ND2: Action potentials

Question 1

What is an action potential and what are its properties?

Answer 1

The change in voltage across a membrane

1. Depends of ionic gradients and relative permeability of the membrane
2. Only occurs if a threshold level is reached
3. All-or-nothing
4. Propogated without loss of amplitude

Question 2

What happens to the membrane potential if the conductance to any ion is increased?

Answer 2

The membrane potential will move closer to the equlibrium potential for that ion

Question 3

Upon what is the conductance of the membrane to a particular ion dependent?

Answer 3

The number of channels for that ion which are open

Question 4

How can it be experimentally shown that Na⁺ is responsible for action potential depolarisation?

Answer 4

Conducting experiments with changing concentrations of extracellular Na⁺

The peak of the action potential will change in a manner parallel to the changes in E_Na

Question 5

What does a voltage clamp enable?

Answer 5

Membrane currents to be measured at a set membrane potential

Question 6

Explain these diagrams.

Answer 6

Diagram 1
The membrane potential is held at –70mV, then increased to –15, 0, and +15 mV in three separate experiments

Diagram 2
The influx of Na⁺ happens more quickly at more positive membrane potentials

The current of Na⁺ influx decreases towards 0 even though V_m is maintained

K⁺ channels take longer to close

Question 7

Why does overshoot occur after an action potential has taken place?

Answer 7

K⁺ channels take longer to close

Question 8

By how much does each action potential increase Na⁺ concentration in the axon?

Answer 8

40 μM

Question 9

What is the axon hillock?

Answer 9

The part of the cell body (soma) of a neuron that connects to the axon

Question 10

Where does depolarisation to threshold initiate an action potential?

Answer 10

At the axon hillock

Question 11

Describe the positive feedback loop during an axonal action potential.

Answer 11

Question 12

Describe channel activity during an axonal action potential.

Answer 12

Question 13

What are the two types of refractory period after an action potential? What is the difference between the two?

Answer 13

Absolute refractory period (ARP): nearly all Na⁺ channels are in the inactivated state

Relative refractory period (RRP): Na⁺ channels are recovering from inactivation; the excitability returns towards normal as the number of channels in the inactivated state decreases

Question 14

Do channels open and close in an ordered or random manner?

Answer 14

Random

Question 15

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

Answer 15

One α-subunit consiting of four similar repeats (I – IV), each containing six α-helix regions, and a pore at the centre

Question 16

How does the action potential propagate along the axon?

Answer 16

• The action potential is initiated at the axon hillock by an EPSP
• It travels down the acon without loss of amplitude
• It goes in one direction only
• The propogation of an action potential depends on local current spread depolarising the membrane potential of adjacent sections of the axon above threshold

Question 17

How do local currents result in a spread of depolarisation along the axon?

Answer 17

They repel any other positively charged ions and trap any negatively charged ions

Question 18

What is the length constant (λ) ?

Answer 18

The distance taken for the potential to fall to 37% of its original value

Question 19

What will be the effects of an injection of current into the axon?

Answer 19

The resulting charge will spread along the axon and cause an immediate local change in the membrane potential

Question 20

How can propogation be maintained?

Answer 20

By local depolarisation ahead of the action potential above threshold initating the positive feedback loop

Question 21

How can action potential conduction speed be increased?

Answer 21

Myelination

Question 22

Why does myelination increase speed of action potential conduction?

Answer 22

The Na⁺ channels are concentrated at unmyelinated regions called the nodes of Ranvier, whereas unmyleinated axons have an even distribution of Na⁺ ions

Question 23

What is saltatory conduction?

Answer 23

The jumping of action potential between nodes of Ranvier

The myelin sheath acts as a good insulator, causing the local currents to depolarise the next node above threshold and initiate an action potential