Pain conveyed to the central nervous begins with nociception.
Nociception refers to the detection of noxious stimuli by specialized sensory receptors called nociceptors[1][5]. Nociceptors are found in most body tissues and respond to potentially damaging stimuli such as extreme heat, intense pressure, or chemical irritants[1].
## Transduction and Activation of Nociceptors
When tissue damage occurs, various substances are released at the site of injury. These substances activate nociceptors, triggering the transduction of the noxious stimulus into electrical signals[1][3]. Some key chemicals involved in this process include:
- Adenosine triphosphate (ATP)
- Protons (changes in tissue pH)
- Bradykinin
- Prostaglandin E2
## Transmission
The electrical signals generated by nociceptor activation are then transmitted to the central nervous system via specialized nerve fibers:
- A-delta fibers: These myelinated fibers transmit fast, sharp pain signals[3].
- C fibers: These unmyelinated fibers carry slower, more diffuse pain signals[3].
These primary afferent nociceptors have their cell bodies in the dorsal root ganglia and send axons that terminate in the dorsal horn of the spinal cord[1][2].
## Spinal Cord Processing
Upon entering the spinal cord, pain fibers synapse with second-order neurons in specific layers of the dorsal horn, primarily Rexed layers I and II[3]. The dorsal horn acts as an important processing center, integrating incoming sensory information and modulating pain signals[4].
## Ascending Pathways
From the spinal cord, pain information is relayed to higher brain centers through several ascending pathways, including:
- Spinothalamic tract
- Spinoreticular tract
- Spinomesencephalic tract
These pathways carry nociceptive information to various regions of the brain, including the thalamus, reticular formation, and periaqueductal gray[1][2].
## Brain Processing and Perception
The pain signals ultimately reach multiple areas of the brain, including:
- Thalamus
- Somatosensory cortex
- Insula
- Anterior cingulate cortex
- Limbic system
These brain regions work together to process and interpret the pain signals, leading to the conscious perception of pain and associated emotional responses[2][4].
It's important to note that the central nervous system doesn't just passively receive pain information. There are also descending modulatory pathways that can inhibit or enhance pain signals at various levels of the nervous system, allowing for complex regulation of pain perception[2].
Citations:
[1] https://www.ncbi.nlm.nih.gov/books/NBK219252/
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3438523/
[3] https://nba.uth.tmc.edu/neuroscience/m/s2/chapter06.html
[4] https://southernpainclinic.com/blog/how-the-nervous-system-detects-and-interprets-pain/
[5] https://www.physio-pedia.com/Nociception
[6] https://www.sciencedirect.com/topics/neuroscience/nociception
[7] https://www.news-medical.net/health/Mechanism-of-Pain.aspx
[8] https://www.sciencedirect.com/topics/immunology-and-microbiology/pain-receptor