Muscle Tissue

Types of Muscle Tissue

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Each type has distinct structural and functional characteristics.

• Skeletal Muscle: Attached to bones or skin, responsible for voluntary movements.

• Cardiac Muscle: Found only in the heart, responsible for pumping blood.

• Smooth Muscle: Located in walls of hollow organs (except the heart), responsible for involuntary movements.

Key Prefixes: Myo-, Mys-, Sarco- are commonly used in muscle terminology (e.g., sarcoplasm, myocyte).

Characteristics & Functions of Muscle Tissue

Muscle tissue exhibits several unique properties and serves essential functions in the body.

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly.

• Extensibility: Ability to stretch.

• Elasticity: Ability to recoil after stretching.

Functions:

• Movement

• Posture and position

• Stability

• Thermogenesis (heat production)

Skeletal Muscle Anatomy

Structure and Organization

Skeletal muscle is organized into hierarchical structures, each surrounded by connective tissue sheaths.

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds fascicles (bundles of muscle fibers).

• Endomysium: Surrounds individual muscle fibers.

Attachments

Muscles attach to bones via tendons or aponeuroses. The origin is the fixed attachment, while the insertion moves during contraction.

Microanatomy of Skeletal Muscle

Muscle Fiber Structure

Each muscle fiber is a multinucleated cell containing specialized organelles and structures.

• Sarcolemma: Plasma membrane of the muscle cell.

• Sarcoplasm: Cytoplasm of the muscle cell, rich in glycosomes and myoglobin.

• Myofibrils: Rod-like structures responsible for contraction.

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions.

• T-Tubules: Invaginations of the sarcolemma that help transmit action potentials.

Striations and Bands

Striations are alternating dark (A band) and light (I band) regions seen in skeletal muscle fibers, resulting from the arrangement of myofilaments.

• I band: Isotropic, light band.

• A band: Anisotropic, dark band.

Sarcomere Structure

Organization of Myofilaments

The sarcomere is the functional unit of muscle contraction, defined by the region between two Z-disks.

• Actin (thin filament): Anchored to Z-disk.

• Myosin (thick filament): Anchored to M-line.

• Titin: Elastic filament contributing to extensibility and recoil.

• H-zone: Region with only thick filaments.

• M-line: Center of the sarcomere.

Molecular Composition of Myofilaments

Myofilaments are composed of contractile and regulatory proteins.

• Thick Filament: Made of myosin molecules with heads that bind actin and ATP.

• Thin Filament: Made of actin, troponin, and tropomyosin.

• Troponin Subunits:

  • TnI: Inserts on actin.

  • TnT: Ties to tropomyosin.

  • TnC: Binds calcium.

Sarcoplasmic Reticulum and T-Tubules

Calcium Storage and Release

The sarcoplasmic reticulum (SR) stores calcium ions and releases them during muscle contraction. T-tubules help transmit action potentials deep into the muscle fiber.

• Terminal cisternae: Enlarged regions of SR adjacent to T-tubules.

• Triad: Structure formed by a T-tubule flanked by two terminal cisternae.

Sliding Filament Model of Contraction

Mechanism of Muscle Contraction

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

• Cross Bridge Formation: Myosin head binds to actin.

• Power Stroke: Myosin head pivots, pulling actin.

• Detachment: ATP binds to myosin, causing it to release actin.

• Cocking: ATP hydrolysis re-energizes myosin head.

Muscle Contraction: The Big Picture

Steps in Skeletal Muscle Contraction

Four main steps are required for skeletal muscle contraction:

1. Events at the neuromuscular junction

2. Muscle fiber excitation

3. Excitation-contraction coupling

4. Cross bridge cycling

Neuromuscular Junction (NMJ)

The NMJ is the site where a motor neuron communicates with a muscle fiber. Acetylcholine (ACh) is released, binds to receptors, and initiates an action potential in the muscle fiber.

• Action potential arrives at axon terminal.

• Ca2+ enters, triggering ACh release.

• ACh binds to receptors, opening Na+ channels.

• End plate potential is generated.

• Acetylcholinesterase degrades ACh.

Excitation-Contraction Coupling

Excitation-contraction coupling links the action potential to muscle contraction via calcium release from the SR.

• Action potential travels along sarcolemma and T-tubules.

• Ca2+ is released from SR.

• Ca2+ binds to troponin, shifting tropomyosin and exposing myosin-binding sites.

• Cross bridge cycle begins.

Energy for Muscle Contraction

ATP and Muscle Contraction

ATP is essential for muscle contraction, powering cross bridge cycling, calcium pumping, and restoring ionic gradients.

• ATP is depleted in 4-6 seconds and must be regenerated.

• Three mechanisms for ATP regeneration:

  • Direct phosphorylation by creatine phosphate (CP)

  • Anaerobic glycolysis (produces lactic acid)

  • Aerobic respiration (produces CO2, H2O, and ATP)

Equation for Aerobic Respiration:

Muscle Contraction Types and Responses

Types of Contraction

Muscle contractions can be classified as isometric (no change in length) or isotonic (change in length).

• Isotonic: Concentric (shortening) and eccentric (lengthening).

• Isometric: Tension increases, but muscle does not shorten.

Motor Units and Muscle Twitch

A motor unit consists of a motor neuron and all the muscle fibers it innervates. A muscle twitch is a single contraction event with three phases: latent, contraction, and relaxation.

• Latent period: EC coupling, no tension.

• Contraction period: Cross bridge formation, tension increases.

• Relaxation period: Ca2+ reenters SR, tension drops.

Graded Muscle Responses

Muscle responses can be graded by changing the frequency or strength of stimulation.

• Temporal Summation: Increased frequency leads to tetanus.

• Recruitment: Increased strength activates more motor units.

Clinical Considerations

Duchenne Muscular Dystrophy (DMD)

DMD is a genetic disorder caused by a defective dystrophin gene, leading to progressive muscle weakness.

Rigor Mortis

Rigor mortis occurs postmortem due to increased intracellular calcium and lack of ATP, resulting in sustained muscle contraction.

Disuse Atrophy

Muscles must be active to remain healthy. Disuse atrophy results from immobilization or loss of neural stimulation, leading to muscle degeneration.

Problems to Ponder

• Why does the A band size remain the same during sarcomere shortening?

• What step of the crossbridge cycle is triggered when inorganic phosphate detaches from the myosin head?

• What would happen if the SR does not reuptake Ca2+?

Medical Problems to Ponder

• How does dantrolene inhibit skeletal muscle contraction and affect body temperature?

• How does botulinum toxin affect the neuromuscular junction?

• What symptoms might be seen in a patient exposed to curare?

Review: Sequence of Events in Muscle Contraction

1. Motor neuron action potential

2. Neurotransmitter release

3. Muscle cell action potential

4. Ca2+ release from SR

5. ATP-powered power stroke

6. Sliding of myofilaments

Additional info: This study guide covers the essential structure, function, and physiology of muscle tissue, focusing on skeletal muscle. It includes clinical considerations and review questions for exam preparation.