ELECTROLYTE PHYSIOLOGY FOR THE USMLE

by / Friday, 04 March 2011 / Published in Physiology

In this post we are going to discuss some of the most high-yield electrolyte physiology concepts that most students simply DO NOT understand properly.  After reading this you will have a much better understanding of this highly important concept.

Concentration gradient (the main driving force behind electrolyte movement)

Electrical gradient (which way will it move to drive membrane resting of -90 to their nernst number)

Driving force (how fast the ions move based on their nernst and the membrane’s resting potential)

Nernst number (E-ion) –-> says that’s the membrane potential at which concentration and electrical gradient are equal and opposite –> there will be no net movement.

Conductance (G-ion) –> is the actual movement across the membrane (need access via channels… every ion’s channel is 100% voltage regulated, except for K+(half are voltage-regulated and the other half are wide open)… at rest the ion with the least flux is Cl-, at rest the electrolyte with the most flux is K+(bc it has access and permeability)…  so this means K+ does the most to maintain the electricity of the membrane b/c it can move at will.

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ELECTROLYTE MOVEMENT: terminology

Depolarize: to become positive from baseline

Overshoot: more positive than the threshold potential

Repolarization: to become negative from a positive potential

Hyperpolarization: (aka undershoot) – to become more negative than baseline potential

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SODIUM CHANNELS:


– There is an ‘m gate’ on the outside facing the plasma, while the ‘h gate’ is on the outside

– When the membrane wants to depolarize, m gate opens up slightly, then GNa(remember this is conductance) moves in, then by concentration alone, some Na leaks into the cytoplasm through concentration gradient (remember concentration is strongest driving force).

Hit threshold potential —> all Na channels open up (pictured above) –> voltage reached and the flood gates open up.

– Once the gates open up because we’ve reached a certain voltage –> the fast sodium channels are open

– As the membrane depolarizes(becomes more negative), what happens to driving force of Na?  it is going to drop because we’re moving closer to our eNa (+65 –> more on this later), meanwhile were moving far away from K+

#1 reason why gNa wont reach eNa below…

**Problem –> gNa is rushing to reach eNa (+65), but won’t ever get there…

There are 2 reasons for this: 1. as membrane depolarizes, the driving force of sodium drops, and 2. we’re creating a huge driving force for potassium (-96)

SO –> once we get close to +35 there’s only a 30 driving force for Na, but there’s a huge force for K+, so K+ is going to leave the cell, making it more negative and thus repolarizing.

 

Inactivated  channel- h gate open, m gate closed

*The K+ is the only ion that has the ability to bring the potential to it’s own E (-96) because it has the ability to leak freely through the channels.

Q: what factor is responsible for re-establishing the concentration gradient?  The Na/K+ pump

–> problem is that it kicks out 3 Na and takes in 2 K+ … so it makes it more negative… but, we also have Na/Ca exhange –> 3 na in while 1 calcium moves out (and this is how we get to re-establish the resting membrane potential)

********** remember –> the Na/K pump re-establishes the gradient, but not the membrane potential —> only the Na/Ca exchange is going to re-establish the membrane potential

 

If you’re given a question that talks about phases, here are some helpful tips:

– question about depolarization (Na moving out) phase 0

– question about repol (K+ moving out)  phase 3

– re-setting membrane potential is called “automaticity” (Na moving in), phase 4

*only 1 membrane in body uses calcium to depolarize, that is the atrium

–> this is why we use a CCB to block an atrial arrhythmia, and why we use Na-channel blockers to treat ventricular arrhythmias.

** every membrane in our body has a phase 0, phase 3, and phase 4

*every membrane has that phase 0, phase 3, and phase 4 –> but the heart is different

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Absolute refractory period(ARP) – the time during which no action potential can propogate, the time required to reset enough channels to reach threshold again… no action potential can propogate no matter how strong or large will do it (ie no signal can propogate)

Relative refractory period(RRF) – everything after ARP adds to the amplitude.. 1 – there are enough channels to reach threshold, and 2 – you can thus fire off another signal.  But 3 – amplitude will be less than the normal

 

A cool concept that relates pain and this concept of ARP

The TENS UNIT – this is a nerve stimulating mechanism that acts to lock up pain fibers in the ARP, so no longer can someone feel any pain in their back.  It functions to keep the pain fibers locked up in the ARP for a certain period of time so that no pain can be felt because no action potential will occur.  So if someone is experiencing significant pain in their lower back (just an example), and we stimulated the nerve fibers that relay this pain, all of a sudden someone goes from experiencing maybe just 1/10 of the pain because the fibers are locked up in the ARP for the majority of the time.

 

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