[ 2006-12-31 12:50 ]
The unnerving, separate sensations we feel when a body
part "falls asleep" truly result from nerves under pressure.
Scientists, using the arm as an example of a limb that has fallen asleep,
explain that the person, most likely, slept on the arm. This sleeping position
exerted pressure on the nerves, which squeezed them and, thus, inhibited the
messages the nerves carry to the brain and to the rest of the body. If sleeping
on the arm also squeezed the blood vessels, oxygen normally carried to the
nerves by these vessels never made it to its destination.
The nerves themselves comprise bundles of single celled fibers, with each
fiber conveying a different sensation or message to the brain. The fibers vary
in thickness and in the amounts of myelin, a protective sheath, surrounding them.
These two variables determine the sensitivity of the nerve fiber to pressure and
to oxygen deprivation. As a general rule, the thicker the fiber, and the larger
the myelin sheath, the more sensitive it is.
When one removes the pressure on the nerves and on the blood vessels, in this
case by a change in sleeping position, the nerve fibers awaken in order of their
thickness and of the thickness of their myelin sheaths. Hence, the thickest and
most protected ones awaken last. This gradual awakening process causes the
different sensations we experience as the affected body part returns to its
The first sensation we experience is a tingling sensation, followed by a burning
sensation, as the fibers that control pain and temperature now function and are
again able to transmit these messages to the brain. Not until later, does the
numbness we feel disappear, simply because the fibers that control touch and
position are thicker fibers with thicker myelin sheaths. Similar fibers, known
as motor neurons, travel in the same nerves, but take direct orders from the
brain to the spinal cord to the muscles, and awaken shortly after those
controlling touch and position. For this reason, after the numbness disappears,
we regain our ability to move the affected body part, and life is finally backed
（英语点津 Annabel 编辑）