In humans and other higher vertebrates, there are two types of nerve fibres used in pain transmission:
"Evoked activity demonstrated that there were fast adapting mechanoreceptors and polymodal slowly adapting mechanoreceptors that responded not only to mechanical stimulation but also to thermal and or chemical stimuli."
A & C FIBRES FOUND IN TROUT
"The present study on the trout has demonstrated that the trigeminal nerve has both C and A-delta fibres."
"There were a large number of A-delta fibres in the trigeminal nerve of the rainbow trout, therefore, they have the potential for nociceptive² capabilities."
TRIGEMINAL NERVE PROJECTS INTO THE BRAIN
"The trigeminal nerve projects to the relevant brain areas, the thalamus, cerebellum and medulla, which are involved in pain or nociceptive processing in higher vertebrates. Fish are subject to noxious pollutants, unpalatable or poisonous food and also mouth injuries as a result of aggression, spiny prey and angling. It is conceivable that it would be adaptive for the animal to be able to perceive these potentially injurious stimuli and react appropriately."
PAIN FIBRES SIMILAR TO HUMANS
"The size range of each fibre type was similar to those found in other animals. The cell bodies of the trigeminal ganglion were of a similar size to those found in man."
"This indicates that the basic components of the trigeminal nerve are similar in all vertebrates but only the relative number of fibre types that comprise the afferent nerves differ in fish."
"Nociceptors are slowly adapting polymodal mechanoreceptors and these receptor types were found on the head and face of the rainbow trout in this study."
"Therefore, the physiological recordings mirror the anatomical findings and show that there are fibres that could potentially act as nociceptive nerves."
¹Oncorhynchus mykiss = Rainbow trout
² A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process is called nociception.