Physical activity initiates complex biochemical signaling cascades throughout the body. Exercise is interpreted as a temporary, beneficial stressor, prompting the release of dozens of signaling molecules from multiple organs and tissues. These chemical messengers coordinate an immediate physiological response to sustain exertion and begin a long-term adaptive process. This process involves communication between the brain, muscles, glands, and fat tissue, affecting mood, metabolism, and cognitive function.
Neurotransmitters for Mood and Pain Relief
The body responds to physical strain by immediately releasing endogenous opioids, most notably beta-endorphin, which functions as a natural pain reliever. Endorphins are neurotransmitters produced primarily by the pituitary gland and hypothalamus. They bind to opioid receptors in the brain to reduce pain perception, creating the “runner’s high” phenomenon of euphoria and well-being during prolonged exercise.
Exercise also modulates other key neurotransmitters that regulate emotional state and motivation. Serotonin, known for its role in mood stabilization, increases during activity and contributes to feelings of calmness and general well-being. The release of this chemical helps alleviate symptoms of mild depression and anxiety by enhancing the brain’s internal mood-regulating systems.
Dopamine, a powerful neurotransmitter, is released alongside endorphins and is a core component of the brain’s reward circuitry. This chemical is responsible for feelings of pleasure and satisfaction, enhancing motivation and reinforcing the desire to repeat the activity. The combined action of these chemicals creates a positive feedback loop, linking exercise with immediate emotional benefits.
Hormones for Acute Energy Mobilization
The body sustains physical exertion by releasing catecholamines, which include adrenaline (epinephrine) and noradrenaline (norepinephrine). These hormones are secreted primarily by the adrenal medulla and quickly ready the cardiovascular system for increased demand. Adrenaline causes a rapid increase in heart rate, while noradrenaline constricts blood vessels in non-working tissues, shunting blood flow toward active muscles.
Catecholamines also play a primary role in mobilizing the energy required to fuel muscle contraction. They stimulate the liver to break down stored glycogen into glucose (glycogenolysis) and promote the breakdown of fat stores (lipolysis) to provide fatty acids as an alternative fuel source. This dual action ensures a steady supply of substrates to the muscle cells, especially during high-intensity or prolonged activity.
The hypothalamic-pituitary-adrenal (HPA) axis activates, leading to the temporary release of cortisol, a glucocorticoid hormone. Cortisol levels rise significantly when exercise intensity exceeds a certain threshold. Its main function during a workout is to maintain blood glucose levels by promoting hepatic gluconeogenesis—the creation of new glucose from non-carbohydrate sources—thereby managing substrate availability for the brain and muscles.
Muscle-Derived Messengers (Myokines)
Skeletal muscle functions as an endocrine factory, secreting signaling proteins called myokines during contraction. These myokines act as communicators, traveling through the bloodstream to influence the function of distant organs, including the liver, fat tissue, and brain. This established muscle as an active participant in systemic metabolism and health maintenance.
One highly studied myokine is Interleukin-6 (IL-6), released acutely from contracting muscle, which plays a complex, dual role. While chronic levels of IL-6 are associated with inflammation, the burst released during exercise acts beneficially. It promotes glucose uptake and lipolysis to provide fuel, and also contributes to an anti-inflammatory effect throughout the body, counteracting chronic inflammation linked to metabolic disorders.
Another important myokine is Irisin, which is released into the circulation. Irisin influences energy expenditure by promoting the “browning” of white adipose tissue. This converts energy-storing fat cells into a type of fat that burns energy to produce heat. Myokines released during muscle activity are central to improving whole-body metabolism and regulating body composition.
Chemicals for Brain Health and Cognitive Function
Brain-Derived Neurotrophic Factor (BDNF) is significantly upregulated by physical activity and is often described as a molecular fertilizer for the brain. BDNF promotes the health and survival of existing neurons while stimulating the growth of new neurons, a process called neurogenesis. This occurs particularly in the hippocampus, a region critically involved in memory and learning functions.
The increase in BDNF enhances synaptic plasticity, which is the ability of neural connections to strengthen or weaken over time in response to activity. This strengthening is the cellular basis for improved memory and learning capacity observed after regular exercise. By supporting the structure and function of neurons, BDNF helps protect against age-related cognitive decline and neurodegenerative conditions.