Histology of Muscular Tissue

Study Notes

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

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 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 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

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 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 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

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)

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

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 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