ABSTRACT
The cranial nerves, as the name implies, lie within the cranial fossae, some may travel in the head, neck, while others continue their paths through the thoracic and abdominal cavities, innervating structures in these areas. They are classified according to their functional components and connections into purely sensory nerves (e.g., olfactory and optic); motor nerves (e.g., oculomotor, trochlear, abducens, accessory, and hypoglossal); and mixed nerves, containing both sensory and motor components (e.g., trigeminal, facial, glossopharyngeal, and vagus). The axons of the bipolar neurons in the olfactory mucosa form the olfactory nerve, while that of the Scarpa’s and spiral ganglia form vestibulocochlear nerve. In the same manner the axons of the multipolar neurons of the retina make the optic nerve. Due to its origin from the retina, a telencephalic structure, the optic nerve, is considered as an extension of the central nervous system and is affected by central demyelinating diseases. The general and special sensory fibers are the central processes of the unipolar neurons of the geniculate ganglion of the facial nerve, and the bipolar neurons of the superior and inferior ganglia of the glossopharyngeal and vagus nerves. The motor fibers within the cranial nerves represent the axons of the multipolar neurons. A lesion of a cranial nerve or associated nucleus produces manifestations of lower motor neuron palsy: atrophy, flaccidity, and areflexia or hyporeflexia with the exception of the abducens nerve and nucleus. Affected structures usually deviate to the side of the lesion, with the exception a lesion of the vagus nerve, which produces deviation toward the intact side. Cranial nerve motor nuclei receive evenly distributed bilateral corticobulbar fibers with the exception of the facial motor neurons to the upper face which receives bilateral cortical input compared to neurons of the lower face which receive only contralateral cortical input. A lesion that disupts a cranial nerve and adjacent part of the corticospinal fibers produces alternating hemiplegia. Cranial nerves and associated nuclei can be affected in variety of conditions, such as the lateral medullary syndrome, Benedikt syndrome.
