Cardiac Action Potential

Question 1

what does the SA node do

Answer 1

Maintains an internal clock

Question 2

what does the AV node do

Answer 2

acts as a secondary internal clock if the SAN fails

Question 3

describe action sequence of heart

Answer 3

Electrical pulse generated in SAN
depolarises atrial cardiomyocytes
atria contract
AVN cells get depolarised after delay
bundle of his then purkinje fibres get depolarised in midline of heart
pulse travels back down ventricle wall and ventricles contract

Question 4

cardiac pacemaker cells

Answer 4

do not contain sarcomeres
mainly in SAN
unstable resting membrane potential
funny current
depolarisation
repolarisation
pacemaker cells have automaticity

Question 5

funny current
rapid depolarisation
repolarisation

Answer 5

slow depolarisation from 60mv to 40mv
further depolarisation to 20mv
repolarisation back to -60mv

Question 6

ion movement in pacemaker cells

Answer 6

HCN channel lets in Na+ when potential is less than -40mv to increase potential
at -40mv HCN closes and L-type Ca2+ channel opens and lets in Ca2+ for depolarisation
at +20mv Ca2+ channel closes and K+ channel opens to let out K+ and causes repolarisation

Question 7

Parasympathetic Stimulation

Answer 7

interaction doen by vagus nerve
can slow down heart rate by 3 methods: slowing down funny current (Slower Na+ flow) by blocking HCN
can increase threshold potential for Ca2+ gate to open so it takes longer to reach thus slowing down heart rate
can hyperpolarise the cell, increase maximum potential cell can reach

Question 8

sympathetic stimulation

Answer 8

activates Beta1 receptor
increases heart rate
stimulate HCN
increases funny current

Question 9

action potential propagation

Answer 9

depolarisation of pacemaker cells
depolarising ions(Ca2+) move from pacemaker cells to next cardiomyocyte cell via gap junctions
which causes voltage gated sodium channels to open to let in Na+ and rapidly depolarise the cell
causes depolarisation of cardiomyocyte
will continue like this

Question 10

cardiomyocyte action potential

Answer 10

has a stable resting membrane potential of -90mv
300ms
0 - depolarisation
1 - initial repolarisation
2 - plateau
3 - repolarisation
4- resting membrane potential

Question 11

depolarisation

Answer 11

+ve ions move in by gap junction to open voltage gated sodium channels and cause depolarisation

Question 12

inital repolarisation

Answer 12

voltage gated sodium channels close and open fast potassium channels
K+ move out for repolarisation
this cell is now in absolute refactory period until -80mv

Question 13

plateu

Answer 13

K+ channel closes quickly Ca2+ channel opens to let in Ca2+
delayed rectifier potassium channel opens to let out potassium ions
this causes plateu as membrane potential remains equal due to equal ions moving in and out

Question 14

repolarisation

Answer 14

opens slow potassium channel to let out potassium ions
this repolarises the cell

Question 15

resting membrane potential

Answer 15

NA-K ATPase uses ATP to move 2 K+(sometime Ca2+) in and 3 Na+ out
this drops membrane potential 5mv
inward rectifying K= channels let K= in or out to keep membrane potential at -80mv

Question 16

refactory periods

Answer 16

between depolarisation and end of plateu (180ms) it is absolute refactory period, another action potential cannot be triggered
after that is the relative refactory period, action potential can be triggered but further away the potential is from -80mv the greater the stimulation is needed

Question 17

what are cardiomyocytes

Answer 17

Cardiac cells of the heart