Neurotransmitter

Neurotransmitters are released from one neuron into the synapse and bind receptors on the next, either exciting or inhibiting it. Acetylcholine carries the signal to muscle; dopamine, serotonin, noradrenaline and GABA shape mood, arousal, motivation and sleep.

It matters to training because these systems mediate both the drive to train and much of the mental benefit that comes from it. Exercise reliably increases several of them, and the antidepressant effect of regular exercise is among the better-supported findings in the field.

In practice, this is also territory where supplement marketing runs well ahead of evidence — precursor and 'brain-boosting' products routinely claim effects far beyond what has been demonstrated. Caffeine's mechanism, by contrast, is genuinely well characterised: it blocks adenosine receptors, reducing perceived effort.