Histology of Muscular Tissue

Histology of Muscular Tissue

• Muscle tissue is specialized for producing movement and displays significant contractility
• Muscle fibers are categorized based on structural (striated or non-striated) and functional (voluntary or involuntary) characteristics
• Muscle cells are elongated, narrow structures called muscle fibers
• The cell membrane of a muscle fiber is the sarcolemma
• The cytoplasm is called sarcoplasm
• Smooth endoplasmic reticulum is called sarcoplasmic reticulum
• Mitochondria are called sarcosomes

Types of Muscle Tissue

• There are three main types of muscle fibers: skeletal, cardiac, and smooth
• Skeletal muscle is striated and voluntary, connected to the skeleton
• Cardiac muscle is striated and involuntary, found in the heart
• Smooth muscle is non-striated and involuntary, found in the walls of internal organs and blood vessels

Skeletal Muscle

• Skeletal muscle fibers are long, cylindrical, and multinucleated
• They have a large diameter, typically up to 0.1 mm
• They are non-branching (except in some specific areas like the face and tongue)
• The cytoplasm (sarcoplasm) is acidophilic and contains transverse striations due to myofibrils
• Nuclei are flattened and situated peripherally under the sarcolemma
• Light microscopy (LM) reveals alternating dark (A) and light (I) bands
• The dark Z line bisects the light I band
• The pale H zone bisects the dark A band
• The dark M line bisects the H zone
• The distance between two Z lines is called a sarcomere
• A sarcomere represents the contractile unit

Myofibrils

• Myofibrils are thread-like structures within the sarcoplasm, running parallel to the fiber's long axis
• They show alternating dark (A) and light (I) bands
• Each I band is bisected by a dark Z line
• Each A band is bisected by a pale H zone
• The H zone is bisected by a dark M line
• The region between two Z lines forms the sarcomere
• Myofibrils show the regular arrangement responsible for transverse striations seen under LM
• The sarcomere is the main contractile unit

Electron Microscopy (EM) of Skeletal Muscle Fiber

• The sarcoplasm contains myofibrils
• The sarcoplasm also contains plenty of mitochondria arranged peripherally
• The sarcoplasm contains myoglobin and glycogen
• Transverse triad (T-triad): consists of T-tubules and sarcoplasmic reticulum (sER)

T-tubules, Sarcoplasmic Reticulum (and T-triad)

• T-tubules are invaginations of the sarcolemma (cell membrane) into the sarcoplasm
• They encircle the myofibrils at the junction of A and I bands
• Terminal cisternae of the sarcoplasmic reticulum (sER) surround T-tubules
• One T-tubule + 2 terminal cisternae constitutes the T-triad
• The T-triad plays a critical role in depolarization and calcium ion release during muscle contraction

Myofilaments

• Myofilaments are the two major types of proteins (thick and thin) within the sarcomere, responsible for contraction
• Thick filaments (myosin) are located within the A band, primarily made of myosin molecules
• Thin filaments (actin) are attached to the Z line and extend into the I band and part of the A band, made of actin and associated proteins like tropomyosin and troponin
• Actin is a polymer of globular G-actin molecules wrapped around each other

Molecular Structure of Myofilaments (Actin)

• Formed of two chains of F-actin filaments (polymer of globular G actin)
• Tropomyosin is a 40 nm long coil of polypeptide chains located in the groove between the two twisted actin strands
• Troponin is a complex of three subunits: TnT (attaches to tropomyosin), TnC (binds Ca2+), and TnI (binds to actin)

Molecular Structure of Myofilaments (Myosin)

• Formed of 200-300 myosin II molecules
• Each myosin molecule has a head and a tail
• Composed of two heavy chains and four light chains

Sliding Filament Theory of Muscle Contraction

• Muscle contraction involves the sliding of actin filaments over myosin filaments
• An impulse triggers the release of calcium ions from the sarcoplasmic reticulum
• Calcium ions bind to troponin, altering its conformation
• The altered troponin shifts tropomyosin, exposing myosin-binding sites on actin
• Myosin heads bind to actin, pulling actin filaments towards the M line, shortening the sarcomere
• ATP hydrolysis powers the cycling process of the myosin-actin interactions

Myoglobin

• Pigment protein (red to brown) in muscle fibers
• Provides oxygen to muscle

Connective Tissue Coverings of Skeletal Muscle

• Epimysium: Covers the whole muscle
• Perimysium: Surrounds bundles of muscle fibers
• Endomysium: Surrounds individual muscle fibers

Regeneration of Skeletal Muscle

• Skeletal muscle fibers regenerate through regenerative cells called satellite cells
• Satellite cells are located between the sarcolemma and basement membrane of skeletal muscle fibers
• They differentiate into myoblasts and fuse to form new skeletal muscle fibers

Smooth Muscle

• Smooth muscle is non-striated and involuntary
• Located in the walls of internal organs and blood vessels
• Smooth muscle fibers are short, spindle-shaped, single-nucleated, and lack transverse striations (sarcomeres)
• They lack T-tubules and have less-developed sarcoplasmic reticulum, using caveolae (invaginations of the sarcolemma) for impulse transmission
• Smooth muscle fibers regenerate through mitosis