Skeletal Muscle Flashcards
What causes muscle contraction?
interactions between actin microfilaments and bipolar myosin filaments
are the skeletal muscles attached to our bones are under voluntary or involuntary control?
voluntary
What are the 3 types of muscle tissues?
cardiac
skeletal
smooth
what does contraction of the 3 muscle types depend on?
ATP-driven sliding of actin against myosin
T or F: skeletal muscles are small with similar organization to other muscle cells
FALSE, they are LARGE and UNIQUELY organized
How many nuclei does a single skeletal muscle cell contain?
hundreds
Why does a single skeletal cell contain so many nuclei?
they are formed by the fusion of many pre-muscle cells in the embryo
How thick are skeletal muscles?
10-100 um
How long are skeletal muscles?
can be hundreds of mm in length
What fills up most of a skeletal muscle cell?
mostly cytoskeleton arrays, some cytoplasm
T or F: skeletal muscle cells are mostly composed of cytoplasm
false! mostly cytoskeleton arrays
Briefly describe the structure of skeletal muscles
a single cell contains hundreds of nuclei
is very thin (10-100 um)
is very long (hundreds of mm)
contains very little cytoplasm - mostly cytoskeleton arrays
What is a fascicle?
a bundle of muscle cells
What is a muscle fibre?
an individual muscle cell
What is a bundle of muscle fibres called?
a fascicle
What surrounds a single muscle fascicle?
a connective tissue sheath
What is a bunch of sheath-coated fascicles called?
skeletal muscle tissue
What are myofibrils?
long cylindrical strands inside every muscle cell/fibre
What are myofibrils mostly composed of?
cytoskeleton components (long stretches of actin and myosin)
What does a bunch of myofibrils make?
a single muscle fibre
What does the prefix ‘sarco’ imply?
the component is part of a muscle cell
What is the sarcolemma?
the plasma membrane of skeletal muscle fibres/cells
What is the sarcoplasm?
the cytoplasm of skeletal muscle fibres/cells
What is the sarcoplasmic reticulum?
the smooth endoplasmic reticulum of the muscle cells
T or F: sarcoplasmic reticulum is the smooth ER of skeletal muscle cells
True
What are T tubules?
they are folds made by the sarcolemma invaginating and they surround the myofibrils and contact the sarcoplasmic reticulum at terminal cisternae
What does T tubule stand for?
Transverse tubules
What are the contractile units in myofibrils?
sarcomeres
What are sarcomeres?
the contractile units in each myofibril that shorten when a muscle contracts
How do sarcomeres allow muscle movement?
they contract and then return to their original size
T or F: sarcomeres allow muscle movement by twisting, elongating, bending, or stretching
FALSE!! they only contract and then return to their normal size
Briefly describe a sarcomere structure
each sarcomere is formed from parallel and partly overlapping thick and thin filaments
thick = myosin thin = actin
Which cytoskeleton component makes the thick filaments of sarcomeres?
myosin II
Which cytoskeleton component makes the thin filaments of sarcomeres?
actin
Describe the thick filaments of sarcomeres
myosin II thick filaments consist of two opposed bundles of myosin with the motor heads jutting outwards in staggered alignment and the tails wind together
What are the 5 density bands/lines in sarcomeres?
Z-lines/discs M-line A-band/dark band I-band/light band H zone
Describe the Z-lines/discs
the two lines formed by accessory proteins on the boundaries of a single sarcomere
Describe the M-line
a dense line at the center of thick myosin filament
Describe the A-band/dark band
the entire length of the myosin thick filament
Describe the I-band/light band
where the thin actin is present without myosin
Describe the H zone
the zone where only thick myosin is present without actin
What are the 5 proteins outside of myosin and actin that help form the sacromere?
CapZ
Tropomodulin
Titin
Myomesin
Nebulin
What is the function of CapZ?
A protein outside of myosin and actin that help form the sarcomere
it helps form the Z disc and keeps the actin plus end anchored to it
What is the function of tropomodulin?
A protein outside of myosin and actin that help form the sacromere that caps the actin minus end
What is the function of titin?
A protein outside of myosin and actin that help form the sacromere
it positions the myosin equidistant between the Z discs by connecting myosin filaments to the Z discs and acts as a spring to prevent over-extension during sarcomere relaxtion
What is the function of myomesin?
A protein outside of myosin and actin that help form the sacromere
it joins the myosin tails together at the M line
What is the function of nebulin?
A protein outside of myosin and actin that help form the sacromere
acts a ‘molecular ruler’ to control the length of the actin thin filaments and prevents too many monomers from assembling
What 2 accessory proteins are critical for proper sarcomere contraction?
tropomyosin
troponin
What is the function of tropomyosin?
it is an accessory protein that wraps around the actin filament
in resting position, it blocks the binding site for myosin on actin in order to block contraction
What does tropomyosin do in its resting position?
it blocks the binding site for myosin on actin and blocks contraction
What is the function of troponin?
it is an accessory protein that regulates the position of the tropomyosin
it is bound to both actin and tropomyosin
What is troponin bound to?
both actin and tropomyosin
What happens to actin and myosin filaments during contraction? What causes this?
they slide past each other caused by the myosin heads binding to actin and ‘walking’ toward the + ends of the microfilaments
What changes length during contraction?
the filaments remain the same length but the sarcomere shortens as the filaments overlap more
In a resting state, what is going on in your skeletal muscle cells?
tropomyosin is coiled around actin to block the binding site on actin for myosin = contraction is blocked
What is required to initiate contraction?
calcium needs to bind to troponin
Describe the structure of troponin
a heterotrimer
subunits are (I,T,C)
Inhibitory
Calcium-binding
Tropomyosin-binding
What are the 3 subunits of troponin?
