***By the NAME OF ALLAH***

SA-Node is the pace maker of the heart

What makes the SA-Node the pace maker of the heart? 1) It is leaky to sodium ions (but this is not enough) since other parts of the cardiac

muscle are also leaky to sodium ions. 2) It gives the highest heart beat (the fastest one). major distinguishing point 3) Slow depolarization(related to point 1) 4) Self-excited

1 RMP(polarized) slight leakage of Na+ 2 Depolarization Na+/Ca++ influx through

slow Na-Ca slow channel 3 Platue(narrow range) Na+/Ca++ influx

through slow Na-Ca slow channel 4 Repolarization K+ outflux

*Cardiac cycle duration*

-60

-50

t

m.v

0.8 second

1

2

3

4

From this figure we can see : 1) Heart beat rate = 60/0.8= 72 bpm 0.8 S: time between first beat and second beat. 2) The time taken for depolarization and repolarization is shorter than the one required to reach the threshold . 3) We can reduce the time to reach the threshold by increase the slope (increase the angle). Slope=tan (α) α: the angle between slope and X-axis. This happens in cell by:

• Increase the conc. Of Na+. • Increase the conc. Of Ca++. • Decrease the conc. Of K+.

Thus, we can increase the heart beat by reducing the time to reach the threshold: eg: if the time of Cardiac cycle duration = 0.5 then, Heart beat= 60/0.5=120 bpm. (This happen by sympathetic stimulation which occurs on fear, running……) * The normal heart beat in ADULTS ranges from 60-100. Both Sympathetic and parasympathetic can be controlled by drugs: 1) Sympathetic: by using sympathagonist . 2) Parasympathetic: by using sympathoantagonist. To produce parasympathetic control we must:

• Decrease the conc. Of Na+ • Decrease the conc. Of Ca++ • Increase the conc. Of K+

Thus, Cardiac cycle duration will take more time. * If the heart beat >100 then the patient has Tachicardia. *If the heart beat <60 then the patient has Bradicardia . SA-node has its common intrinsic …….. which means if we cut nervous supply either Parasympathetic or sympathetic SA-node will expert itself without the nervous supply but it become more faster. Can AV-node become pace maker? AV-node is unable to expert is self because it is suppressed by another more active node(SA), BUT since it is leaky to Na+ but much lower than SA-node , it can become a pace maker with less activity. (with slower heart beat 42-55). Cases of AV-node become pace maker: 1) Sick sinus syndrome

ParaSympathetic: if it is near to 60. Sympathetic: if it is near to 100.

2) Myocardial in fraction. 3) Complete ischemia.(atrioventricular block ,so no communication between atria and ventricles because of blockage of valves between them.) 4) roumatic fever(can defect SA-node, AV-node, endocardiac cell)

Why we call AV-node latent pacemaker? From this figure we can see that the RMP of SA-node is not equal to AV-node but the slope of SA-node will reach the threshold earlier than AV-node because ignition of stimulant will reach after SA-node the AV-node and then continue to AV-bundle ….. so AV is supresteared(first to receive) by external stimulation. What will occur in case of AV-node defection? Purkinje cell will take the responsibility as pacemaker but the heart beat will be decreased (20-40) due to very low leakage for Na+, so in this case we must put artificial pacemaker in right atrium.(and it is very dangerous to expose the patient for x-ray radiations if an artificial pacemaker is fixed in the human body). .

-65 -60

-50

AV-node

SA-node Purkinje fiber

-65 -60

-50

SA-node AV-node

Ectopic pacemaker

-Ventricular cell’s are not leaky to Na+(because sodium channels are inactivated at it’s RMP) so it can’t act as a normal pace maker but it will act as an ectopic pacemaker (i.e outside of its normal route)-مھاجرة under special conditions(diseases/defects…) -If we have patient with M.I this will cause ischemia of heart tissues lead to death in 3min. because only ventricular fibers will work as an Ectopic pacemaker and this will cause only contraction of ventricles ventricular fibrillation. ***we conclude that SA-node/AV-node/AV bundle/purkinje fibers all act as pacemakers but with different permeabilities to Na+(leakages) so the one with the highest leakage ability (fastest to reach threshold value) will predominate and act as a main(normal) pacemaker and the rest will act as latent pacemakers

The end

Mohammad Al-Assaf Tamer batsh

We congratulate the new daughter “Rayat AL_noor” for prince

Hashim Ibn AL-Hussien

●Correction for sheet#5: -cell membrane is asymmetrical -capillary membrane is symmetrical ●Correction for sheet#10:

A case in which heart can not relax and continues to contract without relaxing.

We must apply to him dc-shock (2000 v) to inactivate SA node till it works again normally.

-the heart(cardiac muscle fibers) consist of myofibrils of Actin & Myosine opposite to the Sa-node.