2. Physiological properties of the heart

Question 1

Describe how co-ordinated electrical activity occurs?

Answer 1

Pacemaker acitity of the SAN starts the process and depolarisation spreads due to ‘functional syncytium’ (atria –> ventricles)

Question 2

Draw a graph showing cardiac action potentials with values.

Answer 2

Red = sinus node fibre

Green = ventricular muscle fibre

Question 3

Describe the events of phase 0-4 in atrial/ventricular depolarisation

Answer 3

• Phase 0 - Rapid depolarisation due increased gNa+ and fast Na⁺ channels open
• Phase 1 - Repolarisation begins; Na⁺ channels close
• Phase 2 - Effect of Ca²⁺ entry via L-type DHR channels
• Phase 3 - Rapid repolarisation as increased [Ca²⁺] stimulates K⁺ channels to open and gK⁺ increases. L type Ca²⁺ channels close
• Phase 4 - Stable resting potnetial as gNa⁺ exceeds gK⁺ by 50:1

Question 4

Phases of SA node depolarisation

Answer 4

• Phase 1 - gradual increase in resting potential due to increased gNa⁺ as ‘funny’ F-type Na⁺ channels open and decrease gK⁺ as K⁺ channels close slowly
• Phase 2 - Depolarisation due to Ca²⁺ entry via slow L channels
• Phase 3 - Repolarisation as elevated internal Ca²⁺ srimulates opening of K⁺ channels and increase gK⁺​

Question 5

Effect of sympathetic stimulation on pacemaker activity

Answer 5

NA acts on B1 receptors and increase cAMP production, which increases rate of SAN 1 depolarisations.

= positive chrontropic effect

Question 6

Effect of parasympathetic stimulation on pacemaker activity

Answer 6

ACh acts of M2 which results in a decrease in the production of cAMP. This decreases the rate of SAN 1 depolarisations.

Also hyperpolarises membrane to lower starting level (increases gK⁺ for longer).

= negative chronotropic effect

Question 7

What acts as the intrinsic pacemaker and its rate of depolarisations?

Answer 7

Sinoatrial Node (SAN)

~90/min

Question 8

What is used to measure the electrical activity of the heart?

Answer 8

Electrocardiogram (ECG)

Question 9

Where are the electrodes placed for an ECG?

Answer 9

• 4 on the limbs
• 6 across the chest

Question 10

What is the earth lead used for?

Answer 10

To remove background noise

Question 11

What is the function of the limb leads?

Answer 11

Measure the sum of the electrical activity of the heart and the direction the electrical activity is moving

Question 12

What does depolarisation moving towards the “positive” cause?

Answer 12

Trace on the ECG goes up

Question 13

What does depolarisation moving away from the “positive” cause?

Answer 13

Trace on the ECG goes down

Question 14

How are the size of electric signals from the heart determined?

Answer 14

Current (propertional to tissue mass) and direction of signal

Question 15

Observed signal =

Answer 15

electrical event x cosσ

σ = angle between event and ECG lead

Question 16

What type of angle gets the biggest signal?

Answer 16

smallest angle

Question 17

Draw a normal ECG wave

Answer 17

Question 18

Describe the P, QRS and T waves

Answer 18

• P = atrial depolarisation
• QRS = ventrical depolarisation
• T = ventricular repolarisation

Question 19

What is the repolarisation of the SA Node triggered by?

Answer 19

K⁺

Question 20

Why are “funny” Na⁺ channels more likely to open?

Answer 20

Membrane potential has decreased

Question 21

What is the function of transient Ca²⁺ channels

Answer 21

Help with the final push to get over the threshold in SA node depolarisation

Question 22

Give the different functions of L-type Ca²⁺ channels in nodal and muscle tissue

Answer 22

Nodal = depolarisation

Muscle = plateau

Question 23

What is the PR interval?

Answer 23

Delay in AV node

Question 24

What is the QT interval?

Answer 24

plateau phase

Question 25

Why is there no signal for atrial repolarisation?

Answer 25

occurs at the same time as the QRS complex

Question 26

Why are F-type voltage-gated Na⁺ channels funny?

Answer 26

They open as the membrane voltage becomes more negative, which is different from normal Na⁺ channels which open when the membrane potential becomes more positive

Question 27

What does pacemaker potential refer to?

Answer 27

gradual creep caused by the F-type Na⁺ channels in SA node tissue