Spread of excitation through the heart & ECG

Question 1

Whats the path of blood flow through the heart

Answer 1

1. Superior/Inferior vena cava
2. Right atrium – receives blood returning from the systemic circuit
3. Tricuspid valve
4. Right ventricle – pumps blood to the pulmonary circuit
5. Pulmonary Arteries
6. Lungs
7. Pulmonary veins
8. Bicuspid valve
9. Left ventricle
10. Aorta

Question 1

What do the pacemaker cells do

Answer 1

• Spontaneously generate APs
• Located in SAN + AVN
• After an AP, cells depolarise slowly at first until threshold is reached

Question 2

Describe the electrical connections between cardiac muscle cells

Answer 2

• Cardiac cells are connected by gap junctions inside intercalated discs - allows for rapid transmissions of APs
• Intercalated disks contain desmosomes - enables the myocardium to resist stretching

Question 3

Describe the 5 stages for the initiation and conduction of an impulse

Answer 3

1. Impulse begins in SAN
2. Spreads to the AVN via conduction fibres through atrial muscle
3. AVN passes on the impulse - delayed by 0.1 seconds
4. Impulse travels through bundle of HIS
5. Impulse splits into left and right branches
6. Each branch travels through Purkinje fibres on each side of ventricular myocardium

Question 4

What are the stages of cardiomyocyte stimulation

Answer 4

1. Depolarisation - Na enters, cardiomyocytes rise in MP
2. Repolarisation - NA channels deactivate, MP becomes more negative, decreased permeability to K
3. Plateau - K channels close, Ca channels still open, membrane stays depolarised
4. Repolarisation - K leaves, cardiomyocytes MP becomes more negative, Ca channels close, AP ends, membrane repolarised
5. Resting potential - Permeability to K greater than Na and Ca

Question 5

What does an ECG do

Answer 5

Records the spread of electrical current through the heart as a function of time through the cardiac cycle

Question 6

How do you set up an ECG

Answer 6

• Recorded by electrodes placed on skin
• Electrical activity of heart spreads through body as fluid acts as conductors
• More synchronised activity, the larger the amplitude

Question 7

What is Einthoven’s Triangle

Answer 7

• Equilateral triangle of electrodes surrounding the heart
• Electrodes placed placed at right arm, left arm and left leg
• Electrodes connected to an oscilloscope
• Leads I, II and III calculated

Question 8

What is Arrhythmias and when can it occur

Answer 8

• Abnormal sinoatrial node activity can cause fast/slow HR
• Altered conduction through AVN causes a degree of heart block
• Depolarisation can occur from electrical signal outside conduction pathway - causing PAC or PVC
• Ventricular fibrillation - unsynchronised heart beats
• Atrial fibrillation - causes weakness but not deadly