15.2- THE NERVE IMPULSE

Question 1

What can a nerve impulse be defined as?

Answer 1

self-propagating wave of electrical activity that travels along the axon membrane

Question 2

What is a nerve impulse the temporary reversal of?

Answer 2

temporary reversal of electrical pd across axon membrane

Question 3

Between what states id the temporary reversal of electrical pd between? (nerve impulse)

Answer 3

resting potential and action potential

Question 4

How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #1 (hint- bilayer)

Answer 4

phospholipid bilayer of axon plasma membrane prevents sodium + potassium ions diffusing across it

Question 5

How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #2 (hint- channel proteins)

Answer 5

channel proteins span phospholipid bilayer
channel proteins have ion channels which pass through them
some of these have ‘gates’ which can be opened/closed so sodium/potassium ions can move through them by facilitated diffusion at any one time, but not on other occasions

Question 6

How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #3 (hint- carrier proteins)

Answer 6

some carrier proteins actively transport potassium ions into axon + sodium ions out of axon
mechanism can be called sodium-potassium pump

Question 7

What is the charge of the inside of the axon like relative to the outside?

Answer 7

negatively charged

Question 8

Range of values for resting potential and what is it usually like in humans?

Answer 8

50 to 90 millivolts (mV), but usually around 65mV in humans

Question 9

What is the axon said to be when it is at 65mV?

Answer 9

polarised

Question 10

What is the establishment of the pd due to- where are Na+ ions transported and by what?

Answer 10

sodium ions actively transported out of axon by sodium-potassium pumps

Question 11

What is the establishment of the pd due to- where are K+ ions transported and by what?

Answer 11

potassium ions actively transported into axon by sodium-potassium pumps

Question 12

What is the establishment of the pd due to- is the active transport of sodium or potassium greater and what is the ratio like?

Answer 12

active transport of sodium ions greater than that of potassium ions, so 3 sodium ions move out for every 2 potassium ions

Question 13

What is the establishment of the pd due to- how is an electrochemical gradient created?

Answer 13

although sodium + potassium ions both positive, outward movement of sodium ions greater than inward movement of potassium ions
so there’re more sodium ions in tissue fluid surrounding axon than in cytoplasm, + more potassium ions in cytoplasm than in tissue fluid

Question 14

What is the establishment of the pd due to- where do sodium ions begin to diffuse back into?

Answer 14

sodium ions begin to diffuse back naturally into axon while potassium ions begin to diffuse back out of axon

Question 15

What is the establishment of the pd due to- what are the gates for potassium ions and sodium ions like?

Answer 15

most of the gates in channels that allow potassium ions to move through open, while most of gates in channels that allow sodium ions to move are closed

Question 16

What does a stimulus of sufficient size detected by a receptor in the nervous system cause?

Answer 16

its energy causes temporary reversal of charges either side of this part of axon membrane

Question 17

What happens to charge inside the membrane if the stimulus is great enough?

Answer 17

negative charge of -65mV inside membrane becomes positive charge of around +40mV

Question 18

When inside the membrane becomes a positive charge of around +40 mV, what is this called?

Answer 18

action potential

Question 19

When inside the membrane is around +40mV, what is this part of the axon membrane said to be?

Answer 19

depolarised

Question 20

Why does the depolarisation occur?

Answer 20

as channels in axon membrane change shape, + hence open/close, depending on voltage across membrane

Question 21

What are channels in the axon membrane that change shape and hence open/close depending on the voltage across the membrane called?

Answer 21

voltage-gated channels

Question 22

1 At resting potential are the potassium voltage-gated channels and sodium voltage-gated channels open?

Answer 22

some potassium voltage-gated channels open but sodium voltage-gated channels closed

Question 23

2 What does the energy of the stimulus cause the sodium voltage-gated channels to do?

Answer 23

some sodium voltage-gated channels in axon membrane opened + so sodium ions diffuse into axon through channels along their electrochemical gradient
being positively charged, they trigger reversal in pd across membrane

Question 24

3 What happens as the sodium ions diffuse into the axon?

Answer 24

more sodium channels open, causing even greater influx of sodium ions by diffusion

Question 25

4 What happens once the action potential is around +40mV?

Answer 25

voltage gates on sodium ion channels close + voltage gates on potassium ion channels begin to open

Question 26

5 With some potassium voltage-gated channels now open, what happens?

Answer 26

electrical gradient that was preventing further outward movement of potassium ions now reversed, causing more potassium ion channels to open
means more potassium ions diffuse out, starting repolarisation of axon

Question 27

6 What does the outward diffusion of these potassium ions cause?

Answer 27

temporary overshoot of electrical gradient, with inside of axon being more negative than usual
closable gates on potassium ion channels now close + activities of sodium-potassium pumps once again cause sodium ions to be pumped out + potassium ions in
resting potential of -65mV re-established + axon said to be repolarised

Question 28

What is the movement of sodium ions inwards during the action potential called and what kind of process is it?

Answer 28

diffusion- passive process

Question 29

What process maintains the resting potential and what kind of process is it?

Answer 29

active transport- active process