Module 4: Lecture 3 Flashcards
when a myosin head unbinds to an actin, why cannot it no longer bind to the same one?
because the power stroke caused it to physically move away
do your thin and thick filaments change length when they are contracting?
no, they are only sliding relative to one another
what initiated cross-bridge cycling after it enters the cytoplasm?
calcium
what process allows calcium into the muscle?
excitation-contraction coupling
- really the action potential
what is excitation-contraction coupling?
action potential along the plasma membrane of a myofibre
- it has many steps
- triggers cross-bridge cycling
what is a key pathway to excitation-contraction coupling?
transverse tubule / T-tubule
what is the transverse tubule?
a link from the outside to the cell to the inner core of the muscle fiber
- run perpendicularly from the surface of the muscle cell membrane into the central portion of the muscle fiber
- continuous opening in the sarcolemma
what is another name of the surface membrane of muscle fibre?
sarcolemma
what sits on either side of the t-tubule?
a very specialized intracellular organelle called the sarcoplasmic reticulum (SR)
- its technically the meshwork in between each t-tubule because directly beside the t-tubules is the lateral sacs
where is calcium stored in the muscle?
intracellularly in the lateral sacs of the sarcoplasmic reticulum beside the transverse tubules - away from the cytosol
what forms a sleeve-like segment around each myofibril?
the sarcoplasmic reticulum
- at the end of the segments are the lateral sacs that are connected to each other via smaller tubular elements
what is another name for terminal cisternae?
lateral sacs
what is one of the fundamental purposes of the t-tubule/lateral sac arrangement?
the coordinated release of calcium
- without t-tubules, it would take calcium a long time to travel down to the center of the muscle
what is the triad of the muscle?
a single-tubule and 2 lateral sacs on either side
- key regulatory site that is responsible for the release of calcium into the muscle
what ensures even release of calcium throughout the myosite to ensure we get a very coordinated/even contraction of all the myofibrils throughout the muscle fiber?
the fact that action potential can enter deep into the myosite through the t-tubules because of the gated-channels and their continuity with the sarcolemma.
what sits on the outer surface of the SR, close to the T-tubule?
a whole bunch of proteins that have many names. One is a ‘foot protein’
- most commonly called a ryanodine receptor
what happens when your ryanodine receptors open?
they are like gated-ion channels so they are going to release calcium into the cytosol which will increase the calcium concentration in the cytosol so that your thin filaments will be activated, causing muscle contraction
how do the ryanodine receptors sit?
in small clusters of four
what receptor sits on the t-tubule in a cluster of four?
dihydropyridine receptor (DHPR)
- bind to the ryanodine receptors
what is the dihydrophridine receptor?
specialized channel that is voltage sensitive that sits on the t-tubule
- so they will open once a certain threshold of a change in voltage is reached (they open when they sense that the action potential has occurred along the t-tubule)
what is linked with each ryanodine receptor?
a dihydropyridine receptor
- critical for cytosolic calcium concentration
- when DHPR are activated, it actives the ryanodine receptors because they are linked
the transverse tubules bring action potentials into the ________ of the skeletal muscle fibres so that the wave of depolarization passes close to the __________ ____________.
- interior
- sarcoplasmic reticulum
what assures calcium can readily diffuse to all of the troponin sites?
the extensive meshwork of sarcoplasmic reticulum
what are the steps of activation of calcium into the cytosol?
- action potential –> T-tubule from the sarcoplasmic reticulum –> opening calcium channels
- increase in cytosolic calcium concentration
- calcium will bind to the subunit C (calcium) –> induces a change in the shape of the troponin
4.. release of the inhibitory grip on tropomyosin - movement of tropomyosin making available myosin-binding sites
- remove calcium from troponin –> reverse the process –> disconnects the cross bridge, relax the muscle
calcium re-uptake into SR occurs through?
SERCA (SarcoEndoplasmic Reitculum Calcium ATPase)
- SERCA Pump (utilizing ATP)
what allows the binding of the cross-bridge, followed by its flexing to slide the actin filament?
- the calcium-troponin complex “pulls” tropomyosin off the myosin-binding site of action
