Electrical Activation of the Heart

Question 1

what is the resting potential?

Answer 1

negatively charged on the inside of the membrane.

-90mV in the heart

Question 2

what ions are involved in resting potential/electrical activity?

Answer 2

K+
Ca2+
Na+
Cl-

Question 3

how does action potential spread across cells?

Answer 3

via gap junctions (connexions) which activates other myocytes.

Question 4

how does an electrical stimulus lead to myocyte contraction?

Answer 4

by excitation-contracation coupling

Question 5

how many and what are the stages of myocardial action potential?

Answer 5

1. rapid depolarisation
2. partial repolarisation
3. plateau
4. resting potential
5. repolarisation

Question 6

what happens in rapid depolarisation?

Answer 6

• inflow of Na+ into the cell at -60mV
• Na channels open
  depolarises the membrane
• channels close at +30mV

Question 7

what happens in partial repolarisation?

Answer 7

• K+ outflow, Na+ inflow stops

- transient K+ channels open briefly

Question 8

what happens in the plateau?

Answer 8

• Ca2+ slowly moves in

- balanced potassium and calcium channels maintain the plateau

Question 9

what happens at resting potential?

Answer 9

• K+ outflow and Na+ inflow maintained by sodium potassium pump
• K+ diffuses out by selective permeability
• chemical and electrical gradients reach an eqm

Question 10

what happens at repolarisation?

Answer 10

• K+ outflow
• Ca2+ inflow stops
  calcium channels close and potassium channels activate repolarising the membrane

Question 11

what causes the refractory period?

Answer 11

at the end of the plateau when the calcium channels close which prevents tetany incase of recurrent stimuli (absolute refractory period)
after there is a relative refractory period where channels reopen only if needed.

Question 12

which phase allows the heart to fill with blood?

Answer 12

the plateau phase

Question 13

what is propagation?

Answer 13

Na+ ions depolarise adjacent cells and gap junctions.

Question 14

what is automacticity?

Answer 14

the ability of a cell to contract on its own without needing external stimuli.

Question 15

what is the order of pacemakers in the heart?

Answer 15

SA node (primary)
AV node (secondary)
HP system, fibres down the interventricular septum (final)

Question 16

why is the AV node the secondary pacemaker of the heart (will continue if SA node fails)?

Answer 16

it has less pacemaker cells.

Question 17

how does the AV node delay the electrical impulse into the interventricular septum?

Answer 17

delay allows atria to fill.

• has less gap junctions
• AV fibres are smaller than the atrial fibres

Question 18

how does the HP system allow for coordinated ventricular contraction?

Answer 18

• rapid conduction
• large fibres
• high permeability at gap junctions

Question 19

what controls sympathetic stimulation of the heart?

Answer 19

adrenaline, noradrenaline, and type 1 beta adrenoreceptors which increase adenyl cyclase which increases cAMP

Question 20

what controls parasympathetic stimulation of the heart?

Answer 20

acetylcholine, M2 receptors (which inhibit adenyl cyclase and hence reduce CAMP).