Muscle Tissue

Types and Functions of Muscle Tissue

Muscle tissue is a primary tissue responsible for contraction and movement in the human body. There are three main types of muscle tissue, each with distinct 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 entrances and exits, maintaining body temperature, and storing nutrients.

Organization of Skeletal Muscle

Skeletal muscles are composed of muscle tissue, connective tissues, blood vessels, and nerves. The organization of skeletal muscle involves 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 cells (muscle fibers).

The collagen fibers from these layers converge to form tendons (bundles) or aponeuroses (sheets), which attach muscles to bones or other muscles.

Skeletal Muscle Fibers

Skeletal muscle fibers are large, multinucleate, and striated due to the arrangement of myofibrils. The sarcolemma (plasma membrane) surrounds the sarcoplasm (cytoplasm) and has a resting membrane potential. Transverse tubules (T tubules) transmit action potentials deep into the cell, ensuring coordinated contraction.

The sarcoplasmic reticulum (SR) surrounds each myofibril and forms terminal cisternae, which, together with T tubules, form a triad. The SR stores and releases calcium ions, essential for muscle contraction.

Myofibrils are collections of myofilaments responsible for contraction. There are two types of myofilaments:

• Thin filaments: Composed primarily of actin.

• Thick filaments: Composed primarily of myosin.

Sarcomere Structure

Sarcomeres are the smallest contractile units of muscle fibers, consisting of organized thin and thick filaments. The arrangement of these filaments produces visible bands:

• A bands: Dark bands, length of thick filaments.

• M line: Center of A band, stabilizes thick filaments.

• H band: Only thick filaments, either side of M line.

• Zone of overlap: Region where thick and thin filaments overlap.

• I bands: Light bands, only thin filaments.

• Z lines: Bisect I bands, mark boundaries between sarcomeres.

Myofilament Composition

Thin filaments contain:

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

• Tropomyosin: Covers active sites, prevents actin-myosin interaction.

• Troponin: Holds tropomyosin in place, binds calcium ions.

Thick filaments contain myosin molecules with tails (bind to other myosin) and heads (bind to actin). Titin stretches from the M line to the Z line, acting as a molecular spring.

Sliding-Filament Theory

During contraction, thin filaments slide toward the M line alongside thick filaments. This causes:

• H and I bands to narrow

• Zones of overlap to widen

• Z lines to move closer together

• A band width remains constant

Muscle Contraction

Neuromuscular Junction and Excitable Membranes

The neuromuscular junction (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 membrane of the muscle fiber.

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

Excitation–Contraction Coupling

The action potential travels down T tubules to triads, triggering calcium release from the SR. Calcium binds to troponin, moving tropomyosin off actin's active sites, allowing contraction to begin.

Contraction Cycle and Cross-Bridge Formation

The contraction cycle involves:

1. Arrival of calcium ions

2. Active-site exposure

3. Cross-bridge formation

4. Power stroke

5. Detachment of myosin head

Muscle Tension and Relaxation

Muscle tension is the pull force generated during contraction. The amount of tension depends on:

• Number of power strokes

• Resting length of muscle fiber (length-tension relationship)

• Frequency of stimulation

Energy for Muscle Contraction

ATP Generation

ATP is the only direct energy source for muscle contraction. Muscles store ATP and creatine phosphate (CP) for short-term energy. ATP is generated by:

• Glycolysis: Anaerobic breakdown of glucose to pyruvate.

• Aerobic metabolism: Oxygen-dependent process in mitochondria.

Muscle Metabolism at Different Activity Levels

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

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

• Peak activity: ATP demand exceeds oxygen supply; glycolysis and lactic acid production increase.

Muscle Performance

Types of Muscle Fibers

• Fast fibers: Contract quickly, high force, fatigue rapidly.

• Slow fibers: Contract slowly, less force, high endurance.

• Intermediate fibers: Mid-sized, moderate endurance.

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to training.

• Atrophy: Reduction in muscle size due to inactivity.

Cardiac and Smooth Muscle Tissue

Cardiac Muscle Tissue

Cardiac muscle cells are small, branched, striated, and have a single nucleus. They are connected by intercalated discs, which allow synchronized contraction. Cardiac muscle is almost entirely dependent on aerobic metabolism and exhibits automaticity.

Smooth Muscle Tissue

Smooth muscle cells are long, slender, spindle-shaped, and nonstriated. They are found in various organ systems and regulate functions such as blood pressure, airflow, and movement of materials. Smooth muscle contracts via excitation–contraction coupling involving calmodulin and myosin light chain kinase.

Summary Table: Types of Muscle Tissue

Example: Skeletal muscle contraction enables voluntary movements such as lifting weights, while cardiac muscle contraction maintains blood circulation, and smooth muscle contraction regulates blood vessel diameter.