Module 4: section 2 - molecular basis of skeletal muscle contraction

what do the cross bridges do

forms the basis of the “sliding filament mechanism” which starts muscular contraction

what does muscular contraction mean

the activation of tension-generating sites within muscle fibres

what happens when a muscle contracts

it shortens

where do thin filaments go during muscle contraction

move inwards to become overtop of thick filaments

*move the z-lines closer together

when do the z-lines move closer together

during contractions and all at the same time throughout the whole muscle fibre and all to the same degree

what do the thick and thin filaments do during contraction

change how much they overlap, not the length of themselves

what does “power stroke” talk about

the interaction between myosin and actin that leads to a shortening of the sarcomere

when does the power stroke occur

when the cross-bridge bends and pulls the thin myofilament inward toward the centre of the thick filament

how many steps to the cross-bridge cycle

4

what are the steps to the cross-bridge cycle

1. binding

2. power stroke

3. detachment

4. binding 2.0

what happens in the binding stage

the myosin cross-bridge bind to the actin molecule

what happens in the power stroke step

the myosin head bends, pulling the thin filament inward

what happens in the detachment stage

the cross-bridge detaches at the end of the power stroke and returns to original conformation

what happens in the binding 2.0 step

the cross-bridge binds to more distal actin molecule (down the thin filament) and the cycle repeats

how many actin surrounds 1 myosin molecule

6

how many heads does each myosin have

2, each act independently

how do sarcomeres get energy to get power strokes

excitation-contraction coupling

what does excitation-contraction coupling refer to

the process of turning an electrical signal into actual contractions

what is another name for sarcoplasmic reticulum

membranous structure in muscles and endoplasmic reticulum in non-muscle cells

where does the SR run

parallel to the muscle fibres

what does the SR do

store calcium

what helps the skeletal muscles transmit the signal to contract to muscle fibres

sarcoplasmic reticulum and t-tubules

what is a t-tubules

folds inward into the plasma membrane at the junction of A and I bands, run perpendicular to the fibres

what do t-tubules do

spread the depolarization wave through the deeper parts of the cell (sends it to the SR)

how does the t-tubules send depolarization signals to the sarcoplasmic reticulum

due to being in close proximity

what are on the surface of t-tubules

dihydropyridine receptors

what are dihydropyridine receptors

voltage receptors that sense waves of depolarization as it moves down the t-tubules and influences the ryanodine receptors of the SR

what are opposite to dihydropyridine receptors

ryanodine receptors

what happens when ryanodine receptors are infuenced by dihydropyridine receptors

the undergo a conformational change, they open the calcium channel to allow calcium into the cytoplasm of the muscle cell

what are ryanodine receptors

a type of calcium ion channel

why is the release of calcium important

it is the primary trigger to allow skeletal muscles to contract

what happens in a relaxed muscle

the tropomyosin and troponin are positioned specifically to prevent cross-bridge formation due to blocking the myosin binding site on the actin molecules

what happens in an excited muscle

calcium enters the fibre to bind to the troponin to cause a conformational change to move tropomyosin out of the way which exposes the myosin binding site on the actin molecules

what causes muscle relaxation

decreased nerve activity at the neuromuscular junction

what can remove any remaining Ach from the neuromuscular junction when it is no longer released

acetylcholinesterase

what does removing remaining Ach do

stops the generation of action potentials in teh skeletal muscle fibre

what happens when the generation of action potentials are no longer present in skeletal muscle fibres

the sarcoplasmic reticulum is no longer releasing stored calcium and the calcium ATPase pumps on the SR are actively pumping the calcium back into the SR to store

what happens to the skeletal muscle fibre when calcium is not present anymore

the troponin-tropomyosin complex can cover the actin molecules to lengthen and relax the muscle again

what is neeeded for calcium to be reabsorbed

ATP

how many steps are there to how calcium leads to muscle contraction

6

what is step one in how calcium leads to muscle contraction

ATPase binds to ATP to remove one phosphate group turning it into ADP. This releases stored energy and sends it to the myosin cross-bridge

what happens when energy is sent to the myosin cross-bridge

the cross-bridge is now “cocked” and ready to fire when triggered

what happens in step 2 of how calcium leads to muscle contraction

if calcium is present the troponin-tropomyosin complex exposes the actin molecules and the cross-bridge can bind to them. This pulls the trigger and cross-bridge swings to cause a power stroke

*or if calcium isn’t present, the cross bridge remains cocked but a contraction does not happen (step 3)

what happens in step 4 of how calcium leads to muscle contraction

while the power stroke happens, the phosphate removed from ATP is released totally and when the stroke is over, the ADP is released too leaving the ADPase site empty but the cros-bridge is still bound to the actin

what happens in step 5 of how calcium leads to muscle contraction

the binding of a new ATP molecule to cause the cross-bridge to detach and return to its “uncocked” position what

what happens in step 6 of how calcium leads to muscle contraction with no ATP

means death has occured and rigor mortis happens

why does rigor mortis happen

because calcium concentration increases in cells causing them to contract until they run out of ATP, and when that happens, they stay contracted

what happens after sufficient muscle proteins decay

relaxation of the muscles occur (after several days of death)

what are the steps of muscle twitches

1. latent period

2. contraction time

3. relaxation time

what is a muscle twitch

the minimum contraction of a whole muscles caused by a single action potential in the nerve exciting the muscle

what happens in the latent period

a delayed single skeletal muscle action potential happens for about 1-2 miliseconds; this causes the action potential to complete even before contraction happens

what else happens in the latent period

cross-bridge cycling is starting

what happens in the contraction time step

the actin filaments slide along the myosin filaments to create greater tension

what is peak tension

teh greatest tension that can be reached while still creating force against an outside load

when does peak tension happen

between 40-120 miliseconds

why does variation to peak tension happen

due to the type of muscle fibre (fast twitch, slow twitch) and where in the body it happens