Impulse Propagation and Pacemaking

Question 1

decremental current flow

Answer 1

current flow and the resulting voltage changse that decreases with distance from the active area

Question 2

connexins

Answer 2

proteins in gap junctions that contain the electrically conducting channels, varying in resistance

Question 3

space constant (λ)

Answer 3

λ = sqrt(r_m/r_i)

the constant that describes the rate at which voltage declines from the injection site

Question 4

time constant

Answer 4

the time it takes for V to decline by 1/e (~1/3)

Question 5

safety margin for conduction

Answer 5

the likelihood of maintaining a regenerative wave of propagation

promoted by low internal resistance and high membrane resistance

Question 6

M cells

Answer 6

midmyocardial cells in which the contribution of repolarizing K+ currents (especially the delayed rectifier) seems to be reduced

they consequently have a greater action potential duration than the endocardial or epicardial cells

also become disproportionately prolonged in response to factors that influence action potential duration such as cycle length, electrolyte concentration, and some drugs

Question 7

What gives rise to the delay in the AV node?

Answer 7

a smaller margin of error due to the decreased voltage spike

this means that fewer cells are stimulated and the conduction speed is significantly decreased

Question 8

velocity of conduction in the purkinje system

Answer 8

4 m/s

Question 9

velocity of conduction in the atrium

Answer 9

1 m/s

Question 10

velocity of conduction in the AV and SA nodes

Answer 10

0.5 m/s

Question 11

Why is it useful to have a slow response AP in the AV node?

Answer 11

it gives the atria time to contract before the ventricles do

Question 12

location of cells that can function as pacemakers

Answer 12

sinus node

atrial cells

AV nodal cells

purkinje fibers

Question 13

funny current (I_f)

Answer 13

carried out by a non-selective cation channel that conducts both Na and K

activation is slow and it doesn’t inactivate

slowly activates at the end of phase 3

Eq potential is around -20mV

Question 14

What are the driving forces of phase 4 depolarization in the SA node?

Answer 14

membrane voltage clock

sarcolemmal calcium clock

Question 15

membrane voltage clock

Answer 15

gradually increasing inward current I_f

gradually decreasing outward current I_K

activation of T-type and L-type calcium channels in late phase 4

Question 16

sarcolemmal calcium clock

Answer 16

works through the Na/Ca exchanger

rhythmic diastolic release of calcium by the sarcoplasmic reticulum via the ryanodine type 2 receptor activates the exchanger, which pumps out 1 Ca for an influx of 3 Na, thereby depolarizing the membrane

Question 17

periphera SAN cells

Answer 17

serve to insulate the depolarized central cells, which are the dominant pacemakers

this prevents the well-polarized atrial myocardial cells from hyperpolarizing the central cells

done through differential characteristics of the connexins