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During the transmission of nerve impulse through a nerve fibre, the potential on the inner side of the plasma membrane has which type of electric change?
A
first positive, then negative and continue to be positive
B
first negative, then positive and continue to be positive
C
first positive, then negative and again back to positive
D
first negative, then positive and again back to negative
Detailed Solution
In the resting (polarised) state the inner side of the plasma membrane of the nerve fibre is negatively charged with respect to the outside; the resting potential is about -70 mV.
When a stimulus is applied, the membrane at that site becomes freely permeable to $Na^+$, and $Na^+$ ions rush in.
The polarity is reversed: the inner side becomes positive (about +30 mV). This is depolarisation, and the potential difference is the action potential, which lasts about 1 millisecond.
Immediately afterwards the permeability to $K^+$ increases and $K^+$ ions diffuse out, so the inner side becomes negative again. This is repolarisation; the sodium-potassium pump then restores the original ionic distribution.
The same sequence is repeated at each successive point as the impulse travels along the fibre.
So the potential on the inner side is first negative, then positive and again back to negative.
When a stimulus is applied, the membrane at that site becomes freely permeable to $Na^+$, and $Na^+$ ions rush in.
The polarity is reversed: the inner side becomes positive (about +30 mV). This is depolarisation, and the potential difference is the action potential, which lasts about 1 millisecond.
Immediately afterwards the permeability to $K^+$ increases and $K^+$ ions diffuse out, so the inner side becomes negative again. This is repolarisation; the sodium-potassium pump then restores the original ionic distribution.
The same sequence is repeated at each successive point as the impulse travels along the fibre.
So the potential on the inner side is first negative, then positive and again back to negative.