05 The Muscular System

Muscle tissue and its functions

The muscular system produces movement, maintains posture, stabilizes joints, and generates heat. It works with the nervous system, which controls and adjusts muscle activity, and with the skeletal system, which provides the framework and joints that many muscles move.

Three types of muscle tissue

• Skeletal muscle is usually attached to bones by tendons. It is mostly voluntary and moves the skeleton, maintains posture, helps produce heat, and supports actions such as breathing and swallowing.

• Cardiac muscle forms the wall of the heart. It is involuntary and contracts rhythmically to pump blood. Its electrically connected cells help coordinate contractions.

• Smooth muscle is found in the walls of hollow organs and blood vessels. It is involuntary, moves substances through organs, and adjusts the diameter of passageways such as blood vessels.

Skeletal and cardiac muscle cells have visible stripes called striations because their contractile proteins are arranged in repeating patterns. Smooth muscle lacks this striped appearance. All three types use actin, myosin, calcium, and ATP to produce contraction, though their control and cellular mechanisms differ.

How skeletal muscle contracts

A skeletal muscle contains bundles of long muscle cells called fibers. Within each fiber, repeating units called sarcomeres contain thin actin filaments and thick myosin filaments. During contraction, myosin pulls actin inward, making the filaments slide past one another. The filaments themselves do not shorten; sarcomeres shorten, and their combined action can shorten the muscle fiber.

Contraction begins when a motor neuron signals a muscle fiber at a neuromuscular junction. The neuron releases acetylcholine, a chemical messenger that triggers an electrical signal in the muscle fiber. That signal causes calcium to be released inside the fiber. Calcium exposes binding sites on actin, allowing myosin to attach and pull. Myosin uses energy from ATP to repeat this pull-and-release cycle.

When the signal stops, calcium is taken back into storage, binding sites are covered again, and the fiber relaxes. Thus, the sequence of signaling, calcium release, and ATP-powered interactions between actin and myosin produces skeletal muscle contraction.

Muscles create and control movement

A muscle can produce force without visibly shortening. Holding a heavy bag still, for example, requires muscle tension even though the arm’s position stays nearly unchanged. In a shortening contraction, such as lifting the bag, the muscle pulls a bone and moves a joint.

Most skeletal muscles attach to bones through tendons. When a muscle contracts, force passes through its connective tissue and tendon to the bone. Because bones meet at joints, this pull produces movement around a joint. Muscles generally pull rather than push, so opposite movements usually require muscles with opposing actions.

Coordinated muscle groups

Muscles often work in coordinated groups. The agonist, or prime mover, produces the main force for an action; a synergist assists it; an antagonist produces an opposing action and can control or slow movement; and a fixator stabilizes a body part so another muscle can act effectively.

When bending the elbow to lift a cup, the biceps brachii is a prime mover and the brachialis assists it. The triceps brachii, which straightens the elbow, acts as an antagonist. To lower the cup smoothly, the nervous system adjusts the activity of these muscles rather than simply switching one entirely off.

Nervous system control and feedback

The somatic nervous system controls skeletal muscle. The brain and spinal cord plan and coordinate movement, while motor neurons carry signals to muscle fibers. A motor neuron and the fibers it activates form a motor unit. Activating more motor units, or sending signals more frequently, generally increases the force a muscle can produce.

Movement also depends on sensory feedback. Receptors in muscles, tendons, and joints provide information about stretch, tension, and body position. The nervous system uses this feedback to adjust muscle activity, helping maintain balance and posture and support smooth, accurate movement. Some responses, such as a withdrawal reflex, can be coordinated through the spinal cord before the brain processes the sensation in detail.

Muscles work across body systems

Muscular action depends on several systems working together. The nervous system sends commands and receives feedback; the skeletal system provides bones, joints, and attachment points; and the cardiovascular and respiratory systems supply oxygen and nutrients and remove waste products. Muscle contraction also releases heat, which contributes to body-temperature regulation.

Walking illustrates this coordination: the nervous system coordinates alternating muscle activity, leg muscles pull on bones across joints, and the circulatory and respiratory systems support the increased energy demand. No single system produces the movement on its own.

• sarcomere