7—6 The origin, course, and distribution of fibers composing the postsynaptic-posterior column system (upper) and the spinocervicothalamic pathway (lower). Postsynaptic-posterior column fibers originate primarily from cells in lamina IV (some cells in laminae III and V-II also contribute), ascend in the ipsilateral dorsal fasciculi, and end in their respective nuclei in the caudal medulla. Moderate-to-sparse collaterals project to a few other medullary targets.
Fibers of the spinocervical part of the spinocervicothalamic pathway also originate from cells in lamina IV (less so from III and V). The axons of these cells ascend in the posterior part of the lateral funiculus (this is sometimes called the dorsolateral funiculus) and end in a topographic fashion in the lateral cervical nucleus: lumbosacral projections terminate posterolaterally and cervical projections anteromedially. Cells of the posterior column nuclei and the lateral cervical nucleus convey information to the contralateral thalamus via the medial lemniscus.
Neurotransmitters: Glutamate ( + ) and possibly substance P ( + ) are present in some spinocervical projections. Because some cells in laminae III-V have axons that collateralize to both the lateral cervi cal nucleus and the dorsal column nuclei, glutamate (and substance P) may also be present in some postsynaptic dorsal column fibers.
Clinical Correlations: The postsynaptic-posterior column and spinocervicothalamic pathways are not known to be major circuits in the human nervous system. However, the occurrence of these fibers may explain a well known clinical observation. Patients that have received an anterolateral cordotomy (this lesion is placed just ventral to the denticulate ligament) for intractable pain may experience complete or partial relief, or there may be a recurrence of pain perception within days or weeks. Although the cordotomy transects fibers of the antero-lateral system (the main pain pathway), this lesion spares the posterior horn, posterior columns, and spinocervical fibers. Consequently, the recurrence of pain perception (or even the partial relief of pain) in these patients may be explained by these postsynaptic-dorsal column and spinocervicothalamic projections. Through these connections, some nociceptive (pain) information may be transmitted to the ventral posterolateral nucleus and on to the sensory cortex, via circuits that bypass the anterolateral system and are spared in a cordotomy.
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