Module 4: Lecture 3

Question 1

when a myosin head unbinds to an actin, why cannot it no longer bind to the same one?

Answer 1

because the power stroke caused it to physically move away

Question 2

do your thin and thick filaments change length when they are contracting?

Answer 2

no, they are only sliding relative to one another

Question 3

what initiated cross-bridge cycling after it enters the cytoplasm?

Answer 3

calcium

Question 4

what process allows calcium into the muscle?

Answer 4

excitation-contraction coupling
- really the action potential

Question 5

what is excitation-contraction coupling?

Answer 5

action potential along the plasma membrane of a myofibre
- it has many steps
- triggers cross-bridge cycling

Question 6

what is a key pathway to excitation-contraction coupling?

Answer 6

transverse tubule / T-tubule

Question 7

what is the transverse tubule?

Answer 7

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

Question 8

what is another name of the surface membrane of muscle fibre?

Answer 8

sarcolemma

Question 9

what sits on either side of the t-tubule?

Answer 9

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

Question 10

where is calcium stored in the muscle?

Answer 10

intracellularly in the lateral sacs of the sarcoplasmic reticulum beside the transverse tubules - away from the cytosol

Question 11

what forms a sleeve-like segment around each myofibril?

Answer 11

the sarcoplasmic reticulum
- at the end of the segments are the lateral sacs that are connected to each other via smaller tubular elements

Question 12

what is another name for terminal cisternae?

Answer 12

lateral sacs

Question 13

what is one of the fundamental purposes of the t-tubule/lateral sac arrangement?

Answer 13

the coordinated release of calcium
- without t-tubules, it would take calcium a long time to travel down to the center of the muscle

Question 14

what is the triad of the muscle?

Answer 14

a single-tubule and 2 lateral sacs on either side
- key regulatory site that is responsible for the release of calcium into the muscle

Question 15

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?

Answer 15

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.

Question 16

what sits on the outer surface of the SR, close to the T-tubule?

Answer 16

a whole bunch of proteins that have many names. One is a ‘foot protein’
- most commonly called a ryanodine receptor

Question 17

what happens when your ryanodine receptors open?

Answer 17

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

Question 18

how do the ryanodine receptors sit?

Answer 18

in small clusters of four

Question 19

what receptor sits on the t-tubule in a cluster of four?

Answer 19

dihydropyridine receptor (DHPR)
- bind to the ryanodine receptors

Question 20

what is the dihydrophridine receptor?

Answer 20

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)

Question 21

what is linked with each ryanodine receptor?

Answer 21

a dihydropyridine receptor
- critical for cytosolic calcium concentration
- when DHPR are activated, it actives the ryanodine receptors because they are linked

Question 22

the transverse tubules bring action potentials into the ________ of the skeletal muscle fibres so that the wave of depolarization passes close to the __________ ____________.

Answer 22

1. interior
2. sarcoplasmic reticulum

Question 23

what assures calcium can readily diffuse to all of the troponin sites?

Answer 23

the extensive meshwork of sarcoplasmic reticulum

Question 24

what are the steps of activation of calcium into the cytosol?

Answer 24

1. action potential –> T-tubule from the sarcoplasmic reticulum –> opening calcium channels
2. increase in cytosolic calcium concentration
3. 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
4. movement of tropomyosin making available myosin-binding sites
5. remove calcium from troponin –> reverse the process –> disconnects the cross bridge, relax the muscle

Question 25

calcium re-uptake into SR occurs through?

Answer 25

SERCA (SarcoEndoplasmic Reitculum Calcium ATPase)
- SERCA Pump (utilizing ATP)

Question 26

what allows the binding of the cross-bridge, followed by its flexing to slide the actin filament?

Answer 26

• the calcium-troponin complex “pulls” tropomyosin off the myosin-binding site of action