Nociception
According to the International Association for the Study of Pain (IASP), nociception is defined as the neural process of encoding noxious stimuli.
The consequences of encoding may be autonomic (for example elevated blood pressure) or behavioral (motor withdrawal reflex or more complex nocidefensive behavior). However, pain sensation is not necessarily implied. The noxious stimuli are detected and mediated by nociceptive neurons, defined by IASP as a central or peripheral neuron of the somatosensory nervous system that is capable of encoding noxious stimuli. A subset of sensory neurons in dorsal root ganglion and trigeminal ganglion are examples of nociceptive neurons.
Pain
On the other hand, IASP has recently redefined pain in 2020 as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. A central change in the new definition, compared to the old version, is replacing terminology that relied upon a person’s ability to describe the experience to qualify as pain. There are six key notes for the new IASP definition of pain.
- Pain is always a personal experience that is influenced to varying degrees by biological, psychological, and social factors
- Pain and nociception are different phenomena. Pain cannot be inferred solely from activity in sensory neurons
- Through their life experiences, individuals learn the concept of pain
- A person’s report of an experience as pain should be respected
- Although pain usually serves an adaptive role, it may have adverse effects on function and social and psychological well-being
- Verbal description is only one of several behaviors to express pain; inability to communicate does not negate the possibility that a human or a nonhuman animal experiences pain.
The Relationship of Nociception and Pain
A major point from these definitions is that nociception is not equivalent to pain. Nociception starts from the noxious stimulation that activates and depolarizes the unmyelinated C or thinly myelinated Aδ nociceptive nerve fibers. Through their central branches, the signals from nociceptive neurons are conveyed to the spinal cord dorsal horn neurons, and then relayed to motor neurons in ventral spinal cord to produce motor withdrawal reflex, or to preganglionic sympathetic neurons in spinal cord intermediolateral nucleus to autonomic response to produce nociception. In other words, the neural circuitry for nociception is mainly located in the spinal cord without going up to the brain, although in certain circumstances the brain activity can suppress or amplify nociception responses.
Unlike nociception, pain is a perception that requires functional brain activity. When the nociceptive signals are sent from the spinal cord neurons to brain to produce unpleasant sensory and emotional experience, pain perception occurs. Although nociception usually produces pain perception, pain can occur without nociception, and nociception may not lead to pain. For example, when a patient under general anesthesia responds to surgical stimulation as increased heart rate and blood pressure, it is a nociception, not pain. On the other hand, a patient with central neuropathic pain after stroke can experience pain without any nociception.