Function of the Muscular System

There are three main types of muscles in the human body. Skeletal muscles are responsible for movement, stability, heat production, and the regulation of blood glucose levels; cardiac muscles are responsible for pumping blood throughout the body; and smooth muscles are responsible for involuntary movements of internal organs.

Skeletal muscles work together with the skeletal system to support and move the body. Skeletal muscles are composed of striated muscle tissues under control of the somatic, or voluntary, nervous system. In addition to moving the body, skeletal muscles can produce heat through rapid contractions or shivering. Skeletal muscles also regulate blood glucose levels through glucose uptake.

Cardiac muscles are the muscles responsible for the pumping action of the heart. Cardiac muscle is striated like skeletal muscle; but unlike skeletal muscle, it is not under voluntary control. It is controlled by the autonomic, or involuntary, nervous system. Cardiac muscle tissue is also called myocardium and is made of cells called cardiomyocytes. A cardiomyocyte is a muscle cell that is found in cardiac muscle and generates the heart contractions. Cardiomyocytes are connected through junctions, called intercalated discs. These junctions allow the electrical activation that causes all cardiomyocytes to contract as a single unit.

Smooth muscle tissue is also controlled by the autonomic nervous system. Smooth muscle is responsible for the involuntary movement of internal organs. For example, movements of the stomach, intestines, and bladder are all caused through smooth muscle contractions. The walls of blood vessels also contain smooth muscles. Most smooth muscle is organized into tightly packed sheets. This allows it to be rolled into tubes and stretched out to form the walls of just about every hollow organ of the respiratory, reproductive, urinary, and digestive systems. The sheets usually come in pairs, with the muscle fibers positioned at right angles to each other. The first layer, called the longitudinal layer, has muscle fibers that run parallel to the long axis of the organ. Their contraction forces the muscle to shorten in length. The circular layer has fibers that go around the outside area of the organ. When the circular layer contracts, the inside cavity of the organ gets smaller. Smooth muscle contractions help the body expel wastes from the urinary system, the digestive system, and the respiratory system.

Similar to skeletal muscle, cardiac muscle has striations and contracts when the filaments slide past each other. However, cardiac cells have a different appearance than skeletal or smooth muscle cells, in that they interconnected, short, and fat. They also have many branches coming off of them. The number of nuclei also differs, having one or two of them. Some cardiac muscle cells can contract on their own, without any neural input. The contraction is also all or nothing. Either all of the heart muscle cells contract at the same time, or none of them contract at all.