what is a nerve impulse ? how it is conducted hrough a nerve fibre ? mention any 2 factors which can increase the velocity of a nerve fibre ?

The information that are transmitted along the neurons in the form of electric impulses is called as nerve impulse.

Conduction of Nerve Impulse

• At a site (B) that is adjacent to the depolarised site (A), the outer surface of the membrane has positive charge and the inner surface has negative charge.

• Depolarised site (A): Positive (in) and Negative (out)
  Adjacent site (B): Positive (out) and Negative (in)

• Current flow on the inner surface: Site A to B
  Current flow on the outer surface: Site B to A

• This completes the circuit of current flow. Polarity of site B is reversed and action potential is generated at site B.

• Hence, the impulse travels to site B, and we say that this impulse is conducted. This sequence is repeated along the length of the axon.

• Stimulus-induced permeability to Na⁺ is short-lived, and is quickly followed by permeability to K⁺. When this happens, the resting potential is restored, and once more, the site can be stimulated.
  

Third part of your question appears to be incorrect. The velocity of nerve fibre can not be increased. Velocity of nerve impulse is increased by myelin sheath which covers the nerve fibre.

Hope this helps!!