Re: WHat happens to your body when you get pricked by a pin

Well, one side is of course relaying the stimulus (pin prick or other 
painful stimulus) from the periphery (finger, arm, leg) to your central 
nervous system (brain). 
The pin prick is a form of physical energy which stimulates pressure and 
pain receptors in the skin. I will concentrate on the pain receptors: these 
are usually free nerve endings which continue as either thinly myelinated 
A-delta fibers (diameter 1 to 5 micrometers, conduction velocity 5 to 30 
m/s) that relay ‘pricking pain’, i.e. what you sense as stinging pain. 
Burning pain which is longer lasting but has a slower onset is mediated by 
so called unmyelinated C fibers (diameter ranges from 0.2 to 2 micrometers, 
conduction velocity from 0.5 to 2 m/s). The latter are also called 
polymodal nociceptors since they relay chemical and thermal stimuli along 
with the high-intensity mechanical stimuli.
Transmission of the nociceptive stimulus proceeds through the dorsal horn 
of the spinal cord (where pain fibers terminate in the so called marginal 
zone, the outermost laminae of the five-layered dorsal horn). In fact, the 
circuitry is even more complicated but if you’re interested in more details 
I refer you to the textbook by Kandel et al.
The short version is that pain fibers either synapse onto a local circuit 
in the spinal cord to yield a reflex (withdrawal of hand or the like) or 
through the anterolateral system towards other brain centers, esp. 
(neo)cortex (through the thalamus), brain stem, and midbrain. Information 
to and from the limbic system (emotional state in the widest sense) as well 
as the reticular formation (is involved in awareness) figures in, i.e. the 
same pain stimulus might not be felt or experienced the same way in 
different situations. This brings us back to your question: how does the 
brain know? 
As you can see, relaying the painful stimulus to the brain is one thing. 
Awareness and your emotional state play an important role, too (the degree 
of consciousness so to say). In addition, stimuli from other receptors can 
modulate pain. Maybe you have experienced this yourself: you can alleviate 
the irritation caused by let’s say a pin prick by massaging your finger. 
Here the input from the pressure receptors partially blocks out the input 
from the pain receptors. Another example experienced by everyone: you cut 
your finger but don’t notice it until you discover the blood in your sink. 
Here, the pain stimulus was most likely blocked out because you were 
focused on something else, i.e. not aware that you cut yourself. Had you 
been cutting potatoes and seen the knife penetrate your finger the pain 
sensation would most likely have been a little different.
 
I hope this suffices for the moment.
In case you have other questions either refer to the textbook by 
Kandel/Schwartz/Jessell (Principles of Neural Science) or email me.