CA

2-7 Excitation Contraction Coupling in SK Muscle

Excitation-Contraction Coupling in Skeletal Muscle

Overview

  • Excitation-Contraction Coupling: The process connecting an action potential (AP) to muscle contraction, essential for skeletal muscle function.

Sequence of Activation

  1. Action Potential Generation

    • An action potential is generated in the somatic motor neuron.

    • ACh (Acetylcholine) is released at the neuromuscular junction (NMJ).

    • ACh binds to nicotinic ACh receptors on the sarcolemma, opening ligand-gated channels.

    • Na+ ions diffuse into the muscle cell, leading to a depolarizing stimulus.

    • This generates an action potential in the muscle fiber.

  2. Propagation of Action Potential

    • Action potentials travel along the transverse tubules (T-tubules).

    • Voltage-gated Ca2+ channels in T-tubules open as the action potential propagates.

    • The action potential triggers Ca2+ release from the sarcoplasmic reticulum (SR) into the cytoplasm.

  3. Contraction Mechanism

    • Ca2+ ions bind to troponin, resulting in a conformational change that allows actin-myosin interaction.

    • The myosin heads execute a power stroke leading to contraction by sliding along actin filaments.

Calcium Dynamics

  • The sarcoplasmic reticulum (SR) is a modified endoplasmic reticulum storing Ca2+ until muscle activation.

  • Upon stimulation, Ca2+ diffuses out through ryanodine receptors (RyR).

  • At the end of contraction, Ca2+ is actively pumped back into the SR via Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA) pumps.

Phases of Muscle Contraction

Contraction Phase

  • Activation of myosin heads leads to muscle shortening and force generation.

  • Forces are generated based on the summation of muscle twitches which is influenced by:

    • Frequency of action potentials (can vary from twitch to tetany).

    • Sarcomere length can affect tension; optimal tension occurs at 100-120% resting length.

Relaxation Phase

  • The relaxation of muscle fibers occurs when:

    • Action potentials cease, stopping further calcium release from the SR.

    • Ca2+ levels decrease in the cytoplasm as it is pumped back into the SR.

    • Myosin heads disassociate from actin due to the decreasing intracellular Ca2+ concentration, re-covering binding sites with tropomyosin.

Factors Affecting Force Generation

Frequency of Stimulation

  • As the frequency of action potentials increases, tension develops in muscle fibers increases until it reaches a peak (tetany).

  • At tetany, contractions sum to generate maximum force.

Motor Unit Recruitment

  • Motor units consist of a motor neuron and all muscle fibers it innervates.

  • Recruitment of additional motor units increases overall contraction force.

  • Asynchronous recruitment helps to prevent fatigue, allowing different motor units to activate in turns, while synchronous recruitment produces more force quickly.

Muscle Thickness and Fiber Recruitment

  • Muscle strength is influenced by:

    • The thickness of muscle fibers (which can be increased through training).

    • The initial length of the fiber during rest.

    • The number of fibers activated during contraction.

  • The strength of muscular contraction is contingent upon the number of motor units recruited.