Cranial nerves

Cranial nerves are nerves that emerge directly from the brain, including the brainstem, in contrast to spinal nerves, which emerge from segments of the spinal cord. They relay information between the brain and various parts of the body, primarily to the head and neck regions and are responsible for special senses of vision, taste, smell, and hearing.
The cranial nerves emerge from the central nervous system above the level of the first vertebra of the vertebral column. Each cranial nerve is paired and is present on both sides.
There are conventionally twelve pairs of cranial nerves, which are described with Roman numerals I–XII. Some considered there to be thirteen pairs of cranial nerves, including the non-paired cranial nerve zero. The numbering of the cranial nerves is based on the order in which they emerge from the brain and brainstem, from front to back.
The terminal nerves, olfactory nerves and optic nerves emerge from the cerebrum, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain.
The cranial nerves are considered components of the peripheral nervous system, although on a structural level the olfactory, optic, and trigeminal nerves are more accurately considered part of the central nervous system.

Anatomy

Most typically, humans are considered to have twelve pairs of cranial nerves, with the terminal nerve more recently canonized. The nerves are: the olfactory nerve, the optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, and the hypoglossal nerve.

Terminology

Cranial nerves are generally named according to their structure or function. For example, the olfactory nerve supplies smell, and the facial nerve supplies the muscles of the face. Because Latin was the lingua franca of the study of anatomy when the nerves were first documented, recorded, and discussed, many nerves maintain Latin or Greek names, including the trochlear nerve, named according to its structure, as it supplies a muscle that attaches to a pulley. The trigeminal nerve is named in accordance with its three components, and the vagus nerve is named for its wandering course.
Cranial nerves are numbered based on their position from front to back of their position on the brain, as, when viewing the forebrain and brainstem from below, they are often visible in their numeric order. For example, the olfactory nerves and optic nerves arise from the base of the forebrain, and the other nerves, III to XII, arise from the brainstem.
Cranial nerves have paths within and outside the skull. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by which the nerves can exit the skull. All cranial nerves are paired, which means they occur on both the right and left sides of the body. The muscle, skin, or additional function supplied by a nerve, on the same side of the body as the side it originates from, is an ipsilateral function. If the function is on the opposite side to the origin of the nerve, this is known as a contralateral function.

Intracranial course

Nuclei

, all cranial nerves have a nucleus. With the exception of the olfactory nerve and optic nerve, all the nuclei are present in the brainstem.
The midbrain has the nuclei of the oculomotor nerve and trochlear nerve ; the pons has the nuclei of the trigeminal nerve, abducens nerve, facial nerve and vestibulocochlear nerve ; and the medulla has the nuclei of the glossopharyngeal nerve, vagus nerve, accessory nerve and hypoglossal nerve. The olfactory nerve emerges from the olfactory bulb, and depending slightly on division the optic nerve is considered to emerge from the lateral geniculate nuclei.
Because each nerve may have several functions, the nerve fibres that make up the nerve may collect in more than one nucleus. For example, the trigeminal nerve, which has a sensory and a motor role, has at least four nuclei.

Exiting the brainstem

With the exception of the olfactory nerve and optic nerve, the cranial nerves emerge from the brainstem. The oculomotor nerve and trochlear nerve emerge from the midbrain, the trigeminal, abducens, facial and vestibulocochlear from the pons, and the glossopharyngeal, vagus, accessory and hypoglossal emerge from the medulla.
The olfactory nerve and optic nerve emerge separately. The olfactory nerves emerge from the olfactory bulbs on either side of the crista galli, a bony projection below the frontal lobe, and the optic nerves emerge from the lateral colliculus, swellings on either side of the temporal lobes of the brain.

Ganglia

The cranial nerves give rise to a number of ganglia, collections of the cell bodies of neurons in the nerves that are outside of the brain. These ganglia are both parasympathetic and sensory ganglia.
The sensory ganglia of the cranial nerves, directly correspond to the dorsal root ganglia of spinal nerves and are known as cranial nerve ganglia. Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X. There are also a number of parasympathetic cranial nerve ganglia. Sympathetic ganglia supplying the head and neck reside in the upper regions of the sympathetic trunk, and do not belong to the cranial nerves.
The ganglion of the sensory nerves, which are similar in structure to the dorsal root ganglion of the spinal cord, include:

The trigeminal ganglia of the trigeminal nerve, which occupies a space in the dura mater called Meckel's cave. This ganglion contains only the sensory fibres of the trigeminal nerve.
The geniculate ganglion of the facial nerve, which occurs just after the nerve enters the facial canal.
A superior and inferior ganglia of the glossopharyngeal nerve, which occurs just after it passes through the jugular foramen.

