Cardiac Cycle and Electrophysiology

Question 1

what are cardiac myocytes and what is their function

Answer 1

cells in heart tissue that generate electrical impulses causing the heart to contract
connected by intercalated discs

Question 2

pacemaker

Answer 2

the sinoatrial node in the right atria generates action potential to both atria

Question 3

what happens to the action potential that is generated by the SA node

Answer 3

• Depolarisation spreads to atrioventricular (AV) node and is delayed
(~100 msec) before entering the ventricles

Question 4

key features of cardiac muscle

Answer 4

– myogenically active (involuntary)
– have irregular Y shaped fibres
– mostly single nucleated
– striated in appearance
– connected by intercalated discs
(gap junctions)

Question 5

what does the intercalated discs mean

Answer 5

• The intercalated discs mean that the myocardium is a functional syncytium (from the Greek syn kytos: one cell)
– strictly speaking two syncytia as atria AND ventricles

Question 6

how is syncytium important to the heart

Answer 6

• Once one cell depolarises, current will flow to the adjacent cell via intercalated discs
• This is how depolarisation flows through the heart
– it is not directionally dependent, that is set by the refractory period

Question 7

conduction pathways

Answer 7

depolarisation in the right atrium of SA node
depolarisation spreads through intercalated discs through right and left atria
Dp reaches AV node and is slowed to help ventricles fill and atria act as primer pump
Dp spreads from AV node to Bundle of His to Purkinje fibres then ventricular muscle
from endocardium to epicardium

Question 8

what does conduction speed depend on

Answer 8

diameter and number of gap junction
more gap junction - quicker spread
larger diameter - quicker spread

Question 9

speed of conduction pathway through muscle

Answer 9

atria - 0.3m/sec

ventricle - 0.3m/sec

Question 10

speed of conduction through pathways/fibres

i) internodal pathway
ii) purkinje fibres

Answer 10

internodal pathway - 1m/sec

purkinje fibres 3m/sec

Question 11

speed of conduction is faster through muscles and fibres

Answer 11

faster through pathways and fibres

Question 12

why is the SA node is the pacemaker

Answer 12

leaky membrane to cations so resting potential is -55mV

Question 13

What ion channel leaks into the membrane of the SA node

Answer 13

Na+ and K+ slowly depolarisation
Na/K hyperpolarisation activated
cyclic nucleotide gated channels

Question 14

what channels open at around -40mV

Answer 14

voltage-gated L-type Ca2+ channels open and cell depolarises

Question 15

is the sinoatrial node time dependent or voltage dependant

Answer 15

time dependent
after ~200 ms Ca2+ channels close and K+ open leading
to K+ efflux
– cell repolarises and process starts again

Question 16

when does AP occur in ventricular cells

Answer 16

AP occurs when depolarisation spreads from

adjacent cells* via the intercalated discs

Question 17

what is the difference between AP in nerves and AP in cardiac tissue

Answer 17

AP largely similar to that in nerves, except during repolarisation Ca2+
channels open causing a plateau phase as it is as the same time as efflux of K+ ions

Question 18

why is repolarisation by opening Ca2+ channels important

Answer 18

• This is important as it allows the ventricle to fully contract and relax
before the next action potential arrives

Question 19

what is a measure of the right atrial pressure

Answer 19

jugular venous pressure

Question 20

what is a measure of the left atrial pressure

Answer 20

– pulmonary capillary wedge

Question 21

what percentage of the blood directly flows from atria into ventricle

Answer 21

75%

Question 22

aVR meaning and location of positive electrode

Answer 22

Augmented Vector Right, positive electrode right shoulder.

Question 23

aVL meaning and location of positive electrode

Answer 23

Lead aVL Augmented Vector Left, positive electrode left shoulder.

Question 24

aVF meaning and location of positive electrode

Answer 24

Lead aVF Augmented Vector Foot, positive electrode on Food