Skeletal Muscle Tissue

Introduction to Skeletal Muscle Tissue

Skeletal muscle tissue is a specialized tissue composed of muscle cells that enable voluntary movement and play a crucial role in body mechanics. It is one of three types of muscle tissue found in the human body, each with distinct characteristics and functions.

• Skeletal muscle: Responsible for voluntary control and movement; attached to bones.

• Cardiac muscle: Involuntary, found only in the heart.

• Smooth muscle: Involuntary, located in the walls of hollow organs.

Skeletal muscle is considered an organ, consisting of skeletal muscle tissue, connective tissue, nerves, and blood vessels. The basic cell of skeletal muscle is the muscle fiber, which is directly or indirectly attached to bones.

Functions of Skeletal Muscle Tissue

Main Functions and Mechanisms

Skeletal muscle tissue performs several essential functions in the human body. These functions are summarized in the table below:

Example: Shivering is an involuntary contraction of skeletal muscles that helps maintain body temperature in cold environments.

Organization of Skeletal Muscle

Structural Hierarchy

Skeletal muscle is organized from the whole muscle down to the microscopic level, with several layers of connective tissue providing structure and support.

• Epimysium: Dense sheath of collagen fibers surrounding the entire muscle; separates muscle from other tissues and is connected to deep fascia.

• Perimysium: Fibrous layer dividing muscle into compartments called fascicles; contains collagen, elastic fibers, blood vessels, and nerves.

• Endomysium: Thin layer of areolar connective tissue around each muscle fiber; contains capillaries, nerve fibers, and myosatellite cells (stem cells for repair).

• Muscle fascicle: Bundle of muscle fibers within the muscle.

• Muscle fiber: Individual muscle cell containing myofibrils.

Additional info: The hierarchical structure allows for efficient force transmission and repair mechanisms.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

Muscle fibers are large, multinucleate cells containing myofibrils, which are bundles of protein filaments responsible for contraction.

• Myofibrils: Cylindrical structures arranged parallel inside muscle fibers; give skeletal muscle its striated appearance due to alternating light and dark bands.

• Myofilaments: Protein filaments within myofibrils; classified as thin (actin) and thick (myosin) filaments.

• Myosatellite cells: Stem cells in the endomysium that aid in muscle repair.

Example: Muscle fibers can be up to 30 cm in length and 100 micrometers in diameter.

Sarcomere Structure and Function

The sarcomere is the repeating functional unit of a myofibril, responsible for muscle contraction. Each myofibril contains thousands of sarcomeres.

• Z lines: Define boundaries of each sarcomere; anchor thin filaments.

• I band: Region with only thin filaments.

• A band: Dense region containing thick filaments.

• Zone of overlap: Area within the A band where thick and thin filaments overlap.

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

• H band: Lighter region on each side of the M line with only thick filaments (visible in resting sarcomeres).

Additional info: The arrangement of actin and myosin filaments within the sarcomere is essential for the sliding filament mechanism of contraction.

Excitable Membrane and Action Potential

Properties of the Sarcolemma

The sarcolemma is the plasma membrane of a skeletal muscle fiber. Its selective permeability allows for the generation and propagation of electrical impulses (action potentials).

• Reversal of charge across the sarcolemma initiates muscle contraction.

• Action potentials are triggered by nerve impulses and spread across the entire sarcolemma.

• Transverse tubules (T tubules): Extensions of the sarcolemma into the sarcoplasm; encircle each sarcomere and facilitate rapid transmission of action potentials.

Sarcoplasmic Reticulum and Calcium Storage

The sarcoplasmic reticulum (SR) is a specialized organelle that stores calcium ions, which are essential for muscle contraction.

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

• Triad: Pair of terminal cisternae and one T tubule.

• Muscle contraction begins when calcium ions are released from the SR into the cytosol.

Sliding Filament Theory

Thin and Thick Filaments

Muscle contraction is explained by the sliding filament theory, which describes how thin and thick filaments interact within the sarcomere.

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

• Thick filaments: Composed of myosin molecules; each has a tail and two globular heads that form cross-bridges with actin during contraction.

• Titin: Connects thick filaments to Z lines; provides elasticity and recoil.

Troponin-tropomyosin complex regulates access to actin's active sites, controlling contraction.

Mechanism of Contraction

During contraction, thin filaments slide over thick filaments, shortening the sarcomere and producing muscle tension.

• I bands and H bands become smaller; zones of overlap increase.

• Z lines move closer together; A band width remains unchanged.

• All sarcomeres in a myofibril contract simultaneously, shortening the muscle fiber.

Excitable Plasma Membrane and Action Potential

Membrane Potentials

All cells have a membrane potential due to unequal distribution of positive and negative charges across the plasma membrane.

• Resting membrane potential in neurons: ~-70 mV

• Resting membrane potential in skeletal muscle fibers: ~-85 mV

Selective permeability allows movement of ions such as Na+ and K+ through leak channels and sodium-potassium pumps.

• Sodium-potassium pump exports 3 Na+ and imports 2 K+ per cycle.

Action Potential Steps

An action potential is a rapid change in membrane potential that triggers muscle contraction. The process involves five main steps:

1. Small increase in membrane permeability to Na+; membrane potential moves toward threshold (-55 mV).

2. Voltage-gated Na+ channels open; rapid influx of Na+ causes depolarization (membrane potential becomes positive, up to +30 mV).

3. Voltage-gated Na+ channels close; voltage-gated K+ channels open; K+ leaves cell (repolarization).

4. Membrane potential returns to resting level; voltage-gated K+ channels close.

5. Na+/K+ pump restores original ion distribution.

Equation:

Neuromuscular Junction

Structure and Function

The neuromuscular junction (NMJ) is the site where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

• Axon terminal: Contains vesicles with acetylcholine (ACh).

