4 The Muscular System

What muscle tissue does

Muscle tissue produces force by contracting. It supports body movement, helps maintain posture, moves substances through internal organs, and pumps blood through the heart. The body has three types of muscle tissue: skeletal, smooth, and cardiac. They share the ability to contract but differ in structure, location, and control.

The three types of muscle

Skeletal muscle cells are long, cylindrical fibers with many nuclei and visible striations. Skeletal muscle is usually attached to bones and is mostly under voluntary control. It moves the skeleton, maintains posture, and produces heat.

Smooth muscle cells are spindle-shaped, have one nucleus, and lack visible striations. They occur in the walls of many hollow organs and blood vessels and are involuntary. Smooth muscle moves substances, such as food through the digestive tract, and adjusts the diameter of passageways.

Cardiac muscle cells are branched and striated, usually have one nucleus, and connect through intercalated discs. Found in the heart wall, they contract rhythmically to pump blood.

Skeletal muscle generally contracts in response to signals from motor neurons. Smooth and cardiac muscle are involuntary; their activity is regulated by factors including electrical activity, hormones, and the autonomic nervous system. Intercalated discs link cardiac cells and help the heart muscle act as a coordinated unit.

Skeletal muscle structure

A skeletal muscle is an organ containing muscle fibers, connective tissue, blood vessels, and nerves. Its connective-tissue coverings continue into tendons, which attach muscle to bone and transmit pulling force.

The organization is nested:

Whole muscle → fascicles → muscle fibers → myofibrils → sarcomeres → myofilaments

A fascicle is a bundle of muscle fibers. A muscle fiber is a single, elongated muscle cell; its cell membrane is the sarcolemma, and its cytoplasm is the sarcoplasm. Myofibrils run through a muscle fiber and consist of repeating contractile units called sarcomeres.

Each sarcomere contains thin actin filaments and thick myosin filaments. Their repeating arrangement produces the striated appearance of skeletal and cardiac muscle. During contraction, a sarcomere shortens as its thin filaments slide past its thick filaments; the filaments themselves do not significantly shorten.

How skeletal muscle contracts

Skeletal muscle contraction follows a sequence called excitation–contraction coupling: an electrical signal leads to the chemical events that make the muscle fiber contract.

1. A motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction, exciting the muscle fiber.

2. An electrical impulse spreads along the sarcolemma and down inward extensions called T-tubules.

3. The impulse triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.

4. Calcium binds to troponin, shifting tropomyosin and exposing myosin-binding sites on actin.

5. Myosin heads bind to actin and pull the thin filaments toward the center of the sarcomere. This pulling action is the power stroke.

6. ATP allows myosin to detach from actin and prepares the myosin head for another cycle. Repeated cross-bridge cycles generate force and shorten the sarcomere.

When stimulation ends, calcium is pumped back into the sarcoplasmic reticulum. Tropomyosin again blocks the binding sites, and the muscle fiber relaxes. ATP is needed both for cross-bridge cycling and for calcium reuptake.

Contraction in smooth and cardiac muscle

Smooth and cardiac muscle also use actin–myosin interactions, but their contraction is regulated differently from skeletal muscle.

In smooth muscle, calcium binds to calmodulin, which activates enzymes that enable myosin to interact with actin. Smooth muscle does not use troponin to regulate contraction. Its contractions are often slower and sustained, as in the contractions that propel material through the digestive tract.

In cardiac muscle, electrical excitation raises calcium inside heart muscle cells. Calcium entering the cell helps trigger additional calcium release from the sarcoplasmic reticulum, supporting contraction. Electrical connections between cells help coordinate the heartbeat.

Muscle force and movement

Muscles produce force by pulling; they do not push. Skeletal muscles usually act across joints and attach to bones by tendons. When a muscle contracts, its pull can move one bone relative to another.

Muscles commonly work in groups. The agonist, or prime mover, produces a movement. The antagonist produces the opposing movement or controls it. Synergists assist the agonist, while fixators stabilize a body part. When bending the elbow to lift an object, the biceps brachii is a major agonist and the triceps brachii acts as an antagonist.

A muscle can generate force in different ways:

• In a concentric contraction, the active muscle shortens, as when lifting a cup.

• In an eccentric contraction, the active muscle lengthens while resisting a load, as when lowering the cup under control.

• In an isometric contraction, the muscle produces tension without appreciably changing length, as when holding the cup still.

These actions allow muscles to create movement, control movement, and maintain position.