It has been found that impulse conduction depends upon permeability of axon membrane (axolemma) and osmotic equilibrium and electrical equivalence between the axoplasm and extracellular fluid (ECF) present outside the axon.
According to membrane potential theory, conduction of nerve impulse is divisible into two main phases:
(i) Resting membrane potential
(ii) Action membrane potential.
In the resting nerve fibre, the cytoplasm just beneath its membrane is electronegative relative to the layer of extracellular fluid (ECF) just outside the membrane.
If the two sides of the membrane are connected by galvanometer the inner side is seen to possess a negative potential of about 70 mV relative to the outer side.
This is called the resting membrane potential. This results is due to the following facts –
(i) The resting membrane has only a poor permeability for Na+ although it has a higher permeability for K+.
(ii) The membrane carries a sodium pump. The concentration of sodium ions will be about 14 times more in extracellular fluid (outside) and concentration of potassium ions will be about 28 – 30 times more in axoplasm (inside). This actively carries three Na+ ions from the cell to the exterior and in exchange transfers two K+ from the ECF to the cell interior. These two factors, coupled together, result in higher concentration of cations just outside membrane compared to the concentration of cations just inside it.
This state of the resting potential membrane is called polarized state and makes its inner side electronegative to its outer side.
The differential flow of the positively charged ions and the fact that the negatively charged organic ions within the nerve fibre cannot pass out, results in increasing positive charge on the outside of the membrane and negative charge on the inside of the membrane.
This makes the membrane of the resting fibre polarized, its outside being positively charged with respect to the inside.
When the nerve fibre is effectively stimulated, its resting membrane potential undergoes a change.
The inner side of the membrane now becomes electropositive to its outside. This potential change, called action potential is propagated along the membrane of the nerve fibre as the nerve impulse. Its value is 20-30 mV (+), occasionally + 60 mV.
Stimulation of the fibre immediately enhances manifold its membrane permeability to the Na+. So Na+ ions diffuse across the membrane from the ECF where their concentration is higher, to the interior of the fibre where the concentration is much lower.
But the membrane permeabilities to K+ starts rising somewhat later only, so there is no simultaneous rise in the outward diffusion of K+ from the cell interior having a higher K+ concentration to the exterior with a lower K+ concentration.
These effects lower the overall cation concentration outside and enhance that inside the membrane. The membrane is thus depolarised with its interior becoming electropositive.
With the increase of the positive charge inside, further entry of Na+ is prevented and permeability of membrane decreases and Na+ ions are pushed out with the establishment of sodium pump.
The inside of the membrane becomes negative and outside becomes positive and the membrane restores the original resting potential. This is known as repolarization.
When the action potential jumps from node to node and passes along the myelinated axon faster than the series of smaller local currents in a non-myelinated axon. Then this type of conduction is called salutatory conduction.
This occurs because the voltage gated sodium ion channels are found only at the nodes of Ranvier and between the nodes the myelin sheath acts as a good electrical insulation.
It leads to conduction speed upto 120 m/s and is responsible for the high speed impulse transmission.