Question

The heart is said to be automatic. This means that it generates its own action potential and contracts without the need for innervation. The basis for this is that some of its cells show what is called a spontaneous depolarization that moves the membrane potential to threshold. Please give a reasonable explanation in terms of changes of membrane permeability to ions that could explain this spontaneous depolarization termed the pacemaker potential.

Answer 1

SA node is the reason for the automaticity of the heart.

the action potential generated by the SA node is the SA node action potential . it is a slow response action potential because the current generated in small.

SA node action potential have 3 main phases

Phase 4 is called the pacemaker potential or Pre potential

it is the initial potential that trigger the action potential to reach the treshold potential .( from -60mV to -40mV )

Pacemaker potential is due to the effect of

1) Funny current (If) - it is due to the inward depolarizing slow acting Na + channel.( Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels ) this current change the potential from -60 to -50mV . When the potential reaches -50mV another type of channel get open .that is

2) T type Calcium channel : This channel produce inward flux of calcium ion and depolarize the cell to a high potential ( -40mV) .

3) Inward rectifying potassium channel open : this channel produce movement of potassium ion out of the cell and contribute to depolarisation

Next phase phase 0 - it is depolarization of the cell due to the inward flux of calcium ion through the L type calcium channel.

phase 3 - Repolarization of the cell due to the outward flux of potassium ion through transient potassium channel