Lecture 30. Muscle Flashcards
what muscle has multiple peripheral nuclei?
skeletal
which muscle is under voluntary control?
skeletal
what muscles are under involuntary control?
cardiac and smooth
which muscles are striated?
Skeletal and cardiac
which muscle contains intercalated discs?
cardiac
What muscle cells appear branched with 1-3 central nuclei?
cardiac
which muscle cells are uninucleated?
smooth
which muscle cells are not striated?
smooth
which muscle cells are not striated?
smooth
which muscle cells are not striated?
smooth
What attaches muscle to bones?
Tendons
What is the size of muscle fibre?
long up to 35 cm
0,1 mm
can be seen with a naked eye
Structure of the muscle
Muscle-> fascicles-> muscle fibers/cells-> myofibrils->sarcomere-> microfilaments
H zone
lighter middle section only thick filaments
A band
extends the length of myosin filaments
both thin and thick filaments
M line
Line of protein myomesin that connects the thick myosin filaments in the center of sarcomere
Z disc
joins thin filaments
marks the ends of sarcomere
I band
“Light”
only thin filaments
Z disc function
join thin filaments
connect myofibrils to one another
mark the borders of sarcomere
what protein is M line made of?
myomesin
T-tubules
transverse tubules
- deep invaginations are continuous with the sarcolemma (cell membrane) and circle each sarcomere at each of the junctions of the A and I bands.
cover every myofibril
How many t tubules are there per sarcomere?
2
Sarcoplasmic reticulum
The calcium storage site around muscle cells. The terminal cisternae of the SR lie close to the T-tubules.
t-tubules function
conduct AP to the SR
triad
SR-t-tubule-SR structure
for each t-tubule there is SR on each side
What are the contents of T-tubule?
as it is an extention of cell membrane it has the same contents as extracellular fluid on the inside
Thick filaments structure
• Composed of Myosin
• Each myosin has 2 subunits each with a globular head and a tail, the two tails intertwine to form a helix.
Arranged in a polarised fashion- The heads face out/away from M-line and the tails face in.
• The heads have a binding site for actin. The head is an enzyme that hydrolyses ATP( ATPase)
Titin anchors the thick filaments to the Z-line
titin role
Titin anchors the thick filaments to the Z-line
How does the myosin head change shape
ATP hydrolisis
what protein is the thin filament primarily comprised of?
Actin
What is the shape of the thin filament?
The actin filaments are composed of a double stranded helical actin chain (polymers).
What are the regulatory proteins associated with actin in skeletal and cardiac muscle?
troponin and tropomyosin
troponin function
Regulated by Ca2+. When Ca2+ binds to troponin it moves the tropomyosin of myosin binding
tropomyosin function
interacts with the myosin binding site on actin and prevents myosin from binding
Sliding filament theory of muscle contraction
The sarcomere shortens as the thin filaments are pulled over the thick filaments:
• The Z-line is pulled toward the M-line
• The I band and H zone become narrower
A line now takes up a bigger % of the sarcomere
the filaments do not change in length, they just slide over each other
4 major steps of the cross-bridge cycle
- Cross-bridge formation
- Power stroke
- Detachment
- Energization of myosin head
- cross bridge formation
Myosin binds to the actin binding site to form a cross-bridge
Note: cross-bridges can only occur in
the presence of calcium when the
myosin binding site on actin is exposed.
2.The power stroke
• ADP is released
• The myosin head rotates to its low energy state
(about 45° to the actin) pulling with it the thin filament
Myosin head still attached
• The result is shortening of the sarcomere.
- Detachment
• A new ATP molecule binds to the myosin
• The actin-myosin bind is weakened and the
myosin detaches
• (Note: No ATP = no detachment)
- Energization of the myosin head
• Myosin head hydrolyzes the ATP to ADP + Pi
• The myosin head moves back to its “high energy
(cocked)” confirmation (about 90° to the actin)
Why is calcium important?
- “on” switch for cross-brindge cycle to begin
- binds to troponin to move tropomyosin away from the myosin binding site
- the cross bridge cycle will continue as long as calcium levels remain above crtical threshold( 0.001-0.01mM)
- Need Free calcium. Needs to be released from SR
why can heart beat by itself
The heart has its own electrical system that causes it to beat and pump blood.
When supplied with nutrients like Ca2+ and Magnesium can beat outsdie the body
How is Ca2+ removed?
Active transport pumps (Ca2+ ATPase) are constantly
moving Ca2+ from the cytoplasm back into the sarcoplasmic reticulum
Isotonic contraction
Shortening
Tension constant
Velocity variable
(eg bicep curls)
Isometric contraction
No shortening
Length constant
Tension variable
(hold)
Is cross-bridge cycle still occurring in the isometrically contracted muscle?
Yes. Some minimal movement occurs that is immediately counteracted by the opposing force
