Physiological Properties of the Heart

Question 1

Describe the path of the co-ordinated electrical activity in cardiac muscle

Answer 1

– Pacemaker activity of sinoatrial node normally starts the process
– Depolarisation spreads due to “functional syncytium”
– Around atria first, then ventricles

Question 2

What do ECGs measure in cardiac process?

Answer 2

Measures electrical activity of the heart over time
– Is a difference in observing the electrical activity in an individual
fiber re: observing the total electrical activity of the heart
– ECGs measure the latter not the former!

Question 3

Describe the different phases of atrial/ventricular depolarisation

Answer 3

Phase 0
• Rapid depolarisation due to ↑ Na+ permeability (gNa+) as fast Na+ channels open

Phase 1
• Start of repolarisation as fast Na+ channels close

Phase 2
• Effect of Ca2+ entry via L-type channels

Phase 3
• Rapid repolarisation as ↑ [Ca2+]i stimulates K+ channels to open and gK+ ↑
• Ca2+ L-type channels close

Phase 4
• Stable resting membrane potential where gK+ exceeds gNa+ by 50:1

Question 4

Describe the different phases of Sino-atrial node depolarisation

Answer 4

Phase 1
• Gradual drift ↑ in resting membrane potential due to ↑ gNa+ as “funny” F-type Na+ channels open and ↓ gK+ as K+ channels slowly close
• “Pacemaker potential”
• Transient (T) Ca2+ channels help with
the “final push”

Phase 2
• Moderately rapid depolarisation due to Ca2+ entry via slow (L) channels

Phase 3
• Rapid repolarisation as elevated internal Ca2+ stimulates opening of K+ channels and an ↑ in gK+

Question 5

How is pacemaker activity stimulated via the sympathetic autonomic nervous system?

Answer 5

– Noradrenaline acts on β1 receptors to ↑ cAMP production
– Increases rate of SAN phase 1 depolarisation

• ↑ gCa2+
• ↑ gNa+ via “funny” channels

Question 6

How is pacemaker activity stimulated via the parasympathetic autonomic nervous system?

Answer 6

– Acetylcholine on M2 receptors which ↓ cAMP production
– Reduces rate of phase 1 depolarisation
– Hyperpolarises membrane potential to lower starting level
• ↑ extent and duration of opening of K+ channels therefore ↑ gK+

Question 7

What are the basic differences between the sympathetic/parasympathetic nervous systems’ stimulation of pacemaker activity in regards to membrane potential/time

Answer 7

Sympathetic stimulation shows positive chronotropic effect Parasympathetic stimulation shows negative chronotropic effect

Question 8

Name the electrical conduction pathways of the heart

Answer 8

SA Node

Internal tracts

AV node

Bundle of His

Left Bundle branch

Right Bundle branch

Purkinje fibres

Question 9

Discuss rates of depolarisation in electric conduction pathways of the heart

Answer 9

As SAN has the fastest rate, it is the intrinsic pacemaker
Depolarisation spreads from SAN throughout heart before other regions spontaneously depolarise
If conduction blocked, downstream tissues assume their intrinsic rate

Rates decrease passing down the conduction pathway, eg:

Sinoatrial node (SAN) - ~90/min
Atrioventricular node (AV node) - ~60/min
Bundle of His - ~50/min
Purkinje fibers - ~40/min
Ventricles - ~30/min

Question 10

Discuss timings in AV Node conduction

Answer 10

Conduction in atrial and ventrical fibers = 0.3-0.5 m/s
Conduction in internodal pathways = 1 m/s
From SAN to AV node ~0.03s
Signal is delayed in AV node for ~0.09s
Further delay through penetrating portion ~0.04s
Conduction through AV node and bundle of 0.01 m/s
Purkinje fibers 1.5-4 m/s

Question 11

What does an ECG do?

Answer 11

– Measures electrical activity of the heart over time
– Uses multiple electrodes
• Four on the limbs
– One is an “earth”, used to remove background noise
– Three used to create virtual “leads” between each pair of electrodes
• Six across the chest
– To give more specific, localised information about areas of the heart

Question 12

What do the limb leads of an ECG do?

Answer 12

The limb leads measure the sum of the electrical activity of the heart and the direction that electrical activity is moving in
– One end of each lead is designated “positive”
• Depolarisation moving towards the “positive” causes the trace to go up.
• Depolarisation moving away from the “positive” causes the trace to go down

Question 13

What is Einthoven’s triangle?

Answer 13

An imaginary triangle with corners on both shoulders and the pubis

Question 14

How do we calculate the strength of the signal of an ECG?

Answer 14

Size of electrical signals from the heart determined by:
– Current (proportional to tissue mass)
– Direction of signal
Observed signal = E x CosƟ
Smallest angle gets biggest observed signal
E = Electrical event
Ɵ = angle between event and ECG lead

Question 15

Discuss PQRST waves

Answer 15

P wave
– Atrial depolarisation QRS wave
– Ventricular depolarisation T wave
– Ventricular repolarisation Atrial repolarisation?
- This doesn't seem right at all, ignore

On a normal ECG there is:
PR interval from start of P to start of QRS complex
PR segment from end of P to start of QRS complex
QRS complex
QT interval from start of QRS complex to end of T wave
ST segment from end of QRS complex to start of T wave

Question 16

What are the important timing intervals to know for a normal ECG?

Answer 16

– P-R interval (0.12-0.2s)
– QRS complex width (0.06-0.12s)
– Q-T interval (0.25-0.35s)

Question 17

What are the main points of this lecture?

Answer 17

Many parts of the heart exhibit spontaneous, rhythmical depolarisation
The electrical activity spreads in a co- ordinated fashion
The rate of depolarisation is influenced by the ANS
ECG looks at the overall, net activity

Question 18

Learning outcomes

Answer 18

To describe the ionic basis for the different stages of the membrane potential changes in atrial/ventricular muscle, nodal and conducting tissue throughout the heart.
To describe how the sympathetic and parasympathetic nerves modify the spontaneous electrical activity of the heart.
To explain how the spread of electrical activity throughout the heart can be measured non-invasively by means of the ECG, and how the shape and features of the ECG relate to the cellular action potentials.