MadSci Network: Neuroscience
Query:

Re: Why do you feel two flashes of pain when you touch hot water?

Date: Thu Jan 21 15:59:18 1999
Posted By: Salvatore Cullari, Professor and Chair, Lebanon Valley College
Area of science: Neuroscience
ID: 916332800.Ns
Message:


Why do you feel two flashes of pain when you touch hot water?

Sometimes you leave the water running and then when you put your  hands 
forgetting that it's really hot. You feel the first shock of pain, take your 
hands out, and then about half a second later feel a longer pain. 

I think it's probably one of two things. Either the first pain is from the 
skin and the second is from the more sensitive nerves of your bones warming 
up or the first is from your spine's automatic reaction and the second is 
the actual reaction.


Konrad, you have essentially answered your own question correctly.  Simply 
put, you are describing the two systems of pain recognition.  The first, 
acute pain,  travels down fast myelinated pathways; the second, “chronic” 
pain, travels on non-myelinated slower neurons, and results in a delayed 
pain sensation.  

Pain receptors are sensory nerve endings that are located throughout all 
organs and tissues of the body, except the brain.  These respond to various 
types of stimulus extremes, such as temperature, cuts, burns or anything 
that is potentially damaging to the tissue.

This may be a bit more detailed than you want, but when an injury, cut or 
any type of damage occurs, Bradykinin (a pain producing enzyme ) is released 
from the damaged cells. This bradykinin attaches to the pain receptors (free 
nerve endings) causing them to transmit pain impulses.

These pain impulses travel to the central nervous system through two 
different fibres:  The fibers that transmit impulses quickly are called A-
delta fibres. The types of sensations they carry are localized, sharp, 
pricking, brief  sensations. The fibres that transmit impulses more slowly 
are called C fibres. The types of sensations they carry are dull, burning, 
aching, longer lasting  sensations. This results in the two flashes of pain 
that you describe.

Both of these fibres send impulses by releasing a transmission agent called 
Substance P. The thalamus, which serves as a switching device, helps to turn 
on substance P and influence the opening and closing of the pain gate 
(Substance P is a neuropeptide, but “P” actually stands for "preparation.")   
Painful impulses from the pain receptors only reach the brain if the "gate" 
is open.

I hope this answers your very interesting question. 








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