Lecture 9 Flashcards
What is the name of the cell membrane that each muscle fibre enclosed in?
The sarcolemma
What is embedded in the sarcoplasmic matrix of a muscle fibre?
Myofibrils
What is the cytoplasmic matrix (cytoplasm) called in a muscle fibre?
sarcoplasm
What are the main components found in the sarcoplasm?
- Cytosol
- Transverse tubules (T-tubules)
- Sarcoplasmic reticulum
What is cytosol?
Energy source for muscle
- In muscle known as sarcoplasm
- Rich in glycogen, ATP, Creatine Phosphate, glycolytic enzymes
- Myofibrils comprise ⅔ the dry mass of the cells
What are T-tubules?
Transmit electrical signals to depolarise the muscle cells
- Series of membranous folds extending from plasma membrane
- Transmit electrical signal
- Small gap to
What is the sarcoplasmic reticulum?
Stores the calcium
- Flattened vesicles which surround each myofibril
- Sequester (collect) calcium
What are the 2 main filamentous components of the sarcomere?
- Actin (thin filaments)
- Myosin (thick filaments)
What is the sliding filament theory?
- Contraction is due to the actin and myosin filaments sliding between each other
- The force of contraction is caused by the movement of the cross-bridges
- As contraction commences increased overlap between actin and myosin filaments
- As the sarcomeres shorten the muscle fibres shorten
What happens to the A-band, I-band and H-zone during muscle contraction?
- The A-band remains the same length
- The I-band shortens
- The H-zone is reduced or disappears
What are the 5 steps to the ratchet mechanism?
- Actin is uncovered through the binding of Ca2+ to troponin. The Myosin-ADP - Pi complex binds to actin
- This releases Pi, producing the powerstroke at the cross-bridge. The myosin head changes conformation
- Immediately another ATP molecule replaces ADP in the myosin head, binding to the actomyosin complex to form an A-M-ATP complex
- The actin is released. The myosin head reverts to the resting conformation
- ATP is hydrolysed to ADP – Pi; the resting state
What are the two types of muscle relaxants used in anaesthesia?
Depolarizing muscle relaxants and non-depolarizing muscle relaxants
Depolarizing muscle relaxants?
Cause contraction of the muscle once, but prevent further contractions
e.g. Suxamethonium
Non-depolarizing muscle relaxants?
Prevent muscle from contracting
e.g. Tubocurarine
What are anaesthetics used for?
Muscle relaxants
What are actin and myosin responsible for?
They’re the main proteins responsible for muscle movement (contraction)
Myofibrils in striated muscle?
Divided into light and dark bands
- I-band
- A-band
- Z-line
- M-line
- H-zone
I-band?
- Isotropic
- Contain only actin thin filaments. This not concentrated area gives this band it’s light appearance
- Facilitates the passage of light (light coloured)
- Shortens during muscle contraction
A-band
- Anisotropic
- Contains thick myosin filaments and thin actin filaments. This concentrated area gives this band it’s dark appearance
- Avoids the passage of light (dark coloured)
- Remains the same length during muscle contraction
Z-line?
- In the middle of the I-band
- A dark line
What is the space between each Z-line?
- The sarcomere
- The unit of contraction
The sarcomere?
- The unit of contraction
- The space between each Z-line
- Gives skeletal muscle the striated appearance under the light microscope
- Made up of parallel and overlapping proteins, the location of which gives the dense (A-band) or light (I-band) appearance to the myofibril
What are the three different components that the thin filament is made up of?
- A double stranded F-actin protein molecule wound into a double helix,
- A double stranded a-helical protein molecule tropomyosin, lying in the groove of the F-actin helix,
- A third, troponin, consisting of 3 globular proteins (TnC, TnI, TnT) periodically attached to the tropomyosin strand
What are the functions of tropomyosin and troponin?
- Regulate actin
In the resting state the tropomyosin strands cover the myosin binding sites on actin
One of the troponin proteins (TnT) binds to the tropomyosin, (TnI) binds to F-actin, and (TnC) binds to Calcium (Ca2+) ions. When Ca2+ is released from the SR, it binds to TnC. This initiates the moving of tropomyosin from the myosin binding site. Myosin is now able to bind to actin.
What are the 3 troponin proteins?
TnT, TnI and TnC
Structure of myosin (thick filament)?
- Made of polypeptides
- Consists of compact myosin heads and a long tail which is made of two a-helices
What is most of the myosin tail called?
- Light meromyosin
What are myosin heads and part of the tail composed of?
Heavy meromyosin
Where can myosin be found?
In the A-band of the sacromere
Function of myosin head?
- Each head acts as an enzyme binding to ATP and subsequently hydrolysing it (uses it to cause the contraction).
- The myosin head stores energy in the form of ADP and Pi
H-zone?
Is reduced or disappears during muscle contraction
Why does the A-band remain the same length, but the I-band shortens during muscle contraction?
Bc the myosin in the A-band pulls the actin
What chemical is released as the initial signal for muscle contraction?
Acetylcholine (ACh)
What is a powerstroke?
A crossbridge action:
- Attachment of myosin
- ‘Contraction’ of myosin head
- Breaking of actin/myosin interaction
- Re-attachment further along the actin filament at the next active site
