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Pain pathways

Created: 16/6/2004
 

Figure 2: Pain pathways I

Image of the spinothalamic tracts


Pain fibres terminate mainly in the superficial dorsal horn (laminae I- II). Ad fibres enter lamina I (and V) and synapse on a second set of neurons. These neurons will carry the signal to the thalamus and are part of the spinothalamic tract (STT). The C fibres enter the spinal cord and synapse on lamina I cells and lamina II interneurons - neurons that make synaptic connections with other cells within the local environment. The interneurons convey the signal to the STT cells that reside mainly in laminae I, IV and V. The axons of the STT cells project across the spinal cord to the STT, which is located in the ventrolateral quadrant of the contralateral spinal cord white matter. 

Figure 3: Temperature and pain pathways II



The STT transmits information about temperature and pain, as well as “simple” touch (i.e. related to localisation of stimulus) and visceral sensations. It mediates the discriminative and arousal-emotional components of these sensations by separating out the “fast” (discriminative aspect) and “slow” (affective aspect) components of pain into different regions of the tract that are transmitted in parallel to the thalamus. Discriminative pain reaches the thalamus directly without making connections elsewhere in the nervous system, whereas arousal-emotional pain reaches the thalamus indirectly via connections with brainstem regions. Slow pain is also transmitted by other pathways such as the spinoreticular tract.

The STT may be divided into the lateral STT and the anterior STT. Pain and temperature is transmitted mainly in the lateral STT. The lateral-STT transmits the sensations of both fast and slow pain. The anterior STT conveys sensations of simple touch (stimulus localisation). The STT ascends the entire length of the cord and the brainstem, staying in about the same location all the way up. It is here in the brainstem that the different modalities separate out to terminate in different thalamic and brainstem nuclei. The fast pain STT axons terminate in the ventroposterior nucleus, which comprises the ventral posterolateral (VPL) and ventral posteromedial (VPM) and the posterior (PO) nuclei. These axons seem to mediate mainly the sense of “simple touch” and pain. These sensations are separated within the thalamus: neurons in the VPL and VPM do not respond specifically to noxious stimulation, whereas cells in the PO receive inputs from both low- and high-threshold afferents. These cells are associated with the conscious perception of pain.

The slow pain-STT axons innervate the non-specific intralaminar nuclei of the thalamus, and the reticular formation in the brainstem. These axons form at least part of the forebrain pain pathway associated with the affective quality (unpleasantness and fear of further injury) of pain and can be dissociated from the discriminative quality (the type and nature of the injury itself). The projections to the reticular formation may underlie the arousal effects of painful stimuli. The arousal itself may activate noradrenergic neurons in the locus coeruleus, and thus decrease the upward pain transmission. This may be an example of a negative feedback loop in the nervous system.

Table 2: Comparison of central pathways for pain transmission

Direct (fast)
Indirect (slow)
Tract
Lateral-STT
Lateral-STT
Spinoreticular tract (SRT)
Origin
Lamina I & IV, V
Lamina I, IV,V, (and VII, VIII)
Somatotopic organisation
Yes
No
Body representation
Contralateral
Bilateral
Synapse in reticular formation
No
Yes
Sub-cortical targets
None
Hypothalamus
Limbic system
Autonomic centres
Thalamic nucleus
Ventral posterolateral (VPL)
Intra-laminar nuclei
Other midline nuclei
Cortical location
Parietal lobe (SI cortex)
Cingulate gyrus
Role
Discriminative pain (quality intensity, location)
Affective-arousal components of pain
Other functions
Temperature
Simple touch

It has long been known that the STT is an important pain pathway because when it is damaged, pain and temperature sense is abolished on the contralateral side of the body below the lesion. It has been used, as a last resort, by surgeons to relieve intractable cancer pain. However, pain is not permanently abolished because of preservation of one side of the bilateral indirect pathways. Also, the transmission of simple tactile modalities (detection, location) via the anterior STT explains why touch sensation is preserved in people with dorsal column lesions (although they are unable to discriminate the nature of the stimulus).


ArticleDate:20040616
SiteSection: Article
 
   
    
                                            
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