Lecture 27 - Cardiovascular System: Cardiac Action Potential, Conduction, and Contraction

Question 1

Similarities between cardiac myocytes and skeletal myofibres

Answer 1

They are both striated.

They both have the same organelles, but may be located in different locations (ex. nucleus)

Question 2

3 differences between cardiac myocytes and skeletal myofibres

Answer 2

Cardiomyocytes usually have only a single, central nucleus and have a branched structure

Reduced in number of T-tubules and sarcoplasmic reticulum in cardiomyocytes

NMJ is absent in cardiac myocytes

Question 3

How do cardiac myocytes form a functional syncytium?

Answer 3

The syncytium is linked by:

Intercalated discs - physically links the plasma membranes of 2 cells

Gap junctions - allows ions (and thus membrane potential signals) to flow between cells

These linkages mean that myocytes are both physically and electrochemically connected and can act like a large, single unit

Question 4

Why are cardiac action potentials (APs) prolonged?

Answer 4

They involve the opening of L-type VGCC, which are known for long length of opening and the amount if time it takes for them to get open

Question 5

T/F? The difference between the contraction cycle between cardiac myocytes and skeletal myofibres is the lack of sliding filaments in cardiac myocytes

Answer 5

False - The contraction cycle in cardiac myocytes involves interactions between sliding filaments, like myofibres. Once Ca2+ is present, the contraction cycles are similar

Question 6

Describe how Ca2+ plays a role in excitation-contraction coupling in cardiac myocytes

Answer 6

1. Some calcium ions enter the cytoplasm from the ECF
2. The elevated [Ca2+] triggers the release of much, much more calcium from cellular stores (the SR)

Question 7

How is the role of Ca2+ different in cardiac myocytes compared to skeletal myofibres?

Answer 7

Ca2+ in cardiac EC coupling has a different relationship between DHPRs and RyRs

In skeletal muscle cells, it plays a role in mechanical coupling, where a change in voltage causes DHPR to pull open the RyR allowing Ca2+ to leave the SR and enter the cytosol. There is no ion flow through DHPR

In cardiac muscle cells, it plays a role in biochemical coupling, where Ca2+ flows through DHPR into cytosol, and the increase in Ca2+ in the cytosol triggers the opening of the RyR, allowing Ca2+ to escape the SR

Question 8

T/F? Cardiac myocytes cannot produce tetanus

Answer 8

True - due to the duration of the cardiac AP (and its refractory periods). A single AP generates a single contraction (twitch) in both skeletal myofibres and cardiac myocytes, but in cardiac myocytes, a second AP cannot be generated until the twitch is nearly over

Question 9

How is excitation and contraction of heart chambers synchronized?

Answer 9

Conduction pathways - they are formed from highly modified cardiac myocytes and run through the heart wall. These cells lack myofibrils but are highly excitable and connected by gap junctions

Question 10

What is the sinoatrial (SA) node?

Answer 10

Where cardiac rhythm begins; it is a pacemaker cell and generate its own rhythm of depolarization and repolarization

Question 11

What are the 2 parts of the electrical activity of SA node cells?

Answer 11

1. Action potential - depolarization is generated by T-type (transient) VGCCs, not VGNCs. Repolarization is generated by VGKCs
2. Pacemaker potential - a slow depolarization that automatically restarts after every repolarization

These potentials are calcium-based

Question 12

What is the “funny channel” and why is it an important component of the pacemaker potential?

Answer 12

The funny channel is a voltage-gated cation channel that only opens when the membrane is hyperpolarized (allowing Na+ to enter the cell)

During the pacemaker potential, a “funny current” (I_f) flows across the cardiac myocyte plasma membrane

Question 13

Which electrical activity of SA node cells affects the ANS?

Answer 13

Pacemaker potential - changes to this potential affects ANS activity, which causes change in heart rate

Parasympathetic - ACh leads to the opening of additional K+-selective channels. This causes hyperpolarization and slower depolarization (dec. heart rate)

Sympathetic - NE enhances the activation of funny channels. This causes reduced repolarization and more rapid depolarization (inc. heart rate)

Question 14

Describe the depolarization from the SA node

Answer 14

Depolarization from the SA node spreads through the atrial myocardium and the conduction pathway

SA nodes are electrically coupled to neighbouring cardiac myocytes and to the conducting cells of the internodal fibres, which allow depolarization to rapidly spread across both atria

Question 15

T/F? The spread of depolarization is faster through the cardiac myocytes than the conduction system

Answer 15

False - it’s faster through the conduction system than the cardiac myocytes because the conduction pathways cells are specialized for rapid electrical conduction. They can conduct APs up to several metres per second

Question 16

What is the importance of the AV node and how does it affect depolarization?

Answer 16

Since electrical activity cannot spread directly from atrial wall myocytes to ventricle wall myocytes, it must pass through the AV nodes.

The AV nodes has relatively few gap junctions, which slows down AP transmission between its cells and causes a 100ms delay in the spread of depolarization

Question 17

Where does depolarization travel from the interventricular bundle?

Answer 17

It travels rapidly through the interventricular bundle to the Purkinje fibres, then back up to the ventricle wall

The geometry of the bundle and the Purkinje fibres means the apex of the ventricle will be excited (and contract) before the base, allowing for efficient emptying into the arteries

Question 18

What is an electrocardiogram (ECG) and what are the 3 waves involved?

Answer 18

ECGs can detect rhythmic electrical activity in the heart wall

1. P wave - related to atrial depolarization. It’s due to the depolarization from action potentials occurring in cardiac myocytes within the atrial wall
2. and 3. QRS complex and T wave - relate to ventricular depolarization and repolarization. QRS is large because there are many more myocytes in the ventricle walls than in the atrial walls and they depolarize nearly at the same time. T wave is due to repolarization of the ventricle mycoytes

Question 19

Name and describe 4 patterns of arrythmias

Answer 19

1. Sinus arrhythmia - interval between heart beats varies 5% during respiratory cycle and 30% during deep respiratory
2. Premature atrial contractions - occasional shortened intervals between one contraction and the next (frequently occurs in healthy people)
3. Tachycardia - heart rate >100bpm (normal in babies and during exercise; abnormal for adults at rest)
4. Bradycardia - heart rate <60bpm (common in athletes are rest, but should rise with exercise)