There is a group of basic concepts to understand the pain in general and the orofacial pain in particular.

Their knowledge is indispensable to understand the clinical mechanisms of the pain and they are of great help for the diagnosis of the squares of orofacial pain .

The receptors of the pain, called nociceptors, they are those in charge of capturing the nociceptive information.

The stimulation of the nociceptors can take place as a result of mechanical stimulation (pressure), thermal stimulation (heat) or chemical stimulation (substances liberated after the tissular lesion).

These nociceptives inputs are transported to the central nervous system (CNS) for the sensitive nerves corresponding to each territory of the organism.

Each nociceptor is connected to afferent neuron called first order neuron. The nociceptives neurons of first order are of the A-delta or C type.

In orofacial structures, the main sensitive nerve is the Trigeminal nerve or fifth cranial nerve. Nevertheless, also participate the Facial nerve or seventh cranial nerve (concretely their sensitive branch or intermediary nerve of Wrisberg), the Glossophayingeal nerve or ninth cranial nerve, the Vagus nerve or tenth cranial nerve and the first three Upper Cervical nerves.

Trigeminal nerve, with their ophthalmic, maxillary and mandibular division, it arrives to the Gasserian Ganglion and from there he goes to the pons of the CNS, where it has a structure, called Trigeminal Spinal Tract that consists of three nuclei.

In the subncleus caudalis of the Trigeminal Spinal Tract, the trigeminal first order neuron he makes synapse with the second-order neuron, that they are of the type "nociceptive specific " and "wide dynamic range".

Starting from here the second-order neuron follows the anterolateral spino-thalamic tract and by means of two roads it arrives to the thalamus. A quick road, the neo-spino-thalamic tract and a slow road, the paleo-spino-thalamic tract.

The quick road goes directly to the thalamus and it transports basically, mechanical and thermal nociception. Of the thalamus, the nociceptive information is a correspondent to the brain's sensorial cortex for its interpretation and evaluation and to emit an answer that is usually motorboat (motor cortex) and until reflective.

The slow road goes first by the reticular formation, where the impulse can be modulated (to be inhibited or to get excited) to arrive finally to the thalamus. Of the thalamus the nociceptive impulse is not only a correspondent to the sensorial cortex but rather it is also sent simultaneously to the lymbic structures and the hipothalamus. The lymbic system is the responsible for the basic instincts and the behavior. The level of their activity (emotional state) it can influence in the answer from the individual to the pain. The hipothalamus regulates the autonomous nervous system and the hormonal secretion of the hypophysis.

The slow road of the pain is the more implied in the chronic pain.

When exists a continuous input of nociceptive information, like it is habitual in the chronic pain, the second-order neurons are sensitized and it can lose temper the prosecution of the nervous impulses toward the superior centers. When a neuron is sensitized, the normal impulses can be not well interpreted and considered as noxious.

This alteration in the prosecution of the nociceptives impulses is it denominates neuroplasticity and can give place to secondary effects as referred pain, secondary hyparalgesia or autonomous effects.

These secondary effects are specially frequent in the deep somatic pains and frequently complicates the diagnosis.

The neurochemical substances that transmit the impulses in the synaptic space are the neurotransmitters. The neurotransmitters can have excitatory or inhibitory effects of the neuronal synapsis and, therefore, of the transmission of the nociceptive impulse. They are small molecules of quick action or bigger molecules of slow action.

The most important neurotransmitters of excitatory quick action is the acetilcoline, noradrenalin, serotonin, histamine, glutamate and aspartate.

The most important neurotransmitters of inhibitory quick action is the dopamine, glycine and the GABA.

The most important neurotransmitters of excitatory slow action is the P substance and the bradikinine. The neurotransmitters of more important inhibitory slow action they are the endorphins.