IASP Definition of Nociceptor
Definitions You Might Have Seen
Nociceptor or ...
Danger detector or receptor that detects danger and sends danger messages to the brain.
Threat detector or receptor, that sends threat messages to the brain.
Pain receptor that sends pain messages to the brain.
Part of an alarm system warning you of danger.
A sensory neuron that detects pain.
Let's just look a little closer at the definition
These are sensory receptors that detect noxious stimuli from the periphery (peripheral somatosensory nervous system). Here's a neat and quite short video.
What is meant by transduce and encode?
Transduce means to convert (something, such as energy or a message) into another form. Essentially sense organs transduce physical energy into a nervous signal (sensory to electrical) at the sensory receptor.
Encode means to convert from one system of communication into another. The information that gets encoded may include the type, receptive location, duration, and relative intensity of stimulus.
Also known as free nerve endings, the Aδ (delta) and C fibres are the most prevalent, and most studied. But ...
- Not all Aδ (delta) and C fibres are nociceptors
- Not all nociceptors are Aδ (delta) and C fibres
- When Aβ (beta) neurons (fast conductors) are not specialized, they can act as free nerve endings and respond to mechanical stimuli, including noxious stimuli.
Noxious Thermal, Chemical, Mechanical
Nociceptors respond to a variety of high-threshold noxious chemical, thermal, and mechanical agents. They can be specialized to one type of stimuli, but response properties can be altered once they've been activated.
This means that mechanical receptors responding to, say, a hammer blow, may also become responsive to inflammation (chemical).
Consider, if you will
Activation of nociceptors does not imply the sensation of pain. Once pain has been experienced however, it does imply that there was sufficient nociception activated.
“The labelling of nociceptors as pain fibres was not an admirable simplification but an unfortunate trivialization.”
(Wall PD, McMahon SB. The relationship of perceived pain to afferent nerve impulses. Trends in Neurosciences. 1986;9:254–5)
We can see how unfortunate this can be with some of the other definitions in play. Conflation of nociception with pain fibers is like the infinite cycling of the Ouroboros where pain, the experience, and pain the stimulus are both the beginning and the end. We end up with pain receptors that send pain messages that result in pain.
Don't even get me started on danger or threat. These are unhelpful confusing metaphors for a concrete process that could more easily and accurately be described.
Activation of nociceptors by a noxious stimulus (we'll get there soon) is only one of several methods of nociception (we'll get there later).