Neuroanatomy, Cranial Nerve

Excerpt

The cranial nerves provide afferent and efferent innervation principally to the structures of the head and neck. Unlike spinal nerves, whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures. Unlike the spinal nerves, cranial nerve nuclei are functionally organized into distinct nuclei within the brainstem. Typically, the more posterior and lateral nuclei tend to be sensory, and the more anterior tend to be motor. Cranial nerves I (olfactory), II (optic), and VIII (vestibulocochlear) are considered purely afferent. Cranial nerves III (oculomotor), IV (trochlear), VI (abducens), XI (spinal accessory), and XII (hypoglossal) are purely efferent. The remaining cranial nerves, V (trigeminal), VII (facial), IX (glossopharyngeal), and X (vagus), are functionally mixed (sensory and motor).

While this is the classical way of organizing and indexing cranial nerves, the scientific reality is more complex and is still debated in the academic realm, including the classification and identification of the routes of distinct cranial nerves fibers and the presence or absence of other less recognized structures, such as the terminal nerve, also called nerve nulla or cranial nerve zero. The latter could be classified as a pure afferent nerve following the traditional organization of cranial nerves—damage to the cranial nerves, their tracts, or nuclei results in stereotyped clinical syndromes.