Lab 5 - Somatosensory, Viscerosensory and Spinocerebellar Pathways

Spinothalamic Tract - Spinal Cord

The images move from the sacral cord. At each level read the corresponding section of text and identify the bold structures.

Sacral Cord

The 1° spinothalamic afferent fibers (A delta and c fibers) enter the cord and typically branch, sending collaterals to lower and higher cord segments via the tract of Lissauer. At all levels of the cord, these fibers terminate in either the nucleus posteromarginalis (2° neospinothalamics - A-delta fibers) or in the substantia gelatinosa (2° paleospinothalamics - c fibers). In turn, the 2° paleospinothalamic afferents terminate on 3° afferents in the nearby nucleus proprius. Most of the 2° neospinothalamic fibers and 3° paleospinothalamic afferents decussate in the anterior white commissure to ascend in the spinothalamic tract, contralateral to their cells of origin.

In cross sections of the cord, the spinothalamic tract extends from the anterior half of the lateral funiculus into the anterior funiculus. In many other books, this pathway is divided into lateral and anterior components. The lateral part contains the neospinothalamic fibers that terminate in the ventral posterolateral (VPL) while the anterior part contains paleospinothalamic fibers which terminate in the periaqueductal gray (PAG), mesencephalic reticular formation (MRF) and intralaminar (parafascicular-centromedian - PF-CM) thalamic nuclei. The spinothalamic tracts in the anterior and lateral funiculi are often referred to collectively as the anterolateral spinothalamic tract.

Lumbar Cord

The 1° neospinothalamic afferent (A-delta) fibers, which convey sharp, pricking, well-localized pain, terminate in thenucleus posteromarginalis. This nucleus, which constitutes Rexed's Lamina I, forms a thin layer of gray matter capping the substantia gelatinosa. The 1° paleospinothalamic (c fibers) afferents, which convey crude touch, temperature, and dull, poorly localized pain, terminate in the substantia gelatinosa or Rexed’s laminae II. This nucleus is light in appearance because the 1° paleospinothalamic fibers are unmyelinated (Type c). The target of the 2° paleospinothalamic fibers, the ipsilateral nucleus proprius (dorsal funicular gray), appears darker because these fibers are myelinated.

Thoracic Cord

The 1° spinothalamic afferents innervating the upper trunk, chest and back of the arms enter and terminate within the spinal cord. The organization of the spinothalamic pathways is identical with that in the lower spinal cord. The ascending 2° and 3° afferent axons of the spinothalamic tract are mainly contralateral to the side of the body they represent.

Cervical Cord

The 1° spinothalamic afferents innervating the shoulders, upper chest, arms and hands enter and terminate within the cervical spinal cord. The organization of the spinothalamic pathways is identical with that in the lower spinal cord. The information conveyed by the neospinothalamic and paleospinothalamic pathways is traveling toward the thalamus in the spinothalamic tract. 
Locate the approximate positions of the tract of Lissauer, nucleus posteromarginalis, substantia gelatinosa, nucleus proprius, anterior white commissure, spinothalamic tract, and fasciculus proprius.

Clinical Correlation

Syringomyelia is a disease, which causes a softening or cavitation of the spinal cord anterior white commissure. Typically the lesion is around, and most often anterior to the central canal. Syringomyelia produces bilateral deficits in pain and temperature in the body areas innervated by the affected cord segments. For example, syringomyelia affecting spinal segments C8-C4 would produce a bilateral deficit in pain and temperature from the arms, hand and fingers. This results from the destruction of the crossing fibers of the spinothalamic pathways in the anterior white commissure of spinal segments C8-C4