There are different levels in which exercise can affect the brain. Typically, media that conveys exercise neuroscience tends to focus on singular mechanisms, which get widely adopted by the lay population and health & fitness professionals alike. This “mechanistic-focused” approach puts the spotlight on singular neurophysiological events or adaptations, while seemingly neglecting the complex, multi-level effects of exercise on the brain. For example, an article may mention a single growth factor, such as Brain-Derived Neurotrophic Factor (BDNF), a physiological mechanism (increased cerebral blood flow), or a change in brain regions such as increased volume of the hippocampus. While these adaptations and mechanisms hold a lot of weight within the scientific literature, they are only pieces of the “neuro-puzzle” by which exercise affects the brain. Some researchers have posited exercise as a “poly-pill”, and the complex physiology that justifies this also applies to the central nervous system, also qualifying exercise as a “neuro-poly-pill”.
Exercise can modify brain structures, both in terms of total brain volume and specific brain regions, as well as brain functions, which include localized changes in brain activity, improvements in cognitive functions, to modifications in large-scale brain networks. Changes at the synaptic level, the neuronal level, the neuronal network level, and at the structural level of the brain occur at different time points, with functional changes preceding larger and more significant structural changes. These structural changes occurring in brain regions can take several weeks (typically a minimum of 12 weeks depending upon the population and intervention in question) to months and years.
In addition to structural changes, functional changes also occur in the brain, which can occur as the result of acute exercise bouts. These functional changes can be local to a specific brain region, such as seen in changes in prefrontal cortex activity. Changes in brain activity can also be reflected within functional brain networks, which involve multiple brain regions that spontaneously activate. Functional changes may include alterations to brain activity and metabolism, but on a larger scale than that of the micro-level (within smaller networks of neurons or at the synaptic or cellular level). For example, changes in larger neuronal networks (locally in a brain region or within a functional brain network) may take place due to the alteration of neurochemical, metabolic, or electrical activity in various brain regions.