[nudephil (OP)]

Here's a question from listener Mahavir:

While it is known that substance "P" is released and is responsible for pain perception, there are, as far as I know, no scans/methods for localisation of the source. With me being from a medical background I am aware of CT/MRI/nuclear scans which can variably be used to localise - but is there any specific method/scan to accurately localise the tissue where this substance P is released?

[evan_au]

If you know the protein structure of substance "P", then analyzing the messenger RNA expressed in different tissues will identify the cell type(s) which produce substance "P".

But since it is indicated as a pain signal, it probably exists in or near a sensory nerve cell.
- However, some pain originates in the brain (eg phantom limb pain), and may be associated with abnormal patterns of nerve firing, rather than a specific neurotransmitter.

Substance P and the NK1 receptor are widely distributed in the brain and are found in brain regions that are specific to regulating emotion (hypothalamus, amygdala, and the periaqueductal gray)

See: https://en.wikipedia.org/wiki/Substance_P#Receptor
This article also gives the amino acid sequence of this small peptide.

[Petrochemicals]

As a Pain suffer with no medical background, I have often thought of pain being a nerve that slowly becomes less noticeable. Catastrophic sciatica a year ago lingered horrifically for 6 weeks, moving position on the nerve. Affected me mentally, really ground me down with such sharp severe pain so repeatedly over such a long period. I think that the pain died away but the problem did not, I feel a re-emergence from time to time as though the nerve is coming back to life. This cannot be true, my nerves in my leg still work, but something is different about the area where the pain originated, somehow less responsive and sensitive.

 Complete utter guess but substance p may be a long term cell response to pain signals, in reality there is no point to pain as long as the ceature stops injuring or neglecting itself, this includes bad posture, diet etc. Long term pain with no reason just reduces said creature to incapability and limits its survivability.