NERVE IMPULSE | NEET | BIOLOGY

NERVE IMPULSE

Nerve impulse is a wave of bioelectric /electrochemical disturbance that passes along a neuron during conduction of an excitation. The nerve impulse travels along a neuron or across synapse between one neuron and another, or between neuron and an effector.

Nature of nerve impulse or conduction of nerve impulse is an electrochemical process. 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.

Generation and Conduction of Nerve Impulse

When a neuron is not conducting any impulse, ie , resting the axonal membrane is comparatively more permeable potassium ions ( K " ) and nearly impermeable to sodium ions ( Na + ) . Similarly , the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently , the axoplasm inside the axon contains high concentration of K + and negatively charged proteins and low concentration of Na * . In contrast , the fluid outside the axon contains a low concentration of K + , a high concentration of Na + and thus forming a concentration gradient .

The ionic gradients are maintained by sodium - potassium pump which transports 3 Na + outwards for 2 K + into the cell . As a result the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore , is polarised . The electrical potential difference across the resting plasma membrane is called the resting potential . The state of the resting membrane is called polarised state .

When a stimulus is applied , the permeability of the membrane to Nations is greatly increased at the point of stimulation.

It is due to the fact that the Na + channels open and the K * channels remain closed . Thus , sodium channels permit the influx of Nations by diffusion . This results in the positive charge inside and negative charge outside . The change in polarity across the plasma membrane is known as action potential ( nerve impulse ) and the membrane is said to be depolarised .

With the increase of Nations inside the nerve fiber, the membrane becomes less permeable to Nations and more permeable to K + ions.  Na + influx stops and K + outflow begins until the original resting state of ionic concentration is achieved.  Thus, resting potential is restored which is called repolarisation of the membrane. 

When an impulse travels along a myelinated neuron depolarisation occur only at nodes.  The action potential jumps from node to node and impulse transmission occurs along the myelinated axon faster than the series of smaller local currents in a non-myelinated axon.  This type of conduction is called saltatory conduction.

Manish Mevada

M.Sc,M.Phil,B.Ed