By the fourth week of development, nuclei for all 12 cranial nerves are present. All except the olfactory (I) and optic (II) nerves arise from the brainstem, and of these only the oculomotor (III) arises outside the region of the hindbrain. In the hindbrain, proliferation centers in the neuroepithelium establish eight distinct segments, the rhombomeres. These rhombomeres give rise to motor nuclei of cranial nerves IV, V, VI, VII, IX, X, XI, and XII (Figs. 19.17 and 19.41). Establishment of this segmental pattern appears to be directed by mesoderm collected into somitomeres beneath the overlying neuroepithelium.
Motor neurons for cranial nuclei are within the brainstem, while sensory ganglia are outside of the brain. Thus the organization of cranial nerves is homologous to that of spinal nerves, although not all cranial nerves contain both motor and sensory fibers (Table 19.1, p. 473-474).
Cranial nerve sensory ganglia originate from ectodermal placodes and neural crest cells. Ectodermal placodes include the nasal, otic, and four epi-branchial placodes represented by ectodermal thickenings dorsal to the pharyngeal (branchial) arches (Table 19.2, p. 475; see Fig. 15.2). Epibranchial pla-codes contribute to ganglia for nerves of the pharyngeal arches (V, VII, IX, and X). Parasympathetic (visceral efferent) ganglia are derived from neural crest cells, and their fibers are carried by cranial nerves III, VII, IX, and X (Table 19.1).
table 19.1 Origins of Cranial Nerves and Their Composition
Cranial Nerve Brain Region
Olfactory (I) Telencephalon Optic (II) Diencephalon
Oculomotor (III) Mesencephalon
Trochlear (IV) Metencephalon (exits mesencephalon) Trigeminal (V) Metencephalon
Abducens (VI) Facial (VII)
Vestibulo- Metencephalon cochlear (VIII)
Glossopharyn- Myelencephalon geal (IX)
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