Describe how action potential is generated in cardiac autorhythmic cells, with an emphasis on movement of...

Describe how action potential is generated in cardiac autorhythmic cells, with an emphasis on movement of ions across the membranes.

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Expert Solution

Cardiac autorhythmic cells constitutes pacemaker of heart as it discharges rhythmically and automatically.

Automaticity is possible due to spontaneous diastolic depolarization of membrane potential following completion of each action potential.

The resting membrane potential during diastole that depolarizes is called prepotential/pacemaker potential, as it brings the membrane potential to threshold level, which then triggers the action potential.

IONIC BASIS OF PACEMAKER POTENTIAL:

In initial stage:

Towards the end of repolarization, potassium conductance (IK) declines called as potassium decay.
During that time, f channels open (If/In current) i.e., HCN channels and conductance increases.
These events allow membrane to depolarize.

In later stage:

There occurs opening of transient calcium channels (IcaT).
Entry of calcium through T-channels completes pacemaker potential and takes membrane potential to threshold level, which then fires to form action potential.
This upstroke of action potential occurs due to opening of long lasting calcium channels (IcaL).
Calcium from sarcoplasmic reticulum also contributes to pacemaker potential.
It is the resting membrane potential that automatically depolarizes to form pacemaker potential.
Thus, in pace making tissues, there is no resting membrane potential (restless membrane potential).
Sympathetic stimulation makes the slope steeper and increases the heart rate, whereas parasympathetic stimulation makes the slope flat and decreases the heart rate.

Sydney Mullin answered 9 months ago

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