the nicotinic receptors present in the CNS (they are less common than muscarinic receptors), those on the Renshaw cells activated by motor axon collaterals in the spinal cord are the best characterized. Drugs affecting the activity of cholinergic systems in the brain include the acetylcholinesterase inhibitors used in Alzheimer’s disease (eg, rivastigmine) and the muscarinic blocking agents used in parkinsonism (eg, benztropine).