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A Handbook of Biology

GËÑËRÅTÏØÑ & ÇØÑDÜÇTÏØÑ ØF ÑËRVË ÏMPÜLSËS

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Impulse transmission is electrochemical. It has 3 steps:

1. MÅÏÑTËÑÅÑÇË ØF RËSTÏÑG MËMBRÅÑË PØTËÑTÏÅL

6

The electrical potential difference across the resting plasma

membrane is called as the resting potential (-70mV).

5

The ionic gradients are maintained by the active transport of ions

by the Na-K⁺ pump. It transports 3Na⁺ outwards for 2K⁺ into the

cell. As a result, the outer surface becomes positively charged and

inner surface becomes negatively charged (i.e., polarized).

4

The fluid outside the axon contains low concentration of K⁺ and

high concentration of Na⁺. This forms an ionic concentration

gradient across resting membrane.

3

Therefore, concentration of K⁺ and negatively charged proteins in

axoplasm is high and concentration of Na⁺ is low.

2

In a resting neuron (neuron not conducting impulse), the axonal

membrane is more permeable to K⁺ ions and nearly impermeable

to Na⁺ ions. Also, the membrane is impermeable to negatively

charged proteins in axoplasm. (AIPMT 2011)

1

Neural membrane contains various selectively permeable ion

channels.

2. ÅÇTÏØÑ PØTËÑTÏÅL

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When a stimulus is applied, the membrane at the site A becomes

permeable to Na⁺. This causes rapid influx of Na⁺ and reversal of the

polarity at that site (outer negative and inner positive). It is called

depolarization.

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The electrical potential difference during depolarisation across the

plasma membrane is called action potential (a nerve impulse).