Muscle Control

Let’s first look at how the brain controls our muscles. There’s an important circuit from the brain to the muscle, and another back to the brain. This begins in the area of the brain called the motor cortex, from where nerves run down through the spinal cord to specific muscle fibers; the purpose is to send messages to the muscle, stimulating it to either contract or relax. Virtually all physical activity begins as messages from the brain to some specific muscles to contract, and others to relax (these muscle actions are referred to as “facilitation” and “inhibition,” respectively). This same mechanism also controls other features of the muscles, power and strength. The more nerves that stimulate more muscle fibers to contract, the more power the muscle can generate; so it’s not the size of the muscle that determines strength, it’s how many muscle fibers are told to contract by the brain. Even aerobic muscles can be more powerful than anaerobic ones of the same size because of this phenomenon.

The brain is in constant communication with the muscles, and is also continuously receiving information from each muscle fiber. There’s another nerve in the muscle that goes back up to a nearby area of the brain called the sensory cortex. This feature provides important information to the brain about the status of the muscle—everything from its tension and fatigue to energy levels and risk of injury. The brain can then use this information to make appropriate adjustments in the muscle, get help from other muscles, provide more energy or repair, or tell it to slow down or stop working. This “neuromuscular system”—the loop from brain to muscle and muscle to brain—is very precise, with each muscle’s thousands of aerobic muscle fibers having a separate nerve connecting to it from the brain, and one returning to the brain. Thousands of muscle fibers and the same number of nerves participate in each muscle during contraction and relaxation. It’s a very large orchestra with the brain conducting.

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