Skeletal Muscle Flashcards
Characteristics of the skeletal muscle
- voluntary control
- striated
- single long cylindrical cells
- multiple peripheral nuclei
What is the structure of muscle cells
- attatched to bones via tendons
- the cells “muscle fibres” are long and reasonably wide
- cells composed of myofibrils containing highly organised contractile filaments.
Thick filaments
Run the entire length of the A band
Thin filaments
Run the length of the I band and partway into the A band
Z disc
Coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
H zone
Lighter mid-region where filaments do not overlap
M line
Line of protein myomesin that holds adjacent thick filaments together
What are T-tubules
Deep invaginations continuous with the sarcolemma (cell membrane) and circle each sarcomereat each of the junctions of the A and I bands.
Allows action potentials to be carried deep within the cell.
What is the sarcoplasma reticulum
The calcium storage site. The terminal cisternae of the SR lie close to the T-tubules.
What are thick filaments composed of
Myosin, where each myosin has two subunits each with a globular head and a tail, the two tails intertwine to form a helix. The heads have a binding site for actin.
What is myosin
A high molecular weight protein that is formed from two high molecular weight sub-units. These subunits have a ‘tail’ which wind around each other in a double helix and globular head that is capable of hydrolysing ATP/
What is the head of the myosin in the thick filament
- An enzyme that hydrolyses ATP (an ATPase).
How is the head in the myosin of the thick filament arranged
Arranged in a polarised fashion i.e with the myosin heads projection. Head outside, tail in the middle.
What are thin filaments composed of
- composed primarily of globular actin proteins.
- filaments composed of a double stranded helical actin chain (polymers)
What are troponin and tropomyosin
Regulatory proteins associated with actin in skeletal and cardiac muscle
What is troponin
Binding site to Ca2+ which changes shape and forces the tropomyosin off the actin.
What does tropomyosin do
Interacts with the myosin binding sites
What does contraction refer to
The activation of myosin’s cross bridges. As the sarcomere contracts, the thin filaments are pulled over the thick filaments. The Z discs are pulled towards the M line and the I band and H zone narrower.
At what calcium level is muscle typically relaxed and activated in?
Typically relaxed when the calcium level is less than 0.0001 mM and activated at 0.0001mM.
What is the sliding door theory of muscle contraction
The sarcomere shortens as the thin filaments are pulled over the thick filaments
* the Z line is pulled towards the M line
* the I band and H zone become narrower (thin filaments slide over thick filaments)
What are the four major steps in the cross bridge cycle
- cross-bridge formation
- power stroke
- detachment
- energization of myosin head
Cross bridge formation (1)
When the myosin binds to the actin binding site, forming a cross-bridge
What happens in the power stroke step (2)
- ADP is released
- the myosin head rotates to its low energy state (about 45 degrees to actin) pulling with it the thin filament
- the result is shortening of the sarcomere
What happens in the detatchment step (3)
- A new ATP molecule binds to the myosin
- the actin-myosin bind is weakened and the myosin detaches
What happens in the energization of the myosin head step (4)
- myosin head hydrolyzes the ATP to ADP + Pi
- the myosin head moves back to its “high energy (cocked) confirmation - about 90 degrees to the actin.
What is the importance of calcium
Calcium ions provide the “on-switch” for cross-bridge cycle to begin.
Describe the steps into why calcium is needed in the muscles
When the calcium binds with troponin the tropomyosin moves to expose the myosin binding sites on actin
The cross-bridge cycle will continue as long as calcium levels remain above the critical threshold (0.001-0.01mM)
Calcium regulation: What does the opening of the calcium channels in the SR allow
Allows the movement of calcium ions into the cytosol
Calcium regulation: Active transport pumps
Active transport pumps (Ca2+ ATPase) are constantly moving Ca2+ from the cytoplasm back into the sarcoplasma reticulum.
Isotonic
- shortening
- tension constant
- velocity variable
