In a neuron, the stimuli always propagate in the same direction: they are received by the dendrites, travel through the cell body, travel the axon and, from its end, are passed to the next cell (dendrite - cell body - axon).
The nerve impulse that travels 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 is positively charged on the outside (facing out of the cell) and negative on the inside (in contact with the cytoplasm of the cell). When this 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, electric charges establish a potential electrical energy across the membrane: membrane potential or resting potential (difference between electrical charges across the membrane).
When a chemical, mechanical, or electrical stimulus arrives at the neuron, membrane membrane permeability may change, allowing large sodium input to the cell and small potassium output from it. Thus, there is an inversion of the charges around this membrane, which is depolarized generating an action potential. This depolarization propagates through the neuron characterizing the nerve impulse.
Immediately after the impulse passes, the membrane repolarizes, recovering its resting state, and the impulse transmission ceases.
The stimulus that generates the nerve impulse must be strong enough above a certain critical value, which varies between different types of neurons, to induce depolarization that turns resting potential into 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 nerve impulse.
Any stimulus above the threshold generates the same action potential that is transmitted along the neuron. Thus, there is no variation in intensity of a nerve impulse as a function of increased 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 the finger. The larger the burned area, the greater the pain, as more receptors will be stimulated and more neurons will be depolarized.
The transmission of nerve impulse from one neuron to another or to effector organ cells is accomplished through a specialized binding region called the synapse.
The most common type of synapse is chemistry, where the membranes of two cells are separated by a space called the synaptic cleft.
In the terminal portion of the axon, the nerve impulse provides the release of vesicles that contain chemical mediators, called neurotransmitters. The most common are acetylcholine and adrenaline.
These neurotransmitters fall into the synaptic cleft and give rise to nerve impulses in the next cell. Immediately thereafter, neurotransmitters in the synaptic cleft are degraded by specific enzymes, ceasing their effects.
In the nervous system, it appears that neurons are arranged differently to give rise to two distinctly colored regions that can be macroscopically noted: the gray matter, where the cell bodies are, and the white matter, where are the axons. In the brain (with the exception of the bulb) the gray matter is located externally to the white matter, and in the spinal cord and bulb the opposite occurs.
Nerves are sets of nerve fibers arranged in bundles, joined by dense connective tissues.