Chapter 06 Skeletal Muscle Fiber

                         

Chapter 05 🔗:👇🏻👇🏻👇🏻

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Introduction to Muscle Types

The human muscle tissue is broadly classified into three major types based on its structural organization, physiological control, and anatomical location: skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle makes up the vast majority of our body mass and is under voluntary control by the somatic nervous system. Its fibers feature distinct microscopic striations caused by the highly organized, repeating alignment of actin and myosin myofilaments within functional units called sarcomeres. Smooth muscle, on the other hand, lacks these striations because its contractile filaments are arranged in an intricate, lattice-like framework rather than organized sarcomeres. It operates involuntarily under the control of the autonomic nervous system and local hormones, regulating the caliber of blood vessels and propelling contents through hollow visceral organs like the digestive tract. Lastly, cardiac muscle is unique to the heart wall. while it shares the striated appearance of skeletal muscle, it functions involuntarily and features specialized cellular junctions known as intercalated discs. These discs allow rapid electrical coupling between adjacent cells, enabling the entire heart muscle to contract as a single functional syncytium to pump blood continuously throughout the body.

• Skeletal muscle makes up about 40% of body weight.
• Smooth and cardiac muscles make up another 10%.
• Basic contraction principles apply to all three types.

Skeletal Muscle Fiber

• Skeletal muscles contain numerous fibers (10 to 80 micrometers wide).
• Each fiber is made of smaller subunits.
• Fibers usually extend the entire length of the muscle.
• Most fibers have only one central nerve ending.

The Sarcolemma

• The sarcolemma is the thin outer muscle fiber membrane.
• It includes a plasma membrane and polysaccharide-collagen coat.
• Ends of the sarcolemma fuse with tendon fibers.
• Tendon fibers connect muscles to bones.

Myofibrils (Actin and Myosin)

• Each muscle fiber contains hundreds to thousands of myofibrils.
• Each myofibril has 1500 myosin and 3000 actin filaments.
• Myosin filaments are thick; actin filaments are thin.
• Filaments overlap, creating alternating light and dark bands.
• I bands: Light bands containing only actin filaments.
• A bands: Dark bands containing myosin and overlapping action.
• Cross-bridges: Projections on myosin that interact to cause contraction.

The Z Disc and Sarcomere

• Actin filament ends attach to the Z disc.
• Z discs connect myofibrils across the muscle fiber.
• Z discs give muscle its striated (striped) appearance.
• A sarcomere is the area between two Z discs.
• The contracted sarcomere length is about 2 micrometers.
• Full filament overlap produces maximum contractile force.

Tiny Molecules

• Tiny is a large, springy protein holding filaments aligned.
• It is one of the body's largest protein molecules.
• One elastic end attaches to the Z disc.
• The other end connects to the thick myosin filament.
• Tiny acts as a template for building sarcomere filaments.

Sarcoplasm

• Sarcoplasm is the fluid filling spaces between myofibrils.
• It contains potassium, magnesium, phosphate, and enzymes.
• Mitochondria lie parallel to myofibrils to supply ATP energy.

Sarcoplasmic Reticulum

• It is a specialized endoplasmic reticulum surrounding myofibrils.
• It plays a crucial role in controlling muscle contraction.
• Fast-contracting fibers have especially extensive sarcoplasmic reticulum.

Steps of Muscle Contraction

• Action potential travels along a motor nerve.
• Nerve secretes acetylcholine at muscle fiber endings.
• Acetylcholine opens acetylcholine-gated cation channels.
• Sodium ions enter, initiating a muscle action potential.
• Action potential travels along the muscle membrane.
• Depolarization causes sarcoplasmic reticulum to release calcium ions.
• Calcium ions trigger actin and myosin interactions.
• Calcium pumps restore calcium, ending the contraction.

Sliding Filament Mechanism

• Muscle contracts via a sliding filament mechanism.
• Actin filaments slide inward among myosin filaments.
• Z discs are pulled toward myosin filament ends.
• Myosin cross-bridge forces drive the sliding motion.
• Calcium ions activate contractile forces between filaments.
• ATP breakdown to ADP provides required energy.

Structure of Myosin Filaments

• Myosin molecule weight is about 480,000.
• Made of two heavy and four light chains.
• Heavy chains form a double-helix tail.
• Heads contain light chains to control function.
• Filaments contain 200 or more myosin molecules.
• Cross-bridges consist of protruding arms and heads.
• Hinges allow flexible movement at two points.
• Filament length is uniform at 1.6 micrometers.
• Center lacks cross-bridges for 0.2 micrometers.
• Cross-bridge pairs rotate 120 degrees axially.

ATPase Activity of Myosin

• Myosin head acts as an ATPase enzyme.
• Cleaves ATP to energize the contraction process.

Structure of Actin Filaments

• Backbone is a double-stranded F-actin helix.
• Composed of polymerized G-actin molecules.
• Each G-actin has an attached ADP molecule.
• ADP molecules act as active binding sites.
• Active sites occur every 2.7 nanometers.
• Actin filaments measure 1 micrometer in length.
• Bases anchor strongly into the Z discs.

Tropomyosin Molecules

• Tropomyosin wraps spirally around F-actin strands.
• Has a molecular weight of 70,000.
• Measures 40 nanometers in length.
• Blocks active sites during rest.

Troponin Complex

• Troponin attaches intermittently along tropomyosin molecules.
• Composed of three loosely bound protein subunits.
• Troponin I: Binds strongly to actin.
• Troponin T: Binds strongly to tropomyosin.
• Troponin C: Binds strongly to calcium ions.
• Calcium binding initiates the contraction process.