Cardiac Electrical Activity (Part 1)

Question 1

The cardiac myocytes exhibit a range of electrical properties such as..

Answer 1

Excitability
Conductivity
Automaticity

Question 2

Fast Response cells (fast APs) are?

Answer 2

Atria
Ventricles
Fast parts of the specialised conduction system

Question 3

What are the steps of Fast response, and what is the resting and threshold potentials

Answer 3

Phase 0: Depolarisation
Phase 1 : small repolarisation
Phase 2: Plateau
Phase 3: Repolarisation
Phase 4: Resting

Resting Potential = ~-90mV
Threshold Potential = ~-70mV in ventricles (-30 to -60 in atria)

Question 4

Phase 0

Answer 4

Rapid Depolarisation (Upstroke)

• Very fast increase in Na+ permeability, fast inward sodium current
• Electrical AND conc gradients both inwards
• Membrane approaches Nernst potential for sodium

Question 5

Phase 1

Answer 5

Early repolarisation (to near 0mV)

-transient outward K+ current

Question 6

Phase 2

Answer 6

Plateau

• Na+ channels inactive hence cell is refractory, inward and outwards currents nearly balanced
  1) Slow inward calcium current: L-type Ca channels & release of Ca from SR (ca dependent ca release)
  2) Outward K+ current

Both currents decline across the plateau.

Question 7

Phase 3

Answer 7

Repolarization

Outward K+ current

1) iK switched on after a delay (time-dependent)
2) Reactivation of iK1 as membrane potential drops (voltage-dependent)

• • other smaller channels
    3) iK,ATP (a decrease in intracellular ATP activates iK,ATP)
    4) iK,Ach (incr Ach activates)

Question 8

Phase 4

Answer 8

Resting

iK1 : High potassium conductance defines and sets resting potential

Question 9

What’s always going on in the background and setting the potenital?

Answer 9

1) Calcium pump - outward current
2) Na/Ca exchanger - ongoing 3Na in for 1Ca out, so electrogenic (inward/depolarising)
3) Na/K ATPas - 3Na out for 2K in, electrogenic (outward, repolarising)

Question 10

Draw the fast response with its channels..

Answer 10

…

Question 11

Cells with a slow response. Why is Na+ not responsible?

Answer 11

Upstroke is not as rapid, and is due to Calcium ions current reaching TP. Their influx is not as intense and depolarisation not as rapid.

Slow response cells have a higher Resting potential. At this RP (~-55mV), the sodium channels have already been activated, opened and then closed.

Question 12

Slow Response Cells are found in?

Answer 12

Sinoatrial Node
Atrioventricular Node
Some diseased cells

May be pacemaker or non-pacemaker type

Question 13

What are slow response RP and phases? Draw the slow response AP

Answer 13

RP= -55mV
Phases: 0, (2), 3, 4

Phase 0: slow upstroke, slow inward Ca current

Question 14

Absolute (effective) Refractory Period

Answer 14

Time when membrane cannot be re-excited. This is essential for cardiac function to avoid tetanus, in order for contraction/relaxation function.

Question 15

Relative refractory Period

Answer 15

Need a larger then normal stimulus to get a propagated AP (slow propagation)

Question 16

Supernormal Period

Answer 16

Get propagated AP from weaker then normal stimulus (slow propagation)

Question 17

Full Recovery Time

Answer 17

May extend beyond return to RP (time dependence)

Question 18

Interval-duration relationship

Answer 18

As HR increases, most of this change is due to decreasing the diastolic phase. BUT some is from the active phase. Therefore as HR increases the AP shortens.

Question 19

What does it mean that cardiac cells are NOT neurogenic

Answer 19

-They do NOT contract in response to a neural signal
-They are MYOGENIC, signal begins within the heart itself
-Electrical activation spreads through the myocardium from cell to cell, as each cell is electrically coupled to several neighbours.
As one depolarizes, current spreads to those cells

Question 20

How does the electrical signal spread between cardiac myocytes.

Answer 20

Cells branch and connect to neighbours via intercalated disks, the sites of electrical and mechanical coupling.

These cells are in a laminar structure, and the myocardium is NOT a uniformly continuous syncitium.

Question 21

Automaticity? Where is this found?

Answer 21

Ability of cells to initiate an electrical impulse through their own pacemaker activity or diastolic depolarisation.

SA node
Some cells around AV node
His-purkinje network

Question 22

How does automaticity occur?

Answer 22

A combination of decreasing outward currents and increasing inward currents

OUTWARD currents decrease: iK

INWARD currents increase: if (funny current, iCa (slow inward calcium current)

Question 23

Mechanisms for altering the intrinsic rate of pacemaker discharge

Answer 23

• alter rate of depolarization (slope)
• alter threshold potential
• alter maximum diastolic potential

Question 24

How is the heart rate regulated?

Answer 24

By the Autonomic Nervous System

1) PNS slows HR: by release of Ach at vagal endings in the heart. @ SA node Ach increases K+ permeability of cells (iK,Ach)
- > hyperpolarized, decrreased pacemaker slope

PNS also slows conduction through AV node. Very strong vagal stimulation can stop the SA/AV node

2) SNS increases HR: By release of noradrenaline at SA node
- increase pacemaker depolarisation slope

Question 25

Normal HR:
Bradycardia:
Tachycardia:

Answer 25

Normal HR: 60-100bpm

Bradycardia: 100bpm