Saturday, April 13, 2013

Cardiac fast fibers and slow fibers - Why does a less negative membrane potential convert a normally fast fiber into a slow fiber?

Greetings everyone! :)

What are fast fibers?
Fast fibers have functioning fast channels.
Fast fibers include ventricular fibers, atrial fibers and Purkinje fibers.

What are fast channels?
They are sodium channels that quickly open and close on depolarization.
What are slow fibers?
SA node and AV node.
They lack functioning fast channels.
That's why depolarization is so slow in them.

So what will happen if fast fibers lose their fast sodium channels?
They'll convert into slow fibers!

When does this happen? @_@
When the resting membrane potential is less negative.

Why does this happen?
Less negative potential inactivates sodium channels.
Repolarization is necessary for returning the sodium channels to the ready state.

So.. what will depolarize a slow fiber?
L type Calcium channels! =D
Also known as slow channels.
They allow sodium to pass as well.
It's known as the sodium window current.
So you can appropriately call em slow calcium sodium channels.

Other ways of saying the same thing but worth noting:
Capacity of a cell to depolarize depends on the number of sodium channels in ready state.
The more negative resting potential, the faster the response.
(-90mV in atrial and ventricular cells, compared to -60mV in pacemaker cells)
Slow response fibers have no appreciable Na+ current during phase 0 in these cells because the Na channels are either absent or in an inactivate form because of the existing voltage (-60mV, remember?)

That's all!


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