The Nerve Impulse

Question 1

How is the movement of ions such as sodium ions and potassium ions, across the axon membrane is controlled ?

Answer 1

The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it

Channel proteins, span this phospholipid bilayer. They have ion channels which pass through them. Some of these channels have gates, which can be opened or closed so that sodium or potassium ions can move through then by facilitated diffusion at any one time

Some carrier proteins actively transport potassium ions into the axon and sodium ions our of the axon. This mechanism can be called a sodium potassium pump

Question 2

What is the resting potential in humans, what is the axon said to be in this condition

Answer 2

-65mV

Polarised

Question 3

What is the establishment of the potential difference (the difference in charge between the inside and outside of the axon) due to?

Answer 3

Sodium ions me are actively transported out of the axon by sodium potassium pumps

Potassium ions are actively transported into the axon by sodium potassium pumps

The active transport of sodium ions is greater than that of potassium ions, so 3 sodium ions move out for every 2 potassium ions that move in

The outward movement of sodium ions is greater than the inward movement of potassium ions. Therefore there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm and more potassium ions in the cytoplasm than in the tissue fluid this creating an electrochemical gradient

The sodium ions begin to diffuse back naturally into the axon which the potassium ions begin to diffuse back out of the axon

However, most of the gates in the channels that allow the potassium ions to move through are open, while most of the gates in the channels that allow the sodium ions to move through are closed

Question 4

Why in the neurones resting state is the outside of the membrane positively charged compared to the inside

Answer 4

Because there are more positive ions outside the cell then inside

Question 5

What is the action potential

Answer 5

When a stimulus of sufficient size is detected by a receptor in the nervous system, it’s energy causes a temporary reversal of the chargers either side of this part of the axon membrane

If the stimulus is greater enough, the negative charge of -65mV inside the membrane becomes a positive charge of around 40mV.

Question 6

What is the axon said to be during the action potential

Answer 6

Depolarised

Question 7

Describe the action potential

Answer 7

1. At resting potential (polarised) some potassium voltage gates channels are open but the sodium voltage gates channels are closed
2. The energy of the stimulus caused sodium voltage gates channels in the axon membrane to open and therefore sodium ions diffuse into the axon membrane through these channels along their electrochemical gradient. Being positively charged, they trigger a reversal in the potential difference across the membrane
3. As the sodium ions diffuse into the axon, so more sodium channels open, causing an even greater influx of sodium ions by diffusion
4. Once the action potential of around 40mV has been established, the voltage gates in the sodium ion channels close and the voltage gates on the potassium ion channels begin to open
5. With some potassium voltage gates channels now open, the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open. This means that yet more potassium ions diffuse out, starting repolarisation of the axon
6. The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual ( hyperpolarisation). The closable gates on the potassium ion channels now close and the activities of the sodium potassium pumps once again cause sodium ions to be pumped out and potassium ions pumped in. The resting potential of -65mV is re-established and the axon is said to be repolarised

Question 8

What’s the difference between action and resting potential

Answer 8

Action potential means that the axon membrane is transmitting a nerve impulse, whereas the resting potential is not