Physiological properties of the heart

Question 1

What is the meaning of functional synctium

Answer 1

The cells of the atria and ventricles are all connected electrically
The is only one pathway connecting the atria and ventricles
1 AP will spread between the cells causing them to contract synchrously

Question 2

Why are two kinds of AP needed in the heart

Answer 2

one to regulate nodal tissue and one to regulate contraction of heart muscle

Question 3

What events happen in phase 0 of A/V depolarisation

Answer 3

rapid depolarisation

voltage gated Na+ channels open and Na+ permeability increases

Question 4

During what phase does begin repolarisation begin to occur in A/V depolarisation

Answer 4

Phase 1

Na+ channels close

Question 5

during what phase does Ca2+ enter during A/V depolarisation

Answer 5

Phase 2

Question 6

What is the effect of the entry of Ca2+ in A/V depolarisation

Answer 6

causes contraction

plateau occurs to prevent tetany by preventing repolarisation below threshold

Question 7

What occurs during phase 3 of A/V depolarisation

Answer 7

rapid repolarisation

increase in intracellular Ca2+ causes K+ gates to open and K+ to flow out of the cell

Question 8

at what membrane potential does the plateau occur

Answer 8

20mV

Question 9

Define the pacemaker potential and its effects

Answer 9

The pacemaker potential is the gradual positive drift in resting membrane potential during phase 1
Prevent full repolarisation

Question 10

what causes the pacemaker potential

Answer 10

Increase in Na+ due to opening of funny F type Na+ channels

Decrease in K+ due to closing of K+ voltage gated channels

Question 11

What is the role of transient Ca2+ channels in the nodal AP

Answer 11

pushes pacemaker potential over threshold

Question 12

why does the nodal membrane depolarise more slowly

Answer 12

Ca2+ enters via slow L channels during phase 2

Question 13

Describe the pattern in intrinsic rates of depolarisation in different parts of the heart

Answer 13

SAN is quickest = 90/min
AV node = 60/min
decreases by 10/min for each subsequent level to 30/min in the ventricles

Question 14

how can the effect of sympathetic nerves on the rate of SAN depolarisation be described

Answer 14

positive chronotropic

Question 15

by what mechanism does the sympathetic ANS increase SAN depolarisation

Answer 15

noradrenaline acts of B1 receptors to increase cAMP production
increases Ca2+ permeability and Na+ permeability

Question 16

by what mechanism does the parasympathetic decrease rate of SAN depolarisation

Answer 16

acetylcholine acts of M2 to decrease cAMP

hyperpolarises the membrane by modifying the K+ channels

Question 17

what is the purpose of the earth lead

Answer 17

removes background activity

Question 18

what determines the size of electrical response on the ECG

Answer 18

the angle between the lead and the direction of electrical activity
at 90 the ECG will be flat

Question 19

what causes postive movement on the ECG trace

Answer 19

electical activity moving towards the positive end of the lead

Question 20

why is the cardiac AP longer?

Answer 20

opening of slow L type calcium channels
need for extracellular calcium from these channels to cause contraction
five fold decrease in K+ permeability slows outflow during phase 2 plateau