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Definition: cranial nerves from Dictionary of Psychological Testing, Assessment and Treatment

nerves which enter and leave the brain without the intermediary of the spinal cord. The nerves are numbered (traditionally in Roman numerals) from 1 to 12: I Olfactory, II Optic, III Oculomotor, IV Trochlear, V Trigeminal; VI Abducens; VII Facial; VIII Auditory-vestibular, IX Glossopharyngeal; X Vagus, XI Spinal Accessory, and XII Hypoglossal. Often the nerves are referred to by their number (e.g. the facial cranial nerve is the ‘seventh nerve’). Compare with spinal nerves.


Summary Article: Cranial Nerves from The Brain, the Nervous System, and Their Diseases

The cranial nerves are a series of 12 paired nerves that originate from the cranium (head), 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 (smell), vision (sight), gustation (taste), audition (hearing), and equilibrium (balance) 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; while, 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.

Cranial nerves of the brain.

(My-MS.org)

Consider the example of a person eating a strawberry. The perception of the strawberry includes its appearance (visual system), taste (gustatory system), smell (olfactory system), and texture (somatosensory system). 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 different cranial nerves. All these nerves and organs that allow for these sensations and movements are present in the head.

Naming Technique

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 (toward the nose/beak)—the olfactory nerve or cranial nerve I—to the most caudal (toward the tail)—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, are learning techniques used to help retain information. The most famous mnemonic device used to remember the cranial nerves is “On old Olympus 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 have been using 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, sensory (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 big brother say big brains matter most.” For this saying, the first letter of each word indicates if the nerve is sensory, motor, or both. The name, nerve type, and function of each cranial nerve are summarized in the nerve table (Table C.2).

Anatomy and Physiology

Cranial nerves are bundles of axons, slender projections from neurons—which are specialized cells of the nervous system that conduct signals. For sensory cranial nerves, the cell bodies are usually found in specialized structures called ganglia (a collection of cell bodies). 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 (or axons) do not cross the midline and synapse on the same side of the face, head, or neck.

Table C.2 Cranial Nerves

Number

Name

Type

Function

I

Olfactory

Sensory

Smell

II

Optic

Sensory

Vision

III

Oculomotor

Motor

Eye and eyelid movement

Lens shape and pupil size

IV

Trochlear

Motor

Eye movement

V1

Trigeminal

Sensory

Somatosensation (temperature, pressure, pain) from top third of face

(Ophthalmic branch)

V2

Trigeminal

Sensory

Somatosensation from upper jaw

(Maxillary branch)

V3

Trigeminal

Mixed

Somatosensation from lower jaw

(Mandibular branch)

(both sensory and motor)

Movement of muscles involved in chewing

VI

Abducens

Motor

Eye movement

VII

Facial

Mixed

Taste, anterior tongue

Somatosensation from ear

Movement of facial muscles

Tear and saliva secretion

VIII

Vestibulocochlear

Sensory

Hearing and balance

(auditory)

IX

Glossopharyngeal

Mixed

Taste, posterior tongue

Visceral sensation (Carotid bodies)

Somatosensation from posterior tongue

Saliva secretion

X

Vagus

Mixed

Movement of pharynx and larynx (speech)

Autonomic functions of gut, heart, and lungs

Somatosensation from external ear

XI

Accessory

Motor

Movement of shoulder and neck muscles

XII

Hypoglossal

Motor

Movement of tongue

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. These nerves innervate the ipsilateral eye muscles, which are located on the same side as where the axons exit the brainstem. 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, producing both the stimulated and nonstimulated pupils to constrict (get smaller). This is a fast, easy, and noninvasive cranial nerve test that is used to help diagnose concussions. The hypoglossal nerve is used to control the movement of the ipsilateral half of the tongue muscle. To test for the health of this nerve, patients are asked to stick out their tongue, which should point out straight. If the tongue should deviate to one side, then it suggests that the nerve on the opposite side (contralateral) of the tongue direction may be damaged.

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 (the embryological gill 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 (temperature, pressure, and pain) 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 VII are essential for controlling the muscles used for facial expression in the ipsilateral half of the face as well as the stapedius. In humans, the stapedius is the smallest skeletal muscle used to stabilize the stapes in 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 innervates the posterior tongue and carries the sense of taste and somatosensation from the posterior tongue. It, along with the facial nerve, helps control saliva secretion. Portions of the IX 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 where 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. As stated earlier, the motor portion of cranial nerve X controls the muscles used for speech and swallowing along with cranial nerve IX.

Lastly, there are three pure sensory cranial nerve pairs, which are the olfactory (I, the sense of smell), optic (II, sight), and auditory (vestibulocochlear, VIII, hearing and equilibrium) 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 animals. Thus, the 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, crosses at the optic chiasm, and terminates in the back of the cerebrum—in the occipital lobe. It is through the optic nerve where vision is conveyed from the eye to the brain. Many mammals are highly visual and in these animals most 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, if a person becomes “sea sick” they may be able to alleviate/overcome that sensation by looking at the horizon where visually it is more stable.

See also Abducens Nerve; Accessory Nerve; Bell's Palsy (Bell Palsy); Facial Nerve; Glossopharyngeal Nerve; Hypoglossal Nerve; Nerves; Oculomotor Nerve; Olfactory Nerve; Optic Nerve; Trigeminal Nerve; Trochlear Nerve; Vagus Nerve; Vestibulocochlear Nerve

Further Reading
  • 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). Sinauer Associates, Inc Sunderland MA.
  • Herlevich, N. E. (1990). Reflecting on old Olympus’ towering tops. Retrieved from www.ncbi.nlm.nih.gov/pubmed/2254946.
  • Wilson-Pauwels, L.; Akesson, E. J.; Stewart, P. A. (1988). Cranial nerves: Anatomy and clinical comments. B.C. Decker Incorporated Philadelphia PA.
  • C. J. Saunders
    Jennifer L. Hellier
    Copyright 2014 by Jennifer L. Hellier

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