Helmut Segner Fish Nociception and pain A biological perspective

hyperalgesia (Tracey 2005). Also, it is at the level of the brainstem
where nociceptive information can be integrated with arousal or
autonomic processes, thereby linking nociception with overall
homeo static regulation in the organism.
The large distributed network of interconnected cortical, subcortical
and brainstem regions that is accessed during nociceptive
processing is commonly referred to as pain circuit or “pain matrix”
(Figures 3 and 4). The existence of this matrix highlights that pain
perception is not a matter of one specific brain region or brain part,
but arises from a concerted network activity. Based on neuroanatomical,
neurophysiological, pharmacological and neuroimaging
data, the pain matrix is often subdivided into a lateral and medial
pain systems, with both systems containing brainstem, subcortical
as well as cortical components (Schnakers and Zasler 2007,
Tracey and Mantyh 2007). The literature is not always consistent
regarding what regions are to be included in the two systems, but
generally the lateral pain system is considered to consist of the
lateral thalamus, primary and secondary somatosensory cortex (SI
and SII), parietal operculum and insula, while the medial system
includes the medial thalamus, anterior cingulate cortex, prefrontal
cortex, amygdala, hippocampus, hypothalamus, nucleus accumbens,
and periaqueductal grey matter (i. e. mainly components of
the “limbic system” that are known to be connected to affect). In
addition, there exist connections of both systems to motor-related
brain regions such as primary motor cortex, striatum and cerebellum,
as they are also part of the pain matrix (Peyron et al. 2000).
It has been proposed that the lateral and medial pain systems
have distinctive roles in producing pain perception: Activation
of the lateral pain system is thought to be related to the sensorydiscriminative
aspects of pain, while the medial system is believed
to participate primarily in the emotional-cognitive aspects (Peyron
et al. 2000, Price 2000). Already at the thalamic level, a subdivision
into a sensory and emotional-cognitive part appears to be
realized, as specific thalamic nuclei are interconnected with the
lateral system and other nuclei with the medial system (Millan
1999, Perl 2007). Those thalamic nuclei which belong to the medial
system are known to play a role in affective and motivational
reactions, while those of the lateral system are involved in somatosensory
functions.
Pain Matrix
cortex
spinal cord
nociceptors
sensorydiscriminative
thalamus, brain stem,
subcortex
affectivecognitive
Figure 4: Sensory, affective and
cognitive processing of nociceptive
input in the brain results in pain
sensation
The – at least partly artificial – distinction is an attempt to discriminate
between those elements of pain perception which respond
primarily to stimulus intensity, from those parts which reflect primarily
the emotional side (“unpleasantness”) of the pain sensation.
Support for the concept of an at least partial division of pain sensation
into perceptions of different quality comes from clinical observations
which show that selective lesions on the anterior cingulate
cortex alleviate the affective component of pain, while lesions to
SI/SII cortical regions affect the discriminative component of pain
26 Fish. Nociception and pain | Contributions to Ethics and Biotechnology
Fish. Nociception and pain | Contributions to Ethics and Biotechnology
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