L11: Excitation-contraction coupling

Question 1

what are t tubules?

Answer 1

• t tubules - invaginations of sarcolemma deep into structure
• conduct electrical changes deep into cell

Question 2

describe the sarcoplasmic reticulum

Answer 2

• sarcoplasmic reticulum (SR) wraps around each myofibril, also connects
• holds Ca2+
• specialised ER

Question 3

what is the terminal systerne?

Answer 3

bulbous region of the SR

Question 4

what makes up the triad?

Answer 4

Triad = t tubule and 2x sarcoplasmic terminal cisterna (one on each side)

Question 5

Describe how an AP leads to Ca2+ release into the cytoplasm

Answer 5

AP down into t tubule, depolarisation detected by voltage sensor (DHPR) → direct mechanical activation of RyR → Ca2+ released into cytoplasm

Question 6

What does Ca2+ do once released into the cytoplasm?

Answer 6

Ca2+ binds to triponin → exposes active site, cross bridge forms → contraction

Question 7

How does relaxation of a muscle fibre occur?

Answer 7

termination of all signals, Ca2+ off myosin, SERCA pump moves Ca2+ into SR = contraction stops = relaxation

Question 8

What is the first step of the cross-bridge cycle?What is bound where?

Answer 8

ADP bound to myosin head, ready to bind actin

Question 9

What is the second step of the cross-bridge cycle, starting with an AP?

Answer 9

AP → Ca2+ → binds to tryponin → conformational change, exposes binding site on actin moelcule

Question 10

What is the third step in the cross-bridge cycle, after the myosin binding site is exposed?

Answer 10

Myosin binds to the actin binding site, forming CROSS BRIDGES → ADP and Pi released, myosin head changes shape → power stroke

Question 11

What is the fourth step in the cross-bridge cycle, after the power stroke

Answer 11

myosin ATP binding site exposed → if available ATP binds → lowers the affinity of myosin for actin, myosin detaches, the cross bridge is broken

Question 12

What is the fifth step of the cross-bridge cycle, after the cross bridge is broken

Answer 12

myosin ATPase hydrolyse, the ATP to ADP+Pi, and the myosin head is re-cocked in readiness for another cycle

Question 13

What causes latency between the AP and contraction?

Answer 13

delay between electrical and contractile response. Time is take for the following processes:
* AP to spread down the t-tubule.
* Release of Ca2+
* Ca2+ to attach to Troponin C
* Power stroke
* Ca2+ uptake by the ATPase pump in the SR