Muscle Tissue

Overview of Muscle Tissue

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

• 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.

Functions of Skeletal Muscle

• Producing movement by pulling on tendons to move bones

• Maintaining posture and body position

• Supporting soft tissues

• Guarding body entrances and exits (e.g., sphincters in digestive and urinary tracts)

• Maintaining body temperature (muscle activity generates heat)

• Storing nutrients (as a source of proteins/amino acids)

Organization of Skeletal Muscle

Structural Organization

Skeletal muscles are complex organs containing muscle tissue, connective tissues, blood vessels, and nerves. The organization is hierarchical, with several layers of connective tissue:

• Epimysium: Surrounds the entire muscle; dense layer of collagen fibers.

• 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).

The connective tissue layers merge at the ends of muscles to form tendons (bundles) or aponeuroses (broad sheets), 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 contract only when stimulated by the central nervous system (CNS); each fiber is innervated by a branch of a motor neuron.

Skeletal Muscle Fibers

Development and Structure

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic myoblasts. They are also called striated muscle cells due to their banded appearance, which results from the arrangement of contractile proteins.

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

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

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

Myofibrils and Myofilaments

• Myofibrils: Long, cylindrical structures running the length of the muscle fiber; composed of bundles of myofilaments.

• Myofilaments: Protein filaments responsible for contraction; two main types:

  • Thin filaments: Primarily actin

  • Thick filaments: Primarily myosin

Sarcomere Structure

The sarcomere is the smallest functional unit of a muscle fiber, defined by the area between two Z lines. It contains organized arrangements of thick and thin filaments, whose interactions produce muscle contraction.

• A band: Dark band; length of thick filaments

• I band: Light band; contains only thin filaments

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

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

• Z line: Boundary between sarcomeres; anchors thin filaments

• Titin: Elastic protein that helps restore sarcomere length after contraction

Levels of Functional Organization

The organization of skeletal muscle can be summarized as follows:

Thin and Thick Filaments

• Thin filaments contain:

  • F-actin: Twisted strand of G-actin molecules with active sites for myosin binding

  • Nebulin: Holds F-actin strands together

  • Tropomyosin: Covers active sites on actin, preventing myosin binding

  • Troponin: Binds tropomyosin and calcium ions; regulates exposure of active sites

• Thick filaments contain:

  • Myosin: Each molecule has a tail (binds other myosin) and a head (binds actin)

  • Titin: Core protein that provides elasticity

Sliding Filament Theory

During muscle contraction, thin filaments slide toward the M line alongside thick filaments. This shortens the sarcomere, bringing Z lines closer together, while the width of the A band remains constant.

Neuromuscular Junction and Muscle Contraction

Excitable Membranes and Action Potentials

• All cells maintain a resting membrane potential; in skeletal muscle, it is about -85 mV.

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

• Repolarization: Return to resting potential after depolarization.

• Action potentials are electrical impulses that propagate along the membrane.

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber. Key components include:

• Axon terminal: End of the motor neuron, contains vesicles of acetylcholine (ACh).

• Motor end plate: Folded region of the muscle fiber's sarcolemma.

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

Events at the NMJ

1. Action potential arrives at axon terminal.

2. Vesicles release ACh into synaptic cleft.

3. ACh binds to receptors on motor end plate, opening Na+ channels.

4. Na+ influx depolarizes the sarcolemma, generating an action potential.

5. ACh is broken down by acetylcholinesterase (AChE), ending the signal.

Excitation-Contraction Coupling

This process links the action potential in the sarcolemma to the start of muscle contraction:

• Action potential travels down T tubules to triads.

• Triggers release of Ca2+ from the SR.

• Ca2+ binds to troponin, moving tropomyosin and exposing actin active sites.

• Contraction cycle begins.

The Contraction Cycle

1. Ca2+ binds to troponin, exposing active sites on actin.

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

3. Power stroke: Myosin head pivots, pulling actin toward M line (uses ATP).

4. Myosin detaches, re-cocks, and repeats as long as Ca2+ and ATP are available.

Muscle Relaxation

• Contraction ends when neural stimulation stops.

• ACh is broken down, Ca2+ is pumped back into the SR.

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

Rigor Mortis

After death, ATP production ceases, Ca2+ remains in the cytosol, and cross-bridges cannot detach, resulting in muscle stiffness.

Mechanisms of Muscle Tension

Length-Tension Relationship

The amount of tension produced by a muscle fiber depends on the overlap between thick and thin filaments. Maximum tension is generated at an optimal sarcomere length.

Frequency of Stimulation

• Twitch: Single, brief contraction-relaxation cycle.

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

• Wave summation: Increased tension due to stimuli arriving before relaxation is complete.

• Tetanus: Sustained, maximal contraction (incomplete or complete).

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

• Isometric contractions: Muscle develops tension but does not change length.

Energy for Muscle Contraction

ATP Generation

• ATP is the direct energy source for muscle contraction.

• Muscles store limited ATP; must regenerate it rapidly.

• Creatine phosphate (CP) stores energy to quickly regenerate ATP.

• Glycolysis (anaerobic) and aerobic metabolism (mitochondria) provide ATP during activity.

Muscle Metabolism at Different Activity Levels

• At rest: Fatty acids are used for ATP; glycogen and CP are stored.

• Moderate activity: Glucose is used aerobically.

• Peak activity: Glycolysis predominates; lactic acid accumulates, leading to fatigue.

Recovery and Oxygen Debt

• Lactate is recycled in the liver (Cori cycle).

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

Muscle Performance and Fiber Types

Types of Skeletal Muscle Fibers

• Fast fibers: Large, powerful, fatigue quickly; few mitochondria, white in color.

• Slow fibers: Smaller, less powerful, fatigue-resistant; many mitochondria, high myoglobin, red in color.

• Intermediate fibers: Characteristics between fast and slow fibers.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to training.

• Atrophy: Decrease in muscle size due to inactivity.

Aging and Muscle Tissue

• Muscle fibers decrease in diameter and elasticity.

• Fibrosis increases; recovery from injury slows.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle

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

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

• Automaticity: Contracts without neural input (pacemaker cells).

• Longer contractions, resistant to fatigue, no tetanus.

Smooth Muscle

• Found in walls of hollow organs; non-striated, spindle-shaped, single nucleus.

• Contracts via calmodulin and myosin light chain kinase, not troponin.

• Plasticity: Can function over a wide range of lengths.

• Multiunit and visceral types; controlled by neural, hormonal, or chemical signals.