15.2: The nerve Impulse

Question 1

What’s is a nerve impulse

Answer 1

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

Question 2

What is a nerve impulse a reversal of

Answer 2

The electrical potential difference across the axon membrane

Question 3

What two states is the temporary reversal of electrical potential difference between

Answer 3

The Resting Potential and the Action Potential

Question 4

What 3 ways is the movement of Na/K ions controlled

Answer 4

1. The phospholipid bilayer of the axon membrane prevents Na/K diffusing across it
2. Channel proteins span the bilayer, these have ion channels which ions can pass through. Some channels have gates which can be opened or closed so ions can pass through by facilitated diffusion. Some channels always remain open
3. Some carrier proteins actively transport K ions into the axon, and Na our of the axon. This is the sodium potassium pump

Question 5

What is the result of these controls

Answer 5

The inside of the axon becomes negatively charged relative to the outside

Question 6

What is the Resting Potential

Answer 6

Where the inside of the axon is negatively charged relative to the outside.

Question 7

What is the voltage at resting potential

Answer 7

65mV

Question 8

At 65mV what is the axon said to be

Answer 8

Polarised

Question 9

What is the sequence of events leading up to the establishment of a potential difference

Answer 9

1. Sodium ions are actively transported OUT of the axon by the Na/K pumps
2. Potassium ions are actively transported INTO the axon by the Na/K pumps
3. The active transport of Na is GREATER than that of K. Therefore there are more sodium ions in the axons tissue fluid than the cytoplasm, and vice versus for potassium. This forms a chemical gradient
4. The sodium ions begin to diffuse naturally into the axon while the potassium ions diffuse back out of the axon.

However, most Potassium gates are open while the sodium gates are closed

Question 10

What causes a temporary reversal of the changes either side of the axon membrane

Answer 10

When a stimulus of sufficient size is detected by a receptor in the nervous system, it’s energy causes a temporary reversal

Question 11

What happens to the voltage across the membrane

Answer 11

The negative charge of -65mV (inside the membrane) becomes 40mV

Question 12

What is the change in voltage known as

Answer 12

The action potential, the axon membrane is depolarised

Question 13

Why does depolarisation occur

Answer 13

Because the channels in the axon membrane change shape. Hence opening and closing depending on the membranes voltage

Question 14

Sequence of events leading up to an action potential

Answer 14

1. At resting potential, some K gates are open but the sodium channels are closed
2. The energy of the stimulus causes some Na channels to open and therefore Na ions diffuse into the axon along their electrochemical gradient. Because the Na are positivist charged. There is a reversal of potential difference across the membrane
3. As the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx of sodium ions by diffusion
4. Once the action potential of 40mV has been established, the voltage gates on sodium ion channels close (preventing further influx of sodium ions) and the voltage gates on potassium ion channels begin to open
5. With some potassium channels now open, the chemical gradient that was preventing outward movement of K ions is reversed causing more K channels to open. This causes more diffusion of K ions, starting repolarisation of the axon
6. The outward movement of these K ions causes a resting potential of -65mV to be re-established. The axon is now repolarised

Question 15

Action potential

Answer 15

Passive process - purely due to diffusion.

It means the axon is transmitting a nerve impulse

Question 16

Resting potential

Answer 16

Maintained by active transport - an active process.

It means the axon membrane is not transmitting a nerve impulse