Lecture 15

Muscle Tissue I

Learning Outcomes

  1. Functions and Characteristics of Muscle Tissue

    • Describe major functions, characteristics, and distinguishing structural elements of:

      • Skeletal Muscle

      • Cardiac Muscle

      • Smooth Muscle

  2. Arrangement of Skeletal Myocytes

    • How skeletal myocytes and other tissues are organized to form a skeletal muscle organ.

  3. Structural Components of Skeletal Muscle Fibers

    • Describe the structural components of skeletal muscle fibers, the sarcoplasmic reticulum, and myofibrils.

  4. Arrangement of Myofilaments and Contraction Mechanism

    • Explain how myofilaments are arranged in sarcomeres and describe the sliding filament mechanism of contraction.

  5. Membrane Potential

    • Define membrane potential and explain how a resting membrane potential is achieved in skeletal myocytes.

Types of Muscle Tissue

A. Skeletal Muscle

  • Characteristics: Long, cylindrical, striated muscle fibers; multinucleated cells.

  • Location: Mostly attached to the skeleton.

  • Control: Voluntary.

  • Function: Produces body movement.

B. Cardiac Muscle

  • Characteristics: Short, wide, branching striated muscle cells with intercalated discs; single or binucleated.

  • Location: Heart.

  • Control: Involuntary.

  • Function: Produces heart beating.

C. Smooth Muscle

  • Characteristics: Thin, spindle-shaped muscle cells; usually uninucleated and joined by gap junctions.

  • Location: Walls of hollow organs, skin, and eyes.

  • Control: Involuntary.

  • Functions:

    • Changes diameter of hollow organs.

    • Causes hair to stand erect.

    • Adjusts the shape of the lens and size of the pupil.

Characteristics of Muscle Cells (LO1)

  • Contractility: Ability of protein fibers within myocytes to draw together.

  • Excitability: Responsiveness to stimuli (electrical or chemical).

  • Conductivity: Ability to conduct stimuli (electricity).

  • Extensibility: Capability to stretch up to 3 times resting length.

  • Elasticity: Ability to regain original shape after stretching.

Structure of Muscle Cells

Terminology for Myocyte (Muscle Cell)

  • Sarcoplasm: The cytoplasm of a myocyte.

  • Sarcolemma: The plasma membrane of a myocyte.

  • Sarcoplasmic Reticulum (SR): A modified endoplasmic reticulum that forms a web-like network surrounding myofibrils; its structure varies in the three types of muscle tissue.

  • Myofibrils: Unique cylindrical organelles found in all muscle cells, constituting 50-80% of cell volume; made up of bundles of specialized proteins allowing for contraction; mitochondria are packed between myofibrils.

Arrangement of Skeletal Myocytes (LO2)

  • Fascicle: Bundle of muscle fibers (cells).

  • Perimysium: Connective tissue that collects cells and endomysium into fascicles, continues as tendon.

  • Epimysium: Connective tissue that surrounds the entire muscle, continuing as tendon.

The Skeletal Muscle Fiber (LO3)

Comparison with Erythrocyte

  • Size & Shape

    • Skeletal muscle fiber: Long, cylindrical with multiple nuclei.

    • Erythrocyte: Size similar to red blood cells (for comparison).

Key Features

  • Transverse Tubules (T-tubules): Deep inward extensions of sarcolemma surrounding each myofibril; form a tunnel-like network, continuous with the exterior of the cell and filled with extracellular fluid.

  • Terminal Cisternae: Enlarged sections of the sarcoplasmic reticulum that flank each T-tubule forming a triad (two terminal cisternae plus one corresponding T-tubule).

Myofilament Structure (LO3)

  • Types of Myofilaments:

    • Thick Filaments: Bundles of the contractile protein myosin.

    • Thin Filaments: Composed of proteins actin, tropomyosin, and troponin.

    • Elastic Filaments: A single massive, spring-like structural protein (titin); stabilizes myofibril structure and resists excessive stretching.

Myofilament Arrangement and Sarcomeres (LO4)

Structure of Sarcomere

  • I Band: Contains only thin filaments.

  • Z Disc: In the middle of the I band; composed of structural proteins that anchor thin filaments in place.

  • A Band: Contains both thick and thin filaments; generates tension during contraction.

  • H Zone: Middle of the A band where only thick filaments exist.

  • M Line: Dark line in middle of A band; holds thick filaments in place, anchoring point for elastic filaments.

Sliding Filament Mechanism of Contraction

  • Mechanism:

    • Relaxed sarcomere: H zone and I band are present.

    • Contracted sarcomere: Actin filaments slide along myosin towards the M line, pulling Z-discs closer and reducing the I band width.



Learning Objective 5, which discusses "Membrane Potential," is listed under the "Learning Outcomes" section. However, the detailed explanation and content for this objective are not present within the current provided notes.

Muscle Tissue I

Learning Outcomes

  1. Functions and Characteristics of Muscle Tissue

    • Describe major functions, characteristics, and distinguishing structural elements of:

      • Skeletal Muscle

      • Cardiac Muscle

      • Smooth Muscle

  2. Arrangement of Skeletal Myocytes

    • How skeletal myocytes and other tissues are organized to form a skeletal muscle organ.

  3. Structural Components of Skeletal Muscle Fibers

    • Describe the structural components of skeletal muscle fibers, the sarcoplasmic reticulum, and myofibrils.

