Electrical activity of the heart

Question 1

Define auto-rhythmic

Answer 1

capable of spontaneously firing

Question 2

What are the 3 types of auto-rhythmic cells?

Answer 2

sino-atrial node
atrio-ventricular node
bundle of His, bundle branches and purkinje fibres

Question 3

What is the resting membrane potential of a pacemaker cell?

Answer 3

-60mv

Question 4

Describe the mechanism of excitation in the heart?

Answer 4

1) SA node passes wave of excitation through atria rapidly
2) AV node conduction is slow before current passes through bundle of His rapidly
3) Then down bundle branches
4) Then down purkinje fibres
5) Current passes through the ventricular muscle cells

Question 5

What facilitates rapid spread of excitation through the heart?

Answer 5

intercalated discs between fibres

Question 6

How do gap junctions help between fibres?

Answer 6

provide low resistance pathways, therefore cardiac muscle functions as syncytium (single unit)

Question 7

How does contraction and relaxation occur in the ventricles with relation the ca2+?

Answer 7

influx of ca2+ , triggers further release of ca2+ from sarcoplasmic reticulum
free ca2+ causes contraction of myocardial cells (systole)
uptake of ca2+ by s.reticulum and extrusion of ca2+ by na+/ca2+ exchange and outward ca2+ pump
lowers free ca2+ allowing relaxation (diastole)

Question 8

What protects the heart from tetanus?

Answer 8

long refractory period

Question 9

Which direction does excitation and contraction occur in the ventricle?

Answer 9

endocardium to epicardium

apex to base

Question 10

What is activated in the sympathetic nervous system to increase heart rate?

Answer 10

beta 1 adrenoceptors in SA node

Question 11

What is activated in the parasympathetic nervous system to decrease heart rate?

Answer 11

M2 muscarinic receptors in SA node