Neuromuscular Junction

• Definition: The neuromuscular junction is the site where a motor neuron connects with a muscle fiber (skeletal muscle cell).

• Components:

  • Axon Terminal: The end of the motor neuron that interacts with the muscle fiber.

  • Motor End Plate: The part of the muscle fiber's membrane that contains receptors for neurotransmitters.

  • Neurotransmitter: Acetylcholine (ACh) is released from the axon terminal and binds to receptors on the motor end plate.

Muscle Fiber Structure

• Skeletal Muscle Cell: Also called muscle fibers, contains bundles of myofilaments.

• Myofibrils: Bundles of myofilaments (actin and myosin) that are responsible for muscle contraction.

• T Tubules: Extensions of the muscle cell membrane that penetrate into the muscle fiber, enabling electrical signals to reach deep within the cell.

• Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that stores calcium ions; contains terminal cisternae adjacent to T tubules.

• Calcium's Role in Contraction: Calcium ions released from the sarcoplasmic reticulum are essential for muscle contraction processes.

Myofilaments

• Types of Myofilaments:

  • Thick Filaments: Made primarily of myosin. Myosin heads can pivot to interact with actin.

  • Thin Filaments: Made primarily of actin, with active sites for myosin binding. Tropomyosin covers these active sites at rest.

• Troponin: A protein on thin filaments that binds calcium ions, which triggers muscle contraction by moving tropomyosin away from active sites.

Sarcomere and Muscle Contraction

• Sarcomere: The functional unit of muscle contraction where thick and thin filaments overlap.

• Contraction:

  • Process: When muscle contracts, thin filaments are pulled toward the center (M-line), causing the sarcomere to shorten.

  • Z Discs: Structure at both ends of the sarcomere, where thin filaments anchor. Shortening affects the distance between Z discs.

Electrically Excitable Cells

• Membrane Potential: At rest, muscle fibers have a negative membrane potential (~-85 mV). This difference is crucial for action potential initiation.

• Ion Concentration Gradient: Higher concentration of sodium ions outside the cell and potassium ions inside; this creates a potential for ion movement when channels open.

Action Potentials

• Definition: A rapid depolarization and repolarization event that propagates along the muscle fiber membrane, triggering muscle contraction.

• Triggering Action Potentials:

  • Acetylcholine binds to ligand-gated sodium channels, allowing sodium influx, leading to depolarization.

  • Local depolarization opens more voltage-gated sodium channels, creating a chain reaction (domino effect).

Crossbridge Cycle

• Role of Calcium: Released calcium binds to troponin, causing tropomyosin to uncover active sites on actin, allowing myosin heads to attach.

• Steps of Interaction:

  1. Myosin heads attach to actin, forming crossbridges.

  2. Myosin head pivots, pulling thin filament toward center of sarcomere.

  3. ATP binding releases myosin from actin, allowing the cycle to continue with ATP hydrolysis re-cocking myosin heads.

Energy Sources for Muscle Contraction

• ATP Requirement: ATP is needed for maintaining ion gradients (sodium-potassium pump), crossbridge cycling, and muscle relaxation.

• ATP Production Strategies:

  1. Creatine Phosphate: Rapid ATP production but lasts only ~10 seconds.

  2. Glycolysis (Anaerobic Metabolism): Generates 2 ATP per glucose, lasts ~30-40 seconds without oxygen.

  3. Oxidative Metabolism: Requires oxygen, produces large amounts of ATP for prolonged activity, happens in mitochondria.

Muscle Fiber Types

• Type I (Slow Twitch): More mitochondria, darker appearance, slower contraction, high endurance, uses oxidative metabolism.

• Type II (Fast Twitch): Faster, but fatigue quickly. Subtypes:

  • IIa: Intermediate (some oxidative capabilities).

  • IIb/IIx: Fastest, power-oriented, few mitochondria.

Motor Units

• Definition: A motor unit consists of one motor neuron and the muscle fibers it innervates.

• Size of Motor Units:

  • Small Motor Units: Allow for fine motor control (e.g., fingers).

  • Large Motor Units: Generate greater force but less precision.

Smooth and Cardiac Muscle

• Smooth Muscle: Non-striated, less organized myofilament arrangement, contracts involuntarily (found in organs and blood vessels).

• Cardiac Muscle: Striated, involuntary, with unique features like intercalated discs for synchronized contraction.

Summary of Review Concepts

• Review of the neuromuscular junction, muscle fiber anatomy, the contraction mechanism, and different types of muscle fibers.

• Emphasis on the energy mechanisms that enable sustained muscle activity and differences in muscle fiber composition and function.