(Maintained by . E-mail suggestions to crockett@umdnj.edu.)
David P. Crockett, Ph.D.
Department of Neuroscience and Cell Biology
UMDNJ-Robert Wood Johnson Medical School
Room R-306, Telephone: 263-3404
Revised: March 14, 2000
Ascending Pathways:
1. The pathway is primarily associated with low-threshold mechanosensation (fine touch) as well as proprioception. This phylogenetically recent pathway can transmit information to the cortex with a minimum of synapses involving three neurons.
2. In addition to sending several axon terminals to arborize within the dorsal horn, many primary sensory afferents (largely, Aa ; Ab fibers), have a central ascending axon within the dorsal column, which terminate within one of the dorsal column nuclei, located in the dorsal medulla.
3. Primary afferents with cell bodies located in dorsal root ganglia rostral to T6 have axon collaterals traveling in the cuneate fasciculus, terminating in the cuneate nucleus. Caudal to T6, the primary afferents follow the gracile fasciculus, to terminate in the gracile nucleus. Together the gracile and cuneate nuclei make up the dorsal column nuclei.
4. Rostral ly, the two dorsal column fasciculi do not carry identical sensory information: The cuneate fasciculus contains both cutaneous and proprioceptive afferents. The rostral gracile fasciculus carries only cutaneous afferents. Proprioceptive afferents for the lower (caudal) body terminate within Clarke's Column of the spinal grey matter.
5. Postsynaptic dorsal column system: The dorsal column also contains axons of second-order neurons. The cell bodies are located medial aspects of the dorsal horn and their axons terminate in either the cuneate or gracile nucleus, depending on segment of origin. The functions of the PSDC cells are not fully understood. In cats, at least, they have been suggested to play a role in nociception.
6. Second order DCN cells issue axons that cross the midline (internal arcuate fibers), which rostral ly form the medial lemniscus and join up with the trigeminothalamic fibers, terminating in the ventrobasal complex of the thalamus. Fibers originating in the trigeminal system end in the ventroposterior medial nucleus (VPM) while DCN cells end in the ventroposterior lateral nucleus (VPL) of the thalamus.
7. From the ventrobasal complex, somatosensory information is relayed to the primary somatosensory cortex (areas 3a, 3b, 1, 2).
The central pathways associated with pain (nociceptive) and temperature are much more complex than the dorsal column medial lemniscus system.
1. Spinothalamic (also known as the neospinothalamic, lateral spinothalamic) tract (most developed in primates):
Cells of origin: Laminae I and V-VII.
Cellular characteristics: Nociceptive specific and WDR.
Route: Fibers cross midline and ascend the anterior lateral quadrant of the spinal white matter.
2. Spinoreticular tract (Spinoreticulothalamic, paleospinothalamic):
Cells of origin: Laminae VII; VIII.
Cellular characteristics: Nociceptive specific
Route: ascends anterior lateral quadrant; largely crossed with some fibers ascending ipsilaterally. Some fibers terminate only in the reticular formation of the brainstem; others send collaterals to the reticular formation and the thalamus.
3. Spinomesencephalic tract:
Cells of origin: Laminae I and V.
Cellular characteristics: Nociceptive.
Route: Fibers terminate in the midbrain reticular formation, lateral part of the periaqueductal grey (reciprocally interconnected with the limbic system via the hypothalamus) and other midbrain sites.
4. Spinocervical tract:
Cells of origin: Laminae III and IV
Cellular characteristics: Although most cells within these laminae respond to low threshold mechanostimulation a few are also activated by noxious stimulation (WDR).
Route: Axons ascend the ipsilateral dorsal lateral funiculus terminating in the lateral cervical nucleus (LCN). LCN neurons cross the midline and join the medial lemniscal fibers. Collaterals to midbrain nuclei.
5. Spinomedullary (Postsynaptic Dorsal Column: Not part of the ALS) tract:
Cells of origin: Laminae III and IV
Cellular Characteristics: Possibly WDR (in cats)
Route: ipsilateral dorsal columns terminating in the DCN.
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1. Medial Nuclear Group (Intralaminar nuclear complex; central lateral nucleus):
Cells of origin: lamina VI-VIII directly and indirectly from the reticular formation (medullary) This indirect pathway (paleospinothalamic tract) is bilateral. The wide spread projections (basal ganglia; many cortical areas) of these neurons suggest a role in nonspecific arousal as well as nociception.
Cellular characteristics: Large complex receptive fields
2. Lateral Nuclear Group (Ventrobasal Complex; posterior nuclei):
Cells of origin: Laminae I and V (neospinothalamic tract)
Cellular characteristics: WDR.
1. Destruction of the primary somatosensory cortex does not seem to impair pain processing.
2. Recent evidence regions of the Cingulate cortex (area 24) may be important for pain processing (Talbot et al., 1991, Science, 251:1355).