In this paper the main physiological theories related to pain phenomena are reviewed. It could begin with Descartes, who developed the first pain theory called the ''bell alarm''. Later on, Johannes Muller, known for his specific nerve energy theory, postulated that the incoming information from the external environment to the brain came solely via the sensitive nerves, and the activation of these nerves consisted of a code or symbolic data which depended upon the received stimulus. Afterwards, the specific theory of von Prey was elaborated. it comprised physiological, anatomical and psychological aspects; the main concept of this frame of thought was the physiological specialization of the skin receptors. Goldscheider, based upon von Prey's thinking: was the first to suggest that the stimulus intensity and a central adding process were the main factors in pain perception. These principles were comprised In the Pattern Theory. Later, the sensorial interaction theory came forward, postulating the existence of a system of fast conducting fibers that was inhibited by the synaptic transmission of noxious stimuli by means of another slow conducting system. The two systems were respectively named epicritic and protopathic by Head, or fast and slow by Bishop; from a phylogenetic point of view the two systems were called old and new, respectively. All these theories provided the experimental and intelectual field of what we call the modern era in the study of pain. In the sixties, the specific role of sensorial receptors was elucidated as well as the pathways and many cytoarchitectonic relations in the spinal cord. With all this background in mind, Melzack and Wall described, in 1965, a model based on a circuit in the asta dorsalis of the spinal cord responsible for pain transmission. This knowledge was assembled in the gate control system, in which the specific incoming Information from the sensory organs generated a central activity pattern susceptible to modulation. This theory has motivated numerous researchers to work on this field and has provided Improved treatments and a better understanding of pain In the clinical practice. Cervero and Laird suggested that the neurophysiological mechanisms implicated in the pain phenomena can be divided into three stages: 1)short nociceptive stimuli, regardless of their Intensity, 2) Chronic pain such as inflammatory processes or tissue damage, and 3) abnormal pain perception which included central pain and allodynia; the latter comprises pain evoked by the activation of skin receptors through non-nociceptive stimuli. Recently, Melzack has implicated the cingulum cortex, the reticular formation and some limbic structures in the activation of a neuronal network, a neuromatrix. This neuromatrix concept has become relevant in explaining the genesis of the phantom limb and central pain. This mosaic of findings and theories not only has created a new perspective in understanding and treating pathological pain, but it can also open new ways in the comprehension of some psychiatric diseases like schizophrenia, and even enlighten other important research fields such as evolutive psychology and the working of conscience.
