Nerve impulses

Question 1

Draw the structure of a neuron.

Question 2

Why is there a resting potential present when a neuron is in its resting state?

Answer 2

There are more positive ions outside of the cell compared to the outside meaning that the outside of the cell is positively charged compared to the inside.

Question 3

What maintains the resting potential?

Answer 3

The sodium-potassium pumps and potassium ion channels in the membrane.

Question 4

How do the sodium-potassium pumps and the potassium ion channels maintain the resting potential?

Answer 4

1. The sodium-potassium pumps move three sodium ions out of the cell. The potassium ion channels move two ions inside the cell.
2. The cell membrane is impermeable to sodium ions so they can no longer diffuse back into the cell.
3. This leads to a build-up of positive charge outside the cell.
4. Even if potassium ions are moving back into the cell more ions are moving out of the cell.
   5 This means the outside is positively charged compared to the inside.

Question 5

How is an action potential triggered in a neuron?

Answer 5

1. When a stimulus is detected the membrane of the neurone becomes excited and the sodium ion channels open.
2. The sodium ions diffuse into the neurone which increases the cell potential
3. At around -55mv even more sodium ion channels open and the cell potential increases further. ( Depolarisation)
4. At around + 30mv the sodium ion channels close and the potassium ion channels open.
5. The potassium ions diffuse out of the cell and that causes the cell potential to decrease. ( Repolirasation)
6. The potassium ion channels are slow to close so the cell potential decreases below the resting potential. Hyperpolarisation
7. The ion channels reset and return the cell potential to the resting potential.

Question 6

What is the refractory period?

Answer 6

The period of recovery between the two action potentials.

Question 7

Why is there a refractory period between action potentials?

Answer 7

The ion channels are recovering and can’t be forced to open.

Question 8

Why are refractory periods important?

Answer 8

1. It makes sure that the action potentials are discrete and don’t overlap.
2. It makes sure the action potentials are unidirectional and there is a limit to the frequency of the action potentials.

Question 9

How do waves of depolarization form in neurones?

Answer 9

When sodium ions diffuse into the neurone they can then diffuse sideways. This causes the sodium ion channels on the next part of the neurone to open. Sodium ions then diffuse into the next part of the neurone and this causes a wave of depolarisation to be formed in the neurone.

Question 10

What is the all-or-nothing principle?

Answer 10

That an action potential will only be triggered if the generator potential is higher than the threshold level.

Question 11

How does myelination affect the speed of conduction?

Answer 11

The sodium ions can’t diffuse into the parts of the neurone that contain myelination, they can only do so at the nodes. This means that the electrical impulse will jump from node to node. This means the impulse travels much faster across the neurone as it does not have to travel through the whole neurone.

Question 12

How does axon diameter affect the speed of conduction?

Answer 12

As the axon diameter increases the speed of conduction increases. This is because there is less resistance to the movement of ions across the neurone meaning the waves of depolarisation can travel faster.

Question 13

How does temperature affect the speed of conduction?

Answer 13

As temperature increases the speed of conduction increases as the ions have more energy and can diffuse across the membrane faster. The temperature must not be too high as the ion channels will denature and the ions will no longer be able to diffuse across the membrane.