Muscles of vertebrates, including humans, are divided into three types: skeletal muscle, smooth muscle and cardiac muscle. In humans, muscles can account for close to 50 percent of the body's weight. For the most part, smooth muscle and cardiac muscle are arranged in humans much the way they are in other as vertebrates.
Skeletal Muscle
The nearly 650 skeletal muscles in the human body are attached to the skeleton. Unlike cardiac or smooth muscle, they are under voluntary control. Working in opposing pairs, their contractions and extensions make movement possible and control balance and posture. Skeletal muscle is also called striated muscle because, under the microscope, its long, thin fibers are marked crosswise by red and white lines or striations. Their fibers contain multiple nuclei. Skeletal muscles can tighten or contract briefly or hold tension for longer periods.
Smooth Muscle
Smooth muscle contracts slowly and is associated with nearly every organ. It plays key roles in the function of the stomach, intestines, urinary bladder, uterus, respiratory organs and blood vessels. It can stretch and hold the stretch for long periods. Smooth muscles help regulate blood flow, digestion and breathing. These involuntary muscles do not have the striated appearance of skeletal muscle; their cells are shaped like spindles and each has one nucleus.
Cardiac Muscle
The heart consists of cardiac muscle, which is found nowhere else in the body. Its contractions provide the force that moves blood through the circulatory system. Cardiac muscle is not under voluntary control. Like smooth muscle it can stretch, and like skeletal muscle it can contract, but the contractions are short, quick events. This is why cardiac muscle is called a twitch muscle. Most cardiac muscle cells, sometimes called cardiocytes or cardiac myocytes, have one nucleus but they contain many mitochondria. These are needed to support the aerobic metabolism that cardiocytes depend on to keep the heart beating without fail over a lifetime.
Nerve-Muscle Interaction in the Human Muscular System
Muscles are controlled by nerve impulses. As nerve fibers approach a skeletal muscle, for example, they split into multiple branches. The end of each branch contacts a specific region of a muscle at a point called the motor end plate. It is similar to a synaptic connection between two neurons but in the muscular system, the connection is between a nerve ending and a muscle cell. A nerve impulse causes the nerve to release a neurotransmitter. This chemical, acetylcholine in the case of skeletal muscles, causes channels in the muscle cell to open. This leads to the influx of ions that initiate a series of biochemical events that produce muscle contraction.
