● `color{violet}("Neurons")` are excitable cells because their membranes are in a `color{violet}("polarized state.")`

● Different types of `color{violet}("ion channels")` are present on the `color{violet}("neural membrane.")`

● These `color{violet}("ion channels")` are selectively permeable to `color{violet}("different ions.")`

● When a `color{violet}("neuron")` is not conducting `color{violet}("any impulse,")` i.e., resting, the `color{violet}("axonal membrane")` is comparatively more permeable to `color{violet}("potassium ions" (K^+))` and nearly impermeable to `color{violet}("sodium ions" (Na^+))`.

● Similarly, the membrane is impermeable to `color{violet}("negatively charged proteins")` present in the `color{violet}("axoplasm.")`

● Consequently, the `color{brown}("axoplasm")` inside the `color{violet}("axon")` contains high concentration of `color{violet}(K^+)` and `color{violet}("negatively charged proteins")` and low concentration of `color{violet}(Na^+)`.

● In contrast, the `color{violet}("fluid outside the axon")` contains a low concentration of `color{violet}(K^+)`, a high concentration of `color{violet}(Na^+)` and thus form a concentration gradient.

● These `color{violet}("ionic gradients")` across the resting membrane are maintained by the `color{violet}("active transport of ions")` by the `color{brown}("sodium-potassium pump")` which transports `color{violet}(3 Na^+ "outwards for" 2 K^+ "into the cell.")`

● As a result, the outer surface of the `color{violet}("axonal membrane possesses")` a positive charge while its inner surface
becomes `color{violet}("negatively charged")` and therefore is `color{violet}("polarised.")`

● The `color{violet}("electrical potential difference")` across the resting `color{violet}("plasma membrane")` is called as the `color{brown}("resting potential.")`

● When a stimulus is applied at a site on the `color{violet}("polarised membrane")` , the membrane at the site A becomes freely `color{brown}("permeable to" Na^+)` .

● This leads to a `color{violet}("rapid influx")` of`color{violet}( Na^+)` followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes `color{violet}("negatively charged")` and the `color{violet}("inner side")` becomes `color{violet}("positively charged.")`

● The `color{violet}("polarity of the membrane")` at the site A is thus reversed and hence `color{brown}("depolarised. ")`

● The `color{violet}("electrical potential difference")` across the `color{violet}("plasma membrane")` at the site A is called the `color{brown}("action potential")`, which is in fact termed as a `color{brown}("nerve impulse.")`

● At sites immediately ahead, the `color{violet}("axon")` (e.g., site B) membrane has a `color{violet}("positive charge")` on the outer surface and a `color{violet}("negative charge")` on its`color{violet}(" inner surface.")`

● As a result, a `color{violet}("current flows")` on the `color{violet}("inner surface")` from site A to site B

● On the `color{violet}("outer surface")` current flows from site B to site A to complete the `color{violet}("circuit of current flow")`.

● Hence, the `color{violet}("polarity")` at the site is reversed, and an `color{violet}("action potential")` is generated at site B.

● Thus, the `color{violet}("impulse (action potential)")` generated at site A arrives at site B.

● The sequence is repeated along the length of the `color{violet}("axon")` and consequently the `color{violet}("impulse")` is conducted.

● The rise in the stimulus-induced `color{violet}("permeability to" Na^+)` is extremely `color{brown}("short lived.")`

● It is quickly followed by a `color{brown}("rise in permeability to" K^+)`.

● Within a `color{violet}("fraction of a second," K^+)` diffuses outside the membrane and restores the `color{violet}("resting potential")` of the membrane at the site of excitation and the `color{violet}("fibre")` becomes once more `color{brown}("responsive to further stimulation.")`