Action potentials in cardiac muscle. Refractory periods. Rhythmical excitation of the heart.

Question 1

phase 0

Answer 1

DEPOLARISATION

voltage gated sodium ion channels open
- influx of na+ ions
- MP becomes more positive from -90mv to +20mv

Question 2

phase 1

Answer 2

INTIIAL REPOLARISATION = very brief

voltage gated sodium channels close
- no more na+ influx

transient outward potassium channels open
- k+ efflux

Question 3

phase 2

Answer 3

plateau phase

delayed rectifier potassium channels open
- k+ efflux

voltage gated L- type calcium channels open
- ca2+ influx

= maintains prolonged contraction before relaxing

Question 4

phase 3

Answer 4

RAPID REPOLARISATION

delayed rectifier potassium channels remain open
- k+ efflux

voltage gated L type calcium channels close
- no more ca2+ influx

ensures MP is negative enough to reach RMP

Question 5

phase 4

Answer 5

RESTING MEMBRANE POTENTIAL

inward rectifier potassium channels + na+/k+ atp-ase pump = -90mv

• ready for next AP

Question 6

absolute refractory period

Answer 6

cardiac cells cannot generate another AP no matter how strong the stimulus of AP is

reason: no activity of sodium channels

prevents tetanic contractions = ensuring the heart can relax before it contracts again

Question 7

relative refractory period

Answer 7

an AP can occur if there is a stronger than normal stimulus

only some activity of sodium channels

preventing premature contractions

Question 8

effective refractory period

Answer 8

used alongside absolute refractory periods

prevents propagation of premature impulses causing arrythmia

Question 9

Rhythmical excitation of the heart - component 1

Answer 9

sinoatrial node

• found in right atrium near SVC
• primary pacemaker of heart, initiating AP at 60-100bpm
• unstable resting membrane due to pacemaker potential
• gradual depolarisation = spontaneous firing of AP

Question 10

Rhythmical excitation of the heart - component 2

Answer 10

atrial conduction pathway

spreads impulse from SA node to atria = atrial contraction

the bachmann bundle sends signal from right atrium to left atrium

Question 11

Rhythmical excitation of the heart - component 3

Answer 11

AV node

• located at junction of atria-ventricles near tricuspid valve

= delays AP before it goes to ventricle to ensure atria have fully contracted first

Question 12

Rhythmical excitation of the heart - component 4

Answer 12

bundle of His

carries signal from AV node -> bundle of His branches

Question 13

Rhythmical excitation of the heart - component 5

Answer 13

bundle of His branches

signal from AV node -> ventricles

Question 14

Rhythmical excitation of the heart - component 6

Answer 14

purkyne fibres

• ensures synchronised ventricle contraction
• purkyne fibre cells coat ventricles
• acts as pacemaker if SA/AV node fail