Muscles: Skeletal muscle part 1 Flashcards
Is skeletal muscle voluntary?
Yes
What is the function of skeletal muscle
the physical movement of the body
Features of skeletal muscle
It has multiple peripheral nuclei
It has single long cylindrical cells
Striated
What attaches skeletal muscle to bones
tendons
Length of skeletal muscle fibres
35cm
Width of skeletal muscle fibres
0.1mm
What are skeletal muscle fibres composed of that contract
Myofibrils (contractile filaments)
What are the sections of myofibrils
Sacromeres
Thick filaments
run the entire length of A band
Thin filaments
run the entire length of the I band (only thin) and partway into the A band
Z disc/line
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 (no thin filaments) ONLY thick
M line
line of protein myomesin that holds adjacent thick filaments together
Sacromeres
extend from the z line to the next z line. It can contract. Z disc comes closer to the M line. If the Z disc gets closer together,the H zone and I band will get smaller. A band stays the same length
Structural features of T-tubules
deep invaginations continuous with the sarcolemma and circle each sarcomere at each of the junctions of the A and I bands.
Folding in of the sarcolemma, goes around twice per sarcomere (at ends of A band)
Function of T-tubules
Allows action potentials to be carried deep within the muscle cell and allows extracellular fluid can be close to myofibrils
What is the function of the Sarcoplasmic reticulum (SR)
It is a Ca storage site (Ca is on/off switch), takes up Ca so low [Ca] inside cell
What is the terminal cisternae
The sacroplasmic reticiulm adjacent to the t-tubules. The terminal cisternae of the SR lie close to the T Tubules, means if AP comes down T tubule it can quickly tell SR to do something
What is a triad
2 Terminal cisternae around T tubule
What is thick filaments made of
myosin
How many subunits does each myosin have?
2
What are the features of the subunits in myosin
globular head and a tail - this intertwinses with the tail of the other subunit to form a helix
Function of the myosin head
to act as a binding site for actin. It also is an enzyme that hydrolyses ATP (an ATPase) because it moves (with hinge)
What is the resting state of the myosin head
Is being pulled in
Arrangement of thick filaments
The myosin heads project away from the M-line while the tails of the two subunits are intertwined.
Function of titin in thick filaments
Titin anchors the thick filament to the Z-line
What are thin filaments mainly composed of?
Globular actin proteins
Strucutural shape of thin filaments
double stranded helical actin chain
The two regulatory proteins associated with actin in skeletal and cardiac muscle
Troponin and tropomyosin
Function of tropomyosin
It interacts with the myosin binding sites. Initially on top of myosin binding sites (like a plug)
Function of troponin
It is regulated by Ca2+ and pulls of tropomyosin off binding site
Sliding filament theory of muscle contraction
Sarcomere shortens as THIN filaments are pulled OVER the thick filaments
- Z-line is pulled toward the M-line
- I band and H zone become narrower
- Myosin stays still
- Thin actin filaments are pulled OVER myosin
What is the starting point of the cross bridge cycle?
There is no fixed starting point as it is a cycle
What occurs in the “Cross bridge formation” stage of the cross bridge cycle
The myosin binding sites are available due to Ca so myosin binds to the myosin binding sites on actin forming a cross-bridge. Myosin has already hydrolyzed ATP (to ADP) (ENERGISED state), it “stretched out” - wants to relax to resting state = holding lots of energy
What MUST be present for the cross bridge cycle to occur and why
free Calcium in the intercellular space!! so that the myosin binding site on actin is exposed
What occurs in the “Power stroke” stage of the cross bridge cycle
ADP is released (causing myosin head to pull in: resting state). Myosin head rotates to its low energy state (45° to the actin) pulling with it the thin filament. This causes the sarcomere to shorten
What occurs in the “Detachment” stage of the cross bridge cycle
A new ATP molecule binds to the myosin. The actin-myosin bond is weakened and the myosin detaches (head lets go of actin)
What occurs without the presence of ATP in the cross bridge cycle
The myosin head cannot detatch from the myosin binding site in actin > Rigor mortis
What occurs in the “Energization of the myosin head” stage of the cross bridge cycle
Myosin head hydrolyzes the ATP to ADP + Pi
The myosin head moves back to its “high energy (cocked)” confirmation ( 90° to actin)
What is the role of calcium in the cross bridge cycle?
Ca ions provide the “on” switch for the cross-bridge cycle to begin. When Ca binds with troponin, the tropomyosin moves to expose the myosin binding sites on actin
What is the Ca threshold for the cross bridge cycle
0.001-0.01 mM
What will a high concenration of Ca in the intercellular space cause in skeletal muscle?
A contraction
What are the two features of skeletal muscle that regulate calcium?
Ca channels and active transport pumps Ca2+ ATPase
Where are Ca channels located and how do they function?
Ca channels in the sacroplasmic reticulum allow the movement of Ca ions into the cytosol, regulating Ca levels
- when they are open = Ca release so contraction
- when they are closed = Ca cannot be released so not enough Ca for a contraction
Where are Active transport pumps (Ca2+ ATPase) located and how do they function?
They are in the sacroplasmic reticulum and are constantly moving Ca2+ from the cytoplasm back into the sarcoplasmic reticulum. As soon as Ca levels drop - contraction stops
