Skeletal Muscle Tissue: Structure, Function, and Physiology

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Muscle Tissue

Types of Muscle Tissue

Muscle tissue is highly specialized for contraction, which enables movement in the body. There are three main types of muscle tissue, each with distinct locations and functions:

Skeletal Muscle: Attached to the skeleton; responsible for voluntary movements by acting on bones as levers.
Cardiac Muscle: Found only in the heart; responsible for pumping blood through the cardiovascular system.
Smooth Muscle: Located in the walls of hollow organs and blood vessels; moves fluids and solids along internal passageways.

Note: This chapter focuses on skeletal muscle contraction.

Functions of Skeletal Muscles

Produce Movement: By pulling on bones.
Maintain Posture: Stabilize body positions.
Support Soft Tissues: Protect internal organs.
Guard Entrances and Exits: Control openings of digestive and urinary tracts.
Maintain Body Temperature: Generate heat (e.g., shivering).

Anatomy of Skeletal Muscle

Connective Tissue Layers

Epimysium: Surrounds the entire muscle; separates muscle from surrounding tissues; connected to deep fascia.
Perimysium: Divides muscle into bundles called fascicles; contains blood vessels and nerves.
Endomysium: Surrounds individual muscle fibers; contains satellite cells (muscle stem cells), capillaries, and nerves.

All three layers converge at the muscle's end to form a tendon (attaches muscle to bone) or an aponeurosis (attaches muscle to muscle).

Skeletal Muscle Fibers

Large, multinucleated cells formed by fusion of myoblasts during development.
Sarcolemma: Plasma membrane of muscle fiber.
Sarcoplasm: Cytoplasm of muscle fiber.
Highly vascularized and innervated to meet energy and oxygen demands.

Membrane Potential and Action Potentials

The membrane potential across the sarcolemma is essential for muscle contraction. It is maintained by ion gradients, primarily:

High K+ inside the cell; high Na+ outside.
Na+/K+ ATPase pump maintains this gradient by moving 3 Na+ out and 2 K+ in, using ATP.
Resting membrane potential is typically -70 mV (inside negative).

Opening of Na+ channels (chemically or voltage-gated) leads to depolarization, generating an action potential that triggers contraction.

Microscopic Anatomy of Muscle Fibers

T Tubules and Myofibrils

T tubules (Transverse tubules): Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.
Myofibrils: Bundles of myofilaments (actin and myosin) responsible for contraction; hundreds to thousands per muscle fiber.

Sarcoplasmic Reticulum (SR) and Triads

Sarcoplasmic Reticulum: Specialized smooth ER that stores Ca2+ ions.
Terminal Cisterns: Enlarged SR regions adjacent to T tubules; together with a T tubule, form a triad.
Release of Ca2+ from terminal cisterns into sarcoplasm triggers contraction.

Sarcomere Structure

The sarcomere is the smallest functional contractile unit of muscle, composed of organized myofibrils:

A band: Dark region; contains thick filaments (myosin).
M line: Center of A band; stabilizes thick filaments.
H zone: Region with only thick filaments.
Zone of overlap: Area where thick and thin filaments overlap.
I band: Light region; contains only thin filaments (actin).
Z line: Boundary of sarcomere; anchors thin filaments via actinins and titin (elastic protein).

This organization gives skeletal muscle its striated appearance.

Myofilament Structure

Thin Filaments (Actin):
- Composed of F (filamentous) actin, a double helix of G (globular) actin subunits.
- Nebulin holds actin strands together.
- Tropomyosin covers myosin-binding sites on G actin.
- Troponin binds tropomyosin, G actin, and Ca2+; Ca2+ binding exposes active sites for myosin.
Thick Filaments (Myosin):
- Composed of myosin molecules with long tails and two globular heads.
- Heads form cross-bridges with actin during contraction.
- Titin extends from thick filaments to Z line, providing elasticity.

Mechanism of Muscle Contraction

Sliding Filament Theory

During contraction:

H zone and I band decrease in size.
Zone of overlap increases.
Z lines move closer together.
A band width remains constant.

Thin filaments slide past thick filaments, shortening the sarcomere and thus the muscle.

Excitation-Contraction Coupling

Stimulation by a motor neuron at the neuromuscular junction (NMJ) triggers muscle contraction.
Release of acetylcholine (ACh) at the synaptic cleft binds to receptors on the motor end plate, increasing Na+ permeability and generating an action potential in the sarcolemma.
Action potential travels along T tubules, causing Ca2+ release from SR.
Ca2+ binds to troponin, shifting tropomyosin and exposing actin active sites.

Contraction Cycle

Exposure of actin active sites.
Formation of cross-bridges (myosin heads bind actin).
Pivoting of myosin heads (power stroke) powered by ATP hydrolysis.
Detachment of cross-bridges when new ATP binds to myosin head.
Reactivation of myosin head by ATP hydrolysis; cycle repeats if Ca2+ is present.

