Cranial Nerve Development, Distribution, and Function

Abstract

Human evolution has produced a prodigious brain and fashioned an elaborate mechanism of convolution and gyrification to fit the neural tissue inside a rigid boney chamber. Critical systems, from olfaction and vision, to control of muscles for articulated speech, have been concentrated in the head, through a set of cranial nerves and their nuclei, located mainly in the brainstem. This presentation will trace the development, describe the distribution, and explore the functions of those cranial nerves.Early in development, dorsal and ventral columns emerge in the gray matter of the neural tube. Dorsally, the alar plate differentiates into sensory regions in both the spinal cord (dorsal horns) and the brainstem (sensory nuclei). The ventral portion of the neural tube, the basal plate, is the source of motor structures including the spinal cord ventral horn and brainstem motor nuclei. Bilaterally, a longitudinal furrow in the wall of the central canal, the sulcus limitans, divides the alar and basal plates and establishes the spatial segregation of motor and sensory neuronal systems. As the embryo elongates it differentiates into a five‐vesicle structure that is the progenitor of forebrain cerebral hemispheres and diencephalon, and brainstem midbrain, pons, and medulla.With the formation of the five‐vesicle embryo, the lumen of the central canal begins to elaborate rostrally to form the ventricular system. The lateral ventricles expand within the core of the cerebral hemispheres, and the third ventricle opens on the midline with the diencephalon forming its walls. The cerebral aqueduct is the ventricular correlate of the midbrain, connecting the third and fourth ventricles. The fourth ventricle is roughly pyramidal, tapering upward as it receives the cerebral aqueduct, being broadest at the pons‐medulla junction, and tapering again caudally to become continuous with the central canal of the spinal cord. The sulcus limitans persists in the walls of the fourth ventricle, separating alar plate derivative sensory nuclei dorsolaterally from the basal plate‐derived motor nuclei ventromedially. This effectively establishes the layout of brainstem cranial nerve nuclei. Functionally related nuclear columns are distributed throughout the rostro‐caudal length of the brainstem in consistent relative locations.Cranial nerves (CN) I (olfactory) and II (optic) are the only cranial nerves that have no brainstem nuclei, and their circuity is in the forebrain. Nuclei of CN III, the oculomotor nerve, and CN IV, the trochlear nerve occupy the midbrain. Cranial nerves V (trigeminal), VI (abducens) and the motor nucleus of CN VII (facial motor) are found in the pons, and CNn VIII (vestibulocochlear), IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal) develop in the medulla. Extensive cephalization of the brain and cranial nerves has shaped our facial morphology. The functions of the cranial nerves are well known, and predictable and diagnostic deficits appear after injuries.