"Mark Zarella" <NOmarkzarellaSPAM at attbi.com> wrote in message
news:82tZ8.572162$cQ3.54221 at sccrnsc01...
> Even after rereading the analgesia chapter in Kandel, I'm having trouble
> understanding the mechanism in which the sensation of pain can be
> experienced in areas where there's no other sensory activity. For
instance,
> if a peripheral nerve is severed and a limb or digit then becomes numb to
> all forms of sensory activity, how then can pain still be detected in
these
> areas?
>> More specifically, about 2 weeks ago I was in an accident where I
sustained
> a rather deep laceration in the upper wrist / lower hand resulting in the
> ulnar nerve becoming severed, as well as part of the median nerve (and a
> tendon and artery, which were also repaired). The nerves were reattached,
> but theres no feeling (proprioreceptive or cutaneous) in 3 fingers and
parts
> of the upper palm. However, there's the sensation of "shooting pain" at
the
> fingertips and elsewhere in these areas. How is that possible? Could it
be
> a result of feedback with adjacent nerves in the relay sites and cns?
What
> about distal portions of the nerve beginning to regain function but not
yet
> being "remapped" after reattachment? As you can see, I have no ideas.
> Kandel's book touches on this sort of phenomenon, but nothing specific
> enough to answer this type of question.
>> Any insights are appreciated.
>> - Mark Zarella
>>
Well I'll take my own simple stab at this from my meager knowledge of
neuropsychology....
First, perception of pain runs along a different neurological pathway than
other somatic senses. It is quite common in individuals who have suffered
paresis of limbs through brain injury to regain pain sensation prior to
regaining much motor, cutaneous or proprioceptive sensation. So we could
hypothesise that even in severed nerves, the other nerves remaining in the
pathway are sending back information to the brain, which then has to attempt
to localize the sensation.
Even in people with an intact system, we know that the brain is not always a
good locator of pain (i.e. referred pain phenomenon). In the case of
severed limbs, although the appendage is gone, the somatic cortex still has
the area mapped and wired for sensory perception. In addition to any
surviving input that may be received, there is also the case of random
firings within the cortex, which could also cause perception of pain or
sensation where there is no specific nerve. It is just the somatic cortex
interpreting either the incoming sensation from the remaining part of the
pathway or it's own neural "noise"
It sould be illuminating somewhat that phantom pain or sensation is not
reported in individuals with complete spinal cord severing, which would make
sense, as there would be no working sensory connection from the limb to the
brain. That is to say that when amputation takes place, or localized
destruction, the proximal part of the nerve pathway still exists between the
missing limb or nerve connection and the spinal cord to send impulses to the
brain for interpretation.
Also, and I think an earlier responder mentioned this, over time in cases of
amputation, there is evidence that the area that had control of the
amputated limb begins to merge with adjacent areas of the somatic cortex.
In some cases this has resulted in some very bizarre sensory perceptions (I
seem to recall one case of a man who experienced the sensation of his
amputated foot being stroked when his penis was touched.....at least this is
what I seem to recall.....which made sense when you look at the organization
of the somatic cortex and find the foot and penis occupy adjacent zones of
the cortex).
Anyways, don't know if I'm on the mark, but hope something in here helps.
Joseph III
