Acetylcholine is the only neurotransmitter that does two completely different jobs in two completely different systems. In the central nervous system, it modulates attention, memory, and arousal. In the peripheral nervous system, it is the workhorse of the parasympathetic branch (rest and digest) and the sole transmitter at the neuromuscular junction (every voluntary movement). One molecule, two parallel careers.
In the brain, the cholinergic system originates in the basal forebrain — a small region just anterior to and below the basal ganglia. From here, axons project broadly to cortex and hippocampus. This anatomy is, again, the signature of a modulator. A small source sets the climate of large regions.
The function the basal forebrain supports most clearly is the gating of attention onto novelty and the consolidation of new memories into the cortex. When this system fails, the patient loses the ability to lay down new declarative memories and to sustain focus on what is in front of them. The recent past blurs first.
Alzheimer's disease is, among other things, a disease of basal forebrain degeneration. By the time the patient and family notice the memory problem, large portions of the cholinergic projection are already gone. This is the rationale for the cholinesterase inhibitors — donepezil, rivastigmine, galantamine. They do not stop the disease. They preserve the dwindling supply of acetylcholine in the synapse by blocking its breakdown enzyme. The effect is modest. The mechanism is logical.
Now the second-order point that catches clinicians by surprise. Many medications block the muscarinic acetylcholine receptor as a side effect — first-generation antihistamines, tricyclic antidepressants, some bladder antispasmodics, some antipsychotics, scopolamine. Each one, on its own, may be tolerated. But the elderly patient on five medications, three of which have anticholinergic activity, is being asked to run their already-depleted basal forebrain against pharmacological headwind. The clinical result is delirium, falls, and accelerated cognitive decline.
Beers Criteria and the Anticholinergic Cognitive Burden scale exist precisely because of this. The single most common reversible cause of new cognitive impairment in the geriatric clinic is a medication list that, totaled, is anticholinergic. Review the list. Substitute or remove. Watch the patient come back.
Hold the image of the split figure. Same molecule. Brain side: attention and memory. Body side: rest and digest. Drugs that block it punish the brain side hardest in the patient who can least afford it.