PHYSIOLOGY OF NERVE (Generation and conditions of Nerve impulse)
Different stage of nerve conduction
(1) Polarisation (Resting stage)
(2) Depolarisation (Excited stage)
(3) Repolarisation (again resting stage)
1) POLARISATION
Neurons are excitable cells because their membranes are in a polarised state. Do you know why the membrane of a neuron is polarised? Different types of ion channels are present on the neural membrane. These ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e.. resting, the axonal membrane is comparatively more permeable to 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 Nat. In constrast, the fluid outside the axon contains a low concenration of K a high concentration of Na* and thus from a concentration gradient. These ionic gradients across the resting memberance are maintained by the activ transport of ions by the sodium-potassium pumb 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 as the resting potential and this is about -70 mV (the negative sign indicates that inside the cell is negative with respect to the outside). (Range -60 to -85 mV).
2) Depolarisation
Once the event of depolarization has occurred, a nerve impulse or spike is initiated. Action potential is another name of nerve impulse. This is generated by a change in the sodium ion channels. These channels, and some of the potassium ion channels, are known as voltage gated meaning they can opened or closed with change in voltage. In resting state these channels are closed due to binding of Ca
An action potential is generated by a sudden opening of the sodium gates. Opening of gates increases the permeability of the axon membrane to sodium ions which enter by diffusion.
This increases the number of positive ions inside the axon.
A change of +10mV in potential difference from RMP (Resting Membrane Potenial) through influx is sufficiently significant to trigger a rapid influx of Na* lons leading to generation of action potential. This change of +10 mV is called threshold stimulus
At the point where membrane (Axolemma) is completely depolarised due to rapid influx of Nations, the negative potential is first cancelled out and becomes 0 (Depolarisation). This axolemma is called as excited membrane or depolarised membrane. Due to further entry of Na, the membrane potential "over shoots" beyond the zero and becomes positive upto +30 to +45mV. This "over shoot" peak corresponds to maximum concentration of sodium inside the axon. This potential is called as action potential. In this state, the inner surface of axolemma becomes positively charged and outer surface becomes negatively charged.
3) REPOLARISATION
The rise in the stimulus-induced permeability to Na' is extremely short lived. It is quickly followed by a rise in permeability to K. Within a fraction of a second. K diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.
The repolarization period returns the cell to its resting potential (-70 mV). The neuron is now prepared to receive another stimulus and conduct it in the same manner.
Sodium pump starts working to maintain the normal resting membrane potential by expelling Na+ and intaking of K+
The time taken for restoration of resting potential is called refractory period, because during this period the membrane is incapable of receiving another impulse.
Nerve impulse travels as action potential which passes along axon as a wave of depolarization.
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