Muscle Tissue

Introduction

Muscle tissue is a primary tissue type responsible for producing movement and maintaining vital functions in the body. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle, each with distinct structures and roles.

1. Functions and Properties of Muscle Tissue

Types of Muscle Tissue

• Skeletal muscle: Moves the body by pulling on bones; under voluntary control.

• Cardiac muscle: Pumps blood through the cardiovascular system; found only in the heart.

• Smooth muscle: Pushes fluids and solids through internal passageways and organs; found in walls of hollow organs.

Common Properties of Muscle Tissue

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

• Contractility: Ability of cells to shorten and generate force.

• Extensibility: Ability of muscle to stretch without being damaged.

• Elasticity: Ability of muscle to recoil to its resting length after being stretched.

Primary Functions of Skeletal Muscle

• Producing movement: Pulls on tendons to move bones.

• Maintaining posture and body position: Stabilizes joints and maintains body alignment.

• Supporting soft tissues: Supports and protects internal organs.

• Guarding body entrances and exits: Controls openings of digestive and urinary tracts.

• Maintaining body temperature: Muscle activity generates heat.

• Storing nutrients: Stores proteins and amino acids for energy and repair.

2. Organization of Skeletal Muscle

Structural Organization

Skeletal muscles are organs composed of muscle tissue, connective tissues, blood vessels, and nerves. They are attached to bones and are responsible for voluntary movements.

• Epimysium: Dense layer of collagen fibers surrounding the entire muscle; separates muscle from surrounding tissues.

• Perimysium: Surrounds bundles of muscle fibers called fascicles; contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers; contains capillaries, nerve fibers, and myosatellite cells (stem cells for repair).

• At the ends of muscles, collagen fibers from all three layers merge to form tendons (cord-like) or aponeuroses (sheet-like), which attach muscles to bones.

Blood Supply and Innervation

• Skeletal muscles have extensive blood vessels to deliver oxygen and nutrients and remove wastes.

• Muscle fibers are innervated by branches of motor neurons from the central nervous system.

• Most skeletal muscles are under voluntary control, but some (e.g., diaphragm) can function subconsciously.

3. Skeletal Muscle Fibers

Characteristics of Skeletal Muscle Fibers

• Large, multinucleate cells formed by fusion of myoblasts during development.

• Also called striated muscle cells due to visible striations from organized myofibrils.

• Each fiber is surrounded by a plasma membrane called the sarcolemma and contains cytoplasm known as sarcoplasm.

Internal Structures

• Transverse tubules (T tubules): Invaginations of the sarcolemma that transmit action potentials deep into the cell.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions; forms terminal cisternae adjacent to T tubules, creating a triad (two terminal cisternae + one T tubule).

• Myofibrils: Cylindrical structures containing bundles of contractile proteins called myofilaments (thin and thick filaments).

Sarcomere Structure

The sarcomere is the basic contractile unit of muscle fiber, defined by Z lines. It contains:

• Thin filaments: Composed mainly of actin, with regulatory proteins tropomyosin and troponin.

• Thick filaments: Composed mainly of myosin, with heads that bind to actin during contraction.

• A band: Dark region containing thick filaments (with some overlap of thin filaments).

• I band: Light region containing only thin filaments.

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

• M line: Center of the A band; holds thick filaments together.

• Z line: Boundary between sarcomeres; anchors thin filaments.

• Titin: Elastic protein that stabilizes thick filaments and helps restore resting length.

Sliding Filament Theory

During contraction, thin filaments slide past thick filaments toward the M line, shortening the sarcomere. The A band remains constant, while the I band and H band narrow, and Z lines move closer together.

4. Muscle Contraction

Excitable Membranes and Action Potentials

• Muscle fibers maintain a negative resting membrane potential (about -85 mV).

• Depolarization: Membrane potential becomes less negative (Na+ influx).

• Repolarization: Return to resting potential (K+ efflux).

• Action potentials propagate along the sarcolemma and T tubules.

Neuromuscular Junction (NMJ)

• The synapse between a motor neuron and a skeletal muscle fiber.

• Axon terminal: Releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft.

• Motor end plate: Region of sarcolemma with ACh receptors.

• ACh binding opens Na+ channels, causing depolarization and triggering an action potential in the muscle fiber.

• ACh is broken down by acetylcholinesterase (AChE) to terminate the signal.

Excitation-Contraction Coupling

• Action potential travels along sarcolemma and down T tubules to triads.

• Triggers release of Ca2+ from the SR.

• Ca2+ binds to troponin, causing tropomyosin to move and expose active sites on actin.

• Myosin heads bind to actin, initiating contraction.

Contraction Cycle

1. Ca2+ binds to troponin; tropomyosin shifts, exposing actin sites.

2. Myosin heads (energized by ATP hydrolysis) bind to actin, forming cross-bridges.

3. Power stroke: Myosin head pivots, pulling actin toward M line.

4. ATP binds to myosin, causing detachment from actin.

5. ATP hydrolysis re-cocks the myosin head.

6. Cycle repeats as long as Ca2+ and ATP are available.

Muscle Relaxation

• Stimulation ends; ACh is broken down by AChE.

• Ca2+ is pumped back into the SR.

• Troponin and tropomyosin return to resting positions, covering actin sites.

• Muscle fiber returns to resting length.

Rigor Mortis

• After death, ATP production ceases; Ca2+ remains in cytosol, causing sustained contraction.

• Cross-bridges cannot detach, resulting in muscle stiffness.

5. Muscle Tension

Mechanisms of Tension Production

• Tension is the force generated by muscle contraction.

