Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Peripheral Nervous System, systema nervosum periphericum — General Overview
Cranial Nerves

Cranial Nerves belong to the Peripheral Nervous system. They innervate the Skin, Muscles, and glands in the HEAD region, while some extend to the neck, Thorax, and abdominal cavities (Fig. 179).

The first accounts of cranial nerves date back to the works of Erasistratus and Herophilus (4th–3rd centuries BC) and Claudius Galen (2nd century AD). A. Vesalius (1543) described seven pairs of cranial nerves, and during the 17th and 18th centuries, K. Varolio, R. Vieussens, H. Wrisberg, and I. Prochaska clarified many details of their Structure. In the late 18th century, S. Soemmerring numbered the cranial nerves, designating them with Roman numerals from I to XII. Almost a century later, V. M. Bekhterev divided the VIII cranial nerve into two: the cochlear and vestibular nerves. Even later, the VII cranial nerve was divided into the facial and intermediate nerves.

Thus, humans actually have fourteen cranial nerves, but by tradition they are numbered as follows:

The Specific features of cranial nerve formation in phylo- and ontogenesis are associated with The Development of Sense Organs and branchial arches, as well as the reduction of myotomes in the head region. The olfactory and optic nerves are Neural Pathways connecting the olfactory and visual organs to the Brain. The oculomotor, trochlear, and abducens nerves developed in connection with preotic myotomes; they innervate the derivatives of these myotomes—the Muscles of the Eyeball. The trigeminal, facial, and glossopharyngeal nerves are branchial nerves and therefore innervate the skin, muscles, and glands that developed from the corresponding branchial arches. During evolution, the vestibulocochlear nerve separated from the facial nerve to provide specialized innervation for the organs of Hearing and Balance. The accessory nerve evolved in higher mammals through the Separation of the caudal portion of the vagus nerve. The hypoglossal nerve first appears in amniotes As a result of the fusion of roots emerging from Spinal Nerves.

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Fig. 179. Cranial nerves, nervi craniales (general diagram)

Unlike spinal nerves, cranial nerves have lost their segmental distribution pattern and acquired a high degree of Specificity. As a result, functionally integrated organs receive innervation from a single source only, and damage to neural pathways is not compensated for by adjacent nerves. For instance, the facial muscles are innervated by the facial nerve; in case of its neuritis, the recovery of paralyzed Muscle Function is possible only upon the regeneration of the nerve itself.

Unlike spinal nerves, which are formed by the fusion of anterior and posterior roots, cranial nerves III, IV, VI, XI, and XII correspond to the anterior ROOT, while nerves V, VII, VIII, IX, and X can be regarded as homologues of the posterior roots.

Like spinal nerves, cranial nerves (except I and II) possess sensory, motor, and autonomic nuclei located in the Brainstem, which correspond to the nuclei of the posterior, anterior, and lateral horns of the Spinal Cord. Within cranial nerves, one can distinguish somatic sensory nerve fibers, which transmit impulses from exteroceptors of the skin, hearing, balance, and Vision organs (II, V, VIII); visceral sensory fibers, which conduct impulses from the olfactory and taste organs, as well as visceral chemoreceptors (I, V, VII, IX, X); somatic motor fibers, which innervate striated skeletal muscles (III, IV, V, VI, VII, IX, X, XI, XII); and visceral motor fibers, which innervate Glandular Epithelium, smooth muscle, and cardiac muscle (VII, IX, X). Thus, based on their fiber composition, the olfactory, optic, and vestibulocochlear nerves are sensory; the oculomotor, abducens, trochlear, accessory, and hypoglossal are motor; and the trigeminal, intermediofacial, glossopharyngeal, and vagus nerves are mixed.