7 - Electrical Properties of the Heart

Question 1

Why is the potassium hypothesis important?

Answer 1

resting membrane potential is established by the movement of potassium through ion channels (has nothing to do with the Na+-K+ pump)

Question 2

What is meant by the ‘driving force’?

Answer 2

the difference between the electrical gradient and the concentration gradient

Question 3

What is the purpose of the sodium-potassium pump?

Answer 3

To maintain the K+ conc

Question 4

What is meant by equilibrium potentials? and what equation can be used to calculate them for individual ions?

Answer 4

The membrane potential where the net flow through any open channels is 0. Can be calculated using the Nernst equation.

Question 5

What is a better measure of resting membrane potential that takes into account the relative permeabilities of the membrane to different ions?

Answer 5

Goldman-Hodgkin-Katz equation

Question 6

Why is the duration if a cardiac action potential much longer than a nerve action potential?

Answer 6

the durations of APs control the duration of contractions of the heart
low, slow contractions are required to produce an effective pump

Question 7

What does the upstroke of a cardiac AP look like? Why is this?

Answer 7

(exactly the same as in a nerve)

cause by the opening of sodium channels

Question 8

What does repolarisation of a cardiac AP look like? Why?

Answer 8

• small repolarisation - due to sodium channels inactivation and a transient outward potassium current (a brief increase in the permeability to potassium)
• the membrane potential then plateus for a long time due to the activation of long acting L-type calcium channels (the influx of calcium balances the efflux of potassium for a long while and hence maintains a constant membrane potential)
• complete repolarisation eventually occurs due to the inactivation of the L-type calcium channels and opening of another type of potassium channel

Question 9

Why can cardiac muscle not be tetanised?

Answer 9

it has a long absolute refractory period, so the muscle cannot be restimulated again for a long time

Question 10

What are the 5 phases (0-4) of a cardiac action potential?

Answer 10

0 - upstroke
1 - early repolarisation
2 - plateau
3 - repolarisation
4 - resting membrane potential (diastole)

Question 11

What determines the resting membrane potential?

Answer 11

The potassium ion flow out of the cell

Question 12

What causes phase 1 of the cardiac action potential? (early repolarisation)

Answer 12

• inactivation of sodium channels (means no further depolarisation)
• transient outwards potassium current

Question 13

What causes phase 2 of the cardiac action potential? (early plateau)

Answer 13

calcium influx
there is a balance between the calcium influx and the potassium ions gradually beginning to flow out that means the membrane potential doesn’t change much

Question 14

What causes phase 3 of the cardiac AP (repolarisation)?

Answer 14

• gradual activation of K currents

- large activation of IK1 currents (which are closed during depolarisation)

Question 15

The electrical properties of the heart are intrinsic, but are regulated by the nervous system. How does sympathetic and parasympathetic stimulation reach the SA node?

Answer 15

• cardiac sympathetic nerves take sympathetic stimulations from the brainstem and spinal cord to the SA node
• the vagus nerve takes the parasympathetic stimulation to the SA node

Question 16

Where is the SA node located?

Answer 16

it lied just below the epicardial surface at the boundary between the right atrium and the right atrium and the superior VC

Question 17

What is the activity of the SA node like?

Answer 17

constantly oscillating (no resting membrane potential)
the membrane potential is unstable 
because there are no IK1 currents and very few Na+ channels

Question 18

What is the pacemaker potential?

Answer 18

the gradual upwards slope at the start of the sinoatrial cell APs

Question 19

How do sympathetic and parasympathetic stimulation of the heart affect the gradient of the pacemaker potential?

Answer 19

• sympathetic - becomes steeper
  this means that the threshold is reached more quickly
• parasympathetic - becomes shallower
  this means it takes longer for the membrane potential to reach the threshold, thus decreasing heart rate

Question 20

What are the 4 basic components of the conduction system of the heart?

Answer 20

• SA node
• Internodal fibre bundles
• AV node
• ventricular bundles (bundle branches and purkinje fibres)

Question 21

What is the purpose of the internodal fibre bundles?

Answer 21

they conduct the AP to the AV node at a faster velocity than ordinary atrial muscle