Pain: what does pain depend on?

The ache it is a "sensory experience perceived at the level of the central nervous system as an unpleasant" emotion.

The term nociception refers to all those mechanisms of transmission of the painful stimulus from the periphery to the central nervous system; the interaction of the different brain areas that process the nociceptive signal allows it to become aware of, so you have the perception of pain. This pain perception can be divided into different components:

• a component sensitive-discriminative which allows to localize the stimulus and to quantify its quality and intensity;
• a component affective-emotional which consists of the individual reacting to pain emotionally by speaking;
• a component cognitive-evaluative, which consists in the ability to assess pain according to the education received and the social context.

Pain is also divided into acute pain, resulting from an inflammatory process, and chronic pain; the latter lasts for a long time and is caused by an alteration of the nerve which, if prolonged, modifies its physiological structure.
Analyzing in detail the path of the painful stimulus from the periphery to the central nervous system, the following are involved:

• nociceptors: they are the detectors of the painful stimulus at different levels; the nature of the stimulus can be of a chemical, mechanical-physical or thermal type;
• the stimulus is then conducted along the nociceptive fibers, subsequently to the sensory nerve and then to the spinal nerve; nociceptive fibers can be unmyelinated, in which case they transmit the stimulus slowly and are responsible for the tedious pain, or myelinated, or they can transmit the stimulus more quickly and are responsible for stinging pain;
• the spinal nerve reaches the gray matter of the spinal cord, where at the level of the spinal horns it forms two bundles: the spinothalamic bundle, which transmits stimuli of the cutaneous, somatic and visceral type, going up the medulla to the thalamus; and the spinal reticular beam, which transmits somatic and visceral stimuli
• Once in the CNS, the stimulus is analyzed and integrated into different levels:

At the bulbar level some nuclei are involved for the analysis of the stimulus, from here they depart: descending pathways able to modulate painful perception, by means of neurotransmitters; other fibers that reach the muscles to allow the motor reaction to the painful stimulus; and others fibers still reaching the cardiovascular and respiratory systems.
At the mesencephalic level there are emotional reactions to pain, from here the fibers that regulate the neuroendocrine reaction branch off.
At the thalamic level (cerebral cortex) there is the sensory-discriminatory perception of pain, that is, the origin of the stimulus, the quality and quantity are localized, which will correspond to motor and sensory reactions.
These levels represent the descending pathways of pain, which modulate its perception at the supra-spinal level, in the periaqueductal gray matter and in the ventro-medial region of the bulb. The modulation of pain occurs through the release of neurotransmitters, above all endogenous opioids; their production varies from individual to individual, therefore we speak of the "pain threshold". The descending pathways target spinal nociceptive neurons and interneurons (inhibitory or excitatory); the same neurons of the ascending spino-thalamic pathways.

In conclusion, we could summarize by saying that the ascending pathways transmit the stimulus from the periphery to the CNS, while the descending pathways reduce the pain stimulus and bring it back to the central level.

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