6.2. The muscular system

Bones would be virtually useless if there were no muscles. However, only some muscles move bones. Others assist in circulating blood and in digesting food. The body has more than 600 muscles, accounting for about 40% of the body weight of a healthy person.

Functions of muscles

A muscle is an organ made up of many muscle cells. Muscles attached to bones cause movement at joints. Some muscles are always working in the body whether a person is conscious of this effort or not. For example, the hearts beats and the eyelids open and close. Though movement is the chief function of muscles, they also protect some internal organs. Additionally, sitting and standing require some muscles to be active.

Types of muscles

Muscle issue is made of special cells that have the ability to contract and relax. Three types of muscle tissue make up the muscular system: skeletal, smooth, and cardiac. Each differs in structure and task.

Skeletal muscle

Muscles that move bones are called skeletal muscles. They attach to bones either directly or by means of strong bands of no elastic connective tissue called tendons. Because skeletal muscles are generally under a person’s conscious control, they are also called voluntary muscles. However, they sometimes move without conscious control, such as when responding to danger.

Muscle cells are called muscle fibers. Skeletal muscle fibers have a long tapering shape. Each fiber contains many nuclei and 1000 to 2000 full-length protein threads called myofibrils. Tiny units called sarcomeres can be seen forming bands across the myofibrils. These units lie in single file in a way that gives myofibrils a striped, or striated, appearance. For this reason skeletal muscle is also called striated muscle.

Smooth muscle

Smooth muscle is made up of spindle-shaped cells with one nucleus each. Most smooth muscles protect organs of the digestive, respiratory, and circulatory systems. Smooth muscles are not under conscious control, so they are called involuntary muscles. They do not respond as quickly as voluntary muscles, but they do not tire as easily.

Cardiac muscles

Cardiac muscle is involuntary, and found only in the heart. Unlike other types of muscle, cardiac muscle does not receive impulses from the nervous system. Instead, the heart has its own regulator – the sinoatrial node that cause the muscle cells to contract.

How muscles cause movement

When a skeletal muscle contracts, there is creating a pulling action that results in movement. Muscles can only pull, they cannot push. For this reason, muscles work in opposing pairs. A muscle pair is termed antagonistic if, for example, the contraction of one muscle bends a joint and the contraction of the other straightens the joint. A muscle that bends a joint is called a flexor, and a muscle that straightens a joint is called an extensor.

Most skeletal muscles are attached to 2 bones. During contraction one bone serves as an anchor. The point at which the muscle is attached to the anchoring bone is the origin. The point at which the muscle is attached to the moving bone is the insertion. Between these 2 points is a joint. Contraction of a muscle thus causes movement at the joint.

Two muscles of the upper arm – the biceps and the triceps – illustrate how antagonistic muscles produce movement. The biceps has its origin at the shoulder and its insertion on the radius, a bone of the forearm. When the biceps contracts, then the forearm is drawn toward the front of the shoulder. Of no antagonistic muscle opposed the biceps, the arm would remain bent. However, the triceps on the back of the upper arm has its origin on the humerus of the upper arm and its insertion on the ulna. When the triceps contracts, then it straightens the arm.