• Depends on number of power strokes, sarcomere length (length-tension relationship), and frequency of stimulation.

Length-Tension Relationship

• Optimal overlap between thick and thin filaments produces maximum tension.

• If sarcomere is too short or too stretched, tension decreases.

Frequency of Stimulation

• Twitch: Single, brief contraction-relaxation cycle.

• Treppe: Gradual increase in tension with repeated stimulation after relaxation.

• Wave summation: Increased tension from repeated stimuli before relaxation is complete.

• Incomplete tetanus: Rapid cycles of contraction and partial relaxation.

• Complete tetanus: Continuous contraction with no relaxation.

6. Types of Muscle Contractions

Motor Units and Recruitment

• Motor unit: A motor neuron and all the muscle fibers it controls.

• Recruitment: Increasing the number of active motor units to increase tension.

• Muscle tone: Resting tension that stabilizes bones and joints.

Types of Contractions

• Isotonic contraction: Muscle changes length while tension remains constant.

  • Concentric: Muscle shortens (tension > load).

  • Eccentric: Muscle lengthens (tension < load).

• Isometric contraction: Muscle develops tension but does not change length (tension < load).

Load and Speed of Contraction

• Heavier loads result in slower and less shortening.

• Optimal tension and speed depend on the load.

Muscle Relaxation

• Return to resting length is passive, aided by elastic forces, opposing muscles, and gravity.

7. Energy for Muscle Contraction

ATP and Muscle Metabolism

• ATP is the direct energy source for muscle contraction.

• Muscles store limited ATP; must regenerate it rapidly during activity.

ATP Regeneration Pathways

• Creatine phosphate (CP): Stores energy to quickly regenerate ATP; catalyzed by creatine kinase.

• Glycolysis: Anaerobic breakdown of glucose to pyruvate; yields 2 ATP per glucose.

• Aerobic metabolism: Mitochondrial oxidation of pyruvate (from glycolysis) or fatty acids; yields up to 15 ATP per pyruvate.

Muscle Metabolism at Different Activity Levels

• Rest: Fatty acids are primary fuel; ATP is stored as CP and glycogen.

• Moderate activity: Glucose is used aerobically; ATP demand increases.

• Peak activity: Oxygen supply is limited; glycolysis predominates, leading to lactic acid buildup and fatigue.

Recovery and Oxygen Debt

• Recovery period: Muscles return to pre-exertion state; lactic acid is converted to glucose in the liver (Cori cycle).

• Oxygen debt (EPOC): Extra oxygen is required after exercise to restore normal conditions.

• Active muscles release heat, contributing to body temperature regulation.

Hormonal Regulation

• Growth hormone and testosterone promote muscle growth.

• Thyroid hormones increase metabolic rate.

• Epinephrine enhances muscle metabolism and contraction force.

8. Muscle Performance and Conditioning

Types of Muscle Fibers

• Fast fibers: Large, powerful, fatigue quickly; few mitochondria, low endurance ("white muscles").

• Slow fibers: Small, less forceful, fatigue-resistant; many mitochondria, high myoglobin ("red muscles").

• Intermediate fibers: Characteristics between fast and slow fibers; moderate endurance.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to training; more myofibrils, mitochondria, and glycogen.

• Atrophy: Decrease in muscle size and strength due to inactivity; may be irreversible if severe.

Aging and Muscle Tissue

• Muscle fibers decrease in diameter and elasticity (fibrosis increases).

• Exercise tolerance and recovery ability decline with age.

Muscle Fatigue

• Caused by depletion of energy reserves, damage to fibers, and pH changes.

• Results in reduced force and endurance.

Physical Conditioning

• Anaerobic endurance: Improved by brief, intense activity; increases muscle power and size.

• Aerobic endurance: Improved by sustained, low-intensity activity; increases cardiovascular efficiency and slow fiber proportion.

• Cross-training: Combines aerobic and anaerobic exercises for overall fitness.

9. Cardiac Muscle Tissue

Structure and Function

• Found only in the heart; small, branched, striated cells with a single nucleus.

• Connected by intercalated discs (gap junctions and desmosomes) for synchronized contraction.

• Short, wide T tubules; no triads; SR lacks terminal cisternae.

• Relies almost exclusively on aerobic metabolism; abundant myoglobin and mitochondria.

Functional Characteristics

• Automaticity: Can contract without neural stimulation (pacemaker cells).

• Longer contraction and refractory periods than skeletal muscle; resistant to fatigue.

• Requires both intracellular and extracellular Ca2+ for contraction.

• No wave summation or tetanic contractions.

10. Smooth Muscle Tissue

Structure and Location

• Found in walls of hollow organs (digestive, urinary, reproductive, blood vessels, airways).

• Long, spindle-shaped cells with a single central nucleus; non-striated (no sarcomeres).

• Thick and thin filaments arranged diagonally; dense bodies anchor thin filaments and transmit force.

• No tendons or aponeuroses.

Functional Characteristics

• Contraction triggered by Ca2+ entry and release from SR; Ca2+ binds to calmodulin, activating myosin light chain kinase.

• Can contract over a wide range of lengths (plasticity).

• Two types: Multiunit (individually innervated) and Visceral (connected by gap junctions, contract as a unit).

• Maintains smooth muscle tone: background level of contraction, modulated by neural, hormonal, or chemical factors.

Roles in Body Systems

• Regulates blood pressure and airflow (cardiovascular, respiratory).

• Moves materials through digestive and urinary tracts; forms sphincters.

• Controls hair erection (arrector pili) and reproductive functions.