Chapter 3 - The Action Potential

Question 1

Define hyperpolarization and depolarization

Answer 1

Hyp : membrane potential is < Resting potential

Depolarization : membrane potential is > resting potential

Question 2

What causes an action potential?

Answer 2

… a depolarization of the membrane beyond the threshold

Question 3

What constitutes the neural code?

Answer 3

Frequency and pattern of APs

Question 4

What are the 4 phases of the AP

Answer 4

Rising phase, overshoot, falling phase and undershoot

Question 5

What can cause a change in membrane potential?

Answer 5

• naturally : — adequate stimulus to a sensory receptors
  — chemical neurotransmitters
• experimentally : current injection
• light : optogenetics

Question 6

What are the three types of protein molecules present on the membrane?

Answer 6

Sodium/potassium pump

Sodium channel

Potassium channel

Question 7

What is the net current flow formula?

Answer 7

NCF = conductance x ionic driving force

NCF = g(ion) x (Vm - E(ion))

Question 8

What are the 6 phases of the action potential in an ideal neuron

Answer 8

1) Membrane potential is depolarized to threshold.
2) transient increase in g(Na)
3) Entey of Na+ into the cell dépolarises further the cell
4) More voltage-gated Na+ channels are opened (positive feedback)
5) the time during which these channels are open is short
6) during falling phase, the Na+ gates close whereas the K+ channels stay open. Hyperpolarization of cell follows.

Question 9

What does the voltage clamp do?

How does it work?

Answer 9

It allows to control the membrane potential.

See slide/ book

Question 10

What is the Hodgkin and Huxley formula that calculates the conductance changes?

Answer 10

g(ion) = I(ion) / Vm - E(ion)

Question 11

What is the structure of a voltage-gated Na channel?

Answer 11

Large protein, 4 domains with 6 transmembrane alpha helices

Question 12

What are the two types of components for a domain (sodium channels)

Answer 12

• Voltage sensor S4

- Selectivity filter (for Na+)

Question 13

What are the 4 steps to the activation and deactivation of a sodium channel?

Answer 13

1) Closed channel
2) Open channel upon depolarization
3) Inactivation of gate
4) Deinactivation —> ball removes itself

Question 14

What are the different phases of the AP?

Answer 14

• Threshold : Vm where enough voltage gated Na+ channels are open : PNa > PK
• Rising phase : large driving force on Na+. Na+ rushes into cell
• Overshoot : Vm approaches ENa
• Falling phase : a) voltage gated Na+ channels inactivate
  b) (this starts before falling phase, during rising phase) the v-gated K+ channels finally open, delayed colored to the Na ones, K+ rushes out of cell
• Undershoot : Vm approaches EK as K+ channels remain open.

Question 15

What is the absolute refractory period?

Answer 15

Na* channels remain inactivated until membrane potential goes negative enough to deinactivate the Na+ channels.

Question 16

What is the relative refractory period?

Answer 16

Membrane potential remains hyperpolarized until K+ channels close.

(If these K+ channels didn’t close, more depolarization would be needed to reach threshold, and initiate AP)

Question 17

What are two factors influencing speed of AP

Answer 17

1) Diameter of axon : big diameter entails less contact between ions and membrane, less current leakage, more current down axon, faster propagation
2) Myelination : the specialized glia cells wrap the axon making a very efficient insulator that allows no current leakage

Question 18

What is saltatory conduction?

Answer 18

The AP jumps from node to node. This leads to faster conduction.
At each node, there is an action potential reaction (Na enters, K exits)

Question 19

What are two demyelination guys diseases :

Answer 19

• Multiple sclerosis (CNS)
  —> MS attacks myelin sheath in brain, spinal cord, optic nerves
• Guillain-Barré syndrome (PNS)
  —> immune response against one’s own myelin
  Attacks myelin of nerves