Electrical Properties 7

Question 1

Describe the Potassium hypothesis

Answer 1

Membrane is more permeable to K+ ions than anything else. They move down the gradient until the electrical gradient opposes the concentration gradient. This continues till equilibrium is met. Ions can still cross the membrane but there is no net movement of ions

Question 2

How can resting membrane potential be predicted?

Answer 2

Nernst equation
K+ inside = 120mM
K+ outside = 5mM

Put into equation creates equilibrium of -80mV

Question 3

What is a better equation to calculate the resting membrane potential?

Answer 3

Goldman-Hodgkin-Katz equation

Question 4

How long does an action potential in the heart last?

Answer 4

200-400ms

Question 5

Describe the cardiac action potential:

Answer 5

• Upstroke (opening of sodium channels)
• • Inactivation of sodium channels
• brief increase in potassium permeability due to opening of transient outward channels
• Repolarisation due to K+ efflux
• Absolute refractory period when Na+ channels cannot be opened. This is long in cardiac muscle
• relative refractory period is when a large stimulus can cause depolarisation.
  Increase in permeability to calcium through L-type calcium channels (remain open longer)

Question 6

What does the long absolute refractory period prevent?

Answer 6

Tetany

Question 7

What are the phases of the action potential?

Answer 7

0 = Upstroke
1 = early repolarisation 
2 = plateau
3 = repolarisation 
4 = resting membrane potential (diastole)

Question 8

What can cause inhibition of calcium permeability?

Answer 8

dihydropyridine Calcium channel antagonists e.g. Nifedipine, Nitrendipine, Nisoldipene. They work by blocking the L-type calcium channels

Question 9

What does the IK1 current do?

Answer 9

Large and flows during diastole. It stabilises resting membrane potential and reduces arrythmia risk

Question 10

Word to describe electric potentials of the heart?

Answer 10

INTRINSIC but modulated by sympathetic and parasympathetic activity

Question 11

Function of the Sinoatrial Node

Answer 11

• No IK1 channels
• Constantly depolarising with very little Na+ influx
• Upstroke governed by Ca2+ influx and the pacemaker current is present so gradual upstoke is the pacemaker potential

Question 12

How does Pacemaker Potential relate to heart rate?

Answer 12

Increased sympathetic stimulation (noradrenaline/adrenaline release) steepens pacemaker potential so threshold potential is reached more rapidly.

Increased parasympathetic stimulation (acetylcholine) decreases the pacemaker potential so decreases heart rate

Question 13

Describe the structure of the sinoatrial node

Answer 13

Small bundle of cells situated in the superior aspect of the right atrium. At the anterolateral margin of the orifice between the superior vena cava and the atrium.

Question 14

What are the 4 basic components of heart conduction system?

Answer 14

• Sinoatrial node
• Inter-nodal fibre bundles
• Atrio-ventricular Node
• Ventricular bundles ( bundle branches and purkinje fibres)

Question 15

How long is the AV node delay?

Answer 15

Approximately 0.1s

Question 16

Where do terminal purkinje fibres end?

Answer 16

Extend beneath the endocardium and approximately 1/3rd into the myocardium

Question 17

What allows impulse propagation?

Answer 17

Gap junctions which exist in intercalated disks

Question 18

What happens when a wave of depolarisation moves towards the positive electrode?

Answer 18

Causes an upward deflection. Vice-versa for downwards deflection

Question 19

Describe the excitation sequence?

Answer 19

• Initially, SA fires and action potential propagates across the atria.
• Depolarisation moves towards electrode so small upstroke is seen
• Depolarisation then moves away from electrode so small downstroke
• As it moves down the bundle branches, large upstroke as towards the electrode
• Downstroke seen as it moves along the Prukinje fibres

Question 20

Describe the Repolarisation

Answer 20

They polarise from the endocardium to the epicardium. . T wave signifies ventricular repolarisation.