Cell Columns of the Spinal Cord and Brainstem

Spinal nerves contain 4 functional fiber types: general somatic afferents (GSA), general visceral afferents (GVA), general somatic efferents (GSE) and general visceral efferents (GVE). In cranial nerves there are 3 additional functional components; special somatic afferents (SSA), special visceral (branchial) efferents (SVE) and special visceral (branchial) afferents (SVA). Cranial nerves may contain anywhere from 1 to 5 fiber types. Hence, representatives of their cell columns will only be present at those levels of the brain where there is a cranial nerve that requires them i.e., many cell columns will be discontinuous.

In the spinal cord, the components are represented by columns of cells, extending longitudinally, that are more or less continuous. The basal plate gives rise to the GSEs and GVEs in the ventral gray horn. The GSE cell column is a continuous cell column (supplying skeletal muscle of somite origin) throughout the cord. The GVE cell column is an interrupted one - it is represented by the intermediolateral cell column from a) C8 - L2 and b) S2 - S4. The alar plate gives rise to the a) continuous GSA cell column represented in the dorsal gray by nucleus posteromarginalis, substantia gelatinosa, and nucleus proprius, b) nucleus dorsalis (of Clarke) found between C8 - L3 at the base of the dorsal gray horn, and c) nucleus gracilis and cuneatus which is present in the low medulla but which receives discriminative touch and proprioception from spinal cord levels. Sensory fibers from the viscera enter the cord and synapse on the GVA cell column at the base of the dorsal gray horn between C8 - L2 and S2 - S4.

The brainstem develops similarly to the cord i.e., basal and alar plates, but as the roof of the 4th ventricle thins and the medulla and pons “open up”, alar plate derivatives come to lie laterally to those of the basal plate. Also, the addition of special sensory and special motor components introduce certain changes from the plan of the cord. None-the-less, the concept of cell columns of specific neuronal components, continuous from the cord and extending into the brain stem, is a useful one. The concept allows for the anticipation of the location of even discontinuous cell columns; functional cell columns will appear in approximately the same dorsal-ventral or medial-lateral positions in whatever level of the brainstem they appear. This concept provides a logical scheme to the CNS in spite of what at first appears to be random.

1 - The continuous GSA cell column of the spinal cord dorsal gray horn which handles pain, temperature and touch sensations is continued without interruption into the medulla, pons and midbrain. In these locations it is variably known as the spinal nucleus of V, principal sensory nucleus of (of CN V) and the mesencephalic nucleus (of CN V). Any CN carrying GSA information will terminate in these trigeminal nuclei, derived from the alar plate.

Proprioceptive GSA information destined for the cerebellum from the lower extremity terminated in the nucleus dorsalis of the cord. Similar proprioceptive information from the upper extremity passes to the analagous rostral representation called the accessory cuneate nucleus in the low medulla.

2 - The discontinuous GVA cell column of the spinal cord dorsal gray horn is recognized throughout the medulla when it resumes as the nucleus of the tractus solitarius. Any CN carrying GVA information will terminate in this cell column.

3 - The continuous GSE cell column of the spinal cord ventral gray horn is represented throughout the brainstem by an interrupted cell column of neurons supplying skeletal muscle of head somite origin. In the medulla there is the motor nucleus of CN XII, in the pons there is the motor nucleus of CN VI, in the midbrain it is represented by the the nearly continuous cell column of CNs III and IV.

4 - The discontinuous GVE cell column of the spinal cord ventral gray horn resumes in the medulla as the dorsal motor nucleus of the vagus and the slightly more rostral inferior salivatory nucleus (of CN IX). In the caudal pons this discontinuous cell column is represented by the superior salivatory nucleus (of CN VII). In the midbrain, this interrupted cell column appears again as the nucleus of Edinger-Westphal (of CN III) at the level of the superior colliculus.

5 - Special visceral (branchial) afferents (SVA) are fibers from taste buds and are carried by CNs VII, IX and X. Upon entering the brainstem, they travel with the GVAs, in the tractus solitarius to the most rostral end of its nucleus (in the pons) where it is called the gustatory portion of the solitary nucleus.

6 - Special visceral (branchial) efferents (SVE) are represented in the brainstem by an interrupted cell column that has no equivalent in the cord. These neurons innervate skeletal muscle that developed from the mesoderm of the branchial arches e.g., larynx, pharynx, facial muscles, muscles of mastication. In the medulla it is represented by nucleus ambiguus (CN IX and X) while in the pons it is represented by the motor nuclei of CN VII and V.

7 - Special somatic afferents (SSA) are found only in supraspinal locations and are represented by the laterally-lying vestibular and auditory nuclei in the medulla and pons. They develop from branchial arch structures.

Cranial nerves I and II serve vision (SSA) and olfaction (SVA). However, these are not true nerves. Rather they develop as evaginations of the telencephalon (olfactory nerve) and the diencephalon (optic nerve).

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