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.