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painfully loud sound

Dear List,

while teaching medical students some basic Neuroscience (including
nociceptive pathways and pain) I wondered: what determines whether we
would consider a particular sound to be painfully loud? Does the cochlea
contain specific nociceptive fibres? Or are painfully loud sounds so
intense that they can activate pain fibres in the middle ear?
As I understand it, many people suspect now that one may not need
specific "pain fibre" activation to elicit pain (I guess phantom limb
pain kind of illustrates this possibility quite nicely). Instead,
particular patterns of activation of other types of afferents might also
be able to trigger painful sensations. I wonder whether painfully loud
sounds (or painfully bright lights)  might be examples of this. Have the
experts on middle and inner ear histology among you observed lots of
nociceptive fibres in places that would make them likely to  be
sensitive to intense acoustic stimulation?
What about people with extensive damage to hair cells: if pain responses
to very loud sounds are triggered by separate nociceptors, rather than
by overstimulation of hair cells, then patients with profound deafness
due to hair cell loss should still find exposure to 140 dB plus noise
levels painful, even though they can't hear them. Is that the case? Or
do pain thresholds invariably go up as hearing sensitivity goes down?



Dr Jan Schnupp
University Laboratory of Physiology
Parks Road - Oxford OX1 3PT
Tel +44-1865-272513
email: jan@physiol.ox.ac.uk