Fish have nerve receptors, similar to those found in amphibians, birds, and mammals, which detect painful events. In one experiment, twenty-three pain receptors were found on the face of rainbow trout.
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 one experiment, neural activity was recorded from single cells in the face following the application of stimuli to the head of the fish. Stimulation was: