Crossbridge Cycle- Slide 4 or something
Listen to the lecture
Because when the person dies they no longer generate ATP, one last power stroke of myosin (?) makes
the muscle stiff or contracted forever. After a while, protein that makes up the muscle, breaks down so
the body doesn’t stay like that forever. So you can approximate the time of death.
N.Sl Excitation- Contraction Coupling:
N.Sl with diagram
N.Sl At the t tubule, you’ll find DHP (which is an l type calcium channel). With the arrival of the AP, there
occurs a conformation change and therefore calcium is released from the sarcoplasmic reticulum.
N.sl- With calcium, the actin binding sites are open, the crossbridge cycle we’ve seen occurs.
N.sl- To terminate the contraction we have to remove the calcium, we’ll use a calcium pump to pump ca
back into the sarcoplasmic reticulum. This is a very fast process for skeletal muscle not for cardiac or soft
tissue muscles because the source of calcium is right there in the sarcoplasmic reticulum
Nsl. Twitch: No numbers of the rise time on the exam. Don’t memorize numbers.
Nsl- Three phases of the twitch- Plotting tension vs time for the twitch
Three things to notice in the third graph : there is a delay (latent period) in latent period, action
ppotential is travelling and ca is being released from the sarcoplasmic reticulum, so therefore the delay.
Next phase: Contraction phase- the calcium is now available and it’s high enough so that the crossbridge
cycle can occur.
Relaxation period: Calcium is actively being pumped back into the reticulum, so fewer and fewer
crossbridges occur and therefore the twitch drops.
Summation and Tetanus: A single twitch is useless, we need more.
Actual action potential is 2 s and the time needed for relaxation is 100-120 ms. If you fire another action
potential in that time, it will be added.
Nsl- two big ass graphs: Tetanus is the result of repeated stimulation and the responses actually fuse
into one continuous interaction, under this condition the muscle is generating the maximum isometric
force that it can. Notice that fatigue occurs after a complete tetanus.