Week 2 (Myofiber (Structure-Function Relationships: Membranes, PLasmalemma, T-Tubules &SR)

Sarcolemma - membrane that covers myofiber (there is a difference b/w sarcolemma & Plasmalemma)

  • Composed of outer connective layer called basement membrane (connects to endomysium) and the  inner layer called the plasmalemma (aka plasma membrane)

    • Basement membrane - as muscle develops in utero, the basement membrane comes first and is used as scaffolding to build myofiber

      • It can regenerate myofiber as long as the BM stays intact

Plasmalemma - Composed of 3 main components

  • 1) Phospholipids - creates lipid bilayer which is the primary component of plasmalemma

  • Proteins - voltage-gated ion channels, dystrophin protein complex, hormone receptors

    • Dystrophy/Dystrophin - part of critical transmembrane that when missing or altered due to mutations causes repeated bouts of muscle degeneration  & regeneration that eventually results in replacement of muscle w/ fat & connective, muscle weakness, respiratory and cardiac dysfunction & early death by 20s

  • Cholesterol - provides stability and integrity to plasmalemma


T-Tubules & SR: Triad

T-Tubule - extension of plasmalemma that runs in a transverse plane

  • Forms a tube that acts like a membrane tunnel through the cell

    • 2 T-Tubules per sarcomere so in total there are a lot.

  • Function: Conducts the Action Potential from periphery of myofiber to inside of myofiber to insure near simultaneous activation of all the myofibrils inside the myofiber

    • The action potential actives the dihydropyridine receptor (DHPR) protein which is embedded in the T-Tubule membrane; DHPR will in turn activate the Ryanodine Receptor 1 (RyR1) protein that is embedded in the SR membrane

Sarcoplasmic Reticulum (SR) - membrane system that wraps around each myofibril (think lot of wristbands (SR) around an arm (myofibril)

  • Function - Stores, releases, & re-sequesters intracellular calcium (Ca2+) that is used to activate the force-generating proteins - Actin & myosin

Triad - 1 T-Tubule that is sandwiched b/w end of 2 opposing SR (critical site of muscle activation


Cytosol vs Sarcoplasm

Cytosol - (intracellular fluid) is the aqueous portion inside of a cell

Sarcoplasm - (cytoplasm) includes the fluid portion & all enzymes & organelles (other than nuclei) inside the myofiber 

Functions of both - All intracellular components are either dissolved or suspended in the cytosol/sarcoplasm, and it allows for intracellular signal transduction to occur that is needed to keep the cell alive & functioning.


Troponin binds calcium (Ca2+) on the thin to help regulate when forced is produced 


The protein on the thin filament that blocks the protein interaction producing force is tropomyosin.