Skeletal Muscle Tissue: Types and Structure

Skeletal Muscle Tissue Aims

• Understanding
• Microscopic structures refer to the small-scale components of skeletal muscle tissues, including myofibrils, sarcomeres, and specialized organelles that facilitate muscle function.
• Macroscopic anatomy
• Muscle contraction mechanisms
• Motor units
• Types of skeletal muscles

Key Researchers

• Draper & Hodge were researchers in the field.
• Jean Hanson & Huxley

Cell, Tissue, Muscle Types

• Cell is the structural unit of living organisms
• Tissue is a group of cells with similar functions
• Epithelial, Connective, Muscle and Nervous are the 4 main types of tissue
• Skeletal muscle (organ) helps with the locomotor system.
• Cardiac muscle helps with the cardiovascular system.
• Smooth muscle helps with the gastrointestinal system

Skeletal Muscle Characteristics

• Contractility enables muscles to shorten and generate force.
• Excitability allows muscles to respond to stimuli.
• Elasticity enables muscles to return to their original length.
• Extendability allows muscles to be stretched.

Structure and Organizational Levels of Skeletal Muscle

• Muscle (organ) consists of thousands of muscle cells
• Muscle enclosed by the epimysium
• Fascicle is a discrete bundle of muscle cells surrounded by perimysium.
• Muscle fiber is an elongated multinucleate cell surrounded by endomysium.
• Myofibrils are rodlike contractile elements.
• Sarcomere is the contractile unit.
• Z to Z is the distance of the Sarcomere.
• Myofilaments are the thin (actin) and thick (myosin) filament.
• Many muscle fibers are enclosed by perimysium to form a muscle fascicle
• Many muscle fascicles are enclosed by epimysium to form a muscle.
• Skeletal muscle is a composite tissue including muscle and fibrous connective tissue.
• Capillary networks are in the endomysium.

Muscle Attachments

• Origin refers to the attachment of a muscle on the less movable bone.
• Insertion refers to the attachment of a muscle on the more movable bone.
• Epi/Peri/Endo + fibrous connective tissue fuse with outer fibrous layer of periosteum and fuse with bone.
• Direct or indirect attachments to bone are possible

Muscle Structure

• Myofibrils are parts of a muscle fiber that shows striations.
• Dark A bands and light I bands contribute to the striations in muscle fibers.
• Sarcolemma is a part of a muscle fiber that surrounds the myofibril.
• One myofibril is enlarged to show the myofilaments.
• Each sarcomere extends from one Z disc to the next.
• M line is the middle of the sarcomere.
• The sliding of the thin filaments past the thick filaments produces muscle shortening

Sarcomere Structure

• Thin filaments contain actin, troponin, and tropomyosin.
• Thick filaments contain myosin.
• I band contains thin filaments only.
• H zone contains thick filaments only.
• M line contains thick filaments linked by accessory proteins.

Sarcoplasmic Reticulum

• The sarcoplasmic reticulum (SR) runs longitudinally
• Terminal cistern are each located at A - I bands.
• Both store Ca+++++++
• T tubules are deep invaginations of the sarcolemma and run between pairs of terminal cisterns
• T tubules conduct impulses to deep regions of muscle fibers.
• Titan goes from Z to Thick filament (keep it in place).
• Titan unfolds / refolds when muscle/sarcomere stretches & relaxes
• Transverse tubules are also present

Muscle Contraction

• Concentric contraction involves muscle shortening.
• Eccentric contraction involves muscle lengthening.
• Isometric contraction involves constant muscle length.
• Energy is required for muscle contraction.

How Myofilaments Interact

• Myosin-binding sites on actin are hidden.
• Calcium (Ca2+) binds to troponin, which causes tropomyosin to move away from myosin-binding sites
• Energized cross-bridges bind to actin and generates force.

Muscle Innervation and Contraction

• Motor Unit = a motor neuron and all muscle fibers it innervates.
• Nerve impulses stimulate the release of Neurotransmitter acetylcholine (ACh) into the synaptic cleft to stimulate muscle contraction.
• Acetylcholine (ACh) antagonists inhibit the action of acetylcholine receptors.
• Acetylcholinesterase inhibitors block the breakdown of acetylcholine.

Muscle Fiber Types

• Muscle fibers are categorized by how they make energy (ATP) and how fast they contract (by breaking down ATP).

ATP Production and Fiber Types

• Fibers produce ATP using oxygen (O2) which are oxidative fibers
• ATP is also is produced via glycolysis (without using O2) which are glycolytic fibers
• Fast fibers are oxidative / glycolytic.
• Slow fibers are oxidative.

Detailed Features of Each Type

• Fast Oxidative fibers have Oxygen, numerous mitochondria, numerous capillaries, high myoglobin content, fatigue resistance, intermediate fibers, faster contraction.
• Fast Glycolytic fibers are anaerobic, have few mitochondria, few capillaries, low myoglobin content, glycosomes for glycogen, more myofilaments, pale appearance, fast contraction, and fatigue easily.
• Slow Oxidative fibers have Oxygen, numerous mitochondria, numerous capillaries, high myoglobin content, fatigue resistance, thin fibers, and slow contraction.
• Muscles use direct phosphorylation, anaerobic glycolysis, and aerobic respiration to generate ATP.
• Direct phosphorylation provides 1 ATP, lasts 15 seconds.
• Anaerobic pathway provides 2 ATP per glucose, lasts 30-40 seconds.
• Aerobic pathway provides 32 ATP per glucose, and lasts for hours.

Muscle Disorders

• Rhabdomyolysis.
• Muscular dystrophy.
• Fibromyalgia.
• Duchenne muscular dystrophy is X Linked recessive and has a 2-6 year age onset.
• Duchenne is characterized by Absent Dystrophin protein and affects males.
• Skeletal muscles can increase or undergo hypertrophy but never undergo mitosis.
• Muscles respond to androgens
• Sarcopenia.

Learn about skeletal muscle tissue, its microscopic structures, and macroscopic anatomy. Understand muscle contraction mechanisms, motor units, and the different types of skeletal muscles. Key researchers like Draper & Hodge, Jean Hanson & Huxley are mentioned.