Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, stability, and various physiological functions. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle, each with distinct structural and functional characteristics.

• Skeletal Muscle: Long, multinucleated, striated fibers; voluntary control; attached to bones; responsible for movement.

• Cardiac Muscle: Short, branched, uni- or binucleated cells; striated; involuntary; found in the heart; responsible for pumping blood.

• Smooth Muscle: Short, spindle-shaped, uninucleated cells; non-striated; involuntary; found in walls of hollow organs; responsible for various involuntary movements.

Additional info: All muscle tissues generate tension, but differ in their control mechanisms and locations.

Major Functions and Characteristics of Muscle Cells

Muscle cells, or myocytes, share several key properties that enable their function:

• Contractility: Ability of protein fibers within myocytes to draw together, producing force.

• Excitability: Ability to respond to electrical or chemical stimuli.

• Conductivity: Ability to conduct electrical impulses along the cell membrane.

• Extensibility: Ability to stretch up to three times their resting length.

• Elasticity: Ability to return to original shape after stretching.

Structure of Muscle Cells

Muscle Cell Terminology

Muscle cells have specialized structures and terminology:

• Sarcoplasm: The cytoplasm of a muscle cell.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasmic Reticulum (SR): Modified endoplasmic reticulum forming a network around myofibrils; stores and releases calcium ions.

• Myofibril: Cylindrical organelles composed of myofilaments; responsible for contraction.

Additional info: Mitochondria and nuclei are abundant in skeletal muscle fibers due to high energy demands and multinucleation.

Skeletal Muscle Organization

Arrangement of Skeletal Myocytes and Tissues

Skeletal muscle is organized into hierarchical structures:

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Groups muscle fibers into fascicles.

• Epimysium: Encloses the entire muscle; continues as tendon.

• Tendon: Connects muscle to bone.

Example: The biceps brachii muscle is composed of multiple fascicles, each containing bundles of muscle fibers.

Structure of the Skeletal Muscle Fiber

Internal Components

Skeletal muscle fibers contain specialized structures for contraction:

• Transverse Tubules (T-tubules): Deep extensions of the sarcolemma; surround each myofibril and conduct electrical impulses.

• Terminal Cisternae: Enlarged sections of the SR; flank each T-tubule and store calcium ions.

• Triad: Consists of two terminal cisternae and one T-tubule; essential for excitation-contraction coupling.

Myofibrils and Myofilaments

Types of Myofilaments

Myofibrils are composed of three types of myofilaments:

• Thick Filaments: Bundles of myosin protein; responsible for force generation.

• Thin Filaments: Composed of actin, tropomyosin, and troponin; interact with myosin during contraction.

• Elastic Filaments: Made of titin; provide structural support and resist excessive stretching.

Additional info: The arrangement of these filaments forms the basis of muscle contraction.

Arrangement in Sarcomeres

The sarcomere is the functional contractile unit of muscle fibers, defined by the region between two Z-discs. It contains distinct bands and zones:

• I Band: Contains only thin filaments; appears light.

• A Band: Contains both thick and thin filaments; appears dark.

• H Zone: Central region of A band with only thick filaments.

• M Line: Middle of A band; anchors thick filaments.

• Z Disc: Anchors thin and elastic filaments; defines sarcomere boundaries.

Example: During contraction, actin filaments slide along myosin, pulling Z-discs closer together and shortening the sarcomere.

Sliding Filament Mechanism of Contraction

Mechanism Overview

Muscle contraction occurs via the sliding filament mechanism:

• Actin (thin) filaments slide past myosin (thick) filaments toward the M line.

• Z-discs are pulled closer together, shortening the sarcomere and generating tension.

• The A band remains constant, while the I band and H zone decrease in width.

Additional info: ATP and calcium ions are essential for this process.

Membrane Potential in Skeletal Myocytes

Definition and Generation

Membrane potential refers to the electrical potential difference across the plasma membrane (sarcolemma) of a cell. In skeletal myocytes, the resting membrane potential is typically -85 mV.

• Electrical Gradient: Separation of charged particles across the membrane.

• Voltage: Difference in electrical potential between two points.

• Resting Membrane Potential: Maintained by ion channels and pumps, especially the sodium-potassium pump.

Additional info: The sodium-potassium pump uses ATP to move 3 Na+ ions out and 2 K+ ions in, maintaining the negative resting potential.

Equation:

Summary Table: Muscle Tissue Types

The following table summarizes the main features of skeletal, cardiac, and smooth muscle tissue: