1.9 Multicellular Electrophysiology

Question 1

What is one action potential characteristic that both nerve cells and heart cells share

Answer 1

Both have an action potential threshold with an ‘all or nothing response’.

Question 2

Give 2 differences in action potential characteristics between nerve and heart cells

Answer 2

Neurons have rapid repolarisation but cardiac myocytes have a prolonged depolarised phase called the plateau (>100ms)

In neuronal cells, only Na and K channels contribute to the action potential but in myocytes, there are many channels and pumps (e.g Ca channels, Na/Ca pumps and many different types of K channels).

Question 3

What is the function of action potential propagation in nerves

Answer 3

The transmission of information, either firing at high spike frequencies or irregular/bursting frequencies. It is limited by conduction velocity and synaptic delay

Question 4

What is the function of action potential propagation in the heart

Answer 4

To allow for the rhythm and timing of atria and ventricle contraction. It is at low frequency to allow for the resting heart rate to be around 70 bpm. This propagation sequence is important for efficient heart pumping

Question 5

What connects the electrical processes to the mechanical contraction required to pump blood in the heart.

Answer 5

Intracellular calcium links the electrical event to the heart contraction. An action potential causes an increase in intracellular calcium levels

Question 6

What is the importance of the orderly sequence of atrial and ventricular contraction

Answer 6

It maximises pump efficiency. It also prevents disorderly conduction which can cause arrhythmias (slowed conduction are implicated in dangerous arrhythmias such as ventricular tachycardia and fibrillation)

Question 7

What are the fastest and slowest nerve axons

Answer 7

Fastest - Aa motor fibres 50-120 m/s
Slowest - C fibres 0.5-2m/s

Question 8

What are the fastest and slowest heart axons

Answer 8

Fastest - purkinje fibres 50-120m/s
Slowest - AV node 0.02-0.1 m/s
(ventricular tissue is 0.3-1 m/s)

Question 9

Amongst different species, does myocyte size vary with heart size

Answer 9

No, the cell size stays the same

Question 10

What connects myocytes electrically

Answer 10

Gap junctions, these allow impulses to travel through cells. Gap junctions are formed by protein subunits called connexins (either Cx40, Cx43 or Cx45 - Cx45 is the main isoform in ventricles).

Question 11

Do heart axons have myelination

Answer 11

No

Question 12

In what direction are myocytes arranged

Answer 12

`Longitudinally

Question 13

Describe the source to sink mechanism of action potential propagation in the heart

Answer 13

In heart, during AP propagation, the excited cell serves as a source which provides sufficient charge to excite the neighbouring cell (the sink), the newly excited cell then becomes the source for it’s other neighbour. This polarisation is going out in multiple directions to rapidly depolarise the myocyte tissue

Question 14

What is the difference between isotropic conduction and anisotropic conduction

Answer 14

Isotropic conduction is when all cells depolarise outwards in different directions at the same speed (in a circular shape). However, anisotropic condiction is when cells depolarise outwards at different directions, propagating in an elliptical shape

Question 15

Does the heart depolarise in an isotropic or anisotropic fashion

Answer 15

Anisotropic

Question 16

Describe 3 further ways conduction velocity in the heart can be affected

Answer 16

-levels of cell coupling through gap junctions (as higher resistance between cells means a smaller current flowing from source to sink)

-cell excitability (affects extent of depolarisation, perhaps due to low sodium current)

-the sodium current level can be reduced by acute ischaemia, class 1 antiarrhythmic drugs (e.g quinidine), genetic mutations of Na channel (e.g brugada syndrome) or TTX poisoning

Question 17

What are 3 ways cell to cell coupling with gap junctions is modulated by

Answer 17

-pH (as acidosis reduces coupling)

-high Ca2+ conc reduces coupling

-dephosphorylation reduces coupling (as a ischaemic state is dephosphorylated which reduces coupling compared to the normoxic phopshorylated state)