#07 Heart II Continued/Heart III

Question 1

What Channel Opens After HCN Channel?

Answer 1

○ Once the HCN channels have opened, that leads into a slight depolarization of the cell, and opening of T-type Calcium channels. T is for transient, or short period of activation. It’s voltage-gated.

Question 2

What Occurs In Conductive Myocytes At Around Threshold?

Answer 2

○ Right around threshold, the L-type Calcium channels open, also voltage gated. L stands for long-lasting, so this channel will have a larger current of calcium ions flowing into the cell. This channel drives cell to complete depolarization.
§ Note that this calcium influx is what is responsible for this peak in potential, not sodium like in skeletal muscle.

Question 3

What Occurs In Conductive Myocytes At Peak of Depolarization?

Answer 3

○ At peak of depolarization, the L-type channels close and potassium channels open for repolarization, just like in skeletal muscle. As cell approaches membrane resting potential, the potassium channels close and the HCN channels start to open to repeat the cycle.

Question 4

Conductive Signal Travel

Answer 4

• From the SA node, the signal travels through intermodal pathways. So this is what carries action potential throughout atria. Current is carried through atria so atria can depolarize. Now signal is at AV node. From AV node, signal will now travel down septum of the heart, passing through the AV bundle, to the apex of the heart. From there, lead into Purkinje fibers, which lead into bond sides of the heart.

Question 5

AV vs. SA Node: Speed

Answer 5

○ AV node is slower than SA node. One of the reasons is to give a delay to give atria time to contract before the ventricles have to contract.

Question 6

AV Bundle

Answer 6

○ AV bundle, or bundle of His, is a small opening in fibrous connective tissue between the heart chambers, to reach intraventricular septum. One pathway then splits into bundle branches, which then takes signal down to apex of the heart.

Question 7

Purkinje Fibers

Answer 7

○ Purkinje fibers, the inferior terminal part of the bundle branches, are made of cardiac muscle fibers. They differ from the rest of the heart muscle by having fewer myofibrils. They also have many intercalated discs and gap junctions, so electrical depolarization travels very fast through them compared to other cardiac muscle cells. They’re autorhythmic. They may be faster compared to other muscle cells, but are still the slowest when compared to the nodes.

Question 8

Why Is SA Node Pacemaker?

Answer 8

• Pace of heart set by SA node because it is the fastest beat. Because of this the AV node and purkinje fibers are always responding to what the SA node fires, and can never create a signal of their own.

Question 9

Bradycardia vs. Tachycardia

Answer 9

• Bradycardia means slow heartbeat; Tachycardia means fast heartbeat.

Question 10

What Regulates SA Node?

Answer 10

• Autonomic Nervous system regulates the heart. So even though we follow pace of SA node, they’re regulated by the parasympathetic nervous system, which reduces the action potential firing frequency of the SA node from 100 to like 80.

Question 11

Contractile Myocytes: Noticeable Graphical Differences

Answer 11

• The defining characteristic of a contracticle myocyte is that it has a plateau when graphing its potential.
• Contractile myocytes also have a steeper rising phase, because we are using sodium channels to make that rise, not the calcium channels like the conductive cells.
• Contractile myocytes have a flat base line, which means they need a stimulus to be excited to threshold. That stimulus is the action potential that is fired from the pacemaker cells.

Question 12

What Happens When Contractile Myocytes Reach Threshold?

Answer 12

• When contractile myocytes reach threshold, like skeletal muscle, we are opening up voltage-gated sodium channels, which produces rising depolarization phase.

Question 13

Contractile Myocytes Use of Calcium Channel

Answer 13

• Contractile myocytes do still have a calcium current that is involved. The L-type calcium channels in particular are still opening. It opens up once membrane depolarization reaches its peak. Sodium channels are inactivated, but calcium channels are still open which brings in a positive current. This is the main reason why the contractile myocyte have a plateau phase, the calcium ions keep the cell depolarized longer.

Question 14

Mechanics of Plateau Phase of Contractile Myocytes

Answer 14

• Potassium channels also open in contractile myocytes, and at the same time. Potassium moves out to try to repolarize the cell. But because of the L-type Calcium channel, repolarization is much slower. When calcium channel closes, repolarization quickly completes.

Question 15

Absolute Refractory Period in Contractile Myocytes

Answer 15

• Because the calcium channel stays open in the myocyte contraction longer, the absolute refractory period is much longer as a result. Remember refractory periods are possible because when sodium channel closes, there is absolutely no way to open them until action potential ends.

Question 16

Do Conductive Myocytes Have A Refractory Period?

Answer 16

• Conductive cells do not have a refractory period.

Question 17

AV Block

Answer 17

• AV Block is a clinical condition where your AV node is not working.
○ SA node is still intact, so atria can still contract. Because AV node is blocked though, cannot pass anything beyond the AV node. On an ECG, this would be identified by a much lower frequency in QRS complexes.

Question 18

Long QT Syndrome

Answer 18

• Long QT Syndrome is where repolarization of the ventricles is prolonged, resulting in a much longer T wave. Ventricles take longer to repolarize, so something is going on with potassium current.

Question 19

Fibrillation

Answer 19

• Fibrillation is when you have uncoordinated action potential firings. Can happen in only atria or only ventricle. Means that cells in each region are contracting at random times.
○ When atrial fibrillation, the ECG still has P waves, but QRS and T are not going to occur as much until the AP of atria lines up with ventricle. Can be treated.
○ When ventricle fibrillation, the ECG shows small waves, small depolarizations that are happening at different times. Because of this, never any real waves. This is lethal. Heart cannot pump blood to rest of body.

Question 20

Hyperacute T-Wave

Answer 20

• Hyperacute T-Wave means T wave becomes very pointed. It’s a sign of being close to dying, because it is a sign of a heart attack. Heart becomes ischemic, blood flow stops toward heart, so tissue runs out of oxygen.
○ Can cause hyperkalemia, where potassium concentration is very high outside, because oxygen can no longer energize exchange of ions to produce ATP. Prevents repolarization.
○ When heart attack actually occurs, S of QRS does not come down either.