lecture 17

Muscle Contraction Overview

• Functional Units: The smallest functional units of muscle contraction are sarcomeres which contain myofilaments.

Myofilaments

• Myosin:

  • Called thick filaments, composed of thousands of myosin molecules.

  • Each myosin is a twisted dimer with heads on the ends.

  • Myosin heads are crucial for binding to actin and facilitating contraction.

• Actin:

  • Consists of twisted chains; represented in diagrams often as dark purple filaments.

  • Yellow dots on purple circles are binding sites for myosin heads.

• Tropomyosin:

  • Light blue strands that block the binding sites on actin when the muscle is at rest.

• Troponin:

  • A protein complex consisting of three subunits that bind calcium, acting as a calcium receptor for skeletal muscle.

Contraction Mechanism

• Calcium's Role:

  • In the presence of calcium, tropomyosin undergoes a conformational change, exposing myosin binding sites on actin.

• Action Potential:

  • Required to initiate muscle contraction.

  • It generates an electric signal which ultimately leads to calcium release.

• ATP:

  • Necessary for muscle function and contraction cycles, acting as the energy currency.

Neuromuscular Junction and Excitation

• Neuromuscular Junction:

  • The point of communication between a neuron and a muscle fiber.

• Excitation:

  • Refers to the stimulation of a muscle fiber that leads to contraction.

• Excitation-Contraction Coupling:

  • The process where excitation of a muscle fiber leads to contraction.

Motor Units

• Motor Unit:

  • Comprises a motor neuron and the muscle cells it controls.

  • Allows for graded force; more motor units contract for heavier lifting.

Action Potential in Muscle Contraction

• Initiation:

  • Action potentials travel down the axon of the neuron to axon terminals, releasing neurotransmitters (e.g., acetylcholine).

• Acetylcholine:

  • Binds to receptors on the muscle cell membrane (sodium channels), causing depolarization.

  • Leads to action potential generation in muscle fibers.

T-Tubules and Sarcoplasmic Reticulum

• T-Tubules:

  • Extensions of the muscle cell membrane, enabling action potentials to penetrate deep into the muscle cell.

• Sarcoplasmic Reticulum (SR):

  • Stores calcium; releases calcium in response to action potentials.

• Calcium Release:

  • Calcium floods into the cytoplasm of the cell, enabling myosin binding to actin.

Cross Bridge Cycle

• Steps in Cycle:

  1. ATP Binding:

     • ATP binds to myosin; myosin is in a low energy state.

  2. Hydrolysis:

     • ATP hydrolyzes to ADP and phosphate, releasing energy and activating myosin heads.

  3. Binding:

     • Myosin heads bind to actin at available binding sites.

  4. Power Stroke:

     • Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.

  5. Release:

     • ATP binds again to myosin, allowing it to release from actin.

• Rigor Mortis:

  • Occurs post-mortem due to lack of ATP, causing muscles to remain contracted.

Termination of Contraction

• Calcium Reuptake:

  • Calcium is actively transported back into the SR, stopping muscle contraction.

• Tropomyosin Coverage:

  • Tropomyosin covers binding sites once calcium is removed, restoring muscle to resting state.

Interesting Muscle Adaptations

• Types of Muscle Fibers:

  • Slow Oxidative Fibers:

    • High endurance, require oxygen, appear red (e.g., marathon runners).

  • Fast Oxidative Fibers:

    • Moderate endurance, thicker fibers, also require oxygen (e.g., sprinters).

  • Fast Glycolytic Fibers:

    • Short bursts of intense activity, low endurance, and less oxygen-dependent.

• Adaptability:

  • Muscle fibers can adapt based on training regimens, changing their functional properties over time.

ATP Production Pathways

• Immediate ATP Pathway:

  • Uses creatine phosphate for stored energy.

• Glycolytic Pathway:

  • Produces ATP without oxygen for short, intensive tasks.

• Oxidative Pathway:

  • Utilizes oxygen for ATP production, efficient for long-term activities.

Lecture Summary

• Understanding the mechanisms of muscle contraction, the biochemical processes involved, and the adaptations of muscle fibers is crucial for comprehending both voluntary movements and various physical activities.

Muscle Contraction Overview

Functional Units

• Sarcomeres: The smallest functional units of muscle contraction, comprised of myofilaments organized in a highly structured manner. Sarcomeres are delineated by Z lines, which mark the boundaries of each unit and anchor the actin filaments.

Myofilaments

• Myosin:

  • Also known as thick filaments, myosin is made up of thousands of myosin molecules that form a dimeric structure.

  • Each myosin molecule has a long tail and a globular head, with the heads crucial for interacting with actin during contraction.

  • Myosin heads contain ATPase activity that helps in the hydrolysis of ATP, providing the energy necessary for muscle contraction.

• Actin:

  • Actin filaments are composed of globular actin (G-actin) monomers that polymerize to form filamentous actin (F-actin), creating a double helix structure.

  • These filaments are often depicted in diagrams as dark purple, and they have binding sites for myosin heads marked with yellow dots.

• Tropomyosin:

  • This protein forms a long filament that wraps around actin, blocking myosin binding sites during muscle relaxation.

