There are more charged proteins and potassium ions (K+) inside the axon than outside and more sodium ions (Na+), chloride ions (Cl-) outside the axon than inside.
This imbalance produces an electrical gradient that is required to create a resting potential.
The membrane is selectively permeable to different ions and it allows sodium. Potassium and chloride ions to diffuse freely across it. Larger charged proteins are restricted to the inside of the membrane by their size. Potassium ions inside the axon outnumber the sodium ions 10:1 and sodium ions outside the axon membrane always outnumber potassium ions 10:1.
These ions need to be transported across their concentration gradients to maintain the ratio and the ion pump is called the sodium-potassium exchange pump does this.
It harnesses energy from ATP to move Na+ out and K+ in; it is a large protein molecule that forms a channel across the membrane. The membrane is 50 times more permeable to K+ than Na+.
3. An action potential is triggered when a stimulus reaches a neurone and a subtle electrical change takes place at the point of contact on the cell body. Small changes in potential spread to the axon hillock and they combine to exceed the threshold level and the membrane potential becomes positive.
The membrane becomes more permeable to sodium ions, some of the extracellular sodium ions enter the cytoplasm by diffusion more easily than before. The ions move down the concentration gradient and are attracted into the cell by the negative charge on the inside of the membrane. This entry of sodium ions disturbs the resting potential. When enough sodium ions have entered the cytoplasm to balance the positive charge outside, the entry of more Na+ ions causes the inside of the cell to become positive relative to the outside.
The membrane is now depolarised.
When enough Na+ ions enter the cell to change the transmembrane potential to a certain threshold level an action potential arises that generates an impulse.
At threshold Na+ gates (pores) open in the membrane and allow a sudden influx of Na+ ions into the cell.
When the action potential reaches its peak the Na+ gates close and no more enter.
If the depolarisation isn’t great enough to reach threshold then the action potential and subsequent impulse aren’t produced.
Rest neurone
Permeable to Na+ at site of stimulation
Na+ ions in membrane potential less negative
Threshold –50mv
Influx of Na+
Inside = +_ve +40mv
Electrical charge reversed
= Depolarisation
4. Electrical impulses are conducted by molecules hitting other molecules and are positively charged
Sodium potassium exchange pump causes the “Mexican wave”type response where once the threshold is reached in an axon it passes from one axon to the next.
5. All or nothing means that is the stimulus is stronger than that necessary to produce an impulse is applied then the frequency of impulse production usually increases. The nervous system discriminates against weak and strong stimuli.
Either the impulse is strong enough to cause the sodium potassium pump to work effectively or the resting potential is maintained.