Table 192 Contributions of Neural Crest Cells and Placodes to Ganglia of the Cranial Nerves

Nerve

Ganglion

Origin

Oculomotor (III) Trigeminal (V)

Facial (VII)

Vestibulocochlear (VIII)

Glossopharyngeal (IX)

Ciliary (visceral efferent)

Trigeminal (general afferent)

Superior (general and special afferent) Inferior (geniculate) (general and special afferent) Sphenopalatine (visceral efferent) Submandibular (visceral efferent) Acoustic (cochlear) (special afferent) Vestibular (special afferent)

Superior (general and special afferent)

Inferior (petrosal) (general and special afferent) Otic (visceral efferent) Superior (general afferent) Inferior (nodose) (general and special afferent)

Vagal parasympathetic (visceral efferent)

Neural crest at forebrain-midbrain junction Neural crest at forebrain-midbrain junction, trigeminal placode Hindbrain neural crest, first epibranchial placode First epibranchial placode

Hindbrain neural crest

Hindbrain neural crest

Otic placode

Otic placode, hindbrain neural crest Hindbrain neural crest

Second epibranchial placode Hindbrain neural crest Hindbrain neural crest Hindbrain neural crest; third, fourth epibranchial placodes Hindbrain neural crest

Once the sympathetic chains have been established, nerve fibers originating in the visceroefferent column (intermediate horn) of the thoracolumbar segments (T1-L1,2) of the spinal cord penetrate the ganglia of the chain (Fig. 19.43). Some of these nerve fibers synapse at the same levels in the sympathetic chains or pass through the chains to preaortic or collateral ganglia (Fig. 19.43). They are known as preganglionic fibers, have a myelin sheath, and stimulate the sympathetic ganglion cells. Passing from spinal nerves to the sympathetic ganglia, they form the white communicating rami. Since the visceroefferent column extends only from the first thoracic to the second or third lumbar segment of the spinal cord, white rami are found only at these levels.

Axons of the sympathetic ganglion cells, the postganglionic fibers, have no myelin sheath. They either pass to other levels of the sympathetic chain or extend to the heart, lungs, and intestinal tract (broken lines in Fig. 19.43).

Sympathetic Neuroblasts

Figure 19.42 Formation of the sympathetic ganglia. A portion of the sympathetic neuroblasts migrates toward the proliferating mesothelium to form the medulla of the suprarenal gland.

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Figure 19.42 Formation of the sympathetic ganglia. A portion of the sympathetic neuroblasts migrates toward the proliferating mesothelium to form the medulla of the suprarenal gland.

Other fibers, the gray communicating rami, pass from the sympathetic chain to spinal nerves and from there to peripheral blood vessels, hair, and sweat glands. Gray communicating rami are found at all levels of the spinal cord.

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