a. F
b. T
c. F
d. T
e.T

Discussion 

a.

• Embrologically SA node developed from structures of right side and AV node from stucturs of left side 
• Therefore the SA node innervated by the right vagus nerve and AV node innervated by the left vagus nerve 

b.

• SA node and AV node made up of modified cardiac muscle and small round cell with few organelles
•  Small round cell act as pacemaker therefore they are called P cells 

c.

• SA node located at the junction of the superior vena cava and the right atrium 
• AV node located in the right posterior portion of the interatrial septum 

d.

Conduction rate of cardiac tissue

Tissue	Conduction rate (m/s)
SA node	0.05
Atrial pathways	1
AV node	0.05
Bundle of His	1
Purkinje system	4
Ventricular muscle	1

e.

Factors that affect the rate of SA nodal firing

1. Changes in autonomic nerve activity

• Increase vagal tone ( increase parasympathetic activity ) reduce firing rate of the SA node 
• Increase sympathetic discharge increase the firing rate of SA node 

2. Changes of circulating hormone level

• Hypothyroidism induces bradycardia 
• Hyperthyroidism induces tachycardia 

3 . Changes of serum ion concentration

• Hyperkalemia induces bradycardia 
• Hypokalemia induces tachycardia 

4. Changes of cellular O2 level

• Hypoixa induce bradycardia 

5. Drugs

• Calcium channel blockers and digitalis induce bradycaria 

6. Other factors that increase heart rate

• Inspiration 

• Excitement 

• Anger 

• Most painful stimuli 

•  Exercise 

• Fever 

7.Other factors that decrease heart rate

• Expiration 

• Fear 

• Grief 

• Increased intracranial pressure 

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