Action potential

Question 1

What is an Action Potential

Answer 1

Area of excitable membrane to is depolarized to threshold
Sodium ions move into cell through voltage gated sodium channels. Raising membrane potential to +30.
Sodium channels close and potassium ions move out of cell through voltage channels, repolarizing the cell
Potassium channels slowly close and membrane moves back to resting potential.

Question 2

What is the resting membrane potential

Answer 2

-70

Question 3

What is the purpose of voltage gated channels

Answer 3

To allow the AP to travel quickly down the axon

Question 4

Describe AP propargation

Answer 4

This is the decision point as to whether the AP will continue. If there is sufficient input it will activate enough Na+ to be sent along the axon.

Question 5

How does the AP spread down the axon

Answer 5

Na+ released at initial segment and polarises the available parts of the axon.

Question 6

At what point is it decided if an AP is carried on

Answer 6

Threshold

Question 7

Describe Depolarization to threshold step of AP release

Answer 7

Change in membrane potential (eg.-60) sufficient enough to depolarize the cell and open the voltage gated Na+ channels

Question 8

Describe Rapid depolarization step of AP release

Answer 8

Voltage gated Na+ channels open and bring in Na+ ions (membrane potential +10)

Question 9

Describe Inactivation and activation stage of AP release

Answer 9

membrane rapidly depolarizes, inactivating Na+ channel and activating K+ channels (+30)

Question 10

Describe Inactivation stage of AP release (repolarization)

Answer 10

Membrane repolarizes to resting. K+ channels close. (-90) This ensures the process is reset.

Question 11

What are the differences between the Na+ and K+ channels

Answer 11

The Na+ channel closes quickly - this limits the amount of NA+ coming into the cell
The K+ channel closes slowly - This means the potential channel may become more negative (-90 in one step)

Question 12

Why does K+ make the membrane potential more negative

Answer 12

The K+ channel takes K+ ions out of the cell. These leave negative charges in enzymes within the cell, leaving a negative charge on the inside.

Question 13

What does refractory mean

Answer 13

Resistant to a stimulus or process

Question 14

What are absolute and relative refractory periods

Answer 14

Absolute - Depolarisation occurs

Relative - Repolarization occurs. Gets all Na+ channels ready for next upsurge

Question 15

Describe AP propagation in an unmyelinated axon

Answer 15

AP develops at initial segment, depolarizing it to +30. Ionic current then spreads to affect neighbouring segments and initiates their own AP, depolarizing them to +30

Question 16

Why does the AP only move forward along the membrane

Answer 16

The pervious segment is being repolarized (in the absolute refractory period) so the Na+ only travels to the segments at resting membrane potential

Question 17

Do small or large organisms have quicker reflexes

Answer 17

Smaller organisms - AP’s have less distance to travel.

Question 18

What else can affect the speed of APs

Answer 18

Diameter of axon - larger diameter leads to quicker AP. (Myelinated)

Question 19

Describe AP propagation in a Myelinated axon

Answer 19

AP is only generated at nodes (areas between Schwann cells where membrane is exposed). Only areas of exposed membrane need to be depolarized

Question 20

Why do APs travel faster in Myelinated axons

Answer 20

Myelin separates the membrane from the outside of the cell so they no longer interact. This means that there is less space to depolarize. Instead of needing to pass down the whole length of an axon now only needs to jump from node to node. The faster travelling creates a higher V

Question 21

What does the axon behave as when myelinated

Answer 21

An excellent conductor - like a biological wire - moves Na+ along to help depolarization