describe how the nerve impulse is propagated in a non mylinated fibre

Generation of Nerve Impulse

• Neurons are excitable cells because their membranes are polarised.

• Neural membranes are polarised because selectively permeable ion channels are present on them.

• At the resting state of a neuron:

  • Axonal membrane is more permeable to K⁺ and nearly impermeable to Na⁺.

  • It is impermeable to negatively charged proteins present inside the axoplasm. Thus, it does not let them go out.

  • As a result, the axoplasm has more concentration of K⁺ and negatively charged proteins, and less concentration of Na⁺. This creates a concentration gradient.

  • Gradient is maintained by the Sodium-Potassium Pump which pumps 3 Na⁺ out and 2 K⁺ in.

  • Hence, the outer surface of the membrane possesses positive charge and the inner surface possesses negative charge. This potential difference is called Resting Potential.

  • Membrane is, therefore, polarised.

• When stimulus is applied at a particular site (say site A), that site becomes freely permeable to the influx of Na⁺; hence, the polarity at that site is reversed.

• The membrane is now depolarised, and the potential difference across the site is called Action Potential (nerve impulse).

Conduction of Nerve Impulse

• At a site (B) that is adjacent to the depolarised site (A), the outer surface of the membrane has positive charge and the inner surface has negative charge.

• Depolarised site (A): Positive (in) and Negative (out)
  Adjacent site (B): Positive (out) and Negative (in)

• Current flow on the inner surface: Site A to B
  Current flow on the outer surface: Site B to A

• This completes the circuit of current flow. Polarity of site B is reversed and action potential is generated at site B.

• Hence, the impulse travels to site B, and we say that this impulse is conducted. This sequence is repeated along the length of the axon.

• Stimulus-induced permeability to Na⁺ is short-lived, and is quickly followed by permeability to K⁺. When this happens, the resting potential is restored, and once more, the site can be stimulated.