Electrical Activity of the Heart

Question 1

Discuss pathways and relative speed with which cardiac action potentials are conducted throughout the heart
Explain why there are diff in conducting velocities

Answer 1

Action potential conduction velocities:

Determined by:
1. size of cells
2. Number of gap junctions between cells

In order of speed:
1. Purkinje (4m/s)
2. Bundle of His and branches (2m/s)
3. Ventricular and atrial muscle (0.5m/s)
4. Nodal cells (0.05m/s)

Question 2

Describe different leads used to record and ECG and how they are configured

Answer 2

Einthoven’s Triangle
- 3 polar leads w/ heart at the centre of equilateral triangle (vertices at arms and legs)

I = Vleftarm - Vrightarm
II = Vleftleg - Vrightarm
III = Vleftleg - Vleftarm

II = I + III

Question 3

Explain the ECG waveform
why does it is appear different on different leads?

Answer 3

Basis for ECG
–> AP does not occur at same time in every region of the heart (some are polarized and others are not –> cration of dipole)
–> dipole moment describes mag and direction of dipole
- mag changes based on charge and tissue

Resting:
- negative membrane potential

Atrial Depolarization:
1. SA node depolarizes (invisible b/c mass is small)
2. Atria depolarization spreads outwards (P wave)
3. Depolarization reaches AV node (invisible b/c mass is small)
- slow conduction velocity = flat line
4. Bundle of HIS depolarize quickly (only left bundle is coupled w/ inter ventricular septum –> depolarization moves left to right)

Ventricular Depolarization:
1. Q-wave:
- polarity of depolarizaiton wave on lead II is reversed (downward deflection)
2. R-wave
- dipole moment passes from endocardium to epicardium (large positive deflection)
- atria repolarize during this time (electric activity mased by ventricular depolarization)
3. S-wave
- depolarization wave moves up the ventricular walls from apex towards AV septum
- reversed current flow relative to lead II (downard deflection)

Repolarization
1. Flat line
- atria repolarize and ventricles are completely depolarized –> no net current flow
2. T-wave
- ventricles repolarize from epi to endocardium (positive deflection)

Question 4

Explain how the funny current affects rate of depolarization of the pacemaker cells and how neural and hormonal input change the heart rate

Answer 4

Funny current:
–> responsible for lack of resting membrane potential in nodal cells
–> highest density in SA node per square micrometer of membrane

Depolarization:
- SA has fastest rate of depolarization and sets the pace for the heart rate: sinus rhythm

Neurohumoral (neuron and hormone)
- changes rate of depolarization of nodal cells –> changes heart rate

Question 5

Discuss how voltage-gated Ca2+ channels contribute to cardiac action potentials

Answer 5

Changing force of contraction:

1. release more Ca2+ from SR
   - SNS stimulation increases amt of Ca2+ stored in SR
   - more Ca2+ is released every beat –> more bounded to Tropomyosin C –> increased contraction force
2. change sensitivity of troponin-C to Ca2+

Question 6

Discuss cardiac output and its regulation

Answer 6

Cardiac output = Heart rate * stroke volume

Regulation:
HR:
- ANS and hormones

SV:
- Preload: wall tension prior to contraction (diastolic pressure / end diastolic volume)

• Afterload: Wall tension during systole (mean arterial pressure)
• Contractility: max force developed during systole and rate at which it devlops

Question 7

Analyse pressure volume loop to assess cardiac function
- changes in preload, afterload, stroke volume, and compliance

Answer 7

Increased Contractility
- change in slope –> change in ventricle P/V relationship (elastance) –> change in contractility
- steeper slope = increased contractility

Increased Preload
- increased SV (overall loop gets larger but slope does not change)

Increased Afterload
- decreased SV (smaller area)

In Vivo Sympathetic Activation
- contractility and preload increases

Question 8

Discuss the key aspects and physiological relevance of different blood vessels:
- pressure, flow rate, diameter

Answer 8

Veins and Venous valves
- low pressure system
- valves prevent backflow
- requries muscular contraction to bring blood back to atrium

Question 9

Discuss factors that regulate blood pressure

Answer 9

Mean Arterial Pressure (MAP)
- const pressure required to produce same flow (provide adequate perfusion of organs)

Baroreceptors embedded in arterial wall in carotid sinuses and aortic arch
- detects when MAP increases (results in decreases SNS and increases PNS) or when MAP decreases (opp)

SNS causes constriction
- (increased BP)
PNS causes dilation
- (decreased BP)

Local factors override neural and hormonal inputs
1. Metabolism
- (decreased pH, pO2, and increase in pCO2 –> arterioral vasodilation)
2. Flow
- increased rate of blood flow –> increased endothelial shear stress
- response: endothelium produces NO —> relaxes arterial smooth muscle cells and increases arteriolar diameter