I read an interesting review article published recently in Physiology. The review discussed how various animals sense pain. Perhaps understanding how animals detect pain will lead to better pain management techniques for animals and humans.
Our bodies have special sensors, called nociceptors, that detect noxious stimuli that could injure tissues. Stimulation of these receptors is what allows you to move your hand away from a hot stove even before your brain makes you aware that you touched, or came close to touching, something hot. The majority of receptors in our skin are nociceptors, each one specializing in detecting a specific type of stimulus such as temperature extremes, pressure, or chemicals that could injure the skin. Once injured, nociceptors become more sensitive to the stimulus, which helps us avoid further injury to the damaged tissue.
Pain is our brain’s perception of tissue injury. Activation of certain areas of the brain can result in the release of opioids, which helps reduce pain. The release of these endogenous opioids can sometimes make animals and people appear unaffected by an injury. For example:
Nociceptors have been found in all animals examined including invertebrates. The first invertebrate that was discovered to have nociceptors was the leech (Hirudo medicinalis). Interestingly, leech nociceptors are less sensitive to capsaicin (i.e. what makes chile peppers hot) than people. Chickens and rabbits are similarly less sensitive to capsaicin. Cephalopods (squid, octopus, cuttlefish, etc) also have nociceptors that respond to mechanical injuries, but do not respond to hot temperatures. It is thought that they are missing heat-sensitive receptors as their environment is consistently cold.
Like cephalopods, many species of fish that have been studied have nociceptors that are less sensitive to temperature changes, whereas amphibians are sensitive to temperatures in the range of mammals. These observations suggest that life on land may have made terrestrial animals more sensitive to temperature fluctuations, which would help them avoid extreme heat or cold.
Birds reportedly have nociceptors in their beak, nose and mouth mucous membranes, as well as their legs. Although, as noted above, they do not seem to be sensitive to capsaicin. I am so jealous. It is thought that their lack of aversion to chile peppers allows them to ingest and disperse the seeds.
Naked mole rats are also interesting as they do not appear to have nociceptors that detect acidic stimuli. This is thought to be because of their underground lifestyle as they are likely exposed to high carbon dioxide concentrations that can cause acidosis.
Sneddon LU. Comparative Physiology of Nociception and Pain. Physiology. 33(1): 63-73, 2018.