Muscle cells are specialized to contract. The cells are similar in structure to the other cells in the body but are elongated and, for this reason, are called muscle fibers. They consist mostly of water (about 75%), proteins and inorganic salts. There are around 700 different muscles in the human body and more than 250 million muscle fibers.
The body contains three distinct types of muscle (skeletal, cardiac and smooth), which will be outlined in this article. The following three articles in the series will address the structure and function of skeletal muscle, the largest muscle mass in the body.
Muscles are excitable or irritable. This means that they are capable of receiving and responding to a stimulus. The stimulus is usually a chemical – a neurotransmitter released by a nerve cell, a hormone or a local change in acidity (pH). In response, muscles generate an electrical impulse that causes the muscle cells to contract:
– Muscles can contract or shorten when they are stimulated. No other type of body tissue can do this.
– Muscles are extensible – that is, they can stretch or extend beyond their resting length.
– Muscle fibers are elastic and can return to their resting length after being stretched.