Muscle structure

Each muscle fibre is a single multinucleated cell, covered in cell membrane (sarcolemma)

No syncytial bridges between cells.

 

Components

Contractile proteins

--> Filaments

--> Myofibrils

--> Muscle fibres

Thick filaments

Myosin (myosin II)

• Twice the diameter of thin filaments
• 2 globular heads and a long tail
• Surrounded by 6 thin filaments in a regular hexagonal pattern

Thin filaments

Actin (with tropomyosin and troponin)

• Actin - double helix
• Tropomyosin - double helix, and lies in the groove of actin double helices
• Troponin - small units located at regular intervals on tropomyosin
• Troponin has 3 units: T, C, I
  * I - inhibits interaction between actin and myosin
  * C - binds to Ca2+
  * T - attach the complex to tropomyosin

Sarcomere

M line - where thick filaments (myosin molecules) are held in proper array

Z line - where thin filaments are connected

A band - length of the thick filaments

I band - length of part of thin filaments that is not overlapped by thick filaments

H band - length of part of thick filaments that is not overlapped by thin filaments

Psedo-H zone - M line plus the small narrow light area on either side of it

 

Sarcotubular system

Sarcoplasmic reticulum (SR)

• Forms terminal cisterns in close contact with T-tubules at A-I junction
• Terminal cistern releases Ca2+ when triggered by action potential

Transverse tubule system

• Space between two layers of T-tubule system is extension of extracellular space
• Transmit AP
• At A-I junction in skeletal muscles
• At Z line in cardiac muscles