Mechanism of conduction of nerve impulse

Most accepted mechanism of nerve impulse conduction is ionic theory proposed by Hodgkin and Huxley. This theory states that nerve impulse is an electro-chemical even governed by differential permeability of neurilemma to Na+ and K+ which in turn is regulated by the electric field.
Transmission of nerve impulse along the nerve fibre
  Polarization (Resting membrane potential-RMP):
In a resting nerve fibre (a nerve fibre that is not conducting art impulse), sodium ions Na+ and Cl- predominate in the extracellular fluid, whereas potassium ions ( K+) predominate in the intracellular fluid (within the fibre). •

 Intracellular fluid also contains large number of negatively charged (anions) protein molecules.
 Na+ are 10 times more outside the neuron and K+ ions are 25 times more inside the cell.
Thus it makes a considerable difference between the ion concentration outside and inside the plasma membrane.
It also causes differerce in electrical charges on either side of the membrane. The plasma membrane is electrically positive outside and negative inside.
This difference  is called potential difference. The potential difference across the plasma membrane is knowm as resting potential.
This potential averages -70 MU (- 40 to — 90 my) in inner side of membrane in respect to outer side_ •
A higher concentration of cations outside the membrane compared to the concentration of cations inside it. This state of the resting  membrane is called polarised state and makes its inner side electronegative to its outside.

(b) Depolarization (Action membrane potential or AMP) :
When the nerve fibre is stimulated mechanically, electrically, thermally or chemically a disturbance felt at the point of stimulation which gives rise to a local excitatory state.
The membrane becomes permeable to sodium ions. The membrane is negatively charged on the outside and positively charged on the inside.
 The membrane with reversed polarity is said to be depolarized.  This wave of depolarization travelling down a nerve fibre is called action potential. Infact, the action potential "moves" in the manner of a spark moving along a fuse.
 This "moving" action potential constitutes the nerve impulse. The action potential (impulse) is the basic means of communication within the nervous system.
• The action potential of + 45  mv on  inner side of axolemma in respect to its outer side is also called spike potential
(c) Repolarization
With the increase of sodium ions inside the nerve cell, the mebrane becomes less permeable to sodium ions whereas the permeability membrane to potassium ions increases.
 The sodium ions are pumped out of the cell and potassium ions are pumped into the cell until the original resting state of ionic concentration is achieved.
 Thus this makes the membrane negative on inside and positive on outside.  This process is called repolarization.
The last movement of ions is thought to take Ware by an active transport mechanism called sodium potassium pump (also called sodium potassium exchange pump or sodium pump).
The sodium-potassium pump is a process of expelling out sodium ions and drawing in potassium ions against concentration and electrochemical gradient.
The entire process of repolarization requires some time during which the nerve cannot be stimulated again
This period is called refractory period. During repolarization, as the cell returns to its resting potential, the neuron is ready to receive another stimulus