I C T
Inhibitory
Calcium-binding
Tropomyosin-binding
What does the binding of Ca2+ to troponin cause?
when Ca2+ binds to the C subunit it alters the conformation of the Inhibitory subunit
the conformational change pulls the troponin T subunit into a different position, dragging tropomyosin with it
What does the conformational change in the I subunit of troponin cause?
the change pulls the troponin T subunit into a different position, dragging tropomyosin with it, exposing the myosin binding sites on actin
What happens when the tropomyosin is moved out of the way by troponin?
the myosin binding site on actin is exposed and contraction is possible
Where in muscle cells is calcium sequestered?
in the sarcoplasmic reticulum
How is Ca2+ released from the sarcoplasmic reticulum?
the arrival of an action potential to the synapse of a motor neuron (a stimulus) releases a neurotransmitter (Ach) into the synaptic cleft and binds to the muscle cell receptors to release Ca2+
Where is Ca2+ released from? Where does it go to?
it is released from the sarcoplasmic reticulum into the sarcoplasm
What is the concentration of Ca2+ in the cytosol?
Very low
What are neuromuscular junctions?
The sites where motor neurons reach a muscle fibre
What is the motor plate?
the area of the sarcolemma in the neuromuscular junction
Where does the depolarization travel from and to once the Ca2+ has been released into the sarcoplasm?
it travels along the T tubules to reach the sarcoplasmic reticulum
What does a depolarization in the sarcolemma cause?
a voltage-gated Ca2+ channel to open in the T tubules
What opens the Ca2+ channels in the sarcoplasmic reticulum membrane?
When the T tubule protein’s voltage gated Ca2+ channel opens, it causes a conformational change that opens the Ca2+ channels in the SR membrane
What two components have Ca2+ channels?
T tubule membranes
Sarcoplasmic reticulum membranes
Briefly describe the steps in Ca2+ binding to troponin
Action potential travels across the synaptic cleft of a motor neuron to an NMJ on a muscle cell
T tubules depolarize and voltage gated Ca2+ channels open
conformational change in sarcoplasmic reticulum membrane Ca2+ channels open
Ca2+ is released from SR into the sarcoplasm
Ca2+ binds to troponin
How come contraction is a concerted action?
Once Ca2+ enters the sarcoplasm, every myofibril and sarcomere has Ca2+ available to initiate contraction all at once
What happens to Ca2+ that doesn’t bind to troponin?
it is quickly pumped back into the sarcoplasmic reticulum by a Ca2+ ATPase
How are muscle fibers arranged?
into motor units
Describe a motor unit
A group of cells that are all innervated by one nerve
What is the function of motor units?
they ensure more contraction occurs when necessary or less contraction occurs when necessary
ex. lifting heavy objects vs. light objects doesn’t require the same amount of contraction
What is a crossbridge?
what forms when a myosin head binds to actin
T or F: there will be multiple crossbridges - why/why not?
true because there are multiple myosin heads which will bind to actin
What do the crossbridges do?
they work together to repeatedly bind and release actin to pull actin filaments closer to the M line
What 3 steps are involved in a cross bridge cycle?
ATP attachment
hydrolysis
release
How does the sliding filament model begin?
myosin head is attached to the binding site on actin but not yet bound to ATP/ADP
What happens when ATP binds to the myosin head?
a conformational change occurs in the head which pulls it away from the binding site on actin (ie., myosin releases actin)
What happens once the myosin head releases actin?
the myosin head hydrolyzes ATP
What does ATP hydrolysis by the myosin head cause?
causes the head to undergo a second conformational change into a ‘cocked’ position which displaces the head along the actin filament + rebinds to actin
After the second conformational change, is the myosin still bound to the actin?
Yes, the myosin with ADP + Pi is weakly bound to actin in its new position
What does the weak binding of myosin to actin cause?
the Pi to release from myosin
What does the release of Pi cause?
causes another conformational change aka the power stroke
What is the power stroke? How is it caused?
the power stroke is caused by the release of Pi causing a third conformational change in the myosin
it moves the myosin head back to its original position which pulls the actin filament closer to the M line at the middle of the sarcomere (bc they are attached)
causes the sarcomere to shrink
When will muscle relaxation occur?
when nerve stimulation stops and acetylcholine is no longer present at the neuromuscular junction
What happens when nerve stimulation stops and Ach is not present at the NMJ?
the sarcolemma repolarizes
What does the repolarization of the sarcolemma cause?
voltage-gated Ca2+ channels in the T tubules close
What happens when the Ca2+ voltage gated channels in the T tubules close?
Ca2+ ATPase pumps Ca2+ back into the sarcoplasmic reticulum
What does the lack of Ca2+ in the sarcoplasm cause?
troponin shifts back to its original conformation where tropomyosin blocks the myosin binding sites on actin
What happens when tropomyosin re-blocks the myosin binding sites on actin?
myosin no longer pulls and titin ‘spring’ pushes back so the sarcomere can relax
