The word "muscle" comes from the Latin for "little mouse." In its overview of how the nervous system works and acts upon other bodily systems, the University of Michigan's Medical School emphasizes that all aspects are always fully coordinated and interrelated. For this reason, subdivisions into "central," "peripheral," "somatic" and "autonomic," as taught in anatomy courses, exist mainly to clarify extremely complex functions.
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The human body contains more than 650 different muscles in three categories, all under the control of the nervous system. Striated, or skeletal, muscles, come under conscious, or voluntary, control. Smooth, or visceral, muscles, such as those found in the digestive tract, are connected to organs and perform their work outside of voluntary control. The cardiac muscle has a single specialized function confined to the heart. On average, about 40 percent of normal body weight consists of muscle.
The nervous system consists of billions of neurons in constant touch with each other for the purpose of monitoring and regulating your internal and sensory functions. The central nervous system, or CNS, includes the brain and spinal cord, and the peripheral nervous system, or PNS, includes all the nerves outside the central nervous system. The spinal cord behaves like an information superhighway, speeding signals from the brain to the PNS and vice versa. Like the muscular system, the PNS, consisting of all the roads that ultimately lead to the superhighway, has a dual function. One part is somatic, meaning under conscious control, and the other is autonomic, or outside of conscious control.
Nerves Communicating With Muscles
Actions such as leaning over and picking up a dropped pen involve the coordinated effort of numerous muscle groups. Your conscious mind relays this command to your CNS, which translates it into electrical impulses. These are then channeled through the somatic part of your PNS to the nerves responsible for controlling the necessary muscles. When the messages arrive, a chemical, acetylcholine, is released from the nerve endings, stimulating the membranes of muscle fibers and causing them to contract. It feels as though this happens instantly, but in fact, it takes about 1 millisecond -- 1/1000 of a second. Normally, your conscious mind is unable to speed up or slow down your heart rate, digestion or other visceral muscles because these are regulated autonomically.
Whether under somatic or autonomic control, muscles work in opposing groups, and movement occurs when one part contracts and the other relaxes. During contraction, muscle fibers shorten by 30 to 40 percent. Skeletal muscles are attached to bones either directly or indirectly, via tendons, and must rest and re-oxygenate after vigorous exercise. Visceral and cardiac muscles are influenced by exercise, usually positively, and also require a steady supply of fresh oxygen from the blood, but they keep on working non-stop throughout your life.