Can someone explain the features of the myosin-actin complex and what we should we know?


Membership Revoked
Account on Hold
Aug 4, 2013
this thing always confused me with I, Z, H, and A bands. what are they and what do we need ot know abotu them? i havent found a single review book that explained it in a way that made sense to me.


~Water Tribe~
Oct 10, 2013
Medical Student

When Calcium flows into the muscle cell, it binds to troponin which is attached to tropomyosin covering the actin strand (blue).
Tropomyosin moves out of the way so that myosin (red) is able to bind to actin.
A power stroke occurs when the actin bound myosin + ADP + Pi releases the Pi (inorganic phosphate). ADP releases eventually too, but the myosin doesn't unbind until a fresh ATP attaches.
Once ATP hydrolyzes on the myosin head, it will reset/cockback.

The I, Z, H, A bands are just distances unique to the filaments as they contract. Note that the A band doesn't change while the others get closer/further apart.

This video explains it better.
Oct 20, 2013
few other things...
calcium is released from the sarcoplasmic reticulum, and then relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum. So if calcium unable to be released or unable to be pumped back, then there's going to be a problem.
then some people don't like learning like this, but H and I are the important things to remember (both get smaller during contraction)
myosin, longer word, thicker fragment- H zone, only myosin (thicker letter)
actin, shorter word, thinner fragment- I band, only actin (thinner letter)