Transmission of nerve impulse

Propagation of impulse along the nerve fibre

The nerve tissue has two properties electrical excitability and conductivity.

Electrical excitability is the ability of nerve cells to generate an electrical impulse in response to a stimulus by altering the normal potential difference across  their  membrane.

Conductivity is the ability  of nerve cells to rapidly  transmit the  electrical impulse as a wave from the site of its origin  along their length in a particular direction.

Impulse propagation  is  established through the  maintenance of resting potential and the action potential.

Membrane or ionic theory of Nerve impulse propagation

This theory was put forwarded by English Neurophysiologists Hodkin and Huxley in the late 1930 s. It states that the electrical events in the nerve  fibres  are governed by the differential permeability of its membrane to sodium and potassium ions and that the permeability are regulated by the  electric field across the  membrane. The interaction of these two factors ,the differential permeability and electric field, makes a critical threshold of charge necessary to excite the  fibre. The resting nerve  fibre  is a long  tube, the plasma membrane of which separates two solutions (ECF and ICF) different chemical composition but having approximately the same total number of  ions. In the external medium( tissue fluid ) sodium ions(Na⁺) and chloride ions  (Cl^-)   predominate, where as within  the fibre  (Intracellular fluid)  potassium ions (K⁺) and large negatively charged  organic (protein) ions  predominates. Due to  the differential  concentration  of  ions on the two sides of the membrane (ECF and ICF), sodium ions tends to passively diffuse  into the nerve fibre (ICF) and potassium ions tend to diffuse out of the nerve fibre (ECF)  down their electro chemical gradients. The membrane of a resting  nerve  fibre  is  however more permeable to potassium ions than to sodium.  Because   of   this selective   permeability  of the membrane potassium leaves the nerve  fibre faster than the sodium enters it. The differential flow of the positively charged ions and the inability of the negatively charged organic (proteins) ions   within  the nerve fibre (ICF)  to pass out cause an increase in positive charge  on the outside of the  membrane. This makes the membrane of the   resting  fibre polarized, its outside (ECF) being electropositive (positively charged)  with respect to the  inside (ICF). The   distribution  of  chloride  ions is ignored  as it plays no significant  role in the resting potential.

Increasing negative charge inside the nerve fibre slows down the exit of potassium ions. Ionic concentration would eventually  change  and reach a new equilibrium if the leakage of ions is not disturbed. The steady state of a resting  nerve  fibre  ie   the normal difference in the ionic concentrations  and electrical potential between its outside (ECF) and inside(ICF)  is maintained by the active transport of  sodium and potassium ions against the concentration and electrochemical gradients.  Thus  sodium  ions are pumped out (ICF to ECF)  and  the potassium  ions are forced into the (ECF to ICF) of the nerve  fibre. The process of expelling out sodium ions   and  drawing in potassium ions  against concentration and electrochemical gradient is termed Sodium pump or better Sodium-Potassium-exchange Pump.The active transport of sodium  and potassium ions across the membrane  requires energy (ATP),which is provided by metabolic processes within the nerve fibre itself with the  help of an enzyme (Sodium - Potassium dependent adenosine tri-  phosphatase )  present in the  nerve membrane. The differential distribution of  ions  on the two sides of the membrane (ECF and  ICF)  produces  a potential difference of ^_60 to  ^_90  millivolts  across the membrane. This difference is the  Resting membrane potential.

Depolarisation

The origin and maintenance of the resting potential depends mainly on the  properties of the intact living  membrane (Plasma membrane). Any disturbance caused to the integrity  (original perfect state) of the membrane by a stimulus results in leakage of sodium ions down an electrochemical gradient into the nerve fibre (ECF to ICF)  at the point of stimulation  and results in  lowering of the potential  difference. As  the  trans -membrane potential  decreases, the membrane become more permeable  to sodium ions than to potassium  ions. It is this property  of  the  nerve membrane that distinguishes it from the ordinary membranes.

Entry of sodium ions leads to depolarization (reversal of polarity ) of nerve  membrane ,so that the inside become electropositive with respect to the  outside. The reversed polarity of an excitatory neuron is sudden and momentary, it lasts for less than 1/1000 of a second.

When  a nerve  fibre  receives  electrical, chemical or mechanical  stimulus ,the potential across its  membrane is momentarily reversed at the point of  excitation. In other words  the inside  of the membrane at the excited site  becomes positively charged with respect to the  outside. This  change in polarity of a membrane is known as Action potential. The membrane with reversed polarity is said to be  Depolarized. The reversed  polarity  (depolarization ) then quickly passes a wave along  the nerve  fibre  .A  wave  of reversed polarity  or depolarization (action potential) moving down an axon is called  a nerve impulse.

Features of Action potential

There are three  important  features  of  an  action  potential : threshold  stimulus, summation, all or none  principle.

1. Threshold stimulus: A stimulus must be of a certain minimum    intensity     in  order  to  produce  an  action  potential. This minimum intensity is called the threshold stimulus.

2. Summation:  A  stimulus  unable  to   generate a nerve impulse is known as sub threshold stimulus. A series of threshold stimuli applied  to  nerve  fibre  in  rapid succession may set up an  impulse. This additive effect  of  several sub threshold stimuli  in the generation of a nerve impulse is called summation.

3.All or None principle: The stimuli with strength below the threshold do not cause any impulse  whereas all stimuli having intensity above the  threshold induce the same action  potential.  Thus   the nerve fibre  function by the  all or none principle. The   size  of the action potential is the same ,no matter how strong the stimulus might become.

Re -polarisation

Now the permeability of membrane to the sodium ions drops rapidly,  there are   as many sodium ions  inside  of  the  membrane (ICF) than  on the  outside (ECF).  At the same time the membrane becomes more permeable to potassium ions than to sodium ions. As a result of this potassium ions pass out much  faster  than sodium ions enter, and this  way  the membrane soon return to its normal condition  ie , internal negative  charge (negative charge in ICF). The return of a nerve fibre to the  internal negative charge  is called the Re -polarisation  of the  membrane.

The above description shows  that  the resting membrane potential is determined largely by K⁺ ions and the action potential is determined largely by Na⁺ ions.

The  sodium  pump starts working once  again, transferring sodium ions out of the fibre  and potassium ions back in.

Saltatory conduction

In vertebrates for the quick conduction of  impulse  there is a mechanism known as the  saltatory conduction. They have myelinated  nerve fibre which carry impulse about 20 times faster than that of the non-myelinated nerve fibre.

In myelinated nerve the fatty myelin sheath acts as a highly insulated covering  and that prevents the flow of ions between the fluid  external to the sheath (ECF) and fluid within the axon (ICF).The ionic flow and reversed polarity(depolarization) occur only at the nodes of Raniver where the insulating sheath is absent.Because the action potential jumps from node to node transmission of impulse is more rapid in myelinated nerve fibres.This is called saltatoty conduction of nerve impulse.Myelinated nerve fibres require less energy for action because only the nodes are depolarized and fewer ions are required to be pumped back into position by the energy requiring active transport mechanism. Myelination has increased impulse velocity many fold even in thin fibres of the vertebrates.