A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.
In this experiment, neural activity was recorded from single cells in the trigeminal ganglion following the application of stimuli to the head of the fish. Stimulation was:
TWENTY-TWO PAIN RECEPTORS FOUND ON FACE AND HEAD
"We located 58 receptors on the face and head of the rainbow trout. Twenty-two of these receptors could be classified as nociceptors."
RECEPTORS SIMILAR TO MAMMALS
"The polymodal nociceptors found here in the trout have similar properties to those found in amphibians, birds and mammals including humans."
"This study demonstrates that the rainbow trout is capable of nociception."
"Receptor diameter, thermal thresholds and mechanical responses are similar to those measured in higher vertebrate groups."
RECEPTORS MORE SENSITIVE THAN IN HUMANS
"Mechanical thresholds were lower than those found in humans: at least 0.6g is required for noxious stimulation in human skin but many of the nociceptors in the fish skin were stimulated by 0.1g. This may be a consequence of the more easily damaged nature of the fish skin requiring the nociceptors to have lower thresholds. Similar thresholds were found in mammalian eye nociceptors."