Relaxation occurs when stimulation ceases, Ca2+ is reabsorbed into SR, and active sites are covered again.

Tension Production in Muscle

All-or-None Principle

A muscle fiber contracts fully or not at all. However, the total tension produced by a muscle depends on:

Number of fibers contracting (recruitment).
Resting length of the muscle at stimulation.
Frequency of stimulation.

Length-Tension Relationship

Tension is maximal when there is optimal overlap between actin and myosin, allowing the greatest number of cross-bridges to form. Too much or too little overlap reduces tension.

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 stimulation before relaxation is complete.
Incomplete Tetanus: Rapid stimulation produces sustained, but wavering, contraction.
Complete Tetanus: Very rapid stimulation eliminates relaxation phase, producing continuous contraction.

Internal vs. External Tension

Internal Tension: Generated within sarcomeres.
External Tension: Transmitted to tendons and bones; builds up more slowly due to elasticity of connective tissues.

Motor Units and Recruitment

Motor Unit: A single motor neuron and all the muscle fibers it controls.
Recruitment of more motor units increases muscle tension.
Smaller motor units allow fine control (e.g., eye muscles); larger units provide power (e.g., leg muscles).
Motor units rotate during sustained contractions to prevent fatigue.

Muscle Tone

Continuous, passive partial contraction of muscles.
Maintains posture and stabilizes joints.
Increases basal metabolic rate.

Types of Contractions

Type	Description	Example
Isotonic	Tension rises, muscle length changes	Lifting a weight
Concentric	Muscle shortens	Biceps curl upward
Eccentric	Muscle lengthens	Lowering a weight
Isometric	Tension rises, muscle length does not change	Holding a heavy object still

Note: Even during isometric contractions, sarcomeres shorten, but the muscle as a whole does not change length.

Resistance and Speed of Contraction

Speed of contraction is inversely related to resistance.
Optimal speed and tension depend on the muscle and the load.

Relaxation and Return to Resting Length

Elastic forces (tendons, connective tissue recoil).
Opposing muscle contractions (antagonists).
Gravity.

Energy Use and Muscle Metabolism

ATP and Creatine Phosphate

ATP is required for contraction but is not stored in large amounts.
Creatine Phosphate (CP) stores energy to rapidly regenerate ATP from ADP.
CP reserves last about 15 seconds during intense activity.
Enzyme: Creatine Phosphokinase (CPK) catalyzes the transfer of phosphate from CP to ADP.

Glycolysis and Aerobic Metabolism

Glycolysis: Anaerobic breakdown of glucose to pyruvic acid; yields 2 ATP per glucose.
Aerobic Metabolism: Occurs in mitochondria; pyruvic acid enters the citric acid (Krebs) cycle, producing NADH and FADH2 for the electron transport chain (ETC).
ETC uses O2 to generate a proton gradient, driving ATP synthesis.
Byproducts: CO2 and H2O.
1 glucose can yield up to 34 ATP aerobically.
Fatty acids are also used for ATP production, especially at rest.

Patterns of Energy Use

Condition	ATP Source	Notes
Resting Muscle	Aerobic metabolism (fatty acids, glucose)	ATP demand low; stores CP
Moderate Activity	Aerobic metabolism	ATP demand increases; no storage
Peak Activity	Glycolysis (anaerobic)	O2 limited; lactic acid produced

Muscle Fatigue

Occurs when muscle can no longer perform required activity.
Causes: depletion of metabolic reserves, decline in pH (lactic acid), lack of O2.
Fatigue can be rapid (anaerobic) or gradual (aerobic/endurance).

Recovery Period

Lactic Acid Removal: Converted back to pyruvic acid (Cori cycle) in liver and muscle when O2 is available.
Oxygen Debt: Extra O2 required after exercise to restore ATP and CP levels.
Heat Loss: Muscle activity generates heat; must be dissipated to maintain body temperature.

Muscle Performance and Fiber Types

Performance Metrics

Power: Maximum tension produced.
Endurance: Duration of sustained activity.
Determined by fiber type and conditioning.

Types of Skeletal Muscle Fibers

Type	Characteristics	Example
Fast (Type II-A, glycolytic)	Large diameter, few mitochondria, high glycogen, rapid fatigue	Eye, hand muscles
Slow (Type I, oxidative)	Small diameter, many mitochondria, high myoglobin, resistant to fatigue	Back, calf muscles
Intermediate (Type IIB, fast oxidative)	Intermediate properties, more capillaries, no myoglobin, moderate resistance to fatigue	Most skeletal muscles

Muscle composition is genetically determined but can be modified by training (increase in intermediate fibers).

Training Effects

Increased blood flow and angiogenesis.
Improved cardiac efficiency.
Increased number of mitochondria.
Enhanced glucose uptake and insulin sensitivity.
Increased muscle fiber diameter (hypertrophy) via synthesis of additional myofibrils.

These adaptations improve muscle efficiency and performance.

Additional info:

Equations for ATP regeneration and glycolysis: