Cardiac Electrical

Question 1

Contractile cells

Answer 1

Cardiomyocytes

make up bulk of the atrial and ventricular tissue

“work horses”

Question 2

Conductile cells

Answer 2

• Specialized cardiomyocytes whose sole purpose is to generate and propagate electril activity and spread it throughout
  
  • SA node
  • Atrial internodal tracts
  • AV node
  • Bundle of His
  • Bundle Branches
  • Purkinje fibers

Question 3

Cardiomyocytes info:

Answer 3

• striated- Dark bands (myosin) light bands (no myosin)
• 50-100 uM long: shorter and thinner than skeletal
• 1-3 nuclei
• reduced SR system for Ca++ but extensive T-tubule system
• Desmasomes (macula adherens)- mechanical coupling
• Gap junctions- electrical coupling
• one capillary per myocyte

Question 4

Pathway of electrical excitation

Answer 4

1. SA node (normal pacemaker)
2. Atrial internodal fibers (atrial kick)-causes Atrial contraction
3. AV node- delayed to allow atrium to empty completely
4. AV bundle- Bundle of His
5. Right and Left Bundle Branches
6. Purkinje fibers- spreads rapidly causing ventricle contraction

Question 5

Normal pacemaker of heart

Answer 5

SA node

≈100/min

Gets slowed to resting rate by autonomic nervous system

Question 6

SA node is connected to AV node by…..

Answer 6

• Atrial Internodal pathway
  
  • specialized conducting cells
  • ≈50 msec
  • stimulus passed to contractile cells which spread it across BOTH atria
  • Stops at atria because myocardium of atria and ventricles are not connected.

Question 7

Overdrive suppression

Answer 7

• faster firing of SA node suppresses the other cells from acting as pacemakers
• AV node paces at 40 bpm
• Perkinjie fibers pace at 20 bpm

Question 8

Bundle of His

Answer 8

• divides into left and right bundle branches

Question 9

Perkinje Fibers

Answer 9

• fires first only if SA and AV node dont fire
• larger cells
• fast conduction
• moves upward from apex toward base, effect is to push blood upward

Question 10

Cardiomyocyte Action Potential

Answer 10

• Phase 0: Depolarization
  
  • threshold -75 mV
  • Na channel open–Na influx
  • partial closing of K channels
• Phase 1: early Repolarization (Na/Ca pump goes backwards here)
  
  • Na channels close
  • K efflux
• Phase 2: Planeau
  
  • Ca channels open
  • Ca influx
• Phase 3: Repolarization
  
  • Ca channels close
  • K channels fully open, efflux of K
• Phase 4: Diastole
  
  • K leak channels remain open

Question 11

How does Digoxin work?

Answer 11

• Digoxin inhibits Na/K pump which stops Na from leaving.
• This causes Na to not want to enter via the Na/Ca pump so Ca will build up inside the cell.

Question 12

Which way does the Na/Ca pump pump?

Answer 12

3 Na in

1 Ca out

Leaving cell net +1

Question 13

How does the absolute refractory period in heart muscle differ from the absolute refractory period in skeletal muscle.

Answer 13

• the cardiac absolute refractory period is much longer than in skeletal muscle.
  
  • This prevents tetanic contractions (tetany).
  • Also prevents an ectopic pacemaker from stimulating contraction
  • Allows time for ventricle to fill

Question 14

How does Ca++ level increase in the cardiomyocyte cytosol?

Answer 14

1. Calcium enters myocyte via the L-Type calcium channels
2. This calcium binds to the ryanodine receptors on SR and stimulates release of Ca++ from the SR

Question 15

How is the Ca++ removed from the cardiomyocyte cytosol?

Answer 15

1. Ca++ pumped back into the SR by the SERCA pump
   
   1. 1 atp = 1 Ca++ back into SR
2. Calcium sent out to ECF via the Na/Ca exchanger

Question 16

Action potential of the SA node

Answer 16

• Phase 4- pacemaker potential
  
  • Na channels open “Funny Current”
  • K channels closed
  • upward drift of membrane potential
• Phase 0- depolarization
  
  • Voltage gated Ca+ channels open
  • Large influx of Ca+
• Phase 3- Repolarization
  
  • Voltage gated Ca+ channels close
  • Influx of Ca stops
  • Voltage gated K channels open
  • efflux of K

Question 17

How does the Autonomic nervous system regulate the SA node?

Answer 17

• Sympathetic Regulation:
  
  • Beta-1 receptors/ Norepi
    
    • cause opening of Na and Ca channels and influx of Na and Ca
    • increase steepness of pacemaker potential- + chronotropic effect
• Parasympathetic Regulation:
  
  • Muscarinic receptors/ ACh
    
    • opening of K channels
    • efflux of K
    • hyperpolarizes cell and decreases steepness of pacemaker potential
    • longer time for SA node to reach threshold and - chronotropic effect

Question 18

what part of ekg gets shortened when heart rate increases?

Answer 18

diastole

T wave to P wave

Question 19

Normal P wave measurement

Answer 19

Atrial depolarization

0.08-0.10 sec

Question 20

Normal QRS complex measurement

Answer 20

Ventricular depolarization

0.06-0.10 sec

Question 21

Normal P-R intermal

Answer 21

Atrial depolarization plus AV nodal delay

0.12-0.20 seconds

Question 22

Normal Q-T interval

Answer 22

Length of depolarization plus repolarization

*Corresponds to action potential duration

0.20-0.40 seconds