Skeletal Muscle Tissue

Skeletal Muscle Tissue

Module 9.1: Overview of Skeletal Muscle Tissue

  • Skeletal muscle tissue plays a crucial role in enabling body movement and other vital functions.

  • Composition of muscle tissue:

    • Predominantly composed of muscle cells, which are highly specialized for contraction.

    • There are three types of muscle tissue:

    • Skeletal Muscle: Focus of this chapter

    • Cardiac Muscle

    • Smooth Muscle

  • Muscle Fiber: Each cell in skeletal muscle tissue is referred to as a muscle fiber.

  • Skeletal Muscles: Defined as organs primarily composed of skeletal muscle tissue, intertwined with:

    • Connective tissue

    • Nerves

    • Blood vessels

    • Attached to bones, hence the term 'skeletal muscle'.

Module 9.1: Types of Muscle Tissue

  • Skeletal Muscle Tissue:

    • Moves the body by pulling on skeletal bones.

    • Functions include voluntary control and exhibiting striations (striped appearance).

  • Cardiac Muscle Tissue:

    • Facilitates blood propulsion through heart contractions.

    • Involuntary and striated.

  • Smooth Muscle Tissue:

    • Responsible for moving fluids and solids within the digestive tract, regulating artery diameter, among other functions.

    • Involuntary and non-striated.

Module 9.1: Functions of Skeletal Muscle Tissue

  1. Produce Skeletal Movement:

    • Muscle contractions cause movement by pulling on tendons connected to bones.

    • Movements range from simple actions (e.g., extending the arm) to complex coordinated activities (e.g., typing).

  2. Maintain Posture and Body Position:

    • Tension in skeletal muscles sustains posture, such as keeping the head still while reading.

    • Essential for maintaining balance and the ability to sit or stand upright.

  3. Support Soft Tissues:

    • The abdominal wall and pelvic cavity's floor include layers of skeletal muscle that support organ weight and shield internal tissues from injury.

  4. Guard Entrances and Exits:

    • Sphincter muscles control openings of the digestive and urinary tracts, enabling voluntary control over swallowing, defecation, and urination.

  5. Maintain Body Temperature:

    • Muscle contractions require energy, generating heat as a by-product.

    • This heat is crucial for sustaining the body's normal temperature.

  6. Store Nutrient Reserves:

    • In cases of protein/caloric deficiency, skeletal muscle can break down contractile proteins into amino acids for energy or glucose synthesis.

Module 9.2: Structure of Skeletal Muscle

  • Composition of Skeletal Muscle: Composed of muscle tissue, connective tissues, blood vessels, and nerves.

  • Muscle Fiber Arrangement:

    • Entire skeletal muscle is an organ containing muscle fibers.

    • Individual muscle fibers are enclosed by endomysium, a layer of areolar connective tissue.

    • Muscle fibers are grouped into bundles termed fascicles, which are surrounded by perimysium, a fibrous connective tissue layer.

    • Both perimysium and endomysium encompass blood vessels and nerves supplying the muscle fibers.

    • An entire muscle is encased by epimysium, a dense collagen fibrous layer.

Module 9.2: Muscle Layer Functionality

  • Epimysium:

    • Separates the muscle from surrounding tissues and binds to the fascia of the muscle.

  • Fascia:

    • Deep fascia: Located between adjacent muscles.

    • Superficial fascia: Lies just beneath the skin and merges with deep fascia closer to the surface.

  • Tendons and Aponeuroses:

    • At muscle ends, collagen fibers from different connective tissue layers merge to form tendons or a broad sheet known as an aponeurosis.

    • Tendons attach the muscle at a specific point, while aponeuroses cover broader areas, influencing more than one bone during contraction.

Module 9.2: Muscle Fiber Specifications

  • Skeletal Muscle Cells: Highly specialized with unique terminology and features.

  • Nucleus: Muscle cells typically possess multiple nuclei located at the cell's periphery, facilitating increased gene copies for protein/enzyme production.

  • Sarcoplasm: Contains myofibrils, mitochondria, and the sarcoplasmic reticulum.

  • Myofibrils: Small cylindrical structures filled with muscle fibers, arranged parallel to the fiber's length.

  • Transverse Tubules: Infoldings of the sarcolemma that extend into the sarcoplasm, facilitating action potential propagation across the muscle fiber.

Module 9.3: Myofibril Composition

  • Myofilaments: Consist of thick (mostly myosin) and thin filaments (mostly actin).

  • Sarcomeres: Small contractile units within myofibrils, each myofibril consists of approximately 10,000 sarcomeres aligned end-to-end.

    • Z Lines: Mark sarcomere boundaries and consist of proteins (actinins) that interconnect thin filaments from adjacent sarcomeres.

    • A Band: Contains thick filaments; the width remains unchanged during contraction.

    • I Band: Houses only thin filaments; it shortens during contraction.

    • H Band: A lighter zone flanking the M line, containing only thick filaments.

Module 9.3: Charge and Membrane Potential

  • Skeletal Muscle Fiber Membrane: Selectively permeable and polarized due to an uneven charge distribution.

    • Resting membrane potential:

    • Neurons ≈ –70 mV

    • Muscle fibers ≈ –85 mV

  • Action Potential: Initiated by nerve impulses causing the membrane potential to temporarily reverse, allowing for muscle contraction.

Module 9.4: Sliding Filament Theory

  • Contractile mechanism involves thin filaments (F-actin) sliding over thick filaments (myosin).

  • Myofilament Interactions: During contraction:

    • H and I Bands: Reduce in size.

    • Zones of Overlap: Become larger.

    • Z Lines: Move closer together.

    • A Band Width: Remains unchanged.

Module 9.5: Excitation-Contraction Coupling

  • Neuromuscular Junction (NMJ): Site where a motor neuron interacts with a muscle fiber, typically with one NMJ per muscle fiber.

  • Skeletal Muscle Fiber Contraction: Triggered by action potentials propagated along T tubules, leading to calcium release from the sarcoplasmic reticulum.

  • Acetylcholine (ACh) is released into the synaptic cleft, binding to receptors on the sarcolemma, initiating the contracting process of muscle fibers.

Module 9.9: Muscle Activation and Fatigue

  • Muscle Twitch: Quick contraction-relaxation cycle in response to a stimulus.

  • Phases of Muscle Twitch:

    • Latent Period: Initial tension generation without visible contraction.

    • Contraction Phase: Development of peak tension as the muscle shortens.

    • Relaxation Phase: Decrease of tension, returning muscle to resting length.

  • Factors Leading to Fatigue:

    • Accumulation of lactic acid and decreased pH causing diminished calcium binding to troponin.

Module 9.14: Skeletal Muscle Fiber Types

  • Three Major Types:

    • Fast Fibers: Predominant fibers in skeletal muscle.

    • Slow Fibers: Suitable for prolonged contraction.

    • Intermediate Fibers: Exhibit fatigue resistance, resembling fast fibers.

  • Muscle composition varies according to function (e.g., slow fibers in back muscles vs. fast fibers in eye muscles).

Module 9.15: Muscle Hypertrophy and Atrophy

  • Hypertrophy: Enlargement through repetitive stimulation, resulting in:

    • Increased mitochondria and glycogen.

    • Enhanced myofibril synthesis, leading to increased fiber diameter.

  • Atrophy: Decrease in muscle size due to various factors, initially reversible unless muscle fibers die.