Fish possess the capacity to feel painful events. They have nerves, as we do, to detect the elements that cause pain - heat, chemicals, and pressure.
Researchers in Scotland (Sneddon, 2003) located fifty-eight receptors in the face and head of rainbow trout. Twenty-two of these were pain receptors. The receptors were similar to those found in amphibians, birds and mammals. Moreover, response to heat and pressure, and the diameter, were similar to those in higher vertebrates. Sensitivity to pressure was higher than in humans, and mammalian eyes.
This research, carried out under deep anaesthetic, shows fish have pain receptors in their eyes.
Substance P is an important element in pain perception. The sensory function of substance P is thought to be related to the transmission of pain information into the central nervous system. Brown ghost knifefish have been found to contain a chemical similar to substance P in their brains (Weld, 1992).
A-delta and C fibres communicate information about pain in ‘higher’ vertebrates.
In vertebrates, the trigeminal nerve, the fifth cranial nerve, conveys sensory signal information from the head and mouth to the brain.
Researchers (Sneddon, 2002) deeply anaesthetized rainbow trout. The head was operated on to expose the trigeminal nerve. The nerve was stimulated by very fine wire, heat and chemicals. Recordings were made.
The trout had nociceptors¹ on the head and other regions, which formed a fine and sensitive array of receptors that were able to detect information about noxious stimuli. A-delta and C fibres were discovered in this research, together with receptors.
The nerve examined, the trigeminal, projects into the thalamus, cerebellum and medulla in the brain. These are all areas for processing pain in higher vertebrates, including human. Cell structures were found to be a similar size to those in humans. The physiological recording made in this experiment underlines previous anatomical findings, and points to this nerve being used to transmit pain signals.
In Ireland, (Dunlop, 2005) goldfish were subjected to heat, pin-pricks, and pressure. Nerve responses were recorded from the spinal cord, cerebellum, mid-brain, and forebrain. A harmful stimulus produced greater neuronal activity than a mechanoreceptive stimulus.
The researchers reported that there is a nociceptive pathway from the periphery to the higher central nervous system of fish.
In research carried out at Manchester University in England (Ashleya 2007), the face of the trout was stimulated while responses in the trigeminal nerve in the brain were recorded. The scientists found that skin receptors of trout are more sensitive to mechanical stimulus than mammals and birds. They conjecture that fish are continuously exposed to water pressure, bacteria and fungus. Fish were also pain sensitive to lower thresholds of heat than mammals.
Russian scientists (Chervova) recorded the responses of various fish to painful electrical shocks. The fish jerked their tails. They were given the fish pain killers and then repeated the shocks.
Analgesics reduced pain sensitivity by up to 89%.
The most sensitive areas to pain were: tail and pectoral fins, skin around the eye, and olfactory sacs. Pain sensitivity was comparable to humans.
¹ 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.