15.2 The nerve impulse

Question 1

What is the definition of a nerve impulse

Answer 1

It is a self propagating wave of electrical activity that travels along axon membrane

It is the temporary reversal of electrical potential difference across the axon membrane. This reversal is between two states called resting potential and action potential

Question 2

How is the movement of Na+ ions and K+ ions across the axon membrane controlled

Answer 2

• Phospholipid bilayer of axon membrane prevents the ions diffusing through it
• Channel proteins span the phospholipid bilayer that can open or close to allow sodium or potassium ions to diffuse through by facilitated diffusion. Different gated channels for sodium and potassium ions.
• Sodium potassium pump allows carrier proteins to actively transport potassium ions into the axon and sodium ions out of the axon

Question 3

What does resting potential mean

What does polarised mean

Answer 3

The inside of the axon is more negatively charged than the outside.

Ranges from 50 to 90 millivolts but is usually 65 in humans.

In this condition the axon is said to be polarised

Question 4

Why is there a difference in potential difference (difference in charge between inside and outside of axon)

Answer 4

• Sodium ions are actively transported out by sodium potassium pumps
• Potassium ions are actively transported in by sodium potassium pumps
• 3 sodium ions move out for every 2 potassium that enter because more active transport occurs for sodium.
• Because the outer movement of sodium is greater than the inward movement of potassium, there are more sodium ions in tissue fluid surrounding it than in the cytoplasm.
  More potassium ions in cytoplasm than in tissue fluid.
  This creates an electrochemical gradient
• This means sodium ions diffuse back into axon down their concentration gradient whilst potassium diffuse out of the axon.
• However most gates in the channels that allow potassium ions to move through are open, whilst most of the gates in the channels that allow the sodium ions to move through are closed.

Question 5

What is the action potential

Answer 5

• If a big stimulus is detected by a receptor in nervous system, its energy causes a temporary reversal of the charges either side of this part of the axon membrane
• If the stimulus is great the negative charge of -65mv inside axon becomes +40mv.
  This is known as action potential and in this condition this part of the axon is depolarised.

The depolarisation occurs because the channels in axon membrane change shape and hence open or close depending on voltage across membrane.

Question 6

Describe stages of resting potential and action potential

Key words:
Resting potential
Action potential
Depolarisation
Repolarisation
Hyperpolarisation

Answer 6

. At resting potential some potassium channels are open but the sodium ones are closed.

. The energy of stimulus causes some sodium channels in axon membrane to open so sodium ions diffuse in along electrochemical gradient.

. As sodium ions diffuse into axon, more sodium channels open causing a greater influx of sodium ions by diffusion

. Once action potential reaches +40mv, the sodium channels close and voltage gates of potassium open

. Electrical gradient that was preventing further outward movement of potassium ions is now reversed when these channels open. This causes even more potassium channel gates to open.
So potassium ions diffuse out, starting re-polarisation of axon.

. Hyperpolarisation then occurs as the potassium channels allow too many potassium ions out so there is a temporary overshoot in electrical gradient.

Then sodium potassium pumps cause sodium ions to leave and potassium ions to enter and eventually the resting potential is reestablished, so axon is repolarised.

Question 7

Difference between resting potential and action potential

Answer 7

Action potential is due to diffusion of sodium ions in and potassium ions out

Resting potential is maintained by active transport.