Lecture 5: Initiating cardiac AP

Question 1

What determines AP propagation

Answer 1

resistance between cells, voltage difference between cells, threshold for AP

Question 2

Describe the channel at the SA node

Answer 2

HCN (hyperpolarized, cyclic nucleotide-gated channel) non-selective positive cation channel

Question 3

Describe 4 differences between the SA node and cardiac myocyte AP

Answer 3

diastolic depolarization - due to HCN
slower upstroke - no Na channels
no plateau - Ca channels inactivated by that point since used for depolarization
higher RMP = lack of inward rectifier

Question 4

Describe how sympathetic and parasympathetic stimulation affects the SA node

Answer 4

NE binds to beta1 receptor, activates Gs which activates adenylyl cyclase, converts more ATP to cAMP which opens more HCN channel and Cav1.2 channels, leads to greater AP frequency, right shift in the IV relationship

Ach binds to M2, activates Gi, causes left shift.

Question 5

Describe the differences at the AV node

Answer 5

slower diastolic depolarization - junction escape
more negative Em
smaller cells
fewer gap junctions

Question 6

Describe some numbers (velocity, time) for electrical conduction

Answer 6

all approximately 1m/s except for purkinje fibers (4m/s) and AV node (0.05 m/s)
takes much longer to go from AVN to bundle of His