Neurotransmitters

Neurotransmitters are chemical messengers synthesized by neurons and released into the synapse — the gap between one neuron and the next — to transmit signals. When an electrical signal reaches the end of a neuron (the axon terminal), it triggers release of neurotransmitter molecules into the synaptic cleft. These molecules bind to receptors on the receiving neuron, either exciting or inhibiting its activity.

There are over 100 known neurotransmitters, varying enormously in structure, synthesis pathway, and function. The major categories include small molecule transmitters (acetylcholine, dopamine, serotonin, norepinephrine, GABA, glutamate), which act quickly at synapses; and neuropeptides (including endorphins, oxytocin, and substance P), which typically act more slowly and have modulatory effects.

Neurotransmitter function depends not just on the transmitter itself but on the density and sensitivity of its receptors, the enzymes that break it down, and the transporters that reuptake it into the releasing neuron. Drugs that affect cognition, mood, and movement typically work by modifying one or more of these elements — increasing neurotransmitter availability, blocking receptors, or inhibiting reuptake.

Several neurotransmitter systems are specifically relevant to cognitive health. Acetylcholine is critical for attention and memory formation — the cholinergic system is one of the earliest and most severely damaged in Alzheimer's disease, which is why cholinesterase inhibitors (donepezil, rivastigmine) that prevent acetylcholine breakdown remain widely prescribed for Alzheimer's symptoms. Dopamine in the prefrontal cortex is essential for working memory and executive function. Glutamate and GABA maintain the excitatory-inhibitory balance that underlies all neural processing.

Neurotransmitter balance disrupts with aging and disease. In Alzheimer's disease, acetylcholine deficiency in the hippocampus and cortex contributes to memory and attention impairments. In depression, which significantly impairs cognitive function, serotonin, norepinephrine, and dopamine systems are dysregulated. Understanding these systems provides rationale for current drug treatments and guides the development of new ones.

Lifestyle factors also modulate neurotransmitter systems. Exercise increases dopamine, serotonin, and norepinephrine release and enhances receptor sensitivity. Sleep is critical for neurotransmitter replenishment — adenosine accumulates during wakefulness, building sleep pressure, and is cleared during sleep. Diet provides precursors for several neurotransmitters: tryptophan for serotonin, tyrosine for dopamine and norepinephrine, choline for acetylcholine.