ELECTRIC SIGNALS IN NEURONS

        The transfer of information from receptors to brain or spinal cord and the appropriate response or command, to effectors is mediated by nerve impluses. Therefore, the generation and conduction of nerve impulses is essential aspect of nerve physiology. the mechanism involves the following terms:

(i) Resting membrane potential: It is the membrane potential of a nerve in the resting state, i.e., when a nerve is not stimulated or excited. It results from

*Unequal distribution of ions: The sodium and potassium ions are distributed unequally across the plasma membrane of a nerve cell.

Extracellular Na+ Na+ Na+

Na+ (High)  plasma membrane

K+ K+ +

K+ (high)  Instracelluar fluid

* Relative permeability of the plasma membrane: The plasma membrane shows differential permeability to sodium and potassium ions. It is more permeable to K+.

* Electrogenic pumps: They expel three sodium ions from the cytosol for every two potassium ions imported, and plus help in maintaining the resting membrane potential

. 

          These three factors create a difference in electrical charges on either side of the membrane. It is positive outside and negative inside creating a potential difference across it. this is known as resting membrane potential and it is about -70mv (millivolts) for nervous tissue. Such a membrane is said to be polarised.

(i) Na+ Ecf

+ + + + + + 

(ii) K+ ICF

(ii) Action Potential: when the nerve fibre is simulated. the membrane potential. It is polarity . It is known as action as potential. It consists of two phases: Depolarization and Repolarisation.

(a) Depolarisation: Depolarisation is brought about by the opening of sodium ion channels.

+ + + Na+

- - -

This causes reversal of the charge distribution or potential difference, which charges from negative potential (-70m V) to positive potential (+30m V)inside the nerve cell. the potential reaches a peak called "spike potential" or action potential.

- - - -         depolarisation

+ + + + 

(b) Reproduction: When the stimulus is over, the membrane of the  nerve fibre returns back to its original state and this phase is called as repolarisation. It's brought about by closure of sodium channels and opening of potassium channels.

- - - K+               + + + 

                                 repolarisation

+ + +               >  - - -