The cranial nerves of vertebrates, when compared with the spinal nerves, show a great diversity in configuration, distribution, and function and the same diversity can also be noted in their development in the embryo.
To start with, the two most anterior nerve pairs, nerves I (olfactory) and II (optic), are completely different in nature and cannot be in any way compared with the spinal nerves.
Nerve I (olfactory) consists of fibers which are processes of the primary nerve cells of the olfactory organ. The fibers are not joined together in a compact bundle (or two bundles), so that one could speak of many olfactory nerves rather than of only a pair of nerves being present. The nerves are very short and enter the olfactory lobe of the telencephalon. The fibers of the olfactory nerves establish connections with nerve cells in the telencephalon.
Nerve II (optic) is not really a peripheral nerve at all but must be regarded as a nerve tract in the central nervous system. The sensory retina of the eye develops at the expense of a lateral outgrowth of the brain wall and is thus essentially a part of the central nervous system. The stalk of the eye vesicle which connects the eye rudiment to the brain serves as the pathway for the optic nerve.
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When the cells of the nervous retina differentiate (the development of the eye), the ganglion cells situated on the distal surface of the sensory retina produce axons which grow along the stalk and enter the brain. In the optic chiasma, the fibers cross to the opposite side and end in the contralateral half of the brain, but in mammals a smaller portion do not cross and thus end in the ipsilateral side.
The following nerve pairs (III to X in fishes and amphibians and III to XII in amniotes) may be compared to a certain extent with spinal nerves, even though the comparison cannot be complete in any case.
The three nerves supplying the eye muscles develop much like the ventral (motor) roots of the spinal nerves; they are mainly constituted of axons growing out from groups of motor neurons located in the ventrolateral plates of the brain.
The mixed and sensory cranial nerves—V, VII, VIII, IX, and X—possess ganglia, and in this respect they resemble the dorsal roots of the spinal nerves. The development of the cranial ganglia differs, however, from that of the spinal ganglia. It has been proved experimentally, and is generally recognized, that the spinal ganglia develop from cells of the neural crest.
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The cranial ganglia, on the other hand, derive their material from two sources – from the neural crest and from thickenings of the epidermis on the sides of the head, known as the placodes. Two rows of such thickenings can be distinguished in the lower vertebrates (fishes and amphibians), an upper row of dorsolateral placodes and a lower row of epibranchial placodes.
The dorsolateral placodes play a part in the development of the lateral line sense organs and also the ear, which may be said to develop from one of the placodes. The epibranchial placodes are located just above the gill slits. In amniotes the placodes are not as distinct as in lower vertebrates, but nevertheless, migration of cells from slightly thickened portions of the epidermis and their contribution to the formation of nerve ganglia have been definitely established.
Simple observation proved to be inadequate to establish the contribution of the two sources to individual ganglia, with the result that widely divergent views were held on the subject by different authors. In recent years, however, experimental methods have been applied, and the question of the origin of the various cranial ganglia has been solved satisfactorily.
The experimental approach consists in cutting out parts of the neural tube with the neural crest in early embryos and observing whether particular ganglia are formed without the neural crest. A counterpart of such an experiment is the removal of a strip of epidermis on the side of the head containing the presumptive material of the placodes.
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Cranial ganglia developing after operations of the first type must be derived from the placodes; those which are found after operations of the second type can be presumed to be derived from the neural crest. Much of the work on the origin of cranial ganglia was done on chick embryos.
Another possible approach to the problem is the transplantation of epidermis between amphibian embryos differing in the amount of pigment in cells. If epidermis of a darkly pigmented species is transplanted to a host having light pigmentation, the ganglia formed from the transplanted epidermis can be recognized at once by their dark pigment (Balinsky, unpublished experiments).
By applying experimental methods, it has been established that the ganglion of the trigeminal nerve (V), the ganglion semi-lunare, is composed of both neural crest cells and cells from a placode (presumably a dorsolateral placode). The cells of placodal origin are mainly responsible for the outgrowth of the purely somatic sensory ophthalmic and maxillary nerves and for the somatic sensory fibers of the mandibular nerve.
The facial nerve (VII) in aquatic vertebrates supplies innervation to the lateral line canals of the head and the muscles in the region of the hyoid arch. In terrestrial vertebrates the lateral line sense organs disappear, and the muscles of the hyoid arch change their function, becoming the facial muscles in mammals and man. Accordingly, the facial nerve becomes mainly a visceral motor nerve in higher vertebrates.
The ganglion of the facial nerve in the higher vertebrates, the geniculate ganglion, is derived almost exclusively from an epibranchial placode, while in fishes it receives a contribution from a dorsolateral placode forming a special upper section of the ganglion. This section disappears with the disappearance of the lateral line sense organs.
The acoustic nerve (VIII) is a special nerve and may be considered as a specially differentiated section of the facial nerve. The roots of the two nerves lie very close to each other in fishes and amphibians, though they are clearly separated in amniotes. The acoustic ganglion is derived from cells of the ear rudiment, the ear vesicle. As the ear vesicle is formed from a thickening of the epidermis belonging to the dorsolateral placodal system, the acoustic ganglion may be said to be of placodal origin.
The glossopharyngeal nerve and the vagus have much in common in their composition and development. In both, the cells of the ganglia are derived from two sources, the neural crest and the placodes, but contrary to what is seen in the semilunar ganglion of the trigeminal nerve, cells from different sources do not join together in one ganglion but, instead, form separate ganglia. The glossopharyngeal nerve (IX) has an upper “root” ganglion, consisting of cells of neural crest origin, and a lower petrosal ganglion, derived from an epibranchial placode.
The vagus nerve (X) also has two main ganglia, the upper ganglion jugulare, corresponding to the root ganglion of the glossopharyngeal nerve and consisting of cells derived from the neural crest, and a lower ganglion nodosum, derived from epibranchial placodes. In addition to these a number of accessory ganglia adjoin the jugular ganglion posteriorly.
These ganglia are connected to the medulla by independent rootlets, but the fibers of the accessory ganglia connecting them with the periphery are joined to the fibers of the main vagus nerve. The presence of several ganglia connected to the vagus is additional proof of the poly-segmental nature of this nerve. The accessory ganglia of the vagus represent a transition from the cranial to the spinal nerve system.
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The spinal accessory nerve (XI) and the hypoglossal nerve (XII) of amniotes are formed of fibers growing out from motor cells lying in the floor of the medulla.