ET : M - Skeletal Muscle Flashcards
What is the structure of skeletal muscles?
- Attached to bones via tendons and is responsible for movement
- Cells “muscle fibres” are long (up to 35cm) and reasonably wide (0.1mm)
What are skeletal muscle cells composed of?
Cells are composed of fibrils containing highly organised contractile filaments (myofibrils)
What are myofibrils made up of?
Made up of alternating bands of actin and myosin filaments which interdigitate
What is the sarcomere?
what does it consist of
where does it extend from and to
- Basic contractile element
- Consists of an array of thick filaments (myosin) which interdigitate with thin filaments (actin), attached to Z discs at each end
- Extends from one Z line to the next Z line
What does a single skeletal muscle cell have many of?
peripheral nuclei and myofibrils
Where are the thick (myosin) filaments of a myofibril?
They run the entire length of an A band (A band is the dark part)
where are the thin (actin) filaments of a myofibril?
They run the length of the I band and partway into the A band (I band is the light part), in the middle of the I band, has a Z disc
What is the Z disc of a myofibril?
A coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another (where thin filaments connect and are held together)
What is the H zone of a myofibril?
Lighter mid-region where filaments don’t overlap (has no thin filaments)
What is the M line of a myofibril?
Line of protein myomesin that holds adjacent thick filaments together (middle of thick filaments)
What are the T-tubules?
- Deep invaginations continuous with the sarcolemma (the surface membrane of the muscle cell) at each junctions of the A and I bands
- Goes around every myofibril
- Allows action potentials to be carried deep within the muscle cell
What is the Sarcoplasma Reticulum (SR)?
- An extensive network of a subcellular membrane-bound compartment surrounding the fibril
- Calcium storage site which releases calcium that activates contraction
- Sacromeres are surrounded by the SR whose terminal cisterns lie close to the T-tubules
What are the thick filaments composed of?
Composed of myosin, where each myosin has 2 high molecular weight sub-units each with a globular head and a tail
In thick filaments, what are the globular heads capable of?
They are an enzyme capable of hydrolyzing ATP
In thick filaments, how are the heads and tails structured/arranged?
The 2 tails intertwine to form a helix. Tails come in and are joined at the middle and the heads are poking out at the ends (away from the M line), arranged in a polarized fashion
In thick filaments, what are the low molecular weight proteins called and where may they be bound?
‘Light chains’ and they may be bound near the myosin globular region
In thick filaments, what do the ‘light chains’ regulate?
They regulate the catalytic ability of myosin to hydrolyse ATP
In thick filaments, what can the myosin molecules also form?
Filaments with the myosin molecules polarised along the filament
What anchors the thick filament to the Z line?
Titin
What are the thin filaments composed of?
Composed of primarily globular actin proteins
How are the thin filaments composed?
They are composed of a double stranded helical actin chain (polymers)
What are the regulatory proteins associated with actin and what do they regulate?
Troponin and tropomyosin, they regulate whether myosin can bind onto the actin
What is the dip in the actin?
Myosin binding site
At rest, what are tropomyosin and troponin doing?
- Tropomyosin is lying on top of the actin binding sites to stop the myosin from binding to the actin
- Troponin is what Ca2+ binds onto
What happens when Ca2+ binds onto the troponin?
It changes shape and pulls the tropomyosin off the binding sites
The M line in the sacromere is best described as an area rich in?
Myosin tails
What is the sliding filament theory of muscle contraction?`
As the sacromere contracts/shortens, the thin filaments are pulled over the thick filaments, where the Z line is pulled towards the M line and the I band and H zone become narrower and the A bands don’t change length
What are the major 4 steps of the cross-bridge cycle?
- Cross-bridge formation
- Power stroke
- Detachment
- Energization of the myosin head
What occurs during cross-bridge formation?
Myosin starts in its high-energy state, where ATP has been hydrolyzed to ADP and a phosphate. Myosin binds to the actin binding site to form a cross-bridge (cross-bridge can only occur in the presence of Ca2+ when the myosin binding site on actin is exposed)
What occurs during the power stroke? where is the energy for this from?
ADP is released. The myosin head rotates to its low-energy state (about 45 degrees to the actin), pulling the thin filament with it, towards the centre of the sarcomere (while it remains attached to the actin). Energy for power stroke is ultimately provided by ATP that binds to the myosin head
What is the result of the power stroke?
Shortening of the sarcomere (Z lines slightly shorten)
What occurs during detachment?
A new ATP molecule binds to the empty ATP-binding site on the myosin head. The ink between the myosin head and actin (actin-myosin bind) is weakened and the myosin detaches. The detached myosin head remains in its low-energy state
What happens if there’s no ATP?
No detachment (myosin remains attached to actin), muscle is stiff
What occurs during the energization of the myosin head?
Myosin heads is stretched out. Myosin head hydrolyzes ATP to ADP and a phosphate. The myosin head moves back to its high-energy state (about 90 degrees to the actin)
What is the only naturally occurring ion that can initiate muscle activation?
Ca2+
How does Ca2+ contribute to the cross-bridge cycle?
Ca2+ ions provide the “on switch” for cross-bridge cycle to begin. It binds to troponin and the tropomyosin moves to expose the myosin binding sites on actin
What is the critical threshold that Ca2+ must remain above for the cross-bridge cycle to continue?
0.001 - 0.01mM
What are the sources for changes in Ca2+ levels?
From outside the cell and/or release from internal Ca2+ stores (SR)
What allows the movement of Ca2+ ions into the cytosol?
Opening of Ca2+ channels in the SR
How will the Ca2+ levels change inside the cytosol?
Ca2+ levels will increase as the Ca2+ levels outside the cell and in the stores are usually higher than inside the cytosol (Ca2+ ions will passively move down its conc. gradient)
What are the active transport pumps (Ca2+ ATPase) doing?
Constantly moving Ca2+ from the cytoplasm back into the SR
How is Ca2+ removed from the cytoplasm?
It is an active process which is ultimately linked to ATP hydrolysis by ion pumps in the surface membrane
How does Ca2+ contribute to muscle relaxation?
Relaxation is brought about by the Ca2+ influx channels closing and the pumps returning the Ca2+ to the stores and/or extracellular space
What is isotonic contraction?
The tension developed by the muscle remains almost constant, while the muscle length changes. It’s velocity variable
What is isometric contraction?
The tension developed doesn’t exceed the resistance of the object (tension variable) and there is no change in muscle length (length constant)
When the muscle doesn’t shorten but develops isometric force, what is the determinant of the no. of attached cross-bridges?
The amount of overlap between thick and thin filaments