BIOL 0800 Reading- Chapter 9 Flashcards
What is a myoblast?
Undifferentiated, mononucleated cells that formed into a single cylindrical and multinucleated cell
What are satellite cells?
Undifferentiated stem cells that repair muscle in response to strain or injury
What are tendons?
Attach muscles to bones; bundles of collagen fibers
What are myofibrils?
Filaments that are part of small cylindrical bundles that make up striations
What are thick filaments of the sarcomere?
Made of myosin
What are the thin filaments of the sarcomere?
Composed mostly of actin, troponin, and tropomyosin
What is the A band?
The wide, dark band produced by thick filamins in the middle of the sarcomere
What is the Z line?
Where the thin filament is anchored to interconnecting proteins
What defines the limits of one sarcomere?
To adjacent Z lines
What is the I band?
The light band that’s between the A bands of adjacent sarcomeres that results from thin filaments and interconnecting proteins, but no overlapping thick filaments
What’s the H zone?
The narrow, light band in the middle of the A band, where the two thin filament sets don’t overlap
What’s the M line?
The proteins that link together the central region of adjacent thick filaments, in the center of the H zone
What is titin?
The elastic protein that extends from the Z line to the M line (interconnecting)
How are thin and thick filaments arrange?
Hexagonal thin around one thick; triangular thick around one thin
What are crossbridges?
Projections that bridge the space between overlapping thick and thin filaments; part of the myosin molecules on the thick filaments; the velcro heads
What is the sliding-filament mechanism of contraction?
When the overlapping thick and thin filaments in each sarcomere move past each other, propelled by crossbridge movement: shortens the sarcomere
What does the ability of a muscle fiver to generate force and movement depend on?
Interaction of the contractile proteins actin and myosin
What comprises the myosin molecule?
Two heavy chains and four light chains: produces two globular heads and a long tail
How are myosin molecules oriented in the thick filament?
Opposite directions, so tails point in, so that when the heads swivel, they drag the thin filaments in towards the center
What is the crossbridge cycle?
Attachment of crossbridge to thin filament, movement of crossbridges to produce tension, detachment of crossbridge from thin filament, energizing the crossbridge so it can attach to a thin filament again and repeat the cycle
What initiates crossbridge cycling?
Entry of calcium into the cytoplasm
What happens when the energized myosin binds to actin?
The strained conformation is released, to produce movement of the bound crossbridge and the release of a phosphate and ADP
How is the analogy of energy storage/release in myosin like a mousetrap?
Cock the spring (ATP hydrolysis), and spring the trap (bind to actin)
Why is ATP needed a second time when actin and myosin are bound?
To break the crossbridge bond to reenergize the crossbridge and start the cycle all over again
What role does ATP play when it breaks the bond between the bridged crossbridge and actin?
Allosteric modulator, NOT an energy source: allosterically modulates the myosin head to weaken the binding of myosin to actin
What happens after ATP allosterically modulates the myosin head?
It gets hydrolyzed into ADP and a phosphate, which reenergizes the myosin so that it can continue the crossbridge cycle
What are the two distinct roles for ATP in the crossbridge cycle?
1) energy release during hydrolysis to move the crossbridge, and 2) binding to allosterically modify myosin to release actin and reenergize the cycle
How do troponin and tropomyosin work?
Tropomyosin cover the myosin binding sites, held by troponin, until Ca binds to the troponin and changes it shape, allowing the tropomyosin to move away from the binding sites
What determines the number of actin sites available for crossbridge binding?
The cytosolic Ca concentration
What is excitation-contraction coupling?
How an action potential leads to crossbridge activity
What is the source for increased cytosolic Ca in a muscle fiber?
The sarcoplasmic reticulum
What are lateral sacs?
The enlarged regions of the SR, connected by tubes
What is the transverse tubule?
T-tubule: separate from SR, but associated with lateral sacs of SR: surround myofibrils where the A bands and I bands meet
How Is the action potential propagated to the sarcomeres?
The membrane of the T tubule can propagate the action potential, and the lumen of the T tubule is continuous with the ECF around the muscle fiber
What are the junctional feet?
Where the T-tubules contact the lateral sacs of the SR, using two integral membrane proteins
What are the two integral membrane proteins in the junctional feet between the lateral sacs of the SR and the T-tubules?
The DHP receptor (modified voltage sensitive Ca channel) and the ryanodine receptor (includes foot process and the calcium channel)
What happens during an T-tubule action potential?
Charged amino acid residues in the DHP receptor protein induce conformational change, using the foot process to open the ryanodine receptor channel, so Ca goes from the lateral sacs of the SR into the cytosol, activating crossbridge cycling
How is contraction terminated?
When Ca is removed from troponin and the tropomyosin slides back into place
Why does contraction continue even after the action potential is over?
It takes longer to get all the calcium back into the SR through active transport, so cytosolic Ca concentration is still elevated
What is the neuromuscular junction?
Where the motor neuron connects with the effector organ, the muscle
What’s the difference between motor neurons and somatic efferent neurons?
There is none! They’re the same! Such magical!
What are the largest diameter axons in the body?
Motor neurons
What happens when the axon of a motor neuron reaches the muscle?
It divides into a bunch of junctions with the muscle fiber: each muscle fiber is controlled by a branch from only one motor neuron
What is a motor unit?
A motor neuron and the muscle fibers it innervates
What is the motor end plate?
The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon
What is the junction of an axon terminal with the motor end plate?
The neuromuscular junction!
What neurotransmitter is contained in the vesicles found at the axon terminals of a motor neuron?
ACh
What kind of receptors does the ACh released by the axon terminal of motor neurons bind to?
Ionotropic (nicotinic type)