MODULE 9
THE SOMATIC EFFERENT CELL COLUMN
- - - The oculomotor (III), trochlear (IV) abducens (VI) nerves innervate the extraocular muscles. The hypoglossal (XII) nerve innervates the intrinsic and extrinsic muscles of the tongue. The nuclei of these cranial nerves together form the SOMATIC EFFERENT CELL COLUMN, shown in Figure 9-1. In addition, the oculomotor nerve conveys parasympathetic preganglionic axons to the ciliary ganglion (GVE component). The trochlear nerve is unusual in that its axons, after leaving the trochlear nucleus (present in the midbrain), arch caudally and dorsally around the edge of the periaqueductal gray, decussate in the anterior medullary velum and emerge from the brainstem on its dorsal surface at the junction of the pons and midbrain just caudal to the inferior colliculus (Figure 9-2). The further course of the trochlear nerve can be seen in a lateral view (Figure 2-16). The axons that will form the oculomotor and abducens nerves pass ventrally through the brain stem and emerge from its ventromedial surface. All 3 nerves then converge on the region of the cavernous sinus, as shown in Blumenfeld Figs. 13.2 and 13.4. The position of the nerves within the sinus is best seen in frontal sections such as Blumenfeld's Figure 13.11.
- - - The hypoglossal nerve is included here not because it has a functional relationship to the other three nerves, but rather because the motor neurons that contribute axons to the nerve are found in the same, somatic efferent cell column. The rootlets of the nerve are attached to the ventral surface of the medulla in the sulcus between the pyramid and the inferior olive.
- - - This is a good time to review some of the figures you have seen in past modules. It will work best if you have one of the brainstem models available as you do so. They are in the Gross lab.
- - - - - - Figure 2-14 - shows the exiting cranial nerves on a ventral view of the brainstem.
- - - - - - Figure 2-47 - a similar view
- - - - - - Figure 2-41, Figure 2-42, Figure 2-43 - views of the nerves as they exit the brainstem
- - - - - - Figure 8-25 - look at the "dissected" dorsal surface of the brainstem model and find the nuclei we are dealing with in this module.
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