Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is a primary tissue in the human body, specialized for contraction and movement. There are three main types:

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

• Cardiac muscle: Found only in the heart; controls rhythmic contractions.

• Smooth muscle: Controls movements inside the body, such as in blood vessels and organs.

Common Properties of Muscle Tissue

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to stretch without damage.

• Elasticity: Ability to return to original shape after stretching.

Functions of Skeletal Muscle

• Producing movement

• Maintaining posture and body position

• Supporting soft tissues

• Guarding body entrances and exits

• Maintaining body temperature

• Storing nutrients

Organization of Skeletal Muscle

Muscle Structure at the Tissue Level

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. The organization is hierarchical:

• Epimysium: Surrounds the entire muscle; connected to deep fascia.

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

• Endomysium: Surrounds individual muscle fibers; contains capillaries, myosatellite cells, and nerve fibers.

Collagen fibers from these layers converge to form tendons (bundles) or aponeuroses (sheets), which attach muscles to bones.

Skeletal Muscle Fibers

Characteristics and Development

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic myoblasts. They are also known as striated muscle cells due to their banded appearance.

Internal Structure

• Sarcolemma: Plasma membrane surrounding the fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

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

• Sarcoplasmic reticulum (SR): Specialized for calcium storage and release; forms terminal cisternae and triads with T tubules.

• Myofibrils: Bundles of protein filaments responsible for contraction; contain thin (actin) and thick (myosin) filaments.

Sarcomere Structure

The sarcomere is the smallest functional unit of a muscle fiber. Its arrangement of thick and thin filaments produces the striated pattern.

• A band: Dark band; contains thick filaments.

• I band: Light band; contains thin filaments.

• H band: Area with only thick filaments.

• M line: Center of A band; stabilizes thick filaments.

• Z line: Boundary between sarcomeres; bisects I bands.

• Titin: Elastic protein maintaining alignment and aiding in recoil.

Thin and Thick Filaments

• Thin filaments: Composed of F-actin (two rows of G-actin), nebulin, tropomyosin (covers active sites), and troponin (binds Ca2+).

• Thick filaments: Composed of myosin molecules (head and tail regions); core of titin for elasticity.

Sliding-Filament Theory

Muscle contraction occurs as thin filaments slide toward the center of the sarcomere, causing H and I bands to narrow, Z lines to move closer, and the A band to remain constant.

The Neuromuscular Junction

Components and Events

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber. The process of muscle contraction is initiated by neural stimulation:

1. Motor neuron releases acetylcholine (ACh) into the synaptic cleft.

2. ACh binds to Na+ channels on the muscle fiber, causing depolarization.

3. An action potential is generated and travels along the sarcolemma and T tubules.

4. Ca2+ is released from the SR, binds to troponin, and exposes active sites on actin.

5. The contraction cycle begins: cross-bridge formation, power stroke, detachment, and reactivation.

Muscle Tension Production

Mechanisms of Tension

The amount of tension produced by a muscle fiber depends on:

• Number of power strokes performed

• Resting length of the fiber at stimulation

• Frequency of stimulation

Length–Tension Relationship

Maximum tension is produced when the maximum number of cross-bridges is formed, which occurs when the zone of overlap between thick and thin filaments is optimal.

Frequency of Stimulation

• A single neural stimulation produces a twitch.

• Sustained contractions require repeated stimuli.

• A myogram shows tension development over time.

Treppe and Wave Summation

• Treppe: Stair-step increase in tension with repeated stimulation after relaxation.

• Wave summation: Increased tension due to repeated stimulation before relaxation ends.

Tetanus

• Incomplete tetanus: Near-maximum tension with rapid cycles of contraction and relaxation.

• Complete tetanus: Continuous contraction with no relaxation phase; all cross-bridges formed.

Types of Muscle Contractions

Motor Units and Recruitment

A motor unit consists of a motor neuron and all the muscle fibers it controls. Recruitment increases the number of active motor units, producing a smooth increase in tension.

Muscle Tone

• Normal tension and firmness at rest

• Stabilizes bones and joints

• Maintains posture

Classification of Contractions

• Isotonic contractions: Muscle changes length (concentric: shortens; eccentric: lengthens).

• Isometric contractions: Muscle develops tension without changing length.

Load and Speed of Contraction

The heavier the load, the slower the contraction and the less the muscle shortens.

Energy for Muscle Contraction

ATP and Metabolic Pathways

ATP is the direct energy source for muscle contraction. Muscles store limited ATP and must regenerate it through:

• Direct phosphorylation by creatine phosphate (CP)

• Anaerobic metabolism (glycolysis)

• Aerobic metabolism (citric acid cycle and electron transport chain)

Muscle Metabolism

• At rest: Metabolize fatty acids, store glycogen and CP

• Moderate activity: Aerobic breakdown of glucose

• Peak activity: Glycolysis produces lactate

Recovery and Oxygen Debt

• Recovery period: Time required for muscles to return to normal after exertion.

• Cori cycle: Lactate is transferred to the liver, converted to pyruvate, then glucose.

• Oxygen debt (EPOC): Increased oxygen demand after exercise to restore metabolic balance.

Heat Production and Hormonal Effects

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

• Hormones (growth hormone, testosterone, thyroid hormones, epinephrine) increase muscle metabolism.

Muscle Performance and Fiber Types

Types of Muscle Fibers

• Fast fibers: Contract quickly, large diameter, few mitochondria, fatigue rapidly.

• Slow fibers: Contract slowly, small diameter, many mitochondria, high oxygen supply, contain myoglobin.

• Intermediate fibers: Mid-sized, little myoglobin, slower to fatigue than fast fibers.

Muscle Hypertrophy and Atrophy

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

• Atrophy: Reduction in muscle size, tone, and power due to inactivity.

Aging and Muscle Fatigue

• Muscle fibers decrease in diameter and elasticity with age.

• Fibrosis increases, exercise tolerance and recovery decrease.

• Muscle fatigue: Inability to sustain performance due to metabolic depletion, damage, and pH changes.

Physical Conditioning

• Anaerobic endurance: Uses fast fibers, stimulates hypertrophy; improved by brief, intensive workouts.

• Aerobic endurance: Supported by mitochondria; improved by sustained, low-intensity activity.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

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

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

• Dependent on aerobic metabolism; contains myoglobin and many mitochondria.

• Cells connected by intercalated discs (gap junctions and desmosomes).

• Automaticity: Contraction without neural stimulation, controlled by pacemaker cells.

Smooth Muscle Tissue

• Long, slender, spindle-shaped cells with single central nucleus.

• No T tubules, myofibrils, or sarcomeres; nonstriated.

• Scattered thick filaments with many myosin heads; thin filaments attached to dense bodies.

• Dense bodies connect cells and transmit contractions; no tendons or aponeuroses.

Functional Characteristics of Smooth Muscle

• Excitation–contraction coupling: Ca2+ binds to calmodulin, activating myosin light chain kinase.

• Length–tension relationship: Can contract even when stretched; plasticity allows function over wide length range.

• Control of contractions: Multiunit (innervated) and visceral (not innervated, controlled by pacesetter cells).

• Smooth muscle tone: Normal background activity, modulated by neural, hormonal, or chemical factors.