6.2.1 Nerve impulses

Question 1

What is the structure of the cell body?

Answer 1

Contains the organelles found in a typical animal cell, including the nucleus.

Question 2

What is the axon?

Answer 2

A long, conductive fibre that carries the nerve impulse along the motor neuron.

Question 3

What do Schwann cells do?

Answer 3

Wrap around the axon to form the myelin sheath.

Question 4

What are the nodes of Ranvier?

Answer 4

Gaps in the myelin sheath.

Question 5

What is the resting potential?

Answer 5

The difference in electrical charge between the inside and outside of a neuron’s cell membrane, when a neuron is NOT conducting an impulse.

Question 6

What is the resting potential of a neuron?

Answer 6

-70mV

Question 7

Why is the resting potential of a neuron negative?

Answer 7

There are more positive ions, Na+ and K+, outside compared to inside - so inside is more negative.

Question 8

What is the resting potential maintained by?

Answer 8

Sodium potassium pump involving active transport and ATP.

Question 9

What does the sodium potassium pump do?

Answer 9

Moves 2 K+ ions in and 3 Na+ out

Question 10

What is an action potential?

Answer 10

When a neuron’s voltage increases beyond a certain point from the resting potential, it generates a nerve impulse.

Question 11

What is depolarisation (increase in voltage) due to?

Answer 11

The neurone membrane becoming more permeable to Na+ ions.

Question 12

Describe the ion channels during a resting potential.

Answer 12

• Some K+ ion channels are permanently open and others are closed.
• Voltage gated Na+ ion channels are closed.
• K+ ions are moving out of axon.

Question 13

What causes the start of depolarisation?

Answer 13

A stimulus provides enough energy to cause voltage-gated Na+ ion channels in axon membrane to open.

Question 14

What do ions do at the start of depolarisation?

Answer 14

• Na+ ions diffuse into axon, at the same time as K+ ions diffusing out.
• Causes a slight increase in voltage.

Question 15

What happens if the increase in voltage is above -55mV?

Answer 15

This provides enough energy to enable more Na+ ion channels to open.

Question 16

What happens during depolarisation?

Answer 16

• More Na+ ions diffuse in.
• Sharp increase in voltage.

Question 17

What does depolarisation always peak at?

Answer 17

+40mV

Question 18

What happens to the Na+ ion channels at +40mV?

Answer 18

They close.

Question 19

What happens during repolarisation?

Answer 19

• K+ ion channels open, meaning that twice the amount of K+ ions diffuse out.

Question 20

What does the K+ ion channels opening during repolarisation cause?

Answer 20

A decrease in voltage - becomes more negative.

Question 21

Where does depolarisation happen?

Answer 21

Nodes of Ranvier.

Question 22

What does the voltage reaching +40mV cause?

Answer 22

The next part of the axon to start depolarisation.

Question 23

What is hyperpolarisation?

Answer 23

Overshoot of voltage beyond resting potential - refractory period.

Question 24

What is the all or nothing principle?

Answer 24

If the depolarisation does not exceed -55mV, then an action potential + impulse are not produced.

action potential is only generated when threshold is reached

high presssure = generates action potenetial

Question 25

What is a refractory period?

Answer 25

- After an action potential has been generated, the membrane enters a refractory period. - Where it cannot be stimulated because Na+ channels cannot be opened. Time where no new generator potential can be produced

Question 26

Why is the refractory period important?

Answer 26

- Ensures that an action potential occurs in one direction only. - Ensures that action potentials are separated from one another. - Limits the number of impulses.

Question 27

What factors affect the speed of action potentials?

Answer 27

1. Myelination & saltatory conduction. 2. Axon diameter. 3. Temperature.

Question 28

What is saltatory conduction?

Answer 28

Action potential jumps from one node of Ranvier to the next.

Question 29

How does axon diameter affect speed of conduction?

Answer 29

- The wider the diameter, the faster the speed of conduction. Less resistance to diffusion of ions is less leakage of ions and therefore action potentials travel faster.

Question 30

How does temperature increase the speed of conduction?

Answer 30

- A higher temperature increases the speed of conduction: 1. Ions diffuse faster -> action potentials are generated by ions moving (via facilitated diffusion + active transport). 2. Enzymes involved in ATP work faster -> there is more ATP for active transport in the Na+/K+ pump. Increased rate of diffusion of ions

Question 31

Explain how a resting potential is maintained across the axon membrane in a neurone.

Answer 31

Sodium potassium pump - Higher concentration of potassium ions inside and higher concentration of sodium ions outside.- electrochemical gradient - Membrane more permeable to potassium ions. - Sodium ions actively transported out and potassium ions in. move from high to low

Question 32

Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.

Answer 32

- Myelination provides insulation. - Saltatory conduction (in myelinated). - In non-myelinated depolarisation occurs along whole/length (of axon).

Question 33

Suggest two advantages of simple reflexes.

Answer 33

- Rapid. - Protect against damage to body tissues.

Question 34

Describe and explain how a resting potential is maintained in a neurone

Answer 34

Higher [k+] in neurone than out, and higher [Na+] outside neurone than in So, k+ diffuse out neurone, and Na+ diffuses in Membrane more permeable to K+ leaving neurone, than Na+ entering neurone Sodium ions activly transported out of neurone, and potassium ions into neurone

Question 35

Describe and explain how an action potential is created in a neurone

Answer 35

Gated sodium ion channels open, so sodium ions diffuse in If threshold potential is reached, voltage-gated sodium ion channels open and more sodium ions diffuse in Section of axon membrane depolarised Voltage gated sodium ion channels close, and votage gated potassium channels open Efflux of potassium ions out of neurone causes repolarisation

Question 36

What name is given to depolarisation at Nodes of Ranvier

Answer 36

saltatory conduction

Question 37

 A scientist investigated the effect of inhibitors on neurones. She added a respiratory inhibitor to a neurone. The resting potential of the neurone changed from –70 mV to 0 mV. Explain why.

Answer 37

- No ATP - No AT - electrochemical gradient not maintained - no net movement of ions