Muscles can be voluntarily controlled by your thoughts, such as moving your arms and legs. Other muscles, such as the heart or digestive system, are involuntary muscles that work without any conscious control.
Voluntary and involuntary muscles are both activated and controlled by the peripheral nervous system. The peripheral nervous system consists of the somatic nervous system and the autonomic nervous system. Voluntary muscles, such as the skeletal muscles, are controlled by the somatic nervous system. Involuntary muscles, such as in the digestive system, are controlled by the autonomic nervous system.
Nerve cells that run throughout the body making connections between the brain and other parts of the body. There are two types of nerve cells: sensory nerves and motor nerves. Sensory nerves collect information from the body, such as body position, light, touch, temperature, and pain, and sends this information to the brain for processing. The brain uses this information to decide what needs to be done, and to activate motor nerves to control muscles of the body. Motor nerve fibers carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.
Skeletal muscles are activated through electrical impulses from motor nerves. This activation triggers a chemical reaction causing muscle fibers to contract. The more muscle fibers that contract, the more powerful the muscle contraction. Since muscles can only provide movement through contraction, they generally operate in pairs. One muscle acts in opposition to the another muscle. When one muscle contracts, the opposing muscle must relax. For example, to bend the arm, the biceps in the front of the upper arm must contract. At the same time, the triceps in the back of the upper arm must relax. Otherwise, the arm would not move.
To perform complex movements, such as walking, jumping or throwing a ball, involves the coordinated activity of many muscles. We are not born with this ability, it must be learned. Learning this coordinated muscle activation is part of our motor system development as we grow from baby to child and adult.