Lecture 7 Cardiac Excitability: Heart Rate and ECG Flashcards
What are the two types of Cardiac Action Potentials?
- Non-pacemaker cell (myocyte)
- Pacemaker (autorhythmic) cells
Describe a type 1 non-pacemaker cell
- Fast response AP
- Contractile cells requiring a firing instruction
- make up most atrial and ventricular muscle wall
Describe type 2 pacemaker cell
- Unstable resting potential
- Non-contractile cells providing firing instructions
- Found in sinoatrial and atrioventricular nodes
How does an AP occur in an autorhythmic cell?
- 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
What role does Na+ play in both cardiac muscle and pacemaker cells?
Muscle cell: rapid depolarization phase
Pacemaker cells: slow depolarizing pacemaker potential (If channels open resulting in net Na+ influx)
What is the role of Ca2+ in both cardiac muscle and pacemaker cells?
Muscle Cell: Ca2+ influx prolongs duration of AP producing plateau phase
Pacemaker cells: Ca2+ involved in initial depolarizing phase of AP
What is the intrinsic conduction system?
- 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
Why is the Sinoatrial Node important?
- Sets pace of the heart at about 70 BPM
- Beginning point of electrical signals propagating contractions
Why is the Atrioventricular node important?
- 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
How are the atrial and ventricular myocyte syncytia separated?
- Inert fibrous tissue
- small penetrating portion of intrinsic conduction system passes through here
- Inert tissue prevents inappropriate depolarization in the heart
How is heart rate regulated?
Sympathetic Nervous System
- Acts through Epinephrine and Noradrenaline
Parasympathetic Nervous System
- Acts through acetylcholine
How do epinephrine (Epi) and noradrenaline (NE) affect SA node AP firing?
-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)
How does Acetylcholine (Ach) affect SA node AP firing?
- 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
How does parasympathetic activity lower heart rate?
- 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
How does sympathetic activity increase heart rate?
- Activation of sympathetic nerves
- innervate SA node releasing NE, AV node
- Innervate Atrial and Ventricular syncytia
How is contractile strength influenced by parasympathetic and sympathetic systems?
Only the sympathetic system can increase the force of contraction
- they innervate SA and AV nodes AND Atria and ventricles themselves
What are the Three major waves in an ECG (EKG)
P wave
QRS complex
T wave
What does the P wave represent?
Atrial Depolarization at the SA node
Why is there a flat line after the P wave before the QRS complex?
The flat line represents the heart completing atrial contractions throughout the atrial syncytia
What is the Q wave in the QRS complex?
Depolarizing current traveling down the bundles of His in the septum of the heart after atrial contractions complete
What is the R wave in the QRS complex?
Represents the current reaching the Purkinje fibers and beginning to activate ventricular myocardium for contraction
What is the S wave in the QRS complex?
The electrical depolarizing current traveling up the ventricular syncytia
What does the flat line after the QRS complex represent?
Contraction of the ventricles
What is the T wave?
Represents repolarization of the ventricles following contractions
