Muscle Tissue: Structure, Function, and Physiology

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.

• Cardiac muscle: Pumps blood through the cardiovascular system.

• Smooth muscle: Pushes fluids and solids through internal passageways and 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.

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., urinary and digestive tracts).

• Maintaining body temperature through heat production.

• Storing nutrients as a source of proteins/amino acids.

Organization of Skeletal Muscle

Structural Organization

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. They are organized into several hierarchical levels:

• Epimysium: Surrounds the entire muscle and separates it from surrounding tissues.

• Perimysium: Surrounds individual fascicles (bundles of muscle fibers).

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

The collagen fibers of these connective tissue layers merge to form tendons (bundles) or aponeuroses (broad sheets) that attach muscles to bones.

Blood Supply and Innervation

• Skeletal muscles have extensive blood vessels to deliver oxygen and nutrients and remove wastes.

• Muscles contract only when stimulated by the central nervous system (voluntary muscles), though some (e.g., diaphragm) work subconsciously.

Skeletal Muscle Fibers

Characteristics of Skeletal Muscle Fibers

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic myoblasts. They are also known as striated muscle cells due to visible striations caused by the arrangement of myofibrils.

Internal Structure

• Sarcolemma: Plasma membrane of a muscle fiber, surrounds the sarcoplasm (cytoplasm).

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

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

Myofibrils and Myofilaments

• Myofibrils: Collections of myofilaments responsible for contraction.

• Myofilaments: Contractile protein filaments; two types:

  • Thin filaments: Primarily actin.

  • Thick filaments: Primarily myosin.

Sarcomere Structure

The sarcomere is the smallest functional unit of a muscle fiber, defined by repeating arrangements of thick and thin filaments. Key components include:

• 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.

• Z lines: Boundaries between sarcomeres, bisect I bands.

• Titin: Elastic protein that maintains alignment and restores resting length.

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 G-actin, preventing myosin binding.

  • Troponin: Binds tropomyosin and calcium ions; moves tropomyosin off active sites when calcium is present.

• Thick filaments contain:

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

  • Titin: Core protein that acts as a molecular spring.

Sliding Filament Theory

During contraction, thin filaments slide toward the M line alongside thick filaments, causing the H and I bands to narrow, the zones of overlap to widen, and Z lines to move closer together. The A band remains constant in width.

Neuromuscular Junction and Muscle Contraction

Excitable Membranes and Action Potentials

• Muscle fibers maintain a negative resting membrane potential (about -85 mV).

• Depolarization (Na+ influx) and repolarization (K+ efflux) allow action potentials to propagate along the sarcolemma.

Neuromuscular Junction (NMJ)

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

• Axon terminal: Releases the neurotransmitter acetylcholine (ACh).

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

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

Excitation–Contraction Coupling

The action potential travels down T tubules, triggering calcium release from the SR. Calcium binds to troponin, moving tropomyosin and exposing actin active sites for contraction to begin.

The Contraction Cycle

1. Calcium ions arrive at the zone of overlap in the sarcomere.

2. Calcium binds to troponin, exposing active sites on actin.

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

4. Myosin heads pivot (power stroke), pulling actin toward the M line.

5. ATP binds to myosin, causing detachment from actin.

6. Myosin reactivation occurs as ATP is hydrolyzed.

Relaxation and Rigor Mortis

• Relaxation occurs when stimulation ends, ACh is broken down, calcium is reabsorbed, and active sites are covered by tropomyosin.

• Rigor mortis is the post-mortem stiffening of muscles due to lack of ATP, preventing cross-bridge detachment.

Muscle Tension and Contraction Types

Mechanisms of Tension Production

• The amount of tension produced depends on the number of power strokes, sarcomere length (length-tension relationship), and frequency of stimulation.

• Maximum tension is achieved at optimal sarcomere length, where the greatest number of cross-bridges can form.

Types of Muscle 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.

• ATP is generated by:

  • Creatine phosphate (CP) system (short-term energy).

  • Glycolysis (anaerobic metabolism).

  • Aerobic metabolism (mitochondrial, oxygen-dependent).

Muscle Metabolism at Different Activity Levels

• At rest: Fatty acids are used, and energy is stored as glycogen and CP.

• Moderate activity: ATP is generated primarily by aerobic breakdown of glucose.

• Peak activity: Glycolysis predominates, leading to lactic acid buildup and fatigue.

Muscle Fiber Types and Performance

Types of Skeletal Muscle Fibers

• Fast fibers: Large, powerful, fatigue quickly (white muscles).

• Slow fibers: Smaller, more endurance, rich in myoglobin (red muscles).

• 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 size and elasticity, and recovery from injury is reduced.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

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

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

• Automaticity: Can contract without neural stimulation (pacemaker cells).

Smooth Muscle Tissue

• Found in walls of hollow organs and blood vessels.

• Cells are spindle-shaped, non-striated, with a single nucleus.

• Contraction is regulated by calcium binding to calmodulin, not troponin.

• Can maintain tone and contract over a wide range of lengths (plasticity).