Electrical and mechanical events of the heart

Question 1

What is the purpose of the SA node?

Answer 1

It can initiate cardiac contraction in the absence of any external control (hormonal or nervous)

Question 2

What is different about the membrane potential in SA node cells compared to skeletal muscle cells?

Answer 2

There is an unstable membrane potential (it is continuously drifting towards threshold line).

Question 3

How does sympathetic stimulation to the SA node lead to an increase in the slope of the drift?

Answer 3

Due to an increase of the permeability of the Na+ current

Question 4

How does parasympathetic stimulation to the SA node lead to a decrease in the slope of the drift?

Answer 4

Due to an overall increase of the inward current, and to hyperpolarisation of the membrane due to increased k+ permeability

Question 5

What 2 factors does rate of firing depend on?

Answer 5

1) the initial value of membrane potential
2) The slope of the drift towards the threshold

Question 6

How does stimulation in the atria spread?

Answer 6

From fibre to fibre through gap junctions

Question 7

Why does the electrical impulse delay at the AV node?

Answer 7

In order to allow atria to empty before ventricular contraction begins

Question 8

When does the tricuspid valve open and close?

Answer 8

o Open when R atrial pressure > R ventricle pressure
o Closed when R atrial pressure < R ventricle pressure

Question 9

When does the pulmonary valve open and close?

Answer 9

o Open when R ventricle pressure > PA pressure
o Closed when R ventricle pressure < PA pressure

Question 10

When does the mitral (bicuspid) valve open and close?

Answer 10

o Open when L atrial pressure > L ventricular pressure
o Closed when L atrial pressure < L ventricular pressure

Question 11

When does the aortic valve open and close?

Answer 11

o Open when L ventricular pressure > Aortic pressure
o Closed when L ventricular pressure < Aortic pressure