Lab 3 (ƒ5) - Somatosensory, Viscerosensory and Spinocerebellar Pathways
Spinal Cord to Diencephalon (continued)
The images move from the brainstem to the diencephalon. View and identify the structures in the illustration.
Spinal Cord
The organization of the 1°, 2°, and 3° afferents of the spinal viscerosensory pathway are similar to that of the paleospinothalamics. The 1° afferent neurons have cell soma in the posterior root ganglia, the 2° afferents - in the substantia gelatinosa and 3° afferents - in the nucleus proprius. The 3° spinal viscerosensory afferents ascend bilaterally in or along the spinothalamic tracts.
Lower Medulla
The 3° spinal viscerosensory afferents are traveling bilaterally with the spinothalamic tract. Note that any of visceral afferents terminate in the reticular formation and on visceral motor neurons and are involved in unconscious visceral reflex control mechanisms. Those visceral afferents that do not reach the thalamus are considered to be non-sensory in function.
Mid Medulla
Viscerosensory afferents in the brain stem arrive from two sources -- the spinal cord and the cranial nerves (primarily IX and X). The 3° spinal visceral afferents ascend in the medulla within the spinothalamic tracts. The 1° cranial nerve visceral afferents travel in the tractus solitarius and synapse in the caudal two-thirds of the nucleus solitarius. The 2° cranial viscerosensory afferents are believed to ascend bilaterally in or near the central tegmental tract, which is posterior to the inferior olive.
Upper (Open) Medulla
At this level, the 1° visceral cranial afferents enter the brainstem via the vagus nerve. The 1° afferent fibers travel in the tractus solitarius to their sites of termination in the caudal two-thirds of the nucleus solitarius.
While the ascending course of the 2° cranial viscerosensory fibers is not known, it is believed the fibers travel in or near the central tegmental tract. The bulk of the fibers in the central tegmental tract terminate in the inferior olive. The ascending 3° spinal viscerosensory afferents continue to ascend bilaterally in the spinothalamic tracts.
Mid Pons
The crossed and uncrossed 3° spinal viscerosensory afferents travel bilaterally in the spinothalamic tracts. The crossed and uncrossed 2° cranial viscerosensory afferents are assumed to travel in or near the central tegmental tract. The central tegmental tract is located posterior to the medial lemniscus in the pons tegmentum.
Midbrain
The 3° spinal viscerosensory afferents continue to travel bilaterally with the spinothalamic tracts. The crossed and uncrossed 2° cranial viscerosensory afferents continue to travel in or near the central tegmental tract. The central tegmental tract has been displaced posteriorly by the decussating fibers of the superior cerebellar peduncle in the pons tegmentum.
Diencephalon
The 3° spinal viscerosensory afferents traveling with the spinothalamic tract and the 2° cranial viscerosensory afferents that traveled with the central tegmental tract end in the intralaminar nuclei of the thalamus. While the intralaminar nuclei project diffusely to the cortex, a viscerosensory area has been identified in the insula cortex.