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20.11: Nociception


20.11: Nociception

Nociception, generally referred to as pain, is the process through which a noxious stimulus is transmitted through the peripheral and central nervous systems to warn of potential harm or injury.

Definitions of Pain

The concept of pain has many definitions, including “an unpleasant sensory and emotional experience associated with actual or potential tissue damage.” Nonetheless, pain is a sensation that integrates multiple pathways of nerve transmission and is a completely subjective phenomenon.

The Process

When a damaging signal is detected, nociceptors—free-nerve endings—are activated. Nearby mast cells release chemicals, such as histamines, while macrophages secrete cytokines—small proteins—that are critical factors in immune signaling.

Simultaneously, the axons of the nociceptors transmit the signal to the dorsal horn of the spinal cord, mainly via two different pain fibers. The first ones to be activated are the thinly myelinated Aδ fibers, which conduct immediate sharp and well-localized pain at a very fast speed to allow the body to withdraw from the harmful stimulus. The other type of axons is C fibers, which conduct slowly, as they are unmyelinated and transmit prolonged, burning pain.

From the spinal cord, the pain signals cross over and travel to the brain stem, followed by the thalamus, and somatosensory cortex, where the pain signals are interpreted to determine the location of the injury.

Additional brain regions process the pain, notably corticolimbic structures, including the amygdala and prefrontal cortex, that relate to emotional memories and cognition. Ultimately, the multiple interactions within the brain contribute to the subjective perception that individuals experience.

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