The sensory component of pain is known as nociception (it is receiving signals in the central nervous system caused by the activation of a specialized sensory receptors called nociceptors), is a complex electrochemical mechanism consisting of four physiological processes different: signal transduction, transmission, modulation and perception (21).
2.3.1 SIGNAL TRANSDUCTION
Conversion of noxious stimuli into electrical signals at the level of the peripheral nerve endings, it gives rise, on the site where the pain starts at the stimulate the nocirreceptores by mechanical, thermal or chemical events. There is initiates the depolarization and the transmission of pain impulses to the bone marrow. The receivers significantly increase its electrical response when stimuli are painful. (22,23). Are involved in that process the NSAIDs.
The chemical stimulus is converted into an electrical stimulation, it starts when there is a tissue injury and activated the cyclooxygenase and the lipoxygenase; (((what causes: a) release of protons, potassium and bradiquininas, which directly activate nociceptive fibers, b) release of substance P, which is an amplifier and changes the permeability, causes vasodilatation and edema, lowers the threshold of nociceptive fibers, incorporates and raises new fibers, thus increasing the field and the intensity of the response, c) prostaglandins cause indirect awareness of system central nervous algic substances(, because also amplifies, d) other mediators, such as histamine, serotonin, Leukotrienes, etc., also have some action, especially in the side effects of some drugs.
2.3.2 TRANSMISSION
Propagation of electrical signals along roads nociceptive.(23). In this process involved local anesthetics.
With some exceptions, all the pain impulses are transmitted through fibers C, with slow conduction velocity (0,5 - 2 m/s) and for the delta, with higher driving speed (4 - 30 m/sec). These fibers, part of the peripheral neuron, have his soma in the spinal ganglion and penetrate to the bone marrow by the posterior Horn (75%) and to a lesser extent (15%). The blocking of a peripheral nerve with local anesthetics or its destruction prevents the transmission of impulses to and from the spinal cord.
2.3.3 MODULATION
Capacity that have analgesic systems endogenous modify the transmission of the nervous impulse, primarily inhibition in the dorsal horns bone marrow, but apparently also to other levels (peripheral, for example).
It describes the alteration of signs nociceptive within the dorsal horn of the spinal cord. Among her intersect nerve endings of several types and release neurotransmitters that inhibit, amplify, or alter the sensory signal.
Bone marrow is the area that has been most studied the phenomenon. Substance P is able to depolarize the second neuron and they may also release other substances neuro moduladoras, cholecystokinin, somatostatin and gamaaminobutirico acid (GABA), glutamate. The latter would act as a presynaptic inhibitor. The many Interneurons of the posterior pole, making synapses with the first or second neuron in the levels described, in turn receive inflectional and descending pathways from the periphery.
There are also side since the A-alfa fibers that release enkephalins which inhibit the release of substance P. The endorphins, a group of endogenous substances, so called because of its morphine-like action, constitute another of the endogenous pain modulation and control systems. The enkephalins, which probably act as neurotransmitters, are especially in areas of high concentration of morfinicos receptors. B-endorphins, a polypeptide of larger size, also has a strong opioid agonist action. is located in the pituitary gland, hypothalamus and peripheral tissues, but degrade more slowly and have the property of acting to distance, is rather considered a hormonal agent.(24)
The theory of Melzack and Wall or the gateway theory, emphasizes the fact that the perception of pain sensation depends not only of the peripheral stimulation and transmission, but the spinal cord and central modulation. Its formulation has stimulated the study of many drugs and analgesic techniques.
Transcutaneous electrical stimulation (TENS) and intrathecal electrical stimulation, are based on the fact that all of the afferent nerve fibers have the ability to influence other afferent impulses, mainly through a presynaptic inhibition. Stimulating a mixed nerve with non-painful impulses, the first fibers to respond are those of larger diameter, and these downloads at the spinal cord level would be capable of inhibiting the cephalic transmission of nociceptive impulses. (24).
2.3.4 PERCEPTION.
Is the process by which the nociceptive impulses are integrated with factors cognitive and emotional to create the subjective experience of pain(22).