Unit 2 Lecture 2: Cardiac Conductance

Question 1

What is the SA node?

Answer 1

• Is it the pacemaker of the heart because it is what is causing contraction of the heart to occur.
• It is what sets our resting heart rate (RHR)

Note: Bundle of His at the end around the ventricles will send out an action potential which allows for an electrical impulse to be generated and cause contraction

Question 2

Explain what happens when depolarization in the SA node occurs?

Answer 2

• A signal is sent out which allows for the depolarization of other cells.
• An action potential will spread through atria via gap junctions to contract both simultaneously

Question 3

What does Atrial depolarization automatically cause? What does this lead to?

Answer 3

• Depolarization of the AV node due to the action potential spreading to the bottom of the atria
• This leads to the eventual beginning of contraction for the ventricles but the AV node will allow for full contraction of the atria to move as much blood into the ventricles

Question 4

After the impulse is sent through the AV node where does it head to?

Answer 4

• Travels down interventricular septum via the Bundle of His
• Purkinje fibres rapidly distribute electrical signals to ventricles

• AV node & Bundle of His are the ONLY electrical connection between atria and ventricles

Question 5

Explain what the picture is showing

Answer 5

• This is a picture of how pacemaker potential works
• Slow rise from rest is pacemaker potential
• -40mV is the threshold potential for the SA node
• Progressive ↓ in K+ permeability ⇒ K+ channels close
• ↑ in Na+ permeability ⇒ opening of F-type Na+ channels
• ↑Ca2+ permeability ⇒ Opening of T-type Ca2+ channels

REMEMBER: SA node is not a muscle it just initiates the initial depolarization to allow contraction of the muscle to occur
* Steeper the rise = quicker the AP frequency

Question 6

Decreased permeability means?

Answer 6

Closing of a channel, increased means opening of a channel

Question 7

Explain what is happening during the opening and closing of certain channels in the pacemaker potential cycle

Answer 7

• K+ channels begin to close preventing K+ from moving out but it keeps +ve charge inside allows initiation from resting potential
• Increase in Na+ permeability allows F-type (funny) channels to open up but they let some sodium in
• Finally Ca2+ opens up while F-type channels of Na+ close at -50mV allowing for Ca2+ to cross through t-type channels to increase membrane potential to threshold

NOTE: At the max potential, K+ permeability starts to increase to allow K+ to flow out of the cell and bring everything back to -60mV.
NOTE: SA node action potential rate = Heart Rate

Question 8

If T-type Ca2+ channels increase membrane potential (in pacemaker potential) to threshold what do L-type Ca2+ channels do?

Answer 8

At the threshold it allows for the action potential to spontaneously occur

Note: L-type stands for long because it takes a longer amount of time to close

Question 9

What does intrinsic AP mean?

Answer 9

It is self-managed (SA node), and does not require an outside source which explains why our heart can beat if taken out of the body

Question 10

Does the AV node also have a pacemaker potential like the SA node?

Answer 10

Yes, but it is much slower than the SA node so it is not recognized or considered that often.

• Since the SA node is faster (100 APs/min) it is relied upon more to keep the heart beating together

• Ventricles beat “on their own”
• If it is not rhythmic with the atria - arrhythmia

Question 11

Difference between pacemaker potential and cardiac action potential?

Answer 11

• Pacemaker potential occurs between the SA and AV node while cardiac action potential is measuring the potential between the atria and ventricles