Muscles and Muscle Tissue: Structure, Function, and Contraction

Muscle Terminology and Types

Muscle Terminology

The scientific study of muscle is called myology. Muscle fibers are synonymous with muscle cells. The word roots myo, mys, and sarco are commonly used in muscle-related terminology.

Three Types of Muscle Tissue

Muscle tissue is classified into three types: skeletal, cardiac, and smooth. These types differ in microscopic anatomy, location, and regulation by the endocrine and nervous systems.

• Skeletal muscle: Voluntary, striated, multinucleate, attached to bones.

• Cardiac muscle: Involuntary, striated, branched, forms heart wall.

• Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Microscopic Anatomy of Muscle Types

Skeletal, Cardiac, and Smooth Muscle Comparison

Each muscle type has distinct structural features observable under the microscope.

• Skeletal muscle: Long, cylindrical, striated, multinucleate.

• Cardiac muscle: Branched, striated, uninucleate, intercalated discs.

• Smooth muscle: Spindle-shaped, non-striated, uninucleate.

Characteristics and Functions of Muscle Tissue

Characteristics of Skeletal Muscle

Skeletal muscle is primarily attached to bones and is under voluntary control. It contracts quickly and fatigues easily, allowing for a wide range of force generation.

• Appearance: Long, cylindrical, striated, multinucleate.

• Function: Movement, posture, stabilization, thermogenesis.

Characteristics of Cardiac Muscle

Cardiac muscle forms the myocardium of the heart, contracts involuntarily, and exhibits autorhythmicity. It is striated and contains intercalated discs for cell-to-cell communication.

Characteristics of Smooth Muscle

Smooth muscle is found in the walls of hollow organs and structures. It contracts slowly and is non-fatigable, with a non-striated appearance.

Functions of Muscle Tissue

• Motion: External (walking, running) and internal (heartbeat, digestion).

• Posture: Maintains body posture.

• Stabilization: Stabilizes joints.

• Thermogenesis: Generates heat through contractions and shivering.

Functional Characteristics of Muscle Tissue

Key Properties

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten.

• Extensibility: Ability to stretch without damage.

• Elasticity: Ability to return to original shape.

• Conductivity: Ability to transmit excitation.

Connective Tissue Wrappings

Muscle Organization

Muscle fibers are wrapped and organized by several layers of connective tissue:

• Endomysium: Wraps individual muscle fibers.

• Perimysium: Groups fibers into fascicles.

• Epimysium: Surrounds the entire muscle.

• Deep fascia: Binds groups of muscles.

• Superficial fascia: Located beneath the skin.

Nerve and Blood Supply

Innervation and Vascularization

Each muscle fiber receives a branch of a motor nerve and is supplied by arteries and veins. Blood vessels branch extensively to provide direct supply to each fiber.

Muscle Attachments

Origin and Insertion

Skeletal muscles attach to bones or connective tissue at two points:

• Origin: Attached to stationary bone.

• Insertion: Attached to moving bone.

Attachment Structure

• Direct attachment: Epimysium fused to periosteum.

• Indirect attachment: Connective tissue forms tendons (cords) or aponeuroses (sheets).

Muscle Fiber Anatomy

Structure of Muscle Fibers

Muscle fibers are long, cylindrical, and multinucleate. The sarcolemma is the cell membrane, and the sarcoplasm is the cytoplasm, rich in myoglobin.

Myofibrils and Sarcomeres

Myofibrils are bundles of contractile protein filaments (myofilaments) arranged in parallel. The repeating unit of contraction is the sarcomere.

Organelles of Muscle Fibers

• Mitochondria: Provide ATP for contraction.

• Sarcoplasmic reticulum (SR): Stores Ca2+ ions for contraction.

Myofilaments and Sarcomere Structure

Types of Myofilaments

• Thin filaments: Actin, tropomyosin, troponin.

• Thick filaments: Myosin.

• Elastic filaments: Titin (connectin).

Sarcomere Bands and Lines

• Z discs: Boundary between sarcomeres, bisect I bands.

• A band: Overlap of thick and thin filaments.

• I band: Thin filaments only.

• H zone: Thick filaments only.

• M line: Anchors thick filaments.

Myosin and Actin Proteins

Myosin Protein

Myosin has a rod-like tail and two heads, each with ATPase and actin-binding sites. The heads point to the Z line, and tails to the M line. ATP hydrolysis powers the "ratchet" action.

Actin Protein

Actin filaments are composed of two helical strands of G actin, each with a myosin-binding site. Tropomyosin spirals around actin, and troponin regulates binding via Ca2+ sites.

Sarcoplasmic Reticulum and T-Tubules

Calcium Storage and Transmission

The sarcoplasmic reticulum (SR) stores Ca2+ ions. Transverse (T) tubules are inward folds of the sarcolemma that help transmit action potentials.

Muscle Fiber Triads

A triad consists of two terminal cisternae of the SR and one T tubule, facilitating rapid calcium release during contraction.

Neuromuscular Junction and Excitation

Structure and Function

The neuromuscular junction is where motor neurons communicate with muscle fibers. It consists of an axon terminal and a motor end plate.

Synapse and Neurotransmitter Release

The synaptic cleft separates the axon terminal and motor end plate. Synaptic vesicles release acetylcholine (ACh), which binds to receptors and opens ligand-gated Na+ channels, depolarizing the sarcolemma.

Action Potential and Muscle Contraction

Generation and Propagation

Depolarization opens voltage-gated Na+ channels, triggering an action potential that spreads across the sarcolemma and down T tubules.

Repolarization and Refractory Period

Repolarization occurs as voltage-gated Na+ channels close and K+ channels open. The refractory period is when the muscle fiber cannot respond to new stimuli.

Excitation-Contraction Coupling

Mechanism

The action potential triggers DHP receptors on T-tubules, which activate ryanodine receptors on the SR, releasing Ca2+ ions. This initiates contraction.

Sliding Filament Model

Muscle Shortening

Thin and thick filaments slide past each other, shortening each sarcomere and myofibril, resulting in muscle contraction.

Calcium Movements and Cross Bridge Cycling

Role of Calcium

Ca2+ released from the SR binds to troponin, shifting tropomyosin and exposing actin binding sites for myosin. Ca2+ pumps remove calcium when contraction ends.

Cross Bridge Cycle

The cross bridge cycle involves myosin binding to actin, performing a power stroke, releasing ADP and Pi, detaching with new ATP, and "cocking" the myosin head for the next cycle.

Destruction of Acetylcholine and Clinical Relevance

Acetylcholinesterase

Acetylcholinesterase rapidly breaks down ACh in the neuromuscular junction, preventing continuous excitation. Disorders such as myasthenia gravis and toxins like curare affect ACh function.

Muscle Contraction Summary

Extent of Contraction

One power stroke shortens a muscle by about 1%. Normal contraction shortens a muscle by about 35%. Cross bridge cycles repeat until Ca2+ levels decrease.

Rigor Mortis

Postmortem Muscle Contraction

After death, Ca2+ leaks from the SR, causing actin and myosin binding and muscle contraction (rigor mortis), which lasts about 24 hours before tissue disintegration.

Summary Table: Muscle Types Comparison

Muscle Type	Control	Appearance	Location	Fatigue
Skeletal	Voluntary	Striated, multinucleate	Bones	Fatigable
Cardiac	Involuntary	Striated, branched, intercalated discs	Heart	Non-fatigable
Smooth	Involuntary	Non-striated, spindle-shaped	Hollow organs	Non-fatigable

Additional info: Expanded explanations and diagrams were added for clarity and completeness.