Exam 1 lecture 3

Question 1

Phase 4 of ventricular action potentials

Answer 1

resting potential. membrane is permeable t potassium through leak channels, outward flow of potassium ions generate the Ik2p current. Inward rectifer outward potassium current Iir. Em approx. for potassium (-96mv)

Question 2

Phase 0

Answer 2

rapid opening of voltage gated sodium channels and inward sodium current, driving membrane potential in direction of sodium (+52)

Question 3

Phase 1

Answer 3

closing of voltage gated sodium channels, coupled with voltage gated transient outward potassium current (Ito). Pulls membrane potential back to resting

Question 4

Phase 2

Answer 4

Plateau, reduced outward potassium current (Ik1 or Iir) coupled with an inward calcium current (Ica (l)) through L type voltage gated calcium channels

Question 5

Phase 3

Answer 5

repolarization. closure of L-type calcium channels, an increase in outward potassium current (Ik1), and rapid and slow voltage-gated delayed outward rectifier potassium current (Iks and Ikr). Delay in reporlarization allows for calcium channels to reactivate

Question 6

why is delayed outward rectifier Ikr for potassium important for repolarization

Answer 6

hERG forms the major portion of voltage gated ion channel protein involved in Ikr. mutations can delay cardiac repolarization. In ECG, effect is LONG QT INTERVAL, or congenital long QT syndrome

Question 7

what can congenital long QT syndrome cause

Answer 7

serious ventricular tachycardia and ventricular fibrillation

Question 8

What else can cause congenital long QT syndrome

Answer 8

mutations in sodium and calcium channels

Question 9

drugs that block hERG cause

Answer 9

acquired long QT syndrome, antiarrhythmics, anti-psychotic agents and antibiotics

Question 10

inward rectifier

Answer 10

channels close as the cell depolarizes, (ooposite potassium channels which close when cell depolarizes). This is why rectifier current is reduced at plateau phase of cardiac action potential and increases again as cell repolarizes

Question 11

directions of rectifier current

Answer 11

downward deflections are inward currents and upward deflections are outward currents

Question 12

effective refractory period

Answer 12

during plateau phase, arrival of second depolarizing impulse has no effect

Question 13

what follows effective refractory period?

Answer 13

relative refractory period: when sodium channels in membrane are unable to open when voltage is changed (when other are reset and ready to respond)

Question 14

why dont cardiac muscle experience tetanus like the muscles do?

Answer 14

because the effective refractory period last quite a long time

Question 15

when does calcium enter myocardial cells,

Answer 15

phase 2, during the plateau phase, triggers a release of more calcium from SR-> calcium concentration int he sarcoplasm increases-> calcium binds troponin which causes conformational change in tropomyosin, allowing mysoin to bind actin

Question 16

what increases inward trigger calcium current

Answer 16

by catecholamines acting via beta-1 adrenegic receptors and cAMP/protein kinase-> leads to phosphorylation of L-type calcium channels

Question 17

why does triger calcuim lead to

Answer 17

increased cardiac inotropic state because more trigger calcium enters the myocyte, more is released from the SR

Question 18

what blocks catecholamines

Answer 18

beta-blockers (bind beta adrenergic)

Question 19

what do all cardiac cells have the capability of doing?

Answer 19

rhythmically and spontaneously depolarizing and repolarizing

Question 20

override supression

Answer 20

normally SA node fires before the slower pacemaker can fire and so its depolarizes them before they can spontaneously depolarize

Question 21

escape rhythm

Answer 21

another pacemaker taking over a defective SA node, fires at slower rate

Question 22

ectopic focus

Answer 22

certain conditions in which there can be an isolated spontaneous depolarization of a cell or group of cells in another region of the heart, ventricular or atrial myocardium, area is referred to as eptopic focus

Question 23

what is funny current

Answer 23

prepotential or pacemaker potential is represented by a slow depolarization of the cell, caused by If “funny current” carried mainly by sodium. ( basically sodium leaking into the cell)

Question 24

how does parasympathetic affect pacemaker frequency

Answer 24

decreases heart rate, Ach (muscarinic receptors) causes potassium permeability of SA nod cells to increase. repolarization Em is dirve closer to Ek and rate of prepontential formation is reduced-> takes longer for prepotential to reach firing threshold-> heart is slowed

Question 25

how does sympathetic affect pacemaker frequency

Answer 25

stimulate increase heart rate. NE (beta 1 adrenegeric) causes potassium channels that are opened at the last phase of AP to close more rapidly, reducing the time taken for prepotential to reach firing threshold-> heart rate is accelerated.

Question 26

annulous fibrosis

Answer 26

electrical depolarization can only reach the ventricles by passing through the AV node-> electrically insulated from each other by fibrous 4 ringed skeleton of heart

Question 27

why is there a delay in the AV node

Answer 27

allows time for atria to contract before ventricular contraction begins

Question 28

P wave

Answer 28

depolarization of atria, indicates SA node function

Question 29

P-R internval

Answer 29

indicative of time it takes for impulse to pass through the AV node into the ventricles (atrioventricular time) normal is .12-.2 seconds
( increases in AV block)

Question 30

QRS

Answer 30

depolarization of the ventricles, normally .12 seconds. indicates the duration of the ventricular depolarization-> Q wave may be exaggerated following myocardial infarction

Question 31

T wave

Answer 31

progressive repolarization of ventricles, after which they are ready to be stimulated again
(inverted or peaked in myocardial infarction or hyperkalemia)

Question 32

S-T segment

Answer 32

when elevated above baseline, indicates the possibility of myocardial infarction, depression below baseline may indicate myocardial ischemia

Question 33

Q-T interval

Answer 33

varies with heart rate: abnormally prolonged in long QT syndrome. Rule of thumb: for heart rates between 65-90 is that QT is normal if half preceding R-R interval

Question 34

U wave

Answer 34

if present-> follows T wave, notable in hyperkalemia, digoxin, quindine and other conditions

Question 35

J point

Answer 35

junction between QRS and ST segment

Question 36

one complete ECG wave

Answer 36

PQRSTP