L-18

Question 1

What is an action potential?

Answer 1

A nerve cell action potential is a reversal of trans-membrane voltage that is completed in 2-3ms

Question 2

What are the three main types of ion channels?

Answer 2

• Chemical gated channels
• Voltage gated channels
• Mechanical gated channels

Question 3

How are chemically gated ion channels opened?

Answer 3

The are triggered to open by the presence of specified chemical neurotransmitters. These create local potentials

Question 4

How are mechanically gated ion channels opened?

Answer 4

Cation channels are opened by applied pressure around the channel and close when the pressure is removed. These create local potentials

Question 5

How are voltage gated channels opened?

Answer 5

Cation channels are opened by a change in voltage called depolarisation. These channels create action potentials

Question 6

How is an action potential created?

Answer 6

• There is a high density of voltage gated Na+ channels at the axon hillock where the action potential is initially created. If the net voltage change (summed from local potentials) at this point is at the threshold (-60mV), voltage gated Na+ channels may open and drive further depolarisation and a large Na+ influx. The membrane potential rises to +30mV and triggered voltage gated K+ channels to open and begin repolarisation. Near threshold the channel return to their original states and the membrane potential is restored.

Question 7

What is the threshold membrane potential?

Answer 7

-60mV

Question 8

What is the approximate maximum membrane potential reached during depolarisation?

Answer 8

+30mV

Question 9

Why can no more action potentials be generated during the absolute refractory period?

Answer 9

To prevent action potentials moving backwards up the axon

Question 10

Can another AP be triggered during the relative refractory period?

Answer 10

Only in response to a very large stimulus

Question 11

What are the steps of action potential propagation in an unmyelinated axon?

Answer 11

1. An action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV
2. Sodium ions diffuse away from open ion channels and quickly bring segment 2 to threshold
3. An action potential develops in segment 2 and the initial segment begins depolarisation
4. This continues down the length of the axon ( can only move forward due to inability to form new AP during the ARP)

Question 12

What speed to action potentials travel down unmyelinated axons?

Answer 12

1-5ms^-1

Question 13

What speed can action potentials travel down myelinated axons?

Answer 13

20-100m/s

Question 14

Why do we need myelin

Answer 14

Action potentials travel very slowly down unmyelinated axons and especially for out long axons it can be dangerous when it takes too long for our bodies to respond to a potentially harmful stimulus.

Question 15

How does myelin effect action potential propagation?

Answer 15

The myelin acts as an insulator and there are no voltage gated channels under myelin so the next region to reach threshold( node 1) is further away/ less regions are necessary to bring to threshold.