OVERVIEW OF THE CRANIAL NERVES
Cranial nerves (CNN) emerge through openings in the skull and are covered by tubular sheaths of connective tissue derived from the cranial meninges. Twelve pairs of cranial nerves are numbered I to XII from rostral to caudal, according to their attachment to the brain. The names of the cranial nerves reflect their general distribution and function. Like spinal nerves, cranial nerves are bundles of sensory and motor neurons that conduct impulses from sensory receptors and innervate muscles or glands.
RAPID REVIEW OF THE NERVOUS SYSTEM
To best understand the cranial nerves the following information is helpful to remember:
Neuron versus nerve. A neuron is a single sensory or motor nerve cell, whereas a nerve is a bundle of neuronal fibers (axons). Cranial nerves have three types of sensory and three types of motor neurons, known as modalities. Therefore, a nerve may be composed of a combination of sensory or motor neurons (e.g., the facial nerve possesses sensory and motor neurons).
Ganglion. A ganglion is a collection of nerve cell bodies in the peripheral nervous system.
Nucleus. A nucleus is a collection of nerve cell bodies in the central nervous system (CNS).
The 12 pairs of cranial nerves may have one or a combination of the following sensory and motor modalities (Figure 17-1; Table 17-1):