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How is a nerve impulse transmitted?

Class 11 BiologyNCERT chapter: Neural Control and Coordinationइसे हिंदी में पढ़ें

Exam-ready answer

At rest, the axon membrane is polarised: outside positive, inside negative. A stimulus makes the membrane freely permeable to Na⁺; Na⁺ rushes in and reverses the polarity, creating an action potential (nerve impulse). This depolarisation spreads to the next site along the axon. At a chemical synapse, neurotransmitters carry the impulse to the next neuron.

Explanation

In a resting neuron, the axonal membrane is more permeable to K⁺ ions and nearly impermeable to Na⁺ ions. The sodium-potassium pump actively transports 3 Na⁺ out for every 2 K⁺ brought in. As a result, the outer surface becomes positively charged and the inner surface negatively charged. The membrane is said to be polarised, and the potential difference across it is the resting potential.

When a stimulus is applied at a site A, the membrane there becomes freely permeable to Na⁺. A rapid influx of Na⁺ reverses the polarity: the outer surface turns negative and the inner surface positive. This is the action potential, or nerve impulse. Current then flows between A and the next site B, on the inner surface from A to B and on the outer surface from B to A, so B depolarises in turn.

The rise in Na⁺ permeability is short-lived. It is quickly followed by a rise in K⁺ permeability; K⁺ diffuses out and restores the resting potential (repolarisation), so the fibre is ready for further stimulation. At a chemical synapse, the impulse makes synaptic vesicles release neurotransmitters into the synaptic cleft. These bind receptors on the post-synaptic membrane, open ion channels and generate a new potential in the next neuron.

Points to remember

  1. Resting potential: membrane polarised, outside positive and inside negative; maintained by the Na⁺–K⁺ pump (3 Na⁺ out, 2 K⁺ in).
  2. Depolarisation: rapid Na⁺ influx reverses polarity; this is the action potential or nerve impulse.
  3. Conduction: local currents between the depolarised site and the next polarised site carry the impulse along the axon.
  4. Repolarisation: K⁺ moves out and restores the resting potential.
  5. Synapses are electrical (current flows directly, faster) or chemical (neurotransmitters cross the synaptic cleft).

Exam tip

Draw the axon with + and − charges at rest and at the stimulated site, label 'resting potential' and 'action potential', and use the terms depolarisation and repolarisation.

Questions students also ask

What is the difference between electrical and chemical synapses?
At an electrical synapse, the membranes of the two neurons are very close and current flows directly from one neuron to the next, so transmission is faster; such synapses are rare in our body. At a chemical synapse, a fluid-filled synaptic cleft separates the neurons, and neurotransmitters carry the impulse across.
Why does an impulse cross a chemical synapse in only one direction?
Neurotransmitters are stored in and released from synaptic vesicles only in the axon terminal of the pre-synaptic neuron, while their receptors lie on the post-synaptic membrane. So at a chemical synapse the signal can pass only from the pre-synaptic neuron to the post-synaptic neuron.
What is the role of the sodium-potassium pump?
The sodium-potassium pump uses energy to move 3 Na⁺ ions out of the axon for every 2 K⁺ ions moved in. This keeps Na⁺ high outside and K⁺ high inside, so the membrane stays polarised and keeps the resting potential needed to fire impulses.

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