Cardiac Action Potential

Question 1

Cardiomyocytes

Answer 1

contractile cells of heart - found in atria & ventricles

Question 2

Pacemaker cells

Answer 2

only present in few areas of heart - mostly in SAN - cells in AVN take over when not working.
no sarcomeres - do not contribute to contractile nature.
set heart rate

Question 3

Funny Current

Answer 3

slow depolarisation from -60mv to -40mv

Question 4

Ectopic pacemaker

Answer 4

pacemaker cells not in SAN

Question 5

Automaticity

Answer 5

pacemaker cells - self depolarisation

Question 6

Role of PNS

Answer 6

reduce heart rate

Question 7

Parasympathetic activity on heart

Answer 7

vagus nerve

Question 8

3 main mechanisms of PNS action on heart rate

Answer 8

• lengthening funny current - longer for pacemaker cells to reach threshold potential = inhibition of HCN channel
• increasing threshold potential for activation of v- gated calcium channels
• decreasing maximum membrane potential the cell can achieve - longer to reach repolarisation potential - increased expression of potassium channels - HYPERPOLARISED MEMBRANE POTENTIAL

Question 9

PNS activity on AV node

Answer 9

slow conduction from atria to ventricles

Question 10

Ivabradine

Answer 10

blocks HCN channel - more time for ventricular filling

Question 11

Verapamil

Answer 11

blocks action of v-gated calcium channels = reduces influx of calcium into cells = reduces contractile force of heart

Question 12

SNS

Answer 12

increases heart rate

Question 13

increased heart rate

Answer 13

• increases speed of funny current + threshold potential reached quicker
• increase calcium conduction through v-gated calcium channels - increases calcium induced calcium release from sarcoplasmic reticulum = MORE FORCEFUL CONTRACTIONS

Question 14

gap junction

Answer 14

link pacemaker cells + cardiomyocytes = leads to localised depolarisation of the membrane
calcium diffuse through - leads to opening of sodium channels = RAPID DEPOLARISATION

Question 15

plateau phase

Answer 15

influx of calcium channels

followed by rapid repolarisation - efflux of potassium

Question 16

Cardiomyocyte action potential phases

Answer 16

depolarisation
initial repolarisation 
plateau
repolarisation
resting membrane potential

Question 17

stage 0

Answer 17

v - gated Na channels open = localised depolarisation - neighbouring channels open as well
fast depolarisation

Question 18

stage 1

Answer 18

+20mV - v-gated Na channels close to an inactivated state = no stimulation will open
ABSOLUTE REFRACTORY PERIOD - no other action potential - prevents tetany
fast potassium channels open - repolarisation = EFLUX

Question 19

stage 2

Answer 19

repolarisation is brief - potassium channels close - v gated Ca channels + delayed rectifier K channels open
MUSCLE CONTRACTION

Question 20

stage 3

Answer 20

Ca channels close - slow K channels open = efflux of K -> RAPID REPOLARISATION
Na channels now closed in voltage dependent manner = no longer refractory state

Question 21

stage 4

Answer 21

Na - K ATPase exchanges 2 extracellular K ions for 3 intracellular Na ions - drop of around 5mV in membrane potential
resting membrane potential mainly mediated by inwardly rectifying K channels - efflux of K
mainly mediated by K
Na - Ca exchanger - swaps intracellular Ca for Na

Question 22

Absolute refractory period

Answer 22

180ms

Question 23

Relative refractory period

Answer 23

membrane depolarises - v-gates Na channels become voltage sensitive
AP can be generated by requires greater stimulation

Question 24

Electrical - Contraction coupling

Answer 24

Ca entry enters at T tubules - activates ryanodine receptor Ca channels = release of large amounts of Ca from sarcoplasmic reticulum
Ca interacts with actin and myosin = contraction
Ca removed from intracellular space through Ca ATPase and through Na-Ca exchanger

Question 25

P wave

Answer 25

depolarisation of cardiomyocytes within atria

Question 26

P-R segement

Answer 26

atrial contraction - no change on electrical conductance on an ECG