Muscle Tissue: Structure, Function, and Physiology

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is responsible for movement, posture, and vital body functions. There are three main types of muscle tissue, each with unique characteristics and locations in the body.

• Skeletal Muscle: Attached to bones and skin; elongated, multinucleate cells with striations; voluntary control.

• Cardiac Muscle: Found only in the heart; striated, branched cells; involuntary control.

• Smooth Muscle: Found in walls of hollow organs (e.g., blood vessels, digestive tract); non-striated; involuntary control.

Characteristics of Muscle Tissue

All muscle tissues share four main characteristics:

• Excitability: 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 after stretching.

Functions of Muscle Tissue

• Movement: Skeletal muscles move bones; cardiac muscle pumps blood; smooth muscle moves substances through organs.

• Posture and Body Position: Skeletal muscles maintain posture and stabilize joints.

• Stabilizing Joints: Muscles reinforce and stabilize joints.

• Heat Generation: Muscle contractions produce heat, important for maintaining body temperature.

Skeletal Muscle Structure

Connective Tissue Sheaths

Skeletal muscle fibers are organized and protected by connective tissue sheaths:

• Epimysium: Dense irregular connective tissue surrounding the entire muscle.

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

• Endomysium: Areolar connective tissue surrounding each individual muscle fiber.

Muscle Attachments

• Origin: Attachment to immovable (or less movable) bone.

• Insertion: Attachment to movable bone.

• Direct Attachment: Epimysium fused to periosteum of bone or perichondrium of cartilage.

• Indirect Attachment: Connective tissue wrappings extend beyond muscle as tendon or aponeurosis.

Microscopic Anatomy of Skeletal Muscle Fiber

• Sarcolemma: Plasma membrane of muscle cell.

• Sarcoplasm: Cytoplasm of muscle cell, contains glycosomes (glycogen storage) and myoglobin (O2 storage).

• Myofibrils: Rod-like elements that run parallel to muscle fiber; contain contractile elements.

• Sarcoplasmic Reticulum (SR): Smooth endoplasmic reticulum; stores and releases Ca2+.

• T Tubules: Invaginations of sarcolemma; conduct electrical impulses deep into muscle fiber.

Myofibrils and Sarcomeres

Myofibrils are composed of repeating units called sarcomeres, the functional contractile unit of muscle.

• Striations: Alternating dark (A) and light (I) bands due to arrangement of myofilaments.

• Sarcomere: Region between two Z discs; contains thick (myosin) and thin (actin) filaments.

Myofilaments

• Thick Filaments: Composed of myosin molecules; each myosin has a tail and two heads.

• Thin Filaments: Composed of actin, tropomyosin, and troponin.

• Z Disc: Coin-shaped sheet of proteins anchoring thin filaments.

• M Line: Line of protein myomesin holding adjacent thick filaments together.

Molecular Composition of Myofilaments

• Myosin: Thick filament; heads form cross bridges with actin during contraction.

• Actin: Thin filament; contains binding sites for myosin heads.

• Tropomyosin: Blocks myosin-binding sites on actin in relaxed muscle.

• Troponin: Binds Ca2+ and moves tropomyosin away from binding sites during contraction.

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic Reticulum (SR): Network of smooth endoplasmic reticulum; regulates intracellular Ca2+ levels.

• T Tubules: Extensions of sarcolemma; conduct impulses to deepest regions of muscle fiber.

• Triad: Grouping of one T tubule and two terminal cisterns of SR; important for excitation-contraction coupling.

Physiology of Skeletal Muscle Contraction

Sliding Filament Model of Contraction

Muscle contraction occurs as myosin heads bind to actin, pulling thin filaments toward the center of the sarcomere, shortening the muscle fiber.

• Z discs move closer together.

• I bands shorten.

• H zones disappear.

• A bands remain the same length.

Events at the Neuromuscular Junction

• Motor neurons stimulate skeletal muscle fibers at the neuromuscular junction (NMJ).

• Acetylcholine (ACh) is released from synaptic vesicles, diffuses across synaptic cleft, and binds to ACh receptors on sarcolemma.

• Binding of ACh opens ion channels, allowing Na+ to enter and K+ to exit, generating an end plate potential.

• Action potential spreads along sarcolemma and down T tubules.

Excitation-Contraction Coupling

Sequence of events by which transmission of an action potential along the sarcolemma leads to sliding of myofilaments.

1. Action potential travels along sarcolemma and down T tubules.

2. SR releases Ca2+ into cytosol.

3. Ca2+ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

4. Myosin heads bind to actin, initiating contraction cycle.

Muscle Fiber Contraction Cycle

1. Cross Bridge Formation: Myosin head attaches to actin.

2. Power Stroke: Myosin head pivots, pulling actin filament toward M line.

3. Cross Bridge Detachment: ATP binds to myosin, causing it to detach from actin.

4. Cocking of Myosin Head: ATP is hydrolyzed, re-energizing myosin head.

Muscle Metabolism and Fatigue

• ATP is required for muscle contraction and relaxation.

• Sources of ATP: stored ATP, creatine phosphate, anaerobic glycolysis, aerobic respiration.

• Muscle Fatigue: Physiological inability to contract; caused by ATP depletion, ionic imbalances, or lactic acid accumulation.

Clinical Correlations

Duchenne Muscular Dystrophy (DMD)

• Most common and severe form of muscular dystrophy.

• Inherited, sex-linked disorder; affects mostly boys.

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

• Progressive muscle weakness and loss of function.

Summary Table: Types of Muscle Tissue

Key Equations

• ATP Hydrolysis (for muscle contraction):

• Sliding Filament Theory (conceptual):

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.