Cardiac electrophysiology (i.e. cardiac action potentials)

Question 1

Depolarization steps in the working myocardium - phase 0

Answer 1

1. First Na+ channels open
2. Inward Na+ flow = rise in membrane potential
3. Membrane potential attempts to reach the nernst eq for Na+ but ENa is not reached

Question 2

Membrane potential

Answer 2

= difference in potential (or voltage) between inside and outside of a cell

Question 3

Why isnt the ENa reached in phase )

Answer 3

1) Na+ channels inactivate before can be reached

2) K+ channels carrying the transient outward current open and partially repolarize the membrane

Question 4

Working myocardium phase 1 steps

Answer 4

1. Fast Na+ channels are inactivated
2. Transient outward current carried by K+
3. Membrane potential repolarizes to about 10 mV

Question 5

Working myocardium phase 2 steps

Answer 5

aka the plateau phase

1. Ca2+ channels are open
2. Membrane permeability to Ca2+ is high so get slow inward current or Ca2+ current
3. Membrane potential attempts to reach ECa but cant because opposed by outward K+ current
4. K+ current inactivates in last part phase 2 and inward Ca2+ current is opposed by delayed rectifiers which also carry outward K+ current (rapid and slow K+ currents)

Question 6

Working myocardium phase 3 steps

Answer 6

1. During this phase Ica inactivates
2. membrane potential rapidly returns to baseline as the delayed rectifiers become fully active
3. During the latter part of phase 3 the inward rectfier opens –> conducts an outward K+ current to comlete repolarization
4. also during this phase all of the channels that were inactivated begin t close (the fast Na+ channels, the Ca2+ channels and the potassium channels that carrying transient outward current)

Question 7

Phase 4 working myocardium steps

Answer 7

• inward rectifier is fully active and pushes membrane potential very close to that of Ek
• remaining inactivated channels close

Question 8

Pacemakers cells in the SA and AV nodes -phase 0

Answer 8

There are no active Na+ channels in these
cells. The rising phase of the action
potential is due entirely to the opening of
calcium channels carrying the slow inward
current (Isi). The rate of rise of the action
potential is therefore slower than in cells
from the working myocardium. Membrane
permeability to Ca2+ is high, but ECa is
never reached because the calcium
channels inactivate, and the delayed
rectifiers (IKr and IKs) activate which carry
outward K+ currents.

Question 9

Pacemaker cells in the SA and AV nodes - phase 3

Answer 9

Calcium channels inactivate and the
delayed rectifiers, IKr and IKs, repolarize the
membrane. The funny current activates
(If). The funny current conducts both Na+
and K+ ions and opposes the repolarization; the funny current becomes more active as the
membrane potential becomes more negative.

Question 10

Pacemaker cells in the SA and AV nodes - phase 4

Answer 10

Also called the diastolic depolarization. Pacemaker cells have no resting membrane potential.
The delayed rectifiers are open during this phase, but close with time. As the delayed rectifiers
close, the funny current depolarizes the membrane until threshold potential is reached at which
point the calcium channels open (Isi).

Question 11

Depolarization in purkinje cells and bundles of his -particularities

Answer 11

These cells also express If and act as pacemakers, but their intrinsic frequency is much slower
than nodal cells. Action potentials in the Purkinje cells are like those of the working myocardium
with the addition of a slow depolarization in phase 4. Action potentials in the Bundle of His have
numerous morphologies that gradually transition across its length, from a nodal phenotype
proximal to the atria to a Purkinje phenotype as the Bundle reaches the ventricles.

Question 12

Absolute refractory period - properties of channels

Answer 12

No AP is possible because the Na+ channels are inactivated

Question 13

Relative refractory period - properties of channels

Answer 13

The magnitude and rate of rise of AP are proportional to the number of Na+ channels that have shifted from an inactivated configuration to closed

Question 14

3 factors that control conduction velocity

Answer 14

1) Size of cells
- smaller the cell = greater the resistance to flow of current and the slower the conduction velocity
2) the number of gap junctions between cells
- the more gap junctions there are the easier the current can flow between cells and the faster the conduction velocity (pacemaker cells in the SA and AV node have the fewest number of gap junctions, purkinje have the largest)
3) The rate of rise of AP
- the faster the rise of AP the faster the speed of conduction of AP
- pacemaker cells have the slowest rate of rise of AP while purkinje have the fastest

Question 15

Normal route of excitation

Answer 15

SA node > atrial cells> AV node > bundle of HIS> Purkinje cells> ventricular cells