Muscles and Muscle Tissue

Introduction

Muscles are essential organs that make up nearly half of the body's mass and are responsible for movement, posture, joint stabilization, and heat generation. Muscle tissue transforms chemical energy (ATP) into mechanical energy, enabling forceful contractions.

Types of Muscle Tissue

There are three primary types of muscle tissue, each with distinct characteristics and functions.

• Skeletal Muscle: Attached to bones or skin, responsible for voluntary movements. Skeletal muscle fibers are long, cylindrical, multinucleate, and striated. They contract rapidly but tire easily and are powerful.

• Cardiac Muscle: Found only in the heart, responsible for pumping blood. Cardiac muscle fibers are branching chains of cells, uni- or binucleate, striated, and involuntary. They contract at a steady rate due to the heart's pacemaker.

• Smooth Muscle: Located in the walls of hollow organs (e.g., stomach, bladder, airways). Smooth muscle fibers are spindle-shaped, uninucleate, non-striated, and involuntary.

Muscle Terminology

• Prefixes myo-, mys-, and sarco- refer to muscle (e.g., sarcoplasm is muscle cell cytoplasm).

• Muscle fibers are elongated cells found in skeletal and smooth muscle.

Characteristics of Muscle Tissue

All muscle tissues share four main properties:

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

Functions of Muscle Tissue

Muscles perform four vital functions:

1. Produce Movement: Responsible for locomotion and manipulation (e.g., walking, digesting, pumping blood).

2. Maintain Posture and Body Position: Stabilizes the body during activity.

3. Stabilize Joints: Reinforces and supports joints.

4. Generate Heat: Muscle contraction produces heat, helping maintain body temperature.

Skeletal Muscle Structure

Gross Structure of Skeletal Muscle

Skeletal muscle is an organ composed of muscle fibers, nerves, blood vessels, and connective tissues.

• Nerves and Blood Supply: Each muscle receives a nerve, artery, and veins. Nerves control activity, and blood vessels supply oxygen and nutrients while removing waste.

• Connective Tissue Sheaths: Support and reinforce muscle. Three layers:

  • Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.

  • Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).

  • Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

• Attachments: Muscles attach to bones or other structures via:

  • Direct (fleshy) attachments: Epimysium fused to periosteum of bone or perichondrium of cartilage.

  • Indirect attachments: Connective tissue wrappings extend beyond muscle as tendons (ropelike) or aponeuroses (sheetlike).

Structure and Organizational Levels of Skeletal Muscle

Microscopic Structure of Skeletal Muscle Fibers

Skeletal Muscle Fiber Anatomy

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm, containing glycosomes (for glycogen storage) and myoglobin (for O2 storage).

• Specialized structures: myofibrils, sarcoplasmic reticulum, and T tubules.

Myofibrils

• Densely packed, rodlike elements; a single muscle fiber can contain thousands.

• Account for ~80% of muscle cell volume.

• Features:

  • Striations: Alternating dark (A bands) and light (I bands) regions.

  • Sarcomeres: Smallest contractile unit, aligned end to end along myofibril.

  • Myofilaments: Actin (thin) and myosin (thick) filaments arranged within sarcomere.

Striations and Sarcomere Structure

• A bands: Dark regions; contain H zone (lighter region in middle) and M line (protein myomesin).

• I bands: Light regions; contain Z disc (coin-shaped sheet of proteins).

• Sarcomere: Area between Z discs; contains A band with half of an I band at each end.

Myofilaments

• Actin myofilaments: Thin filaments; extend across I band and partway in A band, anchored to Z discs.

• Myosin myofilaments: Thick filaments; extend length of A band, connected at M line.

• Sarcomere cross section: One thick filament surrounded by six thin filaments (hexagonal arrangement).

Molecular Composition of Myofilaments

• Thick filaments: Composed of protein myosin (two heavy and four light polypeptide chains). Heavy chains form myosin tail; light chains form globular head. During contraction, heads link thick and thin filaments, forming cross bridges.

• Thin filaments: Composed of fibrous protein actin. Actin is made up of G actin (globular) subunits, which bear active sites for myosin head attachment. G actin subunits link to form F actin (filamentous); two F actin strands twist together. Regulatory proteins tropomyosin and troponin are bound to actin.

Structure and Organizational Levels of Skeletal Muscle (Microscopic)

Clinical Note: Duchenne Muscular Dystrophy (DMD)

• DMD is a muscle-destroying disease, most common and serious form of muscular dystrophies.

• Inherited as a sex-linked recessive disease, almost exclusively in males.

• Caused by defective gene for dystrophin, a protein linking thin filaments to extracellular matrix and stabilizing sarcolemma.

• Symptoms: Clumsiness, frequent falls, progressive muscle weakness, loss of regenerative capacity, increased apoptosis, decreased muscle mass.

• No cure; corticosteroids provide temporary relief. Gene therapy approved in 2023.

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic Reticulum (SR)

• Network of smooth endoplasmic reticulum tubules surrounding each myofibril.

• Most run longitudinally; terminal cisterns form perpendicular cross channels at A–I band junction.

• Functions in regulation of intracellular Ca2+ levels; stores and releases Ca2+.

T Tubules

• Tubular extensions of sarcolemma penetrating deep into cell interior.

• Increase muscle fiber’s surface area; lumen continuous with extracellular space.

• Allow electrical nerve transmissions to reach deep into interior of each muscle fiber.

• Penetrate at each A–I band junction between terminal cisterns; form triads (terminal cistern of one sarcomere, T tubule, terminal cistern of neighboring sarcomere).

Triad Relationships

• T tubule proteins act as voltage sensors, changing shape in response to electrical current.

• SR integral proteins control opening of calcium channels in SR cisterns.

• Electrical impulse causes T tubule proteins to change shape, triggering SR proteins to release calcium into cytoplasm.

Summary Table: Comparison of Muscle Types