• Motor end plate: Region of muscle fiber membrane with ACh receptors and junctional folds.

• Synaptic cleft: Space between axon terminal and motor end plate; contains acetylcholinesterase (AChE) to break down ACh.

Events at the Neuromuscular Junction

1. Action potential arrives at axon terminal.

2. Vesicles release ACh into synaptic cleft (exocytosis).

3. ACh binds to receptors on motor end plate, increasing Na+ permeability.

4. Na+ influx generates action potential in sarcolemma.

5. ACh is broken down by AChE; channels close.

6. Action potential spreads across sarcolemma and down T tubules, triggering Ca2+ release from SR.

Additional info: This process is called excitation-contraction coupling.

Muscle Fiber Contraction Cycle

Steps of Contraction

1. Resting sarcomere: Myosin heads are "cocked" and energized (ATP hydrolysis).

2. Contraction cycle begins: Ca2+ arrives from SR; binds to troponin.

3. Active sites exposed: Troponin changes position, moving tropomyosin and exposing actin's active sites.

4. Cross-bridges form: Myosin heads bind to actin.

5. Myosin heads pivot (power stroke): ADP and Pi released.

6. Cross-bridges detach: New ATP binds to myosin, releasing it from actin.

7. Myosin reactivates: ATP hydrolysis "recocks" myosin head.

The cycle repeats as long as Ca2+ and ATP are available. When stimulation ends, Ca2+ is pumped back into SR, and the troponin-tropomyosin complex covers active sites, ending contraction.

Excitation-Contraction Cycle

Six Steps Linking Excitation to Contraction

1. Neural control: Action potential in motor neuron starts process at NMJ.

2. Excitation: ACh release leads to action potential in sarcolemma.

3. Calcium ion release: Action potential travels through T tubules, triggering Ca2+ release from SR.

4. Contraction cycle begins: Ca2+ binds troponin, exposing actin's active sites; cross-bridges form.

5. Sarcomeres shorten: Sliding filaments contract, shortening sarcomeres and producing muscle tension.

6. Muscle tension produced: Muscle fiber shortens, pulling on tendons and producing movement.

Length-Tension Relationship

Optimal Sarcomere Length

Muscle tension is greatest when fibers are stimulated at optimal sarcomere length, allowing maximum cross-bridge formation. The normal range is 75-130% of optimal length. Excessive stretching or compression reduces tension.

Muscle Twitch and Tension

Phases of Muscle Twitch

A muscle twitch is a single stimulus-contraction-relaxation sequence in a muscle fiber. It consists of three phases:

• Latent period: Action potential stimulates sarcolemma; Ca2+ released; no tension yet.

• Contraction period: Ca2+ binds troponin; cross-bridge cycling; tension rises to peak.

• Relaxation phase: Ca2+ drops; cross-bridges detach; tension returns to resting level.

Example: Fasciculation is an involuntary muscle twitch visible under the skin.

Peak Tension and Summation

Muscle tension depends on the amount of tension produced by each fiber and the number of fibers stimulated. Stimulation frequency affects tension:

• Treppe: Repeated stimulation after relaxation increases tension (staircase effect).

• Wave summation: Stimulation before relaxation ends increases tension.

• Incomplete tetanus: Rapid cycles produce near-peak tension with some relaxation.

• Complete tetanus: High frequency eliminates relaxation; continuous peak tension.

Motor Units and Recruitment

A motor unit consists of a single motor neuron and all muscle fibers it controls. Recruitment is the activation of more motor units to increase tension. Smaller units are activated first, followed by larger, more powerful units. Synchronous activation maintains sustained contraction.

Types of Muscle Contractions

Isotonic and Isometric Contractions

• Isotonic contraction: Muscle tension rises and length changes. Includes concentric (muscle shortens) and eccentric (muscle lengthens) contractions.

• Isometric contraction: Muscle tension rises, but length does not change; tension never exceeds load.

Example: Lifting a dumbbell (concentric), lowering it (eccentric), and holding a posture (isometric).

Energy for Muscle Contraction

ATP Sources and Metabolism

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

• Aerobic metabolism: Occurs in mitochondria; produces 15 ATP per pyruvate; supplies 95% of ATP at rest.

• Creatine phosphate (CP): Supplies energy for short bursts (~15 seconds).

• Glycogen: Main energy reserve in muscle fibers.

Equation:

Muscle Metabolism at Different Activity Levels

• Rest: Low ATP demand; mitochondria produce surplus ATP; fatty acids and glucose absorbed; ATP used to build CP and glycogen.

• Moderate activity: Increased ATP demand; relies on aerobic metabolism of pyruvate; fatigue occurs when reserves are exhausted.

• Peak activity: Maximum ATP demand; mitochondria provide ~1/3 ATP; most ATP from glycolysis; excess pyruvate converted to lactate, causing muscle fatigue.

Muscle Fatigue and Recovery

Causes and Effects of Fatigue

Muscle fatigue occurs when a muscle can no longer perform at the required level. Major factors include decline in pH (lactic acidosis), decreased calcium/troponin binding, and altered enzyme activity.

• Recovery period: Time needed to restore muscle conditions to pre-exertion levels; may take hours to a week.

• Oxygen debt (EPOC): Extra oxygen required to restore ATP, CP, and glycogen levels, and convert lactate to glucose.

• Cori cycle: Lactate produced during peak activity is shuttled to the liver, converted to glucose, and returned to muscles.

Equation:

Skeletal Muscle Fiber Types

Classification and Characteristics

There are three main types of skeletal muscle fibers, each with unique properties:

Most muscles contain a mixture of fiber types, reflecting their function. The percentage of fast versus slow fibers is genetically determined, but intermediate-to-fast ratios can be modified with training.