Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is essential for movement and comprises nearly half of the body's mass. It transforms chemical energy (ATP) into mechanical energy, enabling force generation. There are three primary types of muscle tissue, each with distinct characteristics and functions.

• Skeletal Muscle: Attached to bones or skin, responsible for voluntary movements. Skeletal muscle fibers are long, cylindrical, striated, and multinucleated. They contract rapidly but tire easily.

• Cardiac Muscle: Found only in the heart, responsible for pumping blood. Cardiac muscle cells are striated, branched, and involuntary. They contract at a steady rate regulated by the heart's pacemaker.

• Smooth Muscle: Located in the walls of hollow organs (e.g., stomach, bladder, airways). Smooth muscle cells are non-striated, spindle-shaped, and involuntary. They control movements such as digestion and blood vessel constriction.

Muscle Terminology

• Prefixes: myo-, mys-, and sarco- refer to muscle.

• Sarcoplasm: Muscle cell cytoplasm.

• Muscle Fiber: Elongated muscle cell found in skeletal and smooth muscle.

Characteristics and Functions of Muscle Tissue

Characteristics

All muscle tissues share four main properties:

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after stretching.

Functions

Muscle tissue performs four vital functions:

• Produce Movement: Responsible for locomotion and manipulation (e.g., walking, digesting, pumping blood).

• Maintain Posture and Body Position: Stabilizes the body during activity.

• Stabilize Joints: Reinforces and supports joints.

• Generate Heat: Muscle contraction produces heat, helping maintain body temperature.

Skeletal Muscle Structure

Gross Structure

Skeletal muscle is an organ composed of muscle fibers, nerves, blood vessels, and connective tissues. Each muscle receives a nerve, artery, and veins, ensuring adequate oxygen, nutrients, and waste removal.

Connective Tissue Sheaths

Connective tissue sheaths support and reinforce skeletal muscle:

• Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.

• Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).

• Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Muscle Attachments

• Direct (Fleshy) Attachments: Epimysium fused to periosteum of bone or perichondrium of cartilage.

• Indirect Attachments: Connective tissue wrappings extend beyond muscle as tendons (ropelike) or aponeuroses (sheetlike).

Microscopic Structure of Skeletal Muscle Fibers

Muscle Fiber Anatomy

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei. The sarcolemma is the plasma membrane, and the sarcoplasm is the cytoplasm, containing glycosomes (for glycogen storage) and myoglobin (for O2 storage).

Specialized Structures

• Myofibrils: Densely packed, rodlike elements accounting for ~80% of muscle cell volume. Responsible for striations and contraction.

• Sarcoplasmic Reticulum (SR): Network of smooth endoplasmic reticulum tubules surrounding each myofibril. Regulates intracellular Ca2+ levels, stores, and releases Ca2+.

• T Tubules: Tubes formed by protrusion of sarcolemma deep into cell interior, increasing surface area and allowing electrical nerve transmissions to reach deep into the muscle fiber.

Myofibril Features

• Striations: Alternating dark (A bands) and light (I bands) regions.

• Sarcomeres: Smallest contractile unit, consisting of area between Z discs. Sarcomeres align end to end along myofibril.

• Myofilaments: Actin (thin) and myosin (thick) filaments arranged within sarcomere.

Molecular Composition of Myofilaments

• Thick Filaments: Composed of myosin protein with two heavy and four light polypeptide chains. Myosin heads form cross bridges during contraction.

• Thin Filaments: Composed of actin protein. G actin subunits form F actin strands, which twist together. Regulatory proteins tropomyosin and troponin are bound to actin.

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic Reticulum (SR)

The SR is a network of smooth endoplasmic reticulum tubules surrounding each myofibril. Terminal cisterns form perpendicular cross channels at the A–I band junction. The SR regulates intracellular Ca2+ levels, storing and releasing Ca2+ as needed for muscle contraction.

T Tubules and Triad Relationships

• T Tubules: Allow electrical impulses to penetrate deep into the muscle fiber, triggering Ca2+ release from the SR.

• Triad: Area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere.

• Voltage Sensors: T tubule proteins act as voltage sensors, changing shape in response to electrical current and causing SR proteins to release Ca2+ into the cytoplasm.

Clinical Application: Duchenne Muscular Dystrophy (DMD)

Overview

DMD is the most common and serious form of muscular dystrophies, appearing in childhood and inherited as a sex-linked recessive disease. It is caused by a defective gene for dystrophin, a protein that links thin filaments to the extracellular matrix and stabilizes the sarcolemma.

• Symptoms: Clumsiness, frequent falls, progressive muscle weakness.

• Pathology: Sarcolemma tears easily, allowing excess calcium entry, damaging contractile fibers, leading to inflammation and loss of regenerative capacity.

• Treatment: No cure; corticosteroids provide temporary relief. Supportive care can extend life into the 30s and beyond. Gene therapy was approved in 2023.

Progression

The disease progresses from extremities upward, eventually affecting head, chest, and cardiac muscles.