  4. Arrangement of Myofilaments and Contraction Mechanism

    • Explain how myofilaments are arranged in sarcomeres and describe the sliding filament mechanism of contraction.

  5. Membrane Potential

    • Define membrane potential and explain how a resting membrane potential is achieved in skeletal myocytes.

Types of Muscle Tissue
A. Skeletal Muscle

  • Characteristics: Long, cylindrical, striated muscle fibers; multinucleated cells.

  • Location: Mostly attached to the skeleton.

  • Control: Voluntary.

  • Function: Produces body movement.

B. Cardiac Muscle

  • Characteristics: Short, wide, branching striated muscle cells with intercalated discs; single or binucleated.

  • Location: Heart.

  • Control: Involuntary.

  • Function: Produces heart beating.

C. Smooth Muscle

  • Characteristics: Thin, spindle-shaped muscle cells; usually uninucleated and joined by gap junctions.

  • Location: Walls of hollow organs, skin, and eyes.

  • Control: Involuntary.

  • Functions:

    • Changes diameter of hollow organs.

    • Causes hair to stand erect.

    • Adjusts the shape of the lens and size of the pupil.

Characteristics of Muscle Cells (LO1)

  • Contractility: Ability of protein fibers within myocytes to draw together.

  • Excitability: Responsiveness to stimuli (electrical or chemical).

  • Conductivity: Ability to conduct stimuli (electricity).

  • Extensibility: Capability to stretch up to 3 times resting length.

  • Elasticity: Ability to regain original shape after stretching.

Structure of Muscle Cells
Terminology for Myocyte (Muscle Cell)

  • Sarcoplasm: The cytoplasm of a myocyte.

  • Sarcolemma: The plasma membrane of a myocyte.

  • Sarcoplasmic Reticulum (SR): A modified endoplasmic reticulum that forms a web-like network surrounding myofibrils; its structure varies in the three types of muscle tissue.

  • Myofibrils: Unique cylindrical organelles found in all muscle cells, constituting 50-80% of cell volume; made up of bundles of specialized proteins allowing for contraction; mitochondria are packed between myofibrils.

Arrangement of Skeletal Myocytes (LO2)

  • Fascicle: Bundle of muscle fibers (cells).

  • Perimysium: Connective tissue that collects cells and endomysium into fascicles, continues as tendon.

  • Epimysium: Connective tissue that surrounds the entire muscle, continuing as tendon.

The Skeletal Muscle Fiber (LO3)
Comparison with Erythrocyte

  • Size & Shape

    • Skeletal muscle fiber: Long, cylindrical with multiple nuclei.

    • Erythrocyte: Size similar to red blood cells (for comparison).

Key Features

  • Transverse Tubules (T-tubules): Deep inward extensions of sarcolemma surrounding each myofibril; form a tunnel-like network, continuous with the exterior of the cell and filled with extracellular fluid.

  • Terminal Cisternae: Enlarged sections of the sarcoplasmic reticulum that flank each T-tubule forming a triad (two terminal cisternae plus one corresponding T-tubule).

Myofilament Structure (LO3)

  • Types of Myofilaments:

    • Thick Filaments: Bundles of the contractile protein myosin.

    • Thin Filaments: Composed of proteins actin, tropomyosin, and troponin.

    • Elastic Filaments: A single massive, spring-like structural protein (titin); stabilizes myofibril structure and resists excessive stretching.

Myofilament Arrangement and Sarcomeres (LO4)
Structure of Sarcomere

  • I Band: Contains only thin filaments.

  • Z Disc: In the middle of the I band; composed of structural proteins that anchor thin filaments in place.

  • A Band: Contains both thick and thin filaments; generates tension during contraction.

  • H Zone: Middle of the A band where only thick filaments exist.

  • M Line: Dark line in middle of A band; holds thick filaments in place, anchoring point for elastic filaments.

Sliding Filament Mechanism of Contraction

  • Mechanism:

    • Relaxed sarcomere: H zone and I band are present.

    • Contracted sarcomere: Actin filaments slide along myosin towards the M line, pulling Z-discs closer and reducing the I band width.

Membrane Potential (LO5)

  • Definition: The electrical potential difference (voltage) across the plasma membrane of a cell when the cell is at rest. It's established by an unequal distribution of ions (charged particles) across the membrane.

  • Resting Membrane Potential in Skeletal Myocytes:

    • Achieved by the precise balance of ion gradients and selective membrane permeability.

    • The sarcolemma of skeletal myocytes is more permeable to potassium ions (K+K^+) than sodium ions (Na+Na^+) at rest.

    • Ion Distribution:

      • Higher concentration of Na+Na^+ and Cl−Cl^- ions outside the cell.

      • Higher concentration of K+K^+ and negatively charged proteins (A−A^-) inside the cell.

    • Role of Leak Channels: K+K^+ ions passively diffuse out of the cell through leak channels, making the inside of the cell more negative.

    • Role of the Na+/K+ Pump (Sodium-Potassium Pump):

      • Actively transports 33 Na+Na^+ ions out of the cell for every 22 K+K^+ ions pumped into the cell.

      • This electrogenic pump contributes to maintaining the negative resting membrane potential by moving more positive charge out than in, and by maintaining the ion gradients.

    • The resulting resting membrane potential for