15.2 - The Nerve Impulse

Question 1

What is resting potential

Answer 1

• all membranes are either polarised or exhibit a RESTING POTENTIAL
• this means that there is an unequal distribution of ions on the 2 sides of the nerve cell membrane
• inside the nerve membrane, this is -70 mV (this is relative to the outside)

Question 2

Why is resting potential called ‘resting’ potential

Answer 2

Because it occurs when a membrane is not being stimulated or conducting impulses

Question 3

How is resting potential achieved

Answer 3

• Na+K+ATPase uses energy from ATP to simultaneously actively transport 3 Na+ ions out of the cell, and 2 K+ ions in
• There are Na+ and K+ channels that are closed, but will allow a leak. Therefore allowing a small amount of Na+ ions to return and K+ ions to leave down their respective concentration gradients.
• The reasoning for this is that it means that the Na+k+ATPase pump will therefore be able to keep pumping ions, maintaining the imbalance
• This stable imbalance of Na+ and K+ ions across the membrane causes a potential difference across the membrane called the resting potential difference

Question 4

What is polarisation

Answer 4

Resting potential

Question 5

What is depolarisation

Answer 5

When action potential is created, making -70 mV become +40 mV

Question 6

Describe how the membrane becomes de-polarised (action potential is created)

Answer 6

1) The energy of the stimulus causes a local difference in voltage brought about by receptor cells
2) causing sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient.
3) As the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx of sodium ions by diffusion (positive feedback)
4) Being positively charged, they trigger a reversal in the potential difference across the membrane.
5) Once the action potential of around +40 mV has been established, the voltage gates on the sodium ion channels close (thus preventing further influx of sodium ions) and the voltage gates on the potassium ion channels begin to open.

Question 7

Describe how the membrane become re-polarised (resting potential is restored)

Answer 7

1) the voltage gates on the potassium ion channels begin to open after a delay
2) therefore the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open.
3) This means that yet more potassium ions diffuse out, starting repolarisation of the axon.
4) The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative (relative to the outside) than usual (= hyperpolarisation)
5) 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 in.
6) The resting potential of -70 mV is re-established and the axon is said to be repolarised.