Nerve impulses travel through the body because of neurons.
In a neuron, stimuli always propagate in the same direction: they are received by the dendrites, go through the cell body, travel through the axon and, from the end of it, are passed to the next cell (dendrite - cell body - axon).
The nerve impulse that propagates through the neuron is of electrical origin and results from changes in the electrical charges on the outer and inner surfaces of the cell membrane.
The membrane of a resting neuron has a positive electrical charge on the outside (facing outside the cell) and a negative charge on the inside (in contact with the cell's cytoplasm). When that membrane is in such a situation, it is said to be polarized. This difference in electrical charges is maintained by the sodium and potassium pump. Thus separated, the electrical charges establish a potential electrical energy across the membrane: the membrane potential or resting potential (difference between the electrical charges across the membrane).
When a chemical, mechanical or electrical stimulus arrives at the neuron, there may be a change in the permeability of the membrane, allowing large sodium to enter the cell and small potassium to leave it. With this, there is an inversion of the charges around this membrane, which is depolarized generating an action potential. This depolarization spreads through the neuron, characterizing the nervous impulse.
Immediately after the impulse passes, the membrane undergoes repolarization, recovering its resting state, and the transmission of the impulse ceases.
The stimulus that generates the nervous impulse must be strong enough, above a certain critical value, which varies between different types of neurons, to induce depolarization that transforms the resting potential into an action potential. This is the threshold stimulus. Below this value, the stimulus only causes local changes in the membrane, which soon cease and do not trigger the nervous impulse.
Any stimulus above the threshold generates the same action potential that is transmitted along the neuron. Thus, there is no variation in the intensity of a nervous impulse due to the increase in the stimulus; the neuron obeys the "all or nothing" rule.
Thus, the intensity of the sensations will depend on the number of depolarized neurons and the frequency of impulses. Imagine a burn on your finger. The larger the burned area, the greater the pain, as more receptors will be stimulated and more neurons will be depolarized.
