This question is just out of general interest.

From what I understand, reflexes are initiated by ganglia from the spinal cord. A sensor detects the negative stimuli, that sensory signal travels to the spinal cord, which then sends action potentials back to the effector (the skeletal muscle causing a foot to withdraw from a nail, for example).

Collaterol signals are then sent to the brain, which then perceives pain. However, this usually occurs AFTER the reflex occurs, and hence has no effect on the reflex itself.

This begs the question of why people who cannot consciously perceive pain do not have a reflex withdrawal of their foot after stepping on a nail. If the conscious perception of pain is completely unrelated to a reflex withdrawal, why do these people step on a nail and not have the accompanying reflex withdrawal?

If the conscious perception of pain is completely unrelated to a reflex withdrawal, why do these people step on a nail and not have the accompanying reflex withdrawal?
What makes you think that the reflex action and pain are completely unrelated?

The way I see it is that they are both connected in the spinal cord and all. When one cannot feel pain the electrical currect that usually is received in the brain is out, thus the impulse to withdrawal one's foot doesn't occur.

If the phone doesn't ring you don't pick it up.

Idle:

What makes you think that the reflex action and pain are completely unrelated?

Because a conscious perception of pain comes AFTER the reflex action, not beforehand.

http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/neurosci/ch16f13.gif

I think this is what you are talking about. Pain receptors directly influencing the reflex action.

Now for an answer... :o

spurious:

Pain receptors directly influencing the reflex action.

Understood.

But surely people who are unable to feel pain don't lack pain receptors? I thought they lacked the ability to consciously perceive pain... that is, abnormal neurons in the brain.

I think I kind of know the answer, but I do not know it for sure.

The reflex pathway is indeed quite independent. However, if my memory serves me correct, the CNS not only receives the pain stimulus, it also signals back to the reflex to strengthen it. If that would be true (not sure) then the reflex pathway would diminish without feedback from the brain.

I think the number of true reflexes are limited. There is usually some interference of the CNS.

Unfortunately I have no neuroscience textbooks here. And I did a quick search on pubmed books, but cannot find a confirmation.

But maybe this will get a discussion going.

All I know is that there are relex arcs (neural pathways) in the spine; motor neurones in the spine intercept pain signals before they go the brain and immediately send a feedback to muscles to get a rapid response.

Apparently its linked to the production of certain neurotrophins in the skin muscle and sweat glands. Such abnormalities in pain perceptions are related to mutations/defects in the genes which code for the neurotrophins.
No, this is not quite correct. Congenital Insensitivity to Pain with Anhidrosis (CIPA) is due to mutations in the TRKA gene. This gene codes for a neuronal cell-surface receptor for Nerve Growth Factor (NGF). NGF is a survival factor, so without the ability to detect NGF signaling through the TRKA receptor the neurons in question die off during development. CIPA is an autosomal recessive disorder characterized by anhidrosis (ie. the inability to sweat) and absence of reaction to noxious (or painful) stimuli. This inability to feel any pain or temperature sensations is due to the absence of a specific subset of PNS sensory neurons that normally detect these tissue-damaging stimuli.

CIPA is a rare genetic disorder but it always seems to be the example of defective pain sensation that is cited, probably because it is the most well-understood example. It all goes to show that although no one likes acute pain, it does serve a very important purpose in letting us know when our immediate environment or activity is damaging our body. These poor CIPA children have no idea when they’ve broken bones, gashed themselves open, burnt themselves etc. and will often continue the activity and cause even more damage. They require constant supervision and daily medical inspection to ensure they’re not hurt in some way.

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