Does one pain tract control another? The medial lemniscal tract is related to motor tracts in the posterior columns. The spinoreticular catches overflow burning from the spinothalamic. Who controls?
Neurons, which make up nerves, which make up tracts once they reach the cord, more or less form discrete bundles. The spinothalamic tract is quite split up and the individual components too small to image on MRI, as would be the entire bundle, IF it ran together, which it does not. One of the interleavings is with the spinoreticular tract, which some think is dedicated to transmission of burning pain. In back of the cord are the posterior columns, easily seen large bundles which run right up the cord. The posterior columns have divisions, one of which, in the anterior region, can be used to test for somatosensory evoked potentials. In subtle cord damage, an SSEP can sometimes display cord damage when nothing else can, including MRI. To be sure the poseterior columns are involved with muscle activity, but pain is also transmitted there, especially muscle pain and visceral pain.
In this article, we omit discussion of pain carried via the autonomic system, which includes sympathetic and parasympathetic fibers. Since some think the autonomics or possibly nerves traveling with blood vessels are a conduit to bypass injured cord, the omission should not be interpreted as discounting the possibility that the autonomics are very important, as well. Ultimately, even their pain is fed back into cord, but one cannot be sure the connections function the same in injured cord. Indeed, autonomic dysfunction is common in SCI, but the locations which generate AD are not well understood.
The relationship between the spinothalamic tract, the reticulothalamic tract, and the medial lemniscal tract which receives posterior column input, has long been a question. Pain has been identified as traveling in each of these three tracts which ultimately reach the thalamus, although it has not felt to be exclusive to any of them.
Burning pain has thought to be shunted off specifically to the spinoreticular tract, but tensor analysis MRI has never been used to confirm this tract flow, nor has tensor MRI been used to track pain above the thalamus.
An unfortunate patient who experienced first a stroke in the lateral medulla (when you hear medulla, you must think of both ascending pain excitatory tracts [TNF/BDNF and progeny at synapse] as well as descending inhibitory tracts [acetyl choline and related chemicals at synapse]) which was followed by a stroke in the medial medulla.
Writing in Eur Neurol. 2007 May 4;58(1):41-43, J.S. Kim reported that a lateral infarct in the medulla caused pain in the contralateral (opposite) area subserved by the spinothalamic tract. This pain and associated paresis on that side gradually improved until 26 months later the patient suffered another stroke in the lateral medulla on the same side (ipsilateral). This caused the pain to return to its prior level and also caused deficits of the type associated with the posterior columns (lemniscal, more specifically the medial lemniscal) which can be identified clinically or with somatosensory evoked potentials. (SSEP’s test the posterior columns)
Although doctors speak of ST pain, there are actually TWO ST’s, an anterior and a lateral spinothalamic tract. One would assume that the lateral stroke caught the lateral and the medial caught the anterior, which is in fact medial (near the midline).
However, Kim made no attempt to differentiate the modalities of the central pain to try to thread out the specific central pain qualities, which might have been interesting as an attempt to differentiate the pain of the anterior vs. the pain of the lateral ST tracts. These tracts have never been studied in humans, only in primates. It is typical but unfortunate to lump all nerve injury pains together, because in fact they differ markedly. ST pain is dysesthetic while posterior column pain is not.
This sequence of clinical pain and weakness caused Kim to postulate a hierarchy of the three pain tracts: “hyperexcitation of the spinothalamic pathway by the reticulothalamic system, which in turn is modulated by the medial lemniscus pathway.
This explanation seems consistent with this patient’s course. Whether it can be generalized to all central pain is not known. In the surveys, successful treatment of lemniscal (posteror column) pain does not guarantee successful treatment of ST pain (burning to light touch and temperature change).
As we have said, at least from a leading anatomist from the Univ. of Oklahoma (reported here elsewhere), the burning pain on skin routes through the spinoreticular/reticulothalamic pathway. Further, some patients have benefit to the lancinating pains and muscle pains of the posterior columns, but retain pain in the viscera (distention pain in bladder, bowel, etc.) Existing theory places visceral pain transmission in the posterior columns/medial lemniscus. Therefore, we find Kim’s report interesting but are not certain what to make of it.
Only tensor MRI is capable of detecting flow among tracts and unfortunately this has never been attempted in such a fashion as to sort out the central pains. The ST tracts are so small that it is not certain tensor MRI could see anything. The ST tract(s) are described functionally but not anatomically. One theory that was popular for a while and still is held by some is that central pain results from failure of the three pain tracts to coordinate. This idea has also been stated as the inability of the posterior columns to regulate the ST tracts.