HISTOLOGY OF SKELETAL MUSCLE
Histology is the study of the microscopic structure of animal or plant tissues. The study of human tissue is an example of histology.
The body contains three types of muscle tissue:
- Skeletal muscle
- Smooth muscle
- Cardiac muscle
SKELETAL MUSCLE HISTOLOGY:
Skeletal muscle is an excitable, contractile tissue responsible for maintaining posture and producing body movements through its connection with the skeleton.
It attaches to bones via tendons. The tissue is excitable (responds to stimuli), contractile (generates force), extensible (can be stretched), and elastic (returns to original shape).
TERMINOLOGY:
- Myo-, mys-, sarco-: Prefixes used in muscle terminology.
- Sarcoplasm: Cytoplasm of a muscle cell.
- Sarcolemma: Plasma membrane of a muscle cell.
- Sarcoplasmic reticulum: Specialized endoplasmic reticulum of muscle cells.
- Myofiber: A single muscle fiber or muscle cell.
Skeletal muscle fibers vary in length—from a few millimeters (e.g., stapedius muscle) to a meter (e.g., thigh muscles). The myofibrils inside are composed of actin and myosin filaments organized into sarcomeres, giving the muscle a striated appearance.
Each muscle fiber is multinucleated, formed by the fusion of early cells called myoblasts, creating a structural syncytium. The nuclei lie at the periphery beneath the sarcolemma. Satellite cells (muscle stem cells) aid in muscle regeneration.
STRUCTURE OF SKELETAL MUSCLE:
Skeletal muscle is made up of bundles of muscle fibers wrapped in connective tissue sheaths:
- Endomysium: Surrounds individual muscle fibers; allows exchange of nutrients via small capillaries.
- Perimysium: Surrounds groups of fibers forming fascicles; contains larger blood vessels and nerves.
- Epimysium: Encloses the entire muscle; dense connective tissue with the main neurovascular supply.
TYPES OF SKELETAL MUSCLE FIBERS:
There are three types of skeletal muscle fibers based on contraction speed and metabolism:
1) Type I (Slow Oxidative Fibers):
- Small, red fibers with high myoglobin and mitochondria content.
- Specialized for aerobic activity and resistant to fatigue.
- Found in postural muscles like deep back muscles.
2) Type IIa (Fast Oxidative Glycolytic Fibers):
- Intermediate fibers with many mitochondria and moderate glycogen.
- Capable of both aerobic and anaerobic metabolism.
- Provide sustained power; common in athletes like swimmers.
3) Type IIb (Fast Glycolytic Fibers):
- Large, pale fibers with low myoglobin and mitochondria.
- Rely on anaerobic metabolism; fatigue easily.
- Responsible for quick, powerful movements (e.g., gastrocnemius muscle).
SARCOMERES:
The sarcomere is the functional unit of a skeletal muscle cell (~2.5 μm long), made of actin and myosin filaments arranged in repeating patterns.
Key regions include:
- Z disc: Boundary of each sarcomere; thin actin filaments extend from it.
- H zone: Central region lacking actin filaments.
- M line: Middle of H zone; contains cross-links for myosin filaments.
- A band: Dark region with overlapping actin and myosin.
- I band: Light region with only actin filaments.
SARCOPLASMIC STRUCTURES:
- Sarcolemma: Cell membrane of a muscle fiber.
- Sarcoplasmic reticulum (SR): Stores calcium; surrounds myofibrils.
- Terminal cisternae: Enlarged SR regions that hold calcium; part of the triad.
- T-tubules: Invaginations of sarcolemma; conduct action potentials into the cell.
ACCESSORY PROTEINS:
- Titin: Elastic protein anchoring thick filaments to Z lines, prevents overstretching.
- Tropomodulin: Caps actin filaments to maintain length.
- α-Actinin: Organizes actin filaments into bundles and anchors them to Z lines.
- Desmin: Links sarcomeres together and to the plasma membrane.
- Nebulin: Supports actin alignment and development of muscle tissue.
- Dystrophin: Connects actin filaments to the external lamina; mutations cause muscular dystrophy.
- Myomesin: Anchors thick filaments to the M line.
Detailed Notes:
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