Lecture 7: The excitable heart

Question 1

What are the main features of electrical cells in the heart?

Answer 1

• pale striated appearance, = 1%
• low content of actin & myosin
• function is to CONDUCT
• non-contractile
  e. g purkinge fibres and AV nodal cells

Question 2

What are the main features of contractile cells in the heart?

Answer 2

• clear striated appearance, = 99%
• Hight content of actin and myosin
• Function is to CONTRACT
  e. g working myocardial cells

Question 3

How is impulse spread in cardiac cells?

Answer 3

Impulse is spread by intercalated disks and gap junction which connect cardiac cells
- Intercalated disks contain gap junctions

Question 4

What is the function of gap junctions?

Answer 4

• Connects cardiac cells
• allows current flow btw adjacent cells - rapidly spreads the impulse throughout the heart, so the heart can contract in unison
• –> functional syncytium
• Spreads impulse and increases speed of impulse through out the heart

Question 5

How is an impulse transmitted around the entire heart?

Answer 5

• -> Impulse begins at SA node
• -> SA node signals the atria to contract. SA node also signals the AV node which stores signal
• -> After the atria contracts, the signal stored by the AV node is sent to the AV bundle
• –> single depolarise right and left AV bundles
• —> impulse sent through septum & up the heart walls via purkinge fibres to the ventricles
• —> ventricle contract

Question 6

What are the key parts of the excitation and conduction pathway?

Answer 6

1. Ouiescence
   2 SAN pulse excitation of intra & inter atrial conduction pathway. Start of atrial depolarisation
2. Complete atrial contraction
3. AVN pulse, excitation of AV bundle and parking fibres
4. Atrial repolarisation & ventricular depolarisaiton
5. Start of ventricular repolarisation
6. Complete ventricular repolarisation

Question 7

What happens during the P wave in an ECG trace?

Answer 7

SAN impulse and atrial depolarisation

• atria depolarisation
• atria contraction

Question 8

What happens during the QRS wave in an ECG trace?

Answer 8

Ventricular depolarisation while the signal of atrial depolarisation is obscured.

• Ventricular depolarisation
• Ventricular contraction

Question 9

What happens during the T wave in an ECG trace?

Answer 9

Ventricular repolarisation

• Ventricular repolarisation
• Ventricular relaxation

Question 10

What are the 6 major parts of the conduction pathway?

Answer 10

1. SA node
2. Interatrial bundle
3. Internodal bundle
4. Atrioventricular bundle
5. Right and left branches of AV bundle
6. Purkinje fibres