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 include:

• Excitability: 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.

Skeletal muscle functions include producing movement, maintaining posture, supporting soft tissues, guarding body entrances and exits, maintaining body temperature, and storing nutrients.

Organization of Skeletal Muscle

Connective Tissue Layers

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. There are three layers of connective tissue:

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

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

• Endomysium: Surrounds individual muscle fibers and loosely interconnects them.

The collagen fibers of these layers come together at the ends of muscles to form tendons (bundles) or aponeuroses (broad sheets), which attach skeletal muscles to bones.

Blood Supply and Innervation

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

• They contract only when stimulated by the central nervous system, making them voluntary muscles (with some exceptions, such as the diaphragm).

Skeletal Muscle Fibers

Development and Structure

Skeletal muscle fibers are large, multinucleate cells formed by the fusion of embryonic myoblasts. They are also known as striated muscle cells due to the presence of visible striations, which result from the arrangement of myofibrils (collections of contractile myofilaments).

Cellular Components

• Sarcolemma: The plasma membrane of a muscle fiber, surrounding the sarcoplasm (cytoplasm).

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

• Sarcoplasmic reticulum (SR): A network specialized for the storage and release of calcium ions, forming terminal cisternae that attach to T tubules to create triads.

• Myofibrils: Organized collections of myofilaments responsible for muscle contraction.

• Myofilaments: Bundles of contractile proteins, including thin filaments (actin) and thick filaments (myosin).

Sarcomere Structure

The sarcomere is the smallest contractile unit of a muscle fiber, consisting of repeating units of thin and thick 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, stabilizing 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.

• Titin: Elastic protein that maintains alignment and aids in restoring resting length.

Thin and Thick Filaments

• Thin filaments are composed of filamentous actin (F-actin), nebulin, tropomyosin, and troponin. Troponin binds calcium ions, causing tropomyosin to move and expose active sites on actin for myosin binding.

• Thick filaments are composed of myosin molecules, each with a tail and two globular heads. The heads bind to actin during contraction. Titin runs through the core of thick filaments, providing elasticity.

Sliding Filament Theory

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

Neuromuscular Junction and Muscle Contraction

Excitable Membranes and Action Potentials

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

• Depolarization, hyperpolarization, and repolarization are changes in membrane potential that can generate action potentials in excitable cells.

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.

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

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

• Neurotransmitter (acetylcholine, ACh): Released from the axon terminal, binds to receptors on the motor end plate, causing sodium influx and depolarization.

Excitation-Contraction Coupling

This process links the generation of an action potential in the sarcolemma to the start of muscle contraction. The action potential travels down T tubules, triggering calcium release from the SR. Calcium binds to troponin, moving tropomyosin and exposing actin's active sites for myosin binding.

Contraction Cycle and Cross-Bridge Formation

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

• Power stroke: Myosin head pivots, pulling actin toward the M line, using energy from ATP hydrolysis.

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

Relaxation and Rigor Mortis

• Contraction ends when stimulation ceases, ACh is broken down, and calcium is pumped back into the SR.

• Rigor mortis occurs after death when ATP is depleted, preventing cross-bridge detachment and causing sustained contraction.

Muscle Tension and Contraction Types

Mechanisms of Tension Production

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

• Maximum tension is produced at optimal sarcomere length, where the maximum 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.

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

• ATP is also generated by glycolysis (anaerobic) and aerobic metabolism (mitochondria).

Muscle Metabolism at Different Activity Levels

• At rest: Muscles use fatty acids and store glycogen and CP.

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

• Peak activity: ATP demand exceeds oxygen supply; glycolysis predominates, leading to lactic acid buildup and fatigue.

Muscle Performance and Fiber Types

Types of Skeletal Muscle Fibers

• Fast fibers: Large, contract quickly, fatigue rapidly, few mitochondria, white in color.

• Slow fibers: Small, contract slowly, resist fatigue, many mitochondria, high myoglobin, red in color.

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

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle fiber size due to training.

• Atrophy: Decrease in muscle size and power due to inactivity.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

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

• Connected by intercalated discs, allowing synchronized contraction.

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

Smooth Muscle Tissue

• Found in walls of hollow organs and blood vessels.

• Spindle-shaped, non-striated cells with a single nucleus.

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

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