Students should have a basic understanding of how the functions of cranial nerves are tested clinically. Here are two examples of resources on the web. Although these in-depth exams are probably not practical in most situations, they provide nice demonstrations of testing techniques.
The purpose of this chapter is to summarize the 12 pairs of cranial nerves—truly the “meat and potatoes” of head and neck anatomy. It is important that you understand the big picture of the cranial nerves:
Their names
Their components
For example: Sensory information from skin, motor information to skeletal or smooth muscles, etc.
How they exit or enter the skull
Their attachments to the brain and the nuclei within the brain they are associated with
The regions of the head and neck they supply
The details of the nerves can follow once you learn this foundational material and use it as a scaffolding to build a map of each nerve.
Overview of cranial nerves
The cranial nerves, like the spinal nerves we discussed previously, are part of the peripheral nervous system. There are 12 pairs of cranial nerves and each nerve is identified by a name and a roman numeral (e.g., the third cranial nerve is named the oculomotor nerve—it is also identified as cranial nerve III). Remember that nerves are found only in the peripheral nervous system. There are no nerves in the central nervous system.
Question
Review of spinal nerves
Recall that spinal nerves arise from the spinal cord.
All spinal nerves are mixed nerves.
Each spinal nerve is formed by the union of a ventral (anterior) root and a dorsal (posterior) root.
The ventral root contains only motor (efferent) nerve fibers and the dorsal root contains only sensory (afferent) nerve fibers.
Spinal nerves exit from the vertebral column through intervertebral foramina.
Once outside the vertebral column, most spinal nerves divide into a ventral (anterior) ramus and a dorsal (posterior) ramus.
Cranial nerves and spinal nerves
Differences
Similarities
Figure 29.1 Attachment of cranial nerves to brainstem, ventral view.
Unlike spinal nerves, cranial nerves are NOT formed by the union of ventral roots and dorsal roots. This means that not all cranial nerves are mixed nerves.
Cranial nerves DO NOT attach to the spinal cord—instead they attach to the brain. More specifically, nine of the twelve cranial nerves attach to the brainstem = midbrain, pons, and medulla. Of these nine cranial nerves, all but one attach to the ventral surface of the brainstem. Only the trochlear nerve (cranial nerve IV) attaches to the dorsal surface of the brain. The attachments of cranial nerves to the brainstem is illustrated in Figure 29.1.
Cranial nerves I and II attach to more rostral (cranial) areas of the brain and you should consider these two nerves separately from the other ten because each of these cranial nerves are associated with tracts of nerve fibers—the tracts are parts of the central nervous system that have developed as extensions of the cerebrum (in the case of cranial nerve I) and the diencephalon (in the case of CN II).
Cranial nerves, being attached to the brain, must exit the skull in order to course throughout the head and neck to reach their targets. Cranial nerves leave the skull through bony openings = canals, meatuses, fissures, and foramina. As you study the cranial nerves and their branches you must consider and learn these entrance and exit passageways.
Some branches of cranial nerves connect with other cranial nerves (e.g., chorda tympani nerve of CN VII joins the lingual nerve of V3).
Spinal nerve fibers join some cranial nerves—for example, cervical spinal nerve fibers join the hypoglossal nerve (CN XII).
Like spinal nerves, many of the cranial nerves are associated with ganglia = both sensory ganglia and autonomic (motor) ganglia.
Sensory ganglia of cranial nerves contain cell bodies of pseudo-unipolar neurons—just like spinal nerves have dorsal root (spinal) ganglia. These ganglia are summarized in Table 29.1.
Table 29.1
Cranial nerve
Sensory ganglion
Trigeminal (V)
Trigeminal ganglion
Facial (VII)
Geniculate ganglion
Vestibulocochlear (VIII)
Cochlear ganglion and Vestibular ganglion
Glossopharyngeal (IX)
Superior and inferior ganglia of IX
Vagus (X)
Superior and inferior ganglia of X
Figure 29.2 Cranial nerve nuclei in the brainstem (brainstem is transparent). CLINICALLY ORIENTED ANATOMY, FIGURE 9.5.
Recall that motor information in spinal nerves flows out from the ventral side of the spinal cord via ventral roots. The cell bodies of these motor neurons are in the ventral gray column in the spinal cord. Sensory information in spinal nerves enters the dorsal side of the spinal cord via the dorsal roots. This information is relayed to interneuron cell bodies in the dorsal gray column in the spinal cord. Similarly, motor fibers in cranial nerves originate from motor nuclei in the brainstem, and these tend to be located on the ventral side of the brainstem. Sensory fibers in cranial nerves carry their information into the brainstem to relay on sensory nuclei within. These tend to be located on the dorsal side of the brainstem. Cranial nerves are associated with motor and sensory nuclei inthe brainstem.These are the equivalents of ventral and dorsal columns of gray in the spinal cord, respectively. The cranial nerve nuclei in the brainstem are illustrated in Figure 29.2.
Parasympathetic ganglia associated with cranial nerves
Another name for the parasympathetic division of the ANS is “craniosacral outflow.” Preganglionic parasympathetic neurons whose cell bodies are in the brainstem have axons that “flow out” of the brainstem in cranial nerves III, VII, IX and X. These preganglionic fibers synapse on postganglionic neuron cells bodies located in four parasympathetic ganglia in the head. These ganglia have special names:
CN III
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Preganglionic fibers associated with CN III synapse on postganglionic neurons in the ciliary ganglion.
CN VII
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Preganglionic fibers associated with CN VII synapse on postganglionic neurons in the pterygopalatine ganglion and submandibular ganglion. Note that preganglionic parasympathetic fibers carried in cranial nerve VII synapse in two different parasympathetic ganglia in the head.
CN IX
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Preganglionic fibers associated with CN IX synapse on postganglionic neurons in the otic ganglion.
CN X
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What about cranial nerve X? Preganglionic parasympathetic fibers that course within the vagus nerves synapse on postganglionic neurons in intramural ganglia = ganglia in or near the walls of the target organs.
Sympathetic ganglia and cranial nerves
Like spinal nerves, most of the cranial nerves carry with them postganglionic sympathetic nerve fibers to smooth muscle and glands in the head and neck. The cell bodies of these postganglionic neurons are in the superior cervical ganglia of the sympathetic trunks, which are the upper most of all chain ganglia, located at the level of C-2. Postganglionic sympathetic fibers leave the superior cervical ganglia and form plexuses around the carotid arteries. The arteries and plexuses form the “highways” by which sympathetic fibers reach regions of the head and neck—from the plexuses, sympathetic fibers “hitch- a-ride” on cranial nerve branches to their ultimate destinations. For example, postganglionic sympathetic fibers reach the face along branches of CN V = the major nerve of the face.
Some cranial nerves have purely sensory functions—they carry only special sensory information (sense of smell, visual information, or hearing and balance information):
Some cranial nerves are mainly motor nerves. Most of the fibers within these nerves carry efferent impulses to skeletal muscles in the head and neck:
Oculomotor
Trochlear
Abducens
Spinal Accessory
Hypoglossal.
NOTE: Since these nerves serve skeletal muscles they probably also contain afferent fibers that carry proprioception: information about muscle stretch from muscle spindles. We won’t dote on this detail.
The remaining four cranial nerves are mixed cranial nerves—meaning they contain a mixture of sensory and motor nerve fibers. These nerves are the most complex. Mixed cranial nerves often carry mixtures of efferent fibers that innervate both skeletal and smooth muscle and afferent fibers that relay general somatic and visceral information from skin, mucous membranes, and viscera, and in some cases special visceral information from taste buds. The mixed cranial nerves are:
