Midterm 1 Flashcards
What are fascicles surrounded by?
Heavy connective tissue (i.e.
perimysium)
What is the shape of muscle fibers and how is it helpful?
Fibers are polygonal shape and that allows for greater packing density.
Approximately how many muscle fibers are in a muscle?
Thousands (bicep: ~200,000)
Muscle fibers are _____ into bundles called _______
- bound
2. fascicles
Can a muscle fiber work if it is not in contact with a capillary?
No
Myofibrils ___ the _____ of muscle fibers (HINT: length)
“run the length”
What is the diameter of a myofibril?
1-2 micrometres
What is the whole muscle surrounded by?
Heavy connective tissue layer called epimysium.
How thick are muscle fibers?
Fibers are 10-100 micrometres thick
How long are muscle fibers?
A few mm to several cm long and there is 1 micrometre space between fibers
What surrounds the sarcolemma? Describe it.
The endomysium. It is a mesh-work of loose connective tissue surrounding each muscle fiber.
What percent of the muscle fiber is the myofibril?
Myofibrils make up ~85% of the muscle fiber.
How many myofibrils per fibre?
A few hundred to several thousand
Each muscle fiber is surrounded by _________
sarcolemma (cell membrane)
How are we able to produce force and shorten muscle?
Myofilaments within the myofibril interact with one another. The thin filament, actin, pulls against the thick filament, myosin to shorten. Thus, shortening the muscle.
What is myosin made up of?
Myosin is made of 100-400 myosin proteins. First there are 2 myosin heavy chains and these make up the tail (or rod). The myosin head is made up of 2 heads of the myosin protein which are made of myosin light chains
What is the structure of Actin?
Actin filaments are thin filaments that are made up of small actin proteins (globular shaped) and these strands of actin protein are wound around each other.
On top, tropomyosin (a thin filament) wraps around actin filament. And Troponin globular proteins bind onto tropomyosin.
Why is tropomyosin and troponin importnat?
These are important to ensure muscle contraction and allowing myosin heads to attach to actin filament and pull and cause muscle contraction
Each section of the myofibril is called _________
a sarcomere
Within a single sarcomere, how long is the myosin filament?
the myosin filament is about 1.5 micrometers
Within a single sarcomere, how long is the actin filament?
the actin filament is about 1 micrometer
Approximately how long is a sarcomere?
Actin slides in and out depending on muscle contraction so we don’t have an EXACT length of a sarcomere all the time but it is around 2 micrometers
Z Line
The line that marks the end of one sarcomere and the start of another sarcomere (usually thin dark lines)
A band
Thick dark band (striations) that are seen in muscle fibres), containing mostly myosin but it also contains actin filaments as well (overlapped at this point)
M Line
The middle line down the middle of the sarcomere
H Zone
The lighter section of the A band because it only contains myosin thick filaments and none actin thin filament
I Band
Will straddle overtop 2 sarcomeres and it’s the light area. Only has Actin
What separates sarcomeres in parallel?
Glycogen and sarcoplasmic reticulum
Why do you need many sarcomeres to contract a muscle?
Sarcomere only shorten by a few nanometres so you need many of them to shorten the muscle
Where do Auxiliary proteins attach and what is its importance?
Attach to M line and Z line in order to connect them to adjacent myofibrils
What is the role of Nebulin?
Nebulin’s job is to anchor to Z-line and control length of actin by keeping it consistent
What is Titin and what does it do?
Titin is a protein that attaches the Z line to the M line and acts as a molecular spring to allow stretched sarcomeres to return to their optimal length
- *Titin is the largest protein known in the body
- **1 micrometer in length which is made up of 30,000 or more amino acids
Each myosin head is surrounded by how many actin filaments? What shape is this?
Each myosin head is surrounded by 6 actin filaments to create a hexagonal array
1 Actin has how many myosin filaments around it?
3
What is the WORKING RANGE of a muscle fibre that has 300 sarcomeres in series?
(3.5 um * 300) - (1.5 um * 300) = 600 um
= 0.6 mm
OR
(2um * 300) = 600 um AKA 0.6 mm
What’s the formula for working range of a muscle?
(3.5 um * # of sarcomeres) - (1.5 um * # of sarcomeres)
**convert to mm
OR
(2 um * # of sarcomeres)
What is a crossbridge
A crossbridge is the connection between the myosin head and actin filament
What are the steps of a power stroke
1 - the crossbridge is detached
2 - the cross bridge attaches to actin (myosin head attached to actin) and the power stroke begins
3 - the power stroke proceeds, pulling the actin and Z line
4 - the Cross bridge detaches and springs back into its ready position
What is the Bare Zone? How long is it?
