Question

Can someone please explain.. I don't get the quick and slow parts..

1. During action potentials in the heart,

a. voltage-gated K+ channels slowly repolarize; voltage-gated Na+ channels slowly depolarize; voltage-gated calcium channels quickly depolarize; voltage gated K+ channels slowly repolarize

b. voltage-gated K+ channels quickly repolarize; voltage-gated Na+ channels quickly depolarize; voltage-gated calcium channels quickly depolarize; voltage gated K+ channels quickly repolarize

c. voltage-gated K+ channels quickly repolarize; voltage-gated Na+ channels slowly depolarize; voltage-gated calcium channels slowly depolarize; voltage gated K+ channels quickly repolarize

d. voltage-gated K+ channels quickly repolarize; voltage-gated Na+ channels quickly depolarize; voltage-gated calcium channels slowly depolarize; voltage gated K+ channels quickly repolarize

Answer 1

Option D-

During action potentials in the heart,

In Myocradial cells Action potential:

a.Voltage-gated Na+ channels quickly depolarize .

b.voltage-gated K+ channels quickly repolarize;

In Pacemaker cells-SA node and AV node-Action potential:

a.voltage-gated calcium channels slowly depolarize

b.voltage gated K+ channels quickly repolarize

**IN MYOCARDIAL CELLS.

As stimulus reaches myocardial cells,Some positive ions enter the myocardial cell-----RMP changes from -90 mv to -70 mV(Threshold Potential)--At -70 mV Voltage gated Sodium Channels---Rapid Na+ Influx(+ve Charged Ion)-----This Causes further change in Membrane Potential from -70mV to Postive Voltage(approximately +30mV).

This is Called Depolarization.(Restinng membrane Potential which was Negatice(-90mv)Changes to postive voltage due to entry of Na + Ions.

Now,Depolarization sensitive Ca2+ and K+ Channel opens.

Ca2+ channel pump ca inward

K+ Channel pump K+ Outward.

So there is a Net Balance of charge as Positive charges are moving in and out of the cell---Plateu Phase

Later,Ca2+ channels stops,But K+ channels continue to Pump K+(Positive ion)out of the cell.This Changes RMP from nearly positive value back to -90 MV.

This is called Repolarisation(Here RMP is Changed from positive potential back to Resting Membrane Potential(-90mv) due to Efflux of K+ Ions.

IN PACE MAKER CELLS

Resting membrane potentials(RMP) of Pacemaker Cells are : -60 mV.

Slow phase of Depolarisation occurs due to Entry of Some positive ions (Na+ ions) which changes the RMP from -60 mV - -40 mV.

In this phase ,There is Closure of K+ channel(which prevents outward flow of K+ ions) & Opening of Slow NA + channels(Slow entry of Na+ channel) this changes RMP to -40 mV.

As the RMP becomes -40 mV due to entry of Na+ channels,Ca2+ channels opens.

This -40 mV is called the threshold potential.

When RMP reaches threshold potential----Ca 2+ channels open---Entry of Ca2+ ion---Which again Converts Membrane potential to more poistive Values......This change in Membrane potential by which it loses its negative charge (-Ve resting membrane potential) is called Depolaridation.

Repolarisation occur when these Ca2+ channels closes & K+ channels Opens.

Now there is Efflux (Outward flow)of K+.This loss of positive ions converts the positive membrane potential back to Resting Membrane Potential (that is -60 mV)This is Known as Repolarisation(The membrane attains it Negative resting membrane Potential).

****Since depolarisation due to entry of calcium channel is slow and Due to Na+ channel is Rapid,Action potential occuring in Myocardial cells are rapid and in Pacemaker cells like AV node is slower--So in ECG AV nodal depolarisaton is not detected and causes AV nodal Delay