  • It functions as a regulatory protein, playing a key role in whether contraction can occur.

• Troponin:

  • Comprises three subunits (TnI, TnC, TnT) that bind calcium ions, acting as a calcium receptor critical for muscle contraction regulation.

  • When calcium binds to TnC, it causes a conformational change that moves tropomyosin away from actin’s binding sites, allowing contraction to proceed.

Contraction Mechanism

• Calcium's Role:

  • In the presence of calcium ions, the conformational change of tropomyosin exposes the myosin binding sites on actin, facilitating cross-bridge formation.

• Action Potential:

  • An action potential is a rapid electrical signal required to initiate a muscle contraction, generated by the depolarization of the muscle fiber membrane.

  • This process ultimately leads to calcium release from the sarcoplasmic reticulum.

• ATP:

  • Adenosine triphosphate (ATP) is crucial for muscle function; it powers the myosin head's movement through the cross-bridge cycle, allowing muscle contraction cycles to occur. ATP levels must be maintained for optimal muscle performance.

Neuromuscular Junction and Excitation

• Neuromuscular Junction:

  • The specialized structure where a neuron communicates with a muscle fiber, characterized by the synaptic cleft where neurotransmitters are released.

• Excitation:

  • The process of muscle fiber stimulation via the binding of neurotransmitters (primarily acetylcholine) to receptors on the muscle cell membrane, resulting in depolarization and initiation of an action potential.

• Excitation-Contraction Coupling:

  • This critical process links the excitation of a muscle fiber with the subsequent contraction, involving the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber.

Motor Units

• Motor Unit:

  • Defined as a motor neuron and all the muscle fibers it innervates, a motor unit can vary in size and the number of muscle fibers it controls, allowing for graded muscle force.

  • The recruitment of additional motor units enables smoother and stronger muscle contractions, important for activities requiring varying force levels.

Action Potential in Muscle Contraction

• Initiation:

  • Action potentials travel down the axon of a motor neuron, reaching the axon terminals where they trigger the release of neurotransmitters (acetylcholine) into the synaptic cleft.

• Acetylcholine:

  • This neurotransmitter binds to receptors on the muscle cell membrane, opening sodium channels and leading to depolarization and subsequent action potential generation in muscle fibers.

T-Tubules and Sarcoplasmic Reticulum

• T-Tubules:

  • Extensions of the muscle cell membrane that penetrate deep into the muscle fiber, allowing action potentials to reach the interior and facilitate coordinated contraction.

• Sarcoplasmic Reticulum (SR):

  • A specialized endoplasmic reticulum that stores calcium ions; it releases calcium into the cytoplasm in response to action potentials, playing a crucial role in muscle contraction.

Cross Bridge Cycle

• Steps in Cycle:

  1. ATP Binding: Myosin heads in a low energy state bind ATP.

  2. Hydrolysis: ATP is hydrolyzed into ADP and inorganic phosphate, releasing energy that reorients the myosin head into a cocked high-energy position.

  3. Binding: Myosin heads bind to actin at uncovered binding sites, forming cross-bridges.

  4. Power Stroke: The myosin heads pivot and pull the actin filaments toward the center of the sarcomere, shortening the muscle fiber.

  5. Release: Another ATP molecule binds to the myosin head, allowing it to detach from actin.

• Rigor Mortis:

  • A post-mortem condition where the muscles stiffen due to the lack of ATP, preventing myosin from detaching from actin, thus causing a prolonged contraction.

Termination of Contraction

• Calcium Reuptake:

  • Calcium ions are actively transported back into the sarcoplasmic reticulum, leading to the cessation of contraction as calcium levels drop in the cytoplasm.

• Tropomyosin Coverage:

  • With calcium removed, tropomyosin moves back to its original position, covering the myosin binding sites on actin, thereby restoring the muscle to its resting state.

Interesting Muscle Adaptations

• Types of Muscle Fibers:

  • Slow Oxidative Fibers: High endurance fibers with abundant mitochondria and myoglobin, requiring oxygen; appear red (e.g., muscle fibers in marathon runners).

  • Fast Oxidative Fibers: Intermediate fibers that provide moderate endurance and are rich in myoglobin; suited for activities like sprinting.

  • Fast Glycolytic Fibers: Fast-twitch fibers ideal for short, powerful bursts of activity; low endurance and less reliant on oxygen (e.g., sprinters).

• Adaptability:

  • Muscle fibers can adapt in response to training regimens, evidenced by changes in fiber type composition, cross-sectional area, and biochemical properties over time, enhancing performance in specific activities.

ATP Production Pathways

• Immediate ATP Pathway: Utilizes creatine phosphate to quickly regenerate ATP for short efforts.

• Glycolytic Pathway: Generates ATP through anaerobic glycolysis, beneficial during short, high-intensity activities.

• Oxidative Pathway: Involves aerobic respiration; highly efficient for long-duration activities using oxygen for ATP production, ideal for endurance activities.

Lecture Summary

Understanding the mechanisms of muscle contraction, the biochemical processes involved, and the adaptations of muscle fibers is crucial for comprehending both voluntary movements and various physical activities. Knowledge of muscle physiology informs training methodologies, rehabilitation strategies, and athletic performance optimization.