lecture 5: electrical properties of the heart

Question 1

what is the potassium hypothesis?

Answer 1

• the membrane is more permeable to potassium than anything else
• these diffuse down the concentration gradient carrying a positive charge with them
• as they move the second chamber is increasinging positive compared to the first chamber
• the electrical gradient is opposite to the concentration gradient and equilibrium is achieved

Question 2

what is the resting membrane potential?
potassium
k outside: 5
k inside: 120

Answer 2

• we can predict using the nernst equation
  if the membrane is only permeable to k at rest then the potential across it will equal the K equilibrium potential

all it is is potassium moving out of the cell

-80mV

Question 3

why is the sodium potassium pump important?

Answer 3

• this maintains the sodium potassium pump

Question 4

na outside 140

na inside 10

Answer 4

+66
When the membrane is only permeable to sodium (e.g. during the upstroke of AP), potential
across the membrane will be equal to Na equilibrium potential.

Question 5

what is the overall equation to measure the resting membrane potential?

Answer 5

• the goldman - hodgkin katz equation

Question 6

what is an absolute refractory period?

Answer 6

time during which no

action potential can be initiated (regardless of stimulus).

Question 7

what is a relative refractory period?

Answer 7

period after ARP where

an AP can be elicited but only from larger than normal AP

Question 8

what are refractory periods caused by?

Answer 8

they are a result on the Na channel inactivation which reactivate at repolarisation

Question 9

what is the full recovery time?

Answer 9

the time at which a normal AP

can be elicited with a normal AP.

Question 10

what can happen in a skeletal muscle?

Answer 10

Repolarisation occurs very early in
the contraction and is short. This
means re-stimulation can occur soon
after the first AP.

Question 11

how long is the refractory period of the cardiac muscle?

Answer 11

Has a long refractory period
meaning the muscle cannot be restimulated until the process of
contraction is well underway –
cardiac muscle cannot be tetanised.

Question 12

what is tetany?

Answer 12

a condition marked by intermittent muscular spasms

Question 13

what is the graph of the cardiac action potential

Answer 13

insert graph l

Question 14

what are the stages of a cardiac action potential?

Answer 14

1. AP causes large change in PNa causing rapid upstroke
2. a. Large [Na+] Intracellular inactivates Na channels and thus reduces PNa quickly and this causes a brief increase in PK which gives the characteristic notch on the graph as K leaves the cell. Na channels enter absolute refractory period.
   b. Large [Na+] Intracellular also increases PCa, a. Influx provides trigger for Ca2+ release from intracellular stores for contraction.
3. The Ca2+ intracellular increase combined with the K+ efflux maintains the plateau of the graph.
4. the graph increases when the potassium starts to increase again when the potassium channel starts to open up again
5. the repolarisation occurs due to potassium currents

Question 15

what causes different action potential shapes in the graph?

Answer 15

different parts of the heat have different action potential shapes due to different ionic currents flowing due to the expression of different ionic channels

Question 16

what are the intrinsic properties of the heart?

Answer 16

• the heart has its own independent electrical impulse generation and propagation system
• the nature of the heart means it can beat even without nerves
• the internal electrical activity is modulated by sympathetic and parasympathetic nerves

Question 17

what does the ventricular action potential look like compared to the SA node action potential? show the graph:

Answer 17

insert graph

Question 18

what is present in the SA node?

Answer 18

• most of the types of channels exist in the SA node except IK1

Question 19

what is the function of IK1?

Answer 19

to maintain a stable membrane potential so SA node cells do not have a stable membrane potential.

Question 20

what causes the upstroke in the SA node cells?

Answer 20

• There is very little sodium influx into SA node cells, instead, the upstroke is caused by Ca2+ influx.
• The SA node cells contain T-type Ca2+ Channels (TTCC) which activate at a more negative potential than L type calcium channels

Question 21

what pace maker is present in the SA node?

Answer 21

• The pacemaker current If is present.

Question 22

where are L type calcium channels also found?

Answer 22

in smooth muscle

Question 23

what happens in sympathetic stimulation of the heart?

Answer 23

• it makes the pacemaker potential steeper

- this means the threshold potential is reached more quickly so the heart rate is speeded up

Question 24

what happens in the parasympathetic stimulation of the heart?

Answer 24

• there is a decrease in the steepness of the pacemaker potential
• this means it takes longer for the membrane potential to meet the threshold thus slowing the heart rate
• therefore the heart rate slows down

Question 25

where is the sinoatrial node located?

Answer 25

• located between the right atrium and the superior vena cava

Question 26

what are internodal fibres?

Answer 26

Conduct Action Potential to Atrio Ventricular -node at a greater velocity than ordinary atrial muscle

Question 27

what does the atrio ventricular node do?

Answer 27

this produces a delay of 100 ms

- this is important for a delay between atrial excitation and ventricular excitation

Question 28

what is the bundle of his

Answer 28

rapid conduction cells to transport an insulated wave of excitation

Question 29

where is the bundle of his?

Answer 29

• descends from AV-node and splits into two bundle branches made of Purkinje fibres

Question 30

what are purkinje fibres?

Answer 30

conduct Action Potential at around 6x the velocity of ordinary cardiac muscle and penetrate 1/3rd distance into myocardial wall.

Question 31

what are gap junctions

Answer 31

gap junctions greatly reduce the resistance between one cell to another

Question 32

how are impulses propagates between cells`?

Answer 32

by the use of gap junctions which have a low resistance - the effect of depolarisation gradually decays