7.1 Muscle Tissue Types and Structure

Muscle tissue comes in three flavors: skeletal, cardiac, and smooth. Each type has its own special features that help it do its job. Skeletal muscle moves your bones, cardiac muscle pumps your heart, and smooth muscle works in your organs.

The smallest part of a muscle is called a sarcomere. These tiny units stack up to form bigger structures, like muscle fibers and whole muscles. Understanding how these parts work together is key to grasping how muscles function in your body.

Muscle Tissue Types

Structural and Functional Characteristics

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• Skeletal, cardiac, and smooth muscle tissues have distinct structural and functional properties that enable them to perform specific roles in the body
• Skeletal muscle is striated, multinucleated, and under voluntary control, generating force and movement of the skeleton (biceps, quadriceps)
• Cardiac muscle is striated, uninucleated, and under involuntary control, found exclusively in the heart and responsible for pumping blood throughout the body
• Smooth muscle is non-striated, uninucleated, and under involuntary control, found in the walls of hollow organs (stomach, intestines) and blood vessels, responsible for various functions such as peristalsis and vasoconstriction

Cellular Differences

• Skeletal muscle fibers are long, cylindrical cells with multiple peripherally located nuclei
• Cardiac and smooth muscle cells are typically shorter and have centrally located nuclei
• Skeletal muscle contraction is rapid and powerful, while cardiac muscle contraction is rhythmic and sustained, and smooth muscle contraction is slow and sustained

Skeletal Muscle Organization

Hierarchical Structure

• Skeletal muscle is organized in a hierarchical manner, with each level contributing to the overall structure and function of the muscle
• The smallest functional unit is the sarcomere, composed of thick (myosin) and thin (actin) filaments arranged in a repeating pattern
• Sarcomeres are organized end-to-end to form myofibrils, the contractile units of muscle cells
• Multiple myofibrils are bundled together within the sarcoplasm of a single muscle cell, called a muscle fiber

Connective Tissue Layers

• Muscle fibers are surrounded by the endomysium and grouped together into bundles called fascicles
• Fascicles are surrounded by the perimysium, and multiple fascicles are grouped together to form the whole muscle
• The entire muscle is enclosed by the epimysium, which separates it from surrounding tissues
• Connective tissue layers contain blood vessels and nerves that supply the muscle fibers with oxygen, nutrients, and stimulation for contraction

Sarcomere Function

Structure and Composition

• The sarcomere is the smallest contractile unit of skeletal muscle, essential for muscle contraction
• Sarcomeres are composed of thick filaments made of myosin and thin filaments made of actin, arranged in a repeating pattern of light (I bands) and dark (A bands) bands
• The M line anchors the thick filaments in the center of each sarcomere, while the Z lines anchor the thin filaments and define the boundaries of the sarcomere

Sliding Filament Mechanism

• During muscle contraction, the thick and thin filaments slide past each other, causing the sarcomere to shorten
• The shortening of sarcomeres in series along a myofibril results in the shortening of the entire muscle fiber, generating force and movement
• The length-tension relationship of sarcomeres determines the force-generating capacity of the muscle, with optimal force generated when sarcomeres are at their resting length and decreased force at shorter or longer lengths

Proteins in Muscle Contraction

Key Proteins and Their Functions

• Myosin, the primary protein in thick filaments, consists of a long tail and two globular heads that bind to actin and use ATP to generate force and movement during contraction
• Actin, the primary protein in thin filaments, exists as G-actin (globular) and F-actin (filamentous), with F-actin forming the backbone of the thin filament and providing binding sites for myosin
• Tropomyosin, a long, thin protein, winds around the F-actin filament and covers the myosin-binding sites on actin in the absence of calcium
• Troponin, a complex of three proteins (troponin C, I, and T), is associated with tropomyosin and regulates the exposure of myosin-binding sites on actin in response to calcium binding

Structural Support Proteins

• Titin, a large, elastic protein, extends from the Z line to the M line in the sarcomere, providing structural support and maintaining the organization of the thick and thin filaments
• Nebulin, a long, thin protein, runs along the length of the thin filament and regulates the assembly and length of the actin filaments