The cranial nerves are a series of 12 paired nerves that originate from the cranium, as opposed to the spinal nerves that originate from the spinal cord. These nerves are responsible for conducting both motor and sensory information to and from the head, face, and neck. Additionally, some of these nerves are responsible for conducting the information of the special senses such as olfaction, vision, gustatory, auditory, and equilibrium to the brain. Most cranial nerves carry only sensory or motor information. However, one-third of the cranial nerves are “mixed” because they carry both sensory and motor information. In animals, sensory information is the input from the outside world; motor information causes a reaction in response to the outside world. In most bilaterally symmetrical animals, including humans, many of the specialized senses are found in the cranium.
Consider the example of a person eating a strawberry. The perception of the strawberry includes its appearance, taste, smell, and texture. Each of these sensory modalities is mediated by a different cranial nerve. Additionally, the physical action of eating the strawberry requires specific movements of the tongue and jaw, which are caused by other cranial nerves. All these nerves and organs that allow for these sensations and movements are present in the head.
Cranial nerves are named by the special sense they serve or for the muscles they innervate. In addition to their names, cranial nerves are numbered by Roman numerals starting with the most rostral—the olfactory nerve or cranial nerve I—to the most caudal—the hypoglossal nerve or cranial nerve XII. A popular way to remember the names and the order of the 12 cranial nerves is by using mnemonic devices, which is a learning technique to help retain information. The most famous mnemonic device used to remember the cranial nerves is “On old Olympus’s towering tops, a Finn and German viewed some hops.” For this saying, the first letter of each word represents the first letter of each cranial nerve in order. Students use this device to remember the order of the 12 cranial nerves, which is olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, auditory (or vestibulocochlear), glossopharyngeal, vagus, spinal (or accessory), and hypoglossal.
There are also mnemonic devices for remembering which cranial nerves are sensory, motor, or both. The most common of these phrases is “Some say marry money, but my brother says big brains matter most.” For this saying, the first letter of each word indicates if the nerve is sensory, motor, or both.
Anatomy and Physiology
Cranial nerves are bundles of axons that exit or enter the cranium. For sensory cranial nerves, the cell bodies are usually found in specialized structures called ganglia. These ganglia are generally located between their peripheral target (somewhere on the head and/or neck) and the central nervous system. This is not particularly true for motor cranial nerves. Motor nerves generally have their cell bodies located in the brainstem in structures called nuclei and their axons travel toward their target. Most of these nerves do not cross the midline and synapse on the same side of the face, head, or neck.
The simplest cranial nerves—meaning that only motor information is carried to skeletal muscles—are the oculomotor (III), trochlear (IV), abducens (VI), and hypoglossal (XII). All four of these nerves arise from nuclei located near the midline of the brainstem. Cranial nerves III, IV, and VI are used in concert to control eye movement, particularly during complex actions like tracking moving objects. The oculomotor nerve is also responsible for the shape of the lens and pupil size of the eye, which is evident in the consensual pupillary reflex. This is a normal reflex that is caused when one pupil is stimulated with light, causing both the stimulated and nonstimulated pupils to constrict. This is a fast, easy, and noninvasive cranial nerve test that is used to help diagnose concussions.
More complex cranial nerves are the trigeminal (V), facial (VII), glossopharyngeal (IX), vagus (X), and accessory (XI). In vertebrates, these nerves innervate skeletal muscles that originate from the brachial arches. Of these more complex cranial nerves, all but XI are mixed nerves, meaning that they carry both sensory and motor axons. The accessory nerve consists of purely motor axons that assist in moving shoulder and neck muscles.
The trigeminal nerve receives its name because it has three distinct branches—V1, V2, and V3. Both V1 (ophthalmic) and V2 (maxillary) branches carry only sensory information, specifically somatosensation from the top third of the face and the upper jaw, respectively. The mandibular branch (V3) is mixed and carries somatosensation from the lower jaw as well as movement of the muscles involved in chewing.
The facial nerve (VII) is mixed and brings taste information from the anterior tongue, somatosensation from the ear, and controls tear and saliva secretion. The motor axons of nerve VII are essential for controlling the muscles used for facial expression in the ipsilateral half of the face as well as the stapedius muscle of the middle ear. Damage to cranial nerve VII will cause the ipsilateral half of the face to have weakness and the decreased ability to have facial expressions such that the person cannot smile or frown. Furthermore, tear and saliva secretion may be altered on the same side of the face. This type of damage is generally called Bell’s palsy.
The glossopharyngeal is also a mixed nerve. It carries the sense of taste and somatosensation from the posterior tongue. Along with the facial nerve, it helps control saliva secretion. Portions of the glossopharyngeal nerve called the carotid bodies sense chemical changes in the blood, which are visceral sensations. For the motor component of the glossopharyngeal nerve, it controls the muscles of the larynx and pharynx along with the motor portion of the vagus nerve (X). These muscles are used in the action of swallowing and speech.
The vagus nerve is very thick and large in humans compared to other cranial nerves. This is because the vagus nerve, which means wandering in Latin, exits the medulla from the brainstem down to the abdomen. It is here that cranial nerve X serves autonomic functions for the gut, heart, and lungs. These are the primary preganglionic parasympathetic neurons used in the autonomic nervous system. The sensory portion of the vagus nerve innervates the external ear and carries somatosensory information.
Lastly, there are three pure sensory cranial nerve pairs, which are the olfactory (I), optic (II), and auditory (or vestibulocochlear, VIII) nerves. The axons of olfactory sensory neurons (OSN) make up cranial nerve I and pass through the ethmoid bone between the eye sockets and terminate in the olfactory bulb. OSNs are unique in that they are in direct contact with the outside world in the nasal cavity and synapse directly into the brain. These neurons are also exceptional because they are continually replaced throughout the life of the animal. Thus, newborn OSNs must guide their axons through the ethmoid bone and synapse in the correct location in the olfactory bulb to maintain the sense of smell.
The optic nerve (II) is relatively thick compared to most cranial nerves. It consists of axons from ganglion cells that leave the retina of the eye, cross at the optic chiasm, and terminate in the occipital lobe. Through the optic nerve, vision is conveyed from the eye to the brain. Most mammals are highly visual and in these animals the majority of the brain is wired to understand the outside world through vision, such as location in space, shape and color of an object, and determining brightness.
Finally, the auditory (vestibulocochlear, VIII) nerve is used for both hearing and balance (equilibrium). It travels from the inner ear to the brainstem. Together with the optic nerve, the vestibulocochlear portion provides the perception of orientation and head movement in space. For example, people who become “seasick” may be able to alleviate/overcome that sensation by looking at the horizon where visually it is more stable.
C. J. Saunders and Jennifer L. Hellier
See also: Bell’s Palsy; Facial Nerve; Glossopharyngeal Nerve; Nerves; Olfactory Nerve; Optic Nerve; Trigeminal Nerve; Vagus Nerve; Vestibulocochlear Nerve
Appendix A: The brainstem and cranial nerves. (2004). In D. Purves, G. J. Augustine, D. Fitzpatrick, W. C. Hall, A. LaMantia, J. O. McNamara, & S. M. Williams (Eds.), Neuroscience (3rd ed., pp. 755—761). Sunderland, MA: Sinauer Associates.
Herlevich, N. E. (1990). Reflecting on old Olympus’ towering tops. Retrieved from www.ncbi.nlm.nih.gov/pubmed/2254946