Excitation & Conduction of nerve impulse

What is a nerve impulse and how is it generated & propagated? Why the membrane of a neuron is polarised?

Neurons are excitable cells surrounded by extracellular fluid (ECF). Their membrane is selectively permeable to different ions. In this resting State i.e. when the neuron is not conducting any impulse, it is in a polarised state. At this time the interior of axon is more electronegative (due to less cations) as compared to ECF which is electropositive (due to more cations). The major cations are potassium ions(K+) & sodium ions (Na+).

On application of a stimulus the membrane becomes freely permeable to cause the rapid influx of the Na+. This action reverses the polarity at that site i.e. the inner side becomes positively charged. The change of polarity results in potential difference at the site and the axonal end. As a result local circuit is formed and current flows from dendron to axon. In the same manner sequential local circuits are formed along the length of a nerve fiber. The stimulus induced permeability to Na+ is short lived. Once the impulse is conducted the membrane is depolarized and the fibre becomes responsive to further stimulation once again. Then the impulse is transmitted to next Neuron.