Muscle Tissue

Types and Functions of Muscle Tissue

Muscle tissue is a primary tissue responsible for contraction and movement in the human body. There are three main types of muscle tissue, each with distinct functions and properties.

• Skeletal muscle: Moves the body by pulling on bones; responsible for voluntary movements.

• 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.

• Extensibility: Ability of the muscle to stretch.

• Elasticity: Ability of the muscle to recoil to its resting length.

Primary 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

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. The organization at the tissue level involves three layers of connective tissue:

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

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

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

The collagen fibers from these layers converge to form tendons (attach muscle to bone) or aponeuroses (attach muscle to muscle or bone).

Skeletal Muscle Fibers

Skeletal muscle fibers are large, multinucleate, and striated due to the arrangement of myofibrils.

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

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

• Sarcoplasmic reticulum (SR): Specialized for storage and release of calcium ions; forms terminal cisternae.

Sarcomere Structure

The sarcomere is the smallest contractile unit of muscle fiber, composed of organized thick (myosin) and thin (actin) filaments.

• A bands: Dark bands containing thick filaments.

• I bands: Light bands containing only thin filaments.

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

• H band: Region with only thick filaments.

• Zone of overlap: Area where thick and thin filaments overlap.

• Z lines: Boundaries between adjacent sarcomeres.

Myofilaments: Thin and Thick Filaments

Thin filaments:

• Composed primarily of actin (F-actin: two rows of G-actin).

• Active sites on G-actin bind to myosin.

• Tropomyosin covers active sites, preventing interaction.

• Troponin binds calcium and moves tropomyosin off active sites.

Thick filaments:

• Composed of myosin molecules (tail binds to other myosin, head projects toward thin filament).

• Core of titin stretches from M line to Z line, acting as a molecular spring.

Sliding-Filament Theory

The sliding-filament theory explains muscle contraction: thin filaments slide toward the M line, causing the H and I bands to narrow, zones of overlap to widen, and Z lines to move closer together. The A band remains constant.

Muscle Contraction

Neuromuscular Junction and Excitable Membranes

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a skeletal muscle fiber.

• Axon terminal: End of the motor neuron.

• Motor end plate: Folded membrane of the muscle fiber.

• Synaptic cleft: Space between axon terminal and motor end plate.

• Acetylcholine (ACh): Neurotransmitter released from axon terminal, binds to receptors on motor end plate, opens Na+ channels, and generates an action potential.

Excitation–Contraction Coupling

Excitation–contraction coupling links the action potential in the sarcolemma to muscle contraction.

• Action potential travels down T tubules to triads.

• Triggers release of calcium ions from SR.

• Calcium binds to troponin, moving tropomyosin off actin active sites.

• Contraction begins.

The Contraction Cycle

The contraction cycle involves cross-bridge formation, power stroke, and detachment.

• Myosin heads bind to actin, forming cross-bridges.

• Power stroke: myosin head pivots, pulling actin toward M line, using ATP.

• Cycle repeats as long as calcium and ATP are available.

Muscle Tension

Mechanisms of Tension Production

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

• Number of power strokes performed

• Resting length of the muscle fiber (length-tension relationship)

• Frequency of stimulation

Length–Tension Relationship

Tension is maximal when the optimal overlap between thick and thin filaments allows the maximum number of cross-bridges.

Frequency of Stimulation

• Twitch: Single stimulus-contraction-relaxation sequence.

• Treppe: Increased peak tension with repeated stimulations after relaxation.

• Wave summation: Increased tension due to successive stimuli before relaxation completes.

• Tetanus: Maximum tension; can be incomplete or complete.

Types of Muscle Contractions

Motor Units and Recruitment

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

• Recruitment: Increase in muscle tension due to activation of more motor units.

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

Types of Contractions

• Isotonic contractions: Muscle changes length.

  • Concentric: Muscle shortens as tension exceeds load.

  • Eccentric: Muscle lengthens as tension is less than load.

• Isometric contractions: Muscle develops tension but does not change 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 Generation

• Creatine phosphate (CP): Stores energy for rapid ATP regeneration.

• Glycolysis: Anaerobic breakdown of glucose; net gain of 2 ATP per glucose.

• Aerobic metabolism: Oxygen-dependent; produces 15 ATP per pyruvate.

Recovery and Oxygen Debt

• Recovery period: Time required for muscles to return to pre-exertion conditions.

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

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

Muscle Performance

Types of Muscle Fibers

• Fast fibers: Contract quickly, high force, fatigue rapidly; few mitochondria, large glycogen reserves.

• Slow fibers: Contract slowly, low force, high endurance; many mitochondria, high myoglobin.

• Intermediate fibers: Mid-sized, moderate endurance and force.

Hypertrophy, Atrophy, and Aging

• Hypertrophy: Increase in muscle size due to training.

• Atrophy: Decrease in muscle size due to inactivity.

• Aging: Muscle fibers decrease in diameter, elasticity, and recovery ability.

Muscle Fatigue and Conditioning

• Muscle fatigue: Inability to maintain required performance; caused by depletion of reserves, damage, pH decline.

• Anaerobic endurance: Short, intense activity; improves power.

• Aerobic endurance: Sustained, low-intensity activity; improves cardiovascular performance.

• Cross-training: Combination of aerobic and anaerobic exercises.

Cardiac Muscle Tissue

Structure and Function

• Found only in the heart.

• Small, branched, striated cells with a single nucleus.

• Intercalated discs connect cells, allowing rhythmic contraction.

• Automaticity: contracts without neural stimulation.

• Longer contractions, resistant to fatigue.

Smooth Muscle Tissue

Structure and Function

• Found in walls of hollow organs and blood vessels.

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

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

• Dense bodies connect cells, transmitting contractions.

Functional Characteristics

• Excitation–contraction coupling involves calmodulin and myosin light chain kinase.

• Plasticity: ability to function over a wide range of lengths.

• Multiunit and visceral smooth muscle cells differ in innervation and control.

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

Summary Table: Types of Muscle Tissue

Key Equations

• ATP hydrolysis:

• Creatine phosphate reaction:

• Glycolysis:

• Aerobic metabolism:

Additional info:

• Muscle contraction and relaxation are tightly regulated by calcium ion concentration and ATP availability.

• Rigor mortis occurs when ATP is depleted after death, preventing detachment of myosin heads from actin.