The Bare zone is similar to the H zone. This is the segment of myosin filament where there are no cross bridges.
As sarcomeres shorten, the bare zone is going to become overlapped by actin filament
It is 0.2 - 0.3 um long
How do the bands change as muscle contracts?
The Z lines move closer together. The A band remains the same. The I band gets smaller.
Describe the 3 different type of contractions.
Concentric: when the muscle shortens
Eccentric: When the muscle is lengthening. It is trying to shorten but the external load is greater than the force the muscle can produce so it lengthens.
Isometric: No change ; when the force produced by the muscle is equal to the external load
What does a power stroke always want and what are it’s success criteria?
Power stroke always wants to shorten the sarcomere
Success is influenced by external load imposed by muscle and # of crossbridges cycling
Are cross bridge power strokes synchronous or asynchronous?
ASYNCHRONOUS
Why are cross-bridge power strokes asynchronous?
If the cross bridges let go synchronously, the actin filament would slip away and the muscle would not be able to shorten. At any given time, some cross bridges are attached and some aren’t to prevent the actin from slipping away (no slipping of muscle)
How does the cross bridge get energy?
ATP attaches to the myosin head in the muscle. The myosin head has myosin ATPase which turns the ATP into ADP + Pi and energy
Steps of the cross bridge cycle
- ATP binds to myosin head, causing it to detach from myosin
- ATP splits into ADP + PI so the head springs back into position.
- Release Pi so myosin head can attach tightly to actin
- Power stroke and ADP release
When calcium is present what is able to occur?
When calcium is present it attaches to troponin. Troponin moves tropomyosin aside to expose the actin binding sites so that myosin heads can bind to actin (to form a cross bridge)
How does Troponin and Tropomyosin inhibit CB cycling?
Troponin inhibits binding of actin and myosin. Tropomyosin blocks crossbridge binding where myosin has to attach.
What if calcium is removed
The tropomyosin would slide back over the actin binding sites. The cross bridges cant attach to actin so the cross bridge cycle stops and the muscle relaxes. And this is how muscle contraction and relaxation is controlled.
What controls muscle contraction and relaxation?
The calcium ion availability which is controlled by the sarcoplasmic reticulum
What are the functions of the sarcoplasmic reticulum?
1 - Calcium storage (at rest)
2- Calcium release
3 - Calcium Reuptake
How does the brain communicate with muscles to make them contract?
The CNS communicates via neurons. Neurons communicate via changes in charges. Initially the membrane is polarized (negatively charged).When the charge reaches -55 mV (threshold) then the membrane depolarizes due to sodium rushing in. Next, voltage-gated potassium channels open and potassium rushes out to repolarize the membrane. Sometimes it overshoots and hyperpolarizes. Finally, the sodium-potassium pump brings the charge back to its initial charge.
What are transverse tubules? What are they for?
Transverse tubules are are holes that run into the muscle fiber towards the myofibrils. The action potential runs into the transverse tubules so that it can reach all of the myofibrils.
What is a triad?
A triad is the 2 terminal cisternae of the sarcoplasmic reticulum and the transverse tubule.
There are 2 triads per sarcomere
What is a muscle twitch?
A muscle twitch is the contractile response to a single Action potential (nerve impulse)
What occurs faster, Ca2+ reuptake or release? and why?
Calcium release is faster than calcium reuptake (so the contraction is faster than relaxation) because calcium release is a passive process whereas calcium reuptake is an active process
What is summation?
Summation is when action potentials are sent back to back so the next one is sent before the previous one has relaxed. As a result, there is an increase in force produced and it compounds on itself (up to a certain point)
What components contribute to the SEC
Tendon, Titin, Connective Tissue, Cross bridges
What is positive braking and what contraction does it happen in
Positive braking occurs during eccentric contractions. It is when myosin heads are stretched in the direction opposite to the shortening sarcomere. As a result the resistive force is in the direction that shortens the sarcomere. So resistive forces help the muscle shorten.
Why are eccentric contractions strongest?
1 & 2- Stretch-induced “braking” force by cross-bridges and cytoskeleton (Positive-braking)
3 - More cross-bridges attached (many suspended in state 3)
4 - Titin binds to actin
Rank the types of contractions from strongest to weakest
Eccentric, isometric, concentric
How does the SEC contribute to making concentric contractions the weakest?
During concentric contractions, the SEC tends to go slack so a lot of the force goes to taking up the SEC and less force is transmitted to the bone
