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