Re: What are the chemicals between the spinal cord and brain that signal pain?

Signals are transferred from one neuron to another by a combination of 
electrical and chemical impulses.  Within a single cell, the signal is 
transferred as an electrical signal.  But at a synapse, the connection 
between two different nerve cells, the cell releases a chemical called a 
neurotransmitter that crosses the small gap between the cells, called a 
synaptic cleft.  A receptor on the second cell receives the chemical 
signal and converts it into an electrical impulse that can continue on 
within that second cell.  

There are different types of neurotransmitters that are used in different 
types of connections between nerve cells.  The neurotransmitter for pain 
responsive neurons, also called nociceptors, is a protein 
called “substance P”.   A single nerve cell transmits an electrical signal 
from the source of the pain all the way to the spinal cord where substance 
P is released into the synaptic cleft.  The chemical signal is converted 
into an electrical signal in the spinal cord nerve cell.  A single nerve 
cell then transmits the electrical signal through the length of the spinal 
cord to the brain (surprisingly, a single nerve cell can be as long as 3 
feet.)  Within the brain, there are many different neurotransmitters (such 
as acetylcholine, serotonin, dopamine, epinephrine, etc) that are involved 
in the complex role of interpreting, remembering, and responding to the 
pain.

I hope this helps.