You Asked - Norgren

Question 1

What does “rectifier” mean (in reference to type of ion channel)?

Answer 1

A rectifier channel allows the passage of ions easily in only one direction; there is a preference for a single direction of ion flow

Question 2

Why is the cardiac cell resting membrane potential the same as the K+ eq. pot.?

Answer 2

Membrane potential is determined by permeability of the membrane to different ions, mainly Na+ and K+. The more permeable the membrane is to one of those two ions, the closer the membrane potential will be to that ion’s eq. pot. The resting membrane of a cardiac cell has very low permeability to Na+ and a greater permeability to K+, Thus the resting membrane potential is very similar to the equilibrium potential of K+, which is -90 mV.

Question 3

Inward rectifier K+ channels generally only allow flow of K+ ions into the cell. Why is this important?

Answer 3

It is important for the return to and maintenance of the resting membrane potential after the rapid depolarization event in “fast” response cells (which is due to the large efflux of K+ through the delayed rectifier channels that are allowing K+ to flow out of the cell)

Question 4

How do cardiac parasympathetic fibers control the beating HR?

Answer 4

-Vagus Nerves
-Release acetylcholine
–Inc. permeability of resting membrane to K+
–Decreases diastolic if current through HCN channels
-NET EFFECT = slows HR
“Negative chonotropic effect”
-Slows rate of spontaneous depolarization of membrane
-Initial hyper polarization of resting membrane potential

Question 5

What is the HCN channel?

Answer 5

• Four types expressed in the heart and nervous system
• Activated by membrane hyper polarization at voltages neg. to -50 mV (constitutively open at voltages near resting membrane potential of pacemaker cells!)
• HCN channels are the channels through which the funny current moves

Question 6

What does “diastolic mean in reference to the funny current and depolarization?

Answer 6

Bc the pacemaker potential represents the non-contracting time between heart beats (diastole), it is also called the diastolic depolarization.

Question 7

How to calculate a normal Q-T interval?

Answer 7

QT intervale (msec) /R-R interval (msec) = or

Question 8

How is the Q-Tc interval different?

Answer 8

• Take Q-T interval and divide it by the square root of the R-R interval, allowing for the assessment of the QT interval independent of heart rate
• Normal Q-Tc values are

Question 9

Why are QT intervals important?

Answer 9

Can be diagnostic for susceptibility to certain types of tachyarrhythmias (e.g. Torsades des Pointes)