Lecture 7 Cardiac Excitability: Heart Rate and ECG

Question 1

What are the two types of Cardiac Action Potentials?

Answer 1

1. Non-pacemaker cell (myocyte)
2. Pacemaker (autorhythmic) cells

Question 2

Describe a type 1 non-pacemaker cell

Answer 2

• Fast response AP
• Contractile cells requiring a firing instruction
• make up most atrial and ventricular muscle wall

Question 3

Describe type 2 pacemaker cell

Answer 3

• Unstable resting potential
• Non-contractile cells providing firing instructions
• Found in sinoatrial and atrioventricular nodes

Question 4

How does an AP occur in an autorhythmic cell?

Answer 4

• Funny Current Channels (If) causing unstable resting potential
• Permeable to both K+ (out) and Na+ (in)
  1. Net Na+ influx from If channels causes slow depolarization to threshold (pacemaker potential period)
  2. At threshold, Ca2+ channels open causing depolarization while (If) channels close
  3. K+ channels open allowing for repolarization to below threshold
  4. Once below threshold, K+ channels close and If channels open again

Question 5

What role does Na+ play in both cardiac muscle and pacemaker cells?

Answer 5

Muscle cell: rapid depolarization phase
Pacemaker cells: slow depolarizing pacemaker potential (If channels open resulting in net Na+ influx)

Question 6

What is the role of Ca2+ in both cardiac muscle and pacemaker cells?

Answer 6

Muscle Cell: Ca2+ influx prolongs duration of AP producing plateau phase
Pacemaker cells: Ca2+ involved in initial depolarizing phase of AP

Question 7

What is the intrinsic conduction system?

Answer 7

• Made of autorhythmic cells
• Heavily insulated
• Allows for propagation of electrical signals throughout the heart
• Begins at the Sinoatrial node
• Travels to the atrioventricular node
• Down to both Bundle’s of His
• Lastly to Purkinje fibers which connect to myocytes in left and right ventricle

Question 8

Why is the Sinoatrial Node important?

Answer 8

• Sets pace of the heart at about 70 BPM
• Beginning point of electrical signals propagating contractions

Question 9

Why is the Atrioventricular node important?

Answer 9

• Acts as a back up pacemaker incase SA failure
• 50 BPM
• Routes the direction of electrical signals
• Delays transmission of AP to allow for completion of atrial contraction

Question 10

How are the atrial and ventricular myocyte syncytia separated?

Answer 10

• Inert fibrous tissue
• small penetrating portion of intrinsic conduction system passes through here
• Inert tissue prevents inappropriate depolarization in the heart

Question 11

How is heart rate regulated?

Answer 11

Sympathetic Nervous System
- Acts through Epinephrine and Noradrenaline
Parasympathetic Nervous System
- Acts through acetylcholine

Question 12

How do epinephrine (Epi) and noradrenaline (NE) affect SA node AP firing?

Answer 12

-Epi and NE bind to the Beta-adrenergic receptor
- GPCR -> activating alpha-stimulatory subunit
- Activates Adenylate Cyclase releasing cyclic AMP
- cAMP stimulates funny current channel (HCN)
- cAMP stimulates PKA activating voltage gated calcium channels (Cav1.2,1.3)

Question 13

How does Acetylcholine (Ach) affect SA node AP firing?

Answer 13

• Ach binds to the M2R Muscarinic Receptor
• GPCR -> activating alpha-inhibitory subunit
• Activation of Beta-Gamma Subunit stimulates GIRK (G Protein Inward Rectifying Potassium channel)
• Allows Rapid efflux of K+ from cell -> hyperpolarization
• Inhibits Adenylate Cyclase
• Prevents stimulation

Question 14

How does parasympathetic activity lower heart rate?

Answer 14

• Activates vagus nerve that innervates the SA node
• At rest there is a significant vagal tone on SA node and AV node
• between 60 and 80 BPM

Question 15

How does sympathetic activity increase heart rate?

Answer 15

• Activation of sympathetic nerves
• innervate SA node releasing NE, AV node
• Innervate Atrial and Ventricular syncytia

Question 16

How is contractile strength influenced by parasympathetic and sympathetic systems?

Answer 16

Only the sympathetic system can increase the force of contraction
- they innervate SA and AV nodes AND Atria and ventricles themselves

Question 17

What are the Three major waves in an ECG (EKG)

Answer 17

P wave
QRS complex
T wave

Question 18

What does the P wave represent?

Answer 18

Atrial Depolarization at the SA node

Question 19

Why is there a flat line after the P wave before the QRS complex?

Answer 19

The flat line represents the heart completing atrial contractions throughout the atrial syncytia

Question 20

What is the Q wave in the QRS complex?

Answer 20

Depolarizing current traveling down the bundles of His in the septum of the heart after atrial contractions complete

Question 21

What is the R wave in the QRS complex?

Answer 21

Represents the current reaching the Purkinje fibers and beginning to activate ventricular myocardium for contraction

Question 22

What is the S wave in the QRS complex?

Answer 22

The electrical depolarizing current traveling up the ventricular syncytia

Question 23

What does the flat line after the QRS complex represent?

Answer 23

Contraction of the ventricles

Question 24

What is the T wave?

Answer 24

Represents repolarization of the ventricles following contractions

Question 25

What occurs when you have a wide QRS complex?

Answer 25

• Third-degree block (COMPLETE block)
• Top part of the heart disconnected from bottom

Question 26

What occurs when there is no P wave and irregular QRS?

Answer 26

Atrial fibrillation: Atria begin beating chaotically and irregularly

Question 27

What occurs when there is no P wave and no QRS

Answer 27

Ventricular fibrillation: Ventricles begin contracting extremely fast
- require defibrillator

Question 28

What occurs when there is a normal P, normal QRS but P not triggering QRS?

Answer 28

Second degree heart block: AV node
- Partial disconnect between atria and ventricles
- malfunction in electrical transmission