Skeletal Muscle Tissue

Lecture Overview
- Focus on skeletal muscle tissue, starting from general muscle properties.
- Discussion of muscle terminology and the origins of the word "muscle" (derived from Latin "musculus" meaning little mouse).

Major properties include:
- Contractility: Ability of muscle fibers to shorten and produce force.
- Excitability: Muscle's ability to respond to stimuli.
- Extensibility: Ability to be stretched without being damaged.
- Elasticity: Ability of a muscle to return to its original length after stretching.

Key terms used in muscle anatomy:
- Myo / Mys: Prefix related to muscle.
- Sarco: Refers to flesh, as seen in terms like sarcolemma (muscle cell membrane) and sarcoplasm (muscle cell cytoplasm).

Main functions of skeletal muscles include:
- Movement: Enable the movement of bones and bodily motion.
- Sphincters: Control passageways throughout the body.
- Joint Stabilization: Support joints during movement.
- Heat Production: Generate heat through contractions (e.g., during shivering or exercise).

Composition and Characteristics:
- Comprises about 40% of body weight.
- Cells are striated, indicating the presence of sarcomeres, and are under voluntary control.
- Involuntary movements may lead to unwanted twitches.

Cardiac Muscle: Involuntary and striated, found in the heart.
Smooth Muscle: Involuntary and non-striated, located in walls of hollow organs.

Connective tissue components:
- Epimysium: Sheath that covers the entire muscle.
- Perimysium: Surrounds each fascicle (bundle of muscle fibers).
- Endomysium: Fine connective tissue that wraps each individual muscle fiber.
Connective tissues provide structure, elasticity, and carry blood vessels and nerves to muscle fibers.

Fascicles: Bundles of muscle fibers similar to a bundle of sticks (root of the term comes from Latin "fasces").

Attachments can be classified as:
- Direct Attachment: Connective tissue fibers are short and attach directly to bone.
- Indirect Attachment: Involves tendons or aponeurosis (e.g., abdominal muscles).
Muscle attachments create specific anatomical landmarks (e.g., tubercles, trochanters) where tendons anchor to bone.

Muscle Fiber Characteristics:
- Elongated, cylindrical cells ranging in diameter from 10 to 100 micrometers and lengths of centimeters, forming multinucleate structures due to the fusion of embryonic cells.
- Nuclei are located peripherally to avoid interrupting contraction processes.

Myofibrils: Long, cylindrical organelles making up about 80% of muscle cell cytoplasm; consist of repeating segments called sarcomeres, the functional units of muscles.
Striations: Result from the arrangement of myofibrils into alternating dark and light bands:
- A Bands: Full lengths of thick (myosin) filaments containing some overlapping thin (actin) filaments.
- I Bands: Regions containing thin filaments only, located between A bands.
The sarcomere is delineated by Z discs, where actin filaments attach.

Z Disc: The boundary of each sarcomere.
H Zone: The center area of the A band where there are no thin filaments.
M Line: The middle line aiding in holding the thick filaments together.
Differences in light refraction lead to the designations of A bands (anisotropic) and I bands (isotropic).

Proposed in 1954, involving the interaction of actin and myosin during contraction.
The model explains that:
- Myosin heads attach to binding sites on actin, causing the thin filaments to slide towards the sarcomere center.
- Requires ATP and the release of calcium ions from the sarcoplasmic reticulum, which triggers the contraction process.

Sarcoplasmic Reticulum: Specialized smooth endoplasmic reticulum that stores calcium.
When stimulated, calcium is released into the cytoplasm, activating the sliding filament mechanism.
Structure includes terminal cisterns paired with T tubules, forming a triad essential for contraction.

Concentric Contractions: Muscle shortens to perform work (e.g., lifting a weight).
Eccentric Contractions: Muscle generates force while lengthening (e.g., lowering a weight).

The length of muscle fibers remains unchanged as contraction occurs.
Z discs move closer during contraction, shortening the sarcomere, while A bands remain the same length.
Elastic structures such as titin – a large protein – maintains the arrangement and facilitates muscle elasticity during contraction and stretching.

Review of intricate structures within skeletal muscle tissue highlights the significance of connective tissues, muscle fiber composition, and functional mechanisms behind muscle contraction. Understanding these principles is crucial for grasping how muscles perform their roles in the human body.