Neurons has many characters –
(i) Irretability and Conductivity:-
(ii) Stimulus:- It is change in external or internal environment which excite any nerve or muscle or complete organism is called stimulus.
– Threshold stimulus (Adequate stimulus) : It is able to excite tissue or muscle.
– Subthreshold stimulus (Inadequate stimulus) :
(iii) Nerve impulse:- All the physical and chemical changes occur in a neuron in response to stimulus is called nerve impulse.
– The conduction of nerve impulse occur only in single direction if the impulse is generated in the middle of axon then it is transmitted in both side.
(iv) All or none:-
(v) Refractory period:-
Mechanism of Conduction of Nerve Impulse:-
– The axoplasm is rich in K+ and PO4–3 ions (The Axoplasm has 40 times more K+ than ECF).
While the ECF (tissue fluid) is rich in Na+ and Cl– (The ECF has 10 to 12 times more Na+ than axoplasm).
– In normal stage (resting stage) the permeability of axolemma is very less for Na+ and high for K+ so K+ diffuse out in ECF. Thus in resting stage cation increases in ECF while anion increases in axoplasm.
– This distribution of ions on axolemma generates a potential difference of –70 mV which is called resting membrane potential and this stage of axon is called polarisation stage.
– This resting membrane potential is maintained by active transport and passive diffusion. For this sodium potassium pump become active which transport ions against electro-chemical gradient. During this 1ATP molecule transport 3 Na+ from axoplasm to ECF and 2 K+ from ECF to axoplasm
– At the some time some inorganic ion also transport through axolemma by passive diffusion
Depolarization or action membrane potential/Generation of Nerve Impulse –
– According to Hozkins and Huxley the stimulus increases permeability of axolemma for Na+ and decreases permeability for K+. So the outflux of K+ stops and influx of Na+ starts. Thus the cation increases in axoplasm and anion increases in ECF. It is called Depolarization
– If the stimulus is threshold i.e. it is able to decrease the potential difference from –70 mV to –60 mV then all the protein channels for Na+ opens and depolarisation occur more rapidly. So potential difffrence become +35 mV which is called action Membrane potential. This stage represents nerve impulse.
– In axon this action membrane potential remains for 1 milli second only. Now the sodium channels closes and potassium channel open which show repolarisaiton.
– During this all potassium channel open simultaneously so potassium outflux occur more rapidly and +35 mV reaches to –85 or –90 mV or – 95 mV. It is the stage of Hyperpolarisation
– Now the resting membrane potential is establishes which takes 1 or 2 second. During this new impulse is not generated at this point. It is called Refactory period. It is useful for the transmission of impulse only in one direction i.e. cyton to axon.
Refractory period µ 1/Diamater of neuron
Note– The conduction of nerve impulse is electro-chemical process i.e. the cation and anion of two adjacent part in an axon attract each other which establishes local electrical circuits in two parts
of axon. So this new part of axon stimulated to generate action
membrane potential. This process is continued up to axon ending.
Note – According to Luckas the conduction of nerve impulse follows the phenomenon of all or none.
Note– The rate of impulse conduction depends on diameter of axon. It is higher in thick axon.
Note– The rate of impulse transmission in mammals is 100 -130 meter/sec.
Note– Velocity of nerve impulse µ diamater of neuron
Saltatory Impulse Conduction
– Myelinated nerve fibres has a myelin sheath which is electrical insulating but it is absent in node of ranvier.
– So node of ranvier is not electrical insulated thus in myelinated fibres impulse is generated only on nodes which is called saltatory impulse conduction.
– It increases speed of impulse conduction by 50 times than non myelinated fibres.