Neurobiological effects of physical exercise

The neurobiological effects of physical exercise are numerous and involve a wide range of interrelated neuropsychological changes. A large body of research in humans has demonstrated that consistent aerobic exercise (e.g., 30 minutes every day) induces persistent beneficial behavioral and neural plasticity as well as healthy alterations in gene expression in the brain; some of these long-term effects include: increased neuron growth, increased neurological activity (c-Fos and BDNF signaling), improved stress coping, enhanced cognitive control over behavior, improved declarative and working memory, and structural and functional improvements in brain structures and pathways associated with cognitive control and memory. The effects of exercise on cognition have important implications for improving academic performance in children and college students, improving adult productivity, preserving cognitive function in old age, preventing or treating certain neurological disorders, and improving overall quality of life.

People who regularly participate in aerobic exercise have greater scores on neuropsychological function and performance tests. Examples of aerobic exercise that produce these changes are running, jogging, brisk walking, swimming, and cycling. Exercise intensity and duration are positively correlated with the release of neurotrophic factors and the magnitude of nearly all forms of exercise-induced behavioral and neural plasticity; consequently, more pronounced improvements in measures of neuropsychological performance are observed in endurance athletes as compared to recreational athletes or sedentary individuals. Aerobic exercise is also a potent long-term antidepressant and a short-term euphoriant; consequently, consistent exercise has also been shown to produce general improvements in mood and self-esteem in all individuals.

Neuroplasticity is essentially the ability of neurons in the brain to adapt over time, and most often occurs in response to repeated exposure to stimuli whereas neurogenesis is the postnatal (after-birth) growth of new neurons, a beneficial form of neuroplasticity.Aerobic exercise promotes neurogenesis by increasing the production of neurotrophic factors (compounds which promote the growth or survival of neurons), such as brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-1), and vascular endothelial growth factor (VEGF). Consistent aerobic exercise over a period of several months induces marked clinically significant improvements in executive function (i.e., the "cognitive control" of behavior) and increased gray matter volume in multiple brain regions, particularly those which give rise to cognitive control. The brain structures that show the greatest improvements in gray matter volume in response to aerobic exercise are the prefrontal cortex and hippocampus; moderate improvements seen in the anterior cingulate cortex, parietal cortex, cerebellum, caudate nucleus, and nucleus accumbens.The prefrontal cortex, caudate nucleus, and anterior cingulate cortex are among the most significant brain structures in the dopamine and norepinephrine systems that give rise to cognitive control. Exercise-induced neurogenesis (i.e., the increases in gray matter volume) in the hippocampus is associated with measurable improvements in spatial memory. Higher physical fitness scores (measured by VO2 max) are associated with better executive function, faster processing speed, and greater volume of the hippocampus, caudate nucleus, and nucleus accumbens. Long-term aerobic exercise is also associated with persistent beneficial epigenetic changes that result in improved stress coping, improved cognitive function, and increased neuronal activity (c-Fos and BDNF signaling)

One of the most significant effects of exercise on the brain is the increased synthesis and expression of BDNF, a neuropeptide hormone, in the brain and periphery, resulting in increased signaling through its tyrosine kinase receptor, tropomyosin receptor kinase B (TrkB). Since BDNF is capable of crossing the blood–brain barrier, higher peripheral BDNF synthesis also increases BDNF signaling in the brain.Exercise-induced increases in brain BDNF signaling are associated with beneficial epigenetic changes, improved cognitive function, improved mood, and improved memory. Furthermore, research has provided a great deal of support for the role of BDNF in hippocampal neurogenesis, synaptic plasticity, and neural repair. Engaging in moderate-high intensity aerobic exercise such as running, swimming and cycling, increases BDNF biosynthesis through myokine signaling, resulting in up to a threefold increase in blood plasma and brain BDNF levels; exercise intensity affects the magnitude of increased BDNF synthesis and expression.A meta-analysis of studies involving the effect of exercise on BDNF levels found that consistent exercise modestly increases resting BDNF levels as well.Reviews of neuroimaging studies indicate that consistent aerobic exercise increases gray matter volume in several brain regions associated with memory, cognitive control, motor function, and reward processing; the most prominent gains are seen in the prefrontal cortex and hippocampus, which are primarily associated with cognitive control and memory processing respectively. Moreover, the left and right halves of the prefrontal cortex, which is divided by the medial longitudinal fissure, appear to become more interconnected in response to consistent aerobic exercise. Two reviews indicate that marked improvements in prefrontal and hippocampal gray matter volume occur in healthy adults that engage in medium intensity exercise for several months. Other regions of the brain that demonstrate moderate or less significant gains in gray matter volume during neuroimaging include the anterior cingulate cortex, parietal cortex, cerebellum, caudate nucleus, and nucleus accumbens.

Regular exercise has been shown to counter the shrinking of the hippocampus and memory impairment that naturally occurs in late adulthood. Sedentary adults over age 55 show a 1–2% decline in hippocampal volume annually. A neuroimaging study with a sample of 120 adults revealed that participating in regular aerobic exercise increased the volume of the left hippocampus by 2.12% and the right hippocampus by 1.97% over a one-year period. Subjects in the low intensity stretching group who had higher fitness levels at baseline showed less hippocampal volume loss, providing evidence for exercise being protective against age-related cognitive decline. In general, individuals that exercise more over a given period have greater hippocampal volumes and better memory function. Aerobic exercise has also been shown to induce growth in the white matter tracts in the anterior corpus callosum, which normally shrink with age.

The various functions of the brain structures that show exercise-induced increases in gray matter volume include:

Prefrontal and anterior cingulate cortices – required for the cognitive control of behavior, particularly: working memory, attentional control, decision-making, cognitive flexibility, social cognition, and inhibitory control of behavior; implicated in attention deficit hyperactivity disorder (ADHD) and addiction

Nucleus accumbens – responsible for reward perception, motivation, and positive reinforcement; implicated in addiction

Hippocampus – responsible for storage and consolidation of declarative memory and spatial memory;implicated in depression

Cerebellum – responsible for motor coordination and motor learning

Caudate nucleus – responsible for stimulus-response learning and inhibitory control; implicated in Parkinson's disease, Huntington's disease and ADHD

Parietal cortex – responsible for sensory perception, working memory, and attention

Cognitive control and memory

Concordant with the functional roles of the brain structures that exhibit increased gray matter volumes, exercise has been shown to improve numerous aspects of cognitive control and memory function.In particular, consistent aerobic exercise has been shown to improve attentional control,[note attention span, information processing speed, cognitive flexibility (e.g., task switching), inhibitory control,working memory updating and capacity,[note 3] declarative memory,[note 4] and spatial memory.Individuals who have a sedentary lifestyle tend to have impaired cognitive control relative to other more physically active non-exercisers. A reciprocal relationship between exercise and cognitive control has also been noted: improvements in control processes, such as attentional control and inhibitory control, increase an individual's tendency to exercise.A systematic review of studies conducted on children suggests that some of the exercise-induced improvements in executive function are apparent after single bouts of exercise, while other aspects (e.g., attentional control) only improve following consistent exercise on a regular basis.

ADHD is a developmental neuropsychiatric disorder in which there are deficits in certain aspects of cognitive control, particularly attentional control and inhibitory control.[36] Regular physical exercise, particularly aerobic exercise, is an effective adjunct treatment for ADHD, although the best type and intensity is not currently known.[45][46] In non-randomized trials, physical exercise has been shown to result in better behavior and motor abilities without causing any side effects in ADHD populations.