Physiological Properties of the Heart

Question 1

Phase 0 of atrioventricular depolarisation

Answer 1

rapid depolarisation due to increased sodium ion permeability as fast as the channels open

Question 2

Phase 1 of atrioventricular depolarisation

Answer 2

start of repolarisation as fast as sodium ion channels close

Question 3

Phase 2 of atrioventricular depolarisation

Answer 3

L-type calcium ion channels triggered by change in voltage and calcium enters cell

Question 4

Phase 3 of atrioventricular depolarisation

Answer 4

• rapid repolarisation as the intracellular calcium ion concentration increases
• stimulates potassium ion channels to open and the potassium permeability increases
• calcium ion channels close

Question 5

Phase 4 of atrioventricular depolarisation

Answer 5

stable resting membrane potential where potassium ion permeability exceeds sodium ion permeability

Question 6

Phase 1 of SA node depolarisation to create the pacemaker potential

Answer 6

• gradual increase in resting membrane as sodium ion permeability increases as f-type sodium ion channels open
• and decrease in potassium ion permeability as potassium ion channels close

Question 7

What helps establish the pacemaker potential in phase 1 SA node depolarisation?

Answer 7

transient calcium ion channels

Question 8

Phase 2 of SA node depolarisation

Answer 8

moderately rapid depolarisation due to calcium ion entry by slow L channels

Question 9

Phase 3 of SA node depolarisation

Answer 9

rapid repolarisation as increased intracellular calcium ion concentration stimulates opening of potassium ion channels and an increase in potassium ion permeability

Question 10

What nerve innervates the SA and AV node?

Answer 10

Parasympathetic innervation by the vagus nerve

Question 11

How does sympathetic stimulation affect pacemaker activity?

Answer 11

• increases calcium ion permeability
• increases sodium ion permeability by f-type channels
• increases rate of SAN phase 1 depolarisation (positive chronotropic effect)

Question 12

How does parasympathetic stimulation affect pacemaker activity?

Answer 12

• hyperpolarises membrane potential to lower starting level
• increases extent and duration of opening of potassium ion channels therefore increasing the permeability of potassium ions
• decreases rate of SAN phase 1 depolarisation (negative chronotropic effect)

Question 13

How does the sympathetic NS stimulate pacemaker activity?

Answer 13

noradrenaline acts on B1 receptors to increase cAMP production

Question 14

How does the parasympathetic NS stimulate pacemaker activity?

Answer 14

Acetylcholine on M2 receptors which decrease cAMP production

Question 15

During AV node conduction where is the biggest delay?

Answer 15

When passing the AV node

Question 16

How to measure electrical activity non-invasively

Answer 16

• electrocardiogram (ECG)
• uses electrodes (4 on the limbs, 6 across the chest)
• limb leads measure sum of electrical activity of heart and direction the activity is moving in
• one lead is designated positive

Question 17

How to calculate the strength of the electrical activity

Answer 17

observed signal is equal to the electrical event multiplies by the angle between the event and ECG lead

Question 18

What determines the strength of electrical activity?

Answer 18

• current

- direction of signal