Topic 6- Nerve impulses (resting & action potentials )

Question 1

What are the 4 parts of a motor neuron?

Answer 1

1) Cell body
2) Dendrites
3) Axon
4) Schwann cells

Question 2

What is the function of the cell body?

Answer 2

contains organelles found in an animal cell.

Includes nucleus

Question 3

What is the function of dendrites?

Answer 3

They carry action potentials to surrounding cells

Question 4

What is the function of axons?

Answer 4

This is a long conductive fibre which carries nerve impulses along motor neurone.

Question 5

What is the function of the Schwann cell?

Answer 5

Acts as an insulator
These are wrapped around the axon to form the myelinated sheath.
This is a lipid- no charged ions can pass through
There are also gaps between the cells called the nodes of Ranvier(where action potentials can be generated.)

Question 6

Define the term resting potential

Answer 6

This is the difference between the charge inside and outside a neuron.

Question 7

When does the resting potential happen?

Answer 7

When a neurone is NOT conducting an impulse.

Question 8

What is the resting potential?

Answer 8

-70mV

Question 9

How is the resting potential of -70mV is maintained?

Answer 9

• via the sodium potassium pump.
• Active transport
  1) Pumps 3 Na+ ions OUT of the axon and 2 K+ ions INTO the axon.
  2) Creates electrochemical gradient (build up of Na+ ions outside and excess K+ ions inside)
  3) Within membrane there are Na+ ion channels and K+ ion channels.

4) Facilitated diffusion-
- K+ moves from inside where there is higher conc to outside (lower conc)

• Na+ move from outside (high conc) to inside (lower conc)

5) Why is there no equal number of Na+ and K+ ions inside & outside…
-The cell membrane is more permeable to K+ ions…..
- because…more K+ ion channels in the membrane… leads to more K+ ions diffusing out…
-
some channels= closed.. but K+ ones= mainly open compared to Na+ ones which may only open when there is high voltage.

• this is how -70mV resting potential is maintained.

Question 10

What is an action potential?

Answer 10

When a neurone’s voltage increases beyond the resting potential (-70mV)

Generates nerve impulse.

Question 11

What is another term for an increase in voltage and explain what this means?

Answer 11

Depolarisation

• When voltage starts to be= more +ve.
• Because the axon membrane becomes more permeable to Na+ ions.
• more Na+ move into axon and increase + charge.

Question 12

List the order of stages which the take place during the action potential.

Answer 12

1) Resting potential
2) Start of Depolarisation
3) Depolarisation
4) Repolarisation
5) Hyperpolarisation

Question 13

What happens during the resting potential?

Answer 13

• 70mV
• maintained via Na+ & K+ pump.
• More K+ channels and some are permeable
  meaning more K+ is diffused OUT of the axon.

Na+ voltage gated channels are closed.

Question 14

What happens in the early stages of depolarisation and during depolarisation?

Answer 14

1) Early stages of depolarisation
- When there’s a stimulus
- enough energy is provided to cause the voltage gated Na+ channels to OPEN.
- Na+ ions will diffuse INTO the axon.
K+ will diffuse OUT.
- This causes a slight increase in the p.d…

2) During depolarisation
- the p.d goes above -55mV there is more energy for more Na+ voltage gated channels to open.
- More Na+ diffuses IN at the same time k+ moves OUT

Depolarisation is this sharp increase in the voltage.

Question 15

What happens during repolarisation?

Answer 15

• Graph will peak at +40mV
• Na+ channels close at +40mv
• K+ voltage gated channels will open.
• Lots of K+ diffuses out because there is lots of these channels in the membrane.
• causes a DECREASE in p.d
• There is more +ve ions outside the axon than inside.

Question 16

What happens during hyperpolarisation?

Answer 16

• p.d continually decreases.
• until it overshoots the resting potential so it goes above -70mv.
  Known as refractory period.

Question 17

What is the mexican wave analogy?

Answer 17

• The graph of action potential is not looking at change over distance
  its looking at change over time.
• throughout the graph time= 4 millisecs
• the nodes of Ranvier of an axon goes through the entire process of resting potential, depolarisation, repolarisation, and hyperpolarisation.
• When the +40 mv is reached, the next part of the axon is triggered to start the depolarisation wave.
• keeps happening in all of the nodes of Ranvier in the axon until the end of the axon is reached.

Question 18

Explain the all or nothing principle

Answer 18

• if the depolarisation does NOT exceed -55mV then an action potential and the impulse are not produced (NOTHING).
• 55mV= threshold… if stimulus isn’t large enough it wont provide enough energy to open Na+ gated channels to go above -55mV.
• if you do reach -55mv you do have an action potential (ALL)
  all peaks at +40 mV.

The bigger the stimuli the higher the frequency of action potentials.

Question 19

Why is the all or nothing principle important for animals?

Answer 19

• makes sure animals only respond to large enough stimuli rather than every slight little change in their environment.

Question 20

Explain the refractory period

Answer 20

• After an action potential is generated… the membrane enters a refractory period (-80mV) and it cannot be stimulated as the Na+ channels are recovering and cannot be opened.

Question 21

What are the 3 reasons why the refractory period is important

Answer 21

1) discrete response produced..
an action potential cant be generated immediately… separate responses/ no overlap

2) ensures that action potential travels in ONE DIRECTION.
- if this wasn’t the case then there would be a spread of Na and K ions in different directions and so this would stop the response from happening.

3) Limits number of impulse transmission-
- limits action potentials that can happen over time.

prevents over reaction.