3 Basic Electrophysiology

Question 1

Why is a cell’s resting membrane potential negative?

Answer 1

Normally -60 to -90 mV mainly because of the contribution of the resting potential of K

Question 2

What is an equillibrium potential?

Answer 2

When the ion no longer diffuses (no net flux) in/out of the membrane because the electric potential balances the ion concentration gradient

**Electric force= Chemical forces

Question 3

What is depolarization?

Answer 3

Na/Ca ion influx into the cell, making the membrane potential more positive

Question 4

What is repolarization?

Answer 4

K ion flux out of the cell making the membrane potential return to more negative values

Question 5

Why does an action potential travel in one direction?

Answer 5

Inactivation of Na channels

Question 6

What is the effective refractory period?

Answer 6

Period that extends a little beyond absolute refractory period where depolarization can occur but not enough to generate an AP

**TIME and NEG voltage necessary for inactivated Na channels to open again

Question 7

What is the functinal reason the AV node delays AP transmission?

Answer 7

Allows time for atria to empty their blood into the ventricles

Question 8

What portion of the EKG corresponds to the length of the AP?

Answer 8

QT interval= AP duration

Question 9

What are the ion concentrations in mM that contribute to the resting membrane potential?

Answer 9

• K
  
  • contributes the most!
  • 4 mM outside
  • 140 mM inside
• Na
  
  • 140 mM outside
  • 10 mM inside
• Ca
  
  • 2.4 mM outside
  • ~50 nM inside

Question 10

How do you calculate the equilibrium potential?

Answer 10

The Nernst equation (ion valence z in equilibrium at the nernst equilibrium)

RT/zF= 26 for monovalent

Question 11

What is the Na/K ATPase?

Answer 11

Active transport for the uptake of 2K and extrusion of 3Na against their concentration gradients

Question 12

What is the Na-Ca exchanger?

Answer 12

Forward mode; extrudes 1Ca out of the cell/3Na enter

Reverse mode; 1Ca enters/3Na exit

Question 13

What is the Ca-ATPase?

Answer 13

Ca pump that extrudes Ca out of the cell

Question 14

What is the SERCA?

Answer 14

Sarco/endoplasmic reticulum Ca-ATPase

**returns Ca ions back into the lumen of the SR

Question 15

Contrast the AP of myocytes and nodal cells

Answer 15

Question 16

What are the phases/channels of a myocyte AP?

Answer 16

• phase 0= Na channel
  *depolarization
• phase 1= Kto channel
• phase 2= Ca-L, NaCa, Kr, Ks channels
  *delicate balance, easily perturbed
  *plateau phase
• phase 3= Kr, Ks, K1, KAch/ATP channels
  *repolarization
• phase 4= K1, KAch/ATP channels
  *resting phase

Question 17

What can cause long QT syndrome?

Answer 17

Block or dysfunction of K channels can delay repolarization of the cardiac AP

**prolonged QT can lead to “early after depolarization” (break of threshold leading to ectopic beats/torsades des pointes)

Question 18

What is torsades des pointes?

Answer 18

“Twisting of the points”

Polymorphic ventricular tachycardia that can lead to sudden cardiac death

Question 19

What is the physiological result of persistant Na channels?

Answer 19

Mutant Na channels incompletely close the inactivation gate resulting in an increased level of persistent current

**associated with hyperkalemic periodic paralysis, long QT syndrome, and inherited epilepsy

Question 20

Describe the cardiac pacemaker depolarization

Answer 20

“Funny” Na current is carried by HCN (hyperpolarization-activated cyclic nucleotide-gated)

**HCN is voltage gated and regulated/activated by cAMP

Question 21

What creates the refractory period of nodal cells?

Answer 21

Due to the inactivation of Ca channels (take longer to recover)