In this research, the face of the trout was stimulated while responses in the trigeminal nerve in the brain were recorded.
These experiments, carried out at Manchester and Liverpool Universities in the UK, found in the trigeminal nerve, pain receptors on the face of the trout that were more sensitive than that found in mammals and birds. They were able to respond to pressure as low as one thousandth of a gram. The researchers suggest that fish need to be this sensitive because of their vulnerability to fungi, bacteria, and water pressure.
It was found that the trout were also more susceptible to pain caused by low temperatures - below that of mammals.
"This study examined stimulus–response properties of somatosensory receptors on the head of rainbow trout, using extracellular recording from single cells in the trigeminal ganglion."¹
TROUT SKIN SENSITIVE TO PRESSURE
"Trout cutaneous receptors recorded here are more sensitive overall to mechanical stimuli than those of mammals and birds, with some responding to stimuli as low 0.001g."
SENSITIVITY TO PAIN
"It is likely that fish, continuously exposed to external water pressure and to bacterial and fungal agents, require greater cutaneous sensitivity, including for nociception, than terrestrial animals."
TROUT MORE SENSITIVE TO LOWER TEMPERATURES THAN MAMMALS
"The heat thresholds recorded here from mechanothermal and polymodal receptors ranged from 20°C, though mean values were around 29 and 33°C, respectively. This range is below that considered noxious in mammalian nociception, but can be explained by the proposition that nociceptors in fish have evolved to match habitat temperatures."
¹In vertebrates, the trigeminal nerve, the fifth cranial nerve, conveys somatosensory information from the head and mouth to the brain.