Properties and Structure of Muscle Cells

5 Properties of Muscle Cells

• Contractility: The ability of muscle cells to contract (shorten) when stimulated. This property enables movement and force generation.

• Excitability: The capacity to respond to stimuli, especially electrical signals from nerves.

• Conductivity: The ability to conduct electrical impulses along the cell membrane.

• Distensibility: The ability to stretch without being damaged.

• Elasticity: The ability to return to original shape after being stretched or contracted.

Muscle Cell Anatomy

• Sarcolemma: The plasma membrane of a muscle cell, responsible for conducting action potentials.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and dissolved substances.

• Myofibrils: Bundles of protein filaments (myofilaments) within the muscle fiber, responsible for contraction.

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that stores and releases calcium ions (Ca2+).

• Terminal Cisternae: Enlarged areas of the SR that store Ca2+ and are adjacent to T-tubules.

• T-tubules (Transverse Tubules): Invaginations of the sarcolemma that transmit action potentials into the cell's interior.

• Triad: A structure formed by a T-tubule flanked by two terminal cisternae, crucial for rapid Ca2+ release and muscle contraction.

• Motor End Plate: The region of the muscle fiber's sarcolemma that faces the motor neuron at the neuromuscular junction.

Myofilaments and Sarcomere Structure

• Sarcomere: The basic structural and functional unit of a myofibril, defined by Z-discs at each end.

• Myofilaments: Protein filaments within the myofibril, classified as:

  • Thick Filaments: Composed mainly of myosin molecules.

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

  • Elastic Filaments: Composed of titin, providing elasticity and structural support.

• Contractile Proteins: Myosin and actin, responsible for force generation.

• Structural Proteins: Titin, which stabilizes myofilament position and provides elasticity.

• Regulatory Proteins: Tropomyosin and troponin, which regulate contraction by controlling the interaction between actin and myosin.

Table: Types of Myofilaments and Their Components

Key Proteins and Their Roles

• Troponin: Acts as a 'clip' that holds tropomyosin in place on actin. When Ca2+ binds to troponin, it causes a conformational change that moves tropomyosin, exposing the myosin-binding sites on actin.

• Tropomyosin: Covers the active sites on actin, preventing myosin from binding in a relaxed muscle.

• Actin: The main component of thin filaments; contains binding sites for myosin heads.

• Myosin: The motor protein in thick filaments; its heads bind to actin and use ATP to generate force.

Sliding Filament Mechanism

Overview

The sliding filament mechanism explains how muscle contraction occurs at the molecular level. Tension is generated as thick and thin filaments slide past each other, shortening the sarcomere and thus the muscle fiber.

• Myosin heads bind to actin, forming cross-bridges.

• Using energy from ATP hydrolysis, myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

• This process repeats, resulting in muscle contraction.

Example: During a biceps curl, the sarcomeres in the biceps muscle shorten as actin and myosin filaments slide past each other.

Levels of Organization in Skeletal Muscle

Hierarchy of Structure

• Muscle Fiber (Cell): Surrounded by Endomysium (connective tissue).

• Fascicle: A bundle of muscle fibers, surrounded by Perimysium.

• Muscle: Composed of multiple fascicles, surrounded by Epimysium.

• Fascia: Connective tissue that surrounds and separates muscles from each other and from the skin.

Additional info: This organization allows for efficient force transmission and protection of muscle tissue.

Muscle Cell Electrophysiology

Electrical Gradient and Resting Membrane Potential

• Electrical Gradient: The uneven distribution of charges across the sarcolemma, with more K+ leaving the cell and Na+ entering, resulting in a negative charge inside (~ -90 mV).

• Resting Membrane Potential: The electrical potential difference across the sarcolemma when the cell is at rest, typically around -90 mV.

• Electrical Potential: The potential energy for ion flow when the membrane barrier is removed.

Ion Channels and Gradients

• Leak Channels: Always open, allowing passive ion flow.

• Gated Channels: Closed at rest, open in response to specific stimuli.

• Ligand-Gated Channels: Open when a chemical (ligand) binds.

• Voltage-Gated Channels: Open or close in response to changes in membrane voltage.

Action Potential and Depolarization

• Depolarization: A stimulus opens voltage-gated Na+ channels, allowing Na+ to enter the cell, making the inside more positive.

• Repolarization: Na+ channels close, and voltage-gated K+ channels open, allowing K+ to exit, restoring the negative membrane potential.

• Action Potentials: Rapid changes in membrane potential that propagate along the sarcolemma, triggering muscle contraction.

Equation:

Example: When a nerve impulse reaches the neuromuscular junction, it triggers an action potential in the muscle fiber, leading to contraction.

Summary Table: Key Muscle Cell Components and Functions

Key Terms and Definitions

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Conductivity: Ability to transmit electrical signals.

• Distensibility: Ability to stretch without damage.

• Elasticity: Ability to return to original shape.

• Sarcomere: Functional unit of muscle contraction.

• Myofibril: Bundle of myofilaments within muscle fiber.

• Myofilament: Protein filament (actin, myosin, titin) within myofibril.

• Triad: Structure for rapid Ca2+ signaling.

• Motor End Plate: Specialized region of sarcolemma at neuromuscular junction.