depolarisation/repolarisation?

Question

can anyone tell me about depolarisation and repolarisation of a neuron in short please?

Answer 1

To simplify, a cell has a resting potential due to the unequal separation of ion across its membrane (the cell is ionically neutral ie. equal number of positive and negative ions, but the relative concentrations result in a electrical potential). This is typically due to a relative abuntance of sodium (Na) and calcium (Ca) outside the cell and potassium (K) and sequestered Ca inside the cell. This potential is typically around -70mV
The exact details require learning the Nerst Equation.

Due to some event that occurs to the cell (a chemical signal is sent, an electrical discharge occurs, the cell suffers a mechanical deformation, etc) the potential rises (becomes less negative) in the cell due to localized movement of the ions across the membrane with Na or Ca entering and K leaving. When the potential rises above the threshold (typically about -15 mV above the resting potential) the process starts to snowball and other gates or channels to momentarily open at other areas of the cell allowing many more ions to pour through. This is depolarization and results in a sharp rise in the potential called an action potential or "spike".

After a brief interval, the gates/channels close and the cell tries to restore is resting potential. This is typically accomplished by the Na/K pump or Na/k ATPase which uses energy to pump Na out and K in. There are also Na/Ca pumps that pump Ca out and Na in (Note the net flow of Na is out since there are many more Na/K pumps). While the
cell tries to restore its potential or repolarize, it is insensitive to almost all stimulation (it is refractory to stimulation).

Also many neurons are coated with layers of lipids (they are myleniated). This insulation allows the action potential to skip to the next area that is not myleniated this is known as saltatory conduction. This speeds up the spread of depolarization and allows for a faster reaction time and also allows organisms to grow larger (since the nerves need to be longer) and more complex.

Answer 2

Neurones when stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. This is called depolarisation
The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential. This is called repolarisation.