Anatomy And Heartbeat

Question 1

How is a heartbeat generated

Answer 1

1. Myogenic control
2. Intrinsically generated action potential generated
3. Action potential propagated along the SAN, AVN, Bundle of His and Pukinje fibres

Question 2

SAN structure and function

Answer 2

1. Ion channels within the SAN increases membrane potential beyond threshold potential triggering APs
2. Makes the heart rate go 120bpm naturally without nervous stimulation
3. Autonomic control via the parasympathetic (slow HR) and sympathetic (increase HR)
4. SAN is the pacemaker

Question 3

Describe the ionic reaction pathway of the SAN

Answer 3

1. Na+ influx is coupled with transient VG calcium channels
2. Sodium ion channels close, and the calcium ions drift the membrane potential towards threshold potential
3. Long lasting Ca channels trigger depolarisation
4. K+ channels facilitate repolarisation

Question 4

Importance of the AVN

Answer 4

1. It causes a short delay between 0.1 s between atria and ventricle contraction
2. This allows atria to finish contraction when ventricle starts contraction
3. AVN also passes AP via the bundle of His to both left and right ventricles
4. Purkinje fibres facilitate ventricle contraction

Question 5

Why does AVN not initiate AP first?

Answer 5

1. SAN fires APs at a higher rate than the AVN, which prevents AVN from driving heartbeat - 70bpm
2. If SAN node fails AVN can initiate heart beat albeit at a lower rate - 50bpm
3. If both SAN and AVN fail the Purkinje fibres can initiate heartbeat - 30bpm

Question 6

How does cardiac AP differ in ionic mechanism and shape from SAN AP

Answer 6

1. Resting is at -90 mv
2. Sodium channels open
3. When peak is reached K+ VGPC are opened which leads to efflux
4. But Ca+ VGCs cause calcium influx
5. This creates a plateau
6. More VGPCs are opened and rapidly brings membrane potential back to resting
7. This long refractory period is a protective measure which prevents nervous AP summation of heart contraction which can damage it