Additional ganglia for nerves with parasympathetic function exist, and include the ciliary ganglion of the oculomotor nerve, the pterygopalatine ganglion of the maxillary nerve, the submandibular ganglion of the lingual nerve, a branch of the facial nerve, and the otic ganglion of the glossopharyngeal nerve.

Exiting the skull and extracranial course

After emerging from the brain, the cranial nerves travel within the skull, and some must leave it in order to reach their destinations. Often the nerves pass through holes in the skull, called foramina, as they travel to their destinations. Other nerves pass through bony canals, longer pathways enclosed by bone. These foramina and canals may contain more than one cranial nerve and may also contain blood vessels.

The terminal nerve is a thin network of fibers associated with the dura and lamina terminalis running rostral to the olfactory nerve, with projections through the cribriform plate.
The olfactory nerve passes through perforations in the cribriform plate part of the ethmoid bone. The nerve fibres end in the upper nasal cavity.
The optic nerve passes through the optic foramen in the sphenoid bone as it travels to the eye.
The oculomotor nerve, trochlear nerve, abducens nerve and the ophthalmic branch of the trigeminal nerve travel through the cavernous sinus into the superior orbital fissure, passing out of the skull into the orbit.
The maxillary division of the trigeminal nerve passes through foramen rotundum in the sphenoid bone.
The mandibular division of the trigeminal nerve passes through foramen ovale of the sphenoid bone.
The facial nerve and vestibulocochlear nerve both enter the internal auditory canal in the temporal bone. The facial nerve then reaches the side of the face by using the stylomastoid foramen, also in the temporal bone. Its fibers then spread out to reach and control all of the muscles of facial expression. The vestibulocochlear nerve reaches the organs that control balance and hearing in the temporal bone and therefore does not reach the external surface of the skull.
The glossopharyngeal, vagus and accessory nerve all leave the skull via the jugular foramen to enter the neck. The glossopharyngeal nerve provides sensation to the upper throat and the back of the tongue, the vagus supplies the muscles in the larynx and continues downward to supply parasympathetic supply to the chest and abdomen. The accessory nerve controls the trapezius and sternocleidomastoid muscles in the neck and shoulder.
The hypoglossal nerve exits the skull using the hypoglossal canal in the occipital bone.
Development

The cranial nerves are formed from the contribution of two specialized embryonic cell populations, cranial neural crest and ectodermal placodes. The components of the sensory nervous system of the head are derived from the neural crest and from an embryonic cell population developing in close proximity, the cranial sensory placodes. The dual origin cranial nerves are summarized in the following table:
Contributions of neural crest cells and placodes to ganglia and cranial nerves

Cranial nerve	Ganglion and type	Origin of neurons
CNI – olfactory		Telencephalon/olfactory placode; NCCs at forebrain
CNIII – oculomotor	Ciliary, visceral efferent	NCCs at forebrain-midbrain junction
CNV – trigeminal	Trigeminal, general afferent	NCCs at forebrain-midbrain junction, trigeminal placode
CNVII – facial	-Superior, general and special afferent -Inferior: geniculate, general and special afferent -Sphenopalatine, visceral efferent -Submandibular, visceral efferent	-Hindbrain NCCs, 1st epibranchial placode -1st epibranchial placode -Hindbrain NCCs -Hindbrain NCCs
CNVIII – Vestibulocochlear	-Acoustic: cochlear, special afferent; and vestibular, special afferent	-Otic placode and hindbrain NCCs
CNIX – glossopharyngeal	-Superior, general and special afferent -Inferior, petrosal, general and special afferent -Otic, visceral efferent	-Hindbrain NCCs -2nd epibranchial placode -Hindbrain NCCs
CNX – vagus Superior laryngeal branch; and recurrent laryngeal branch	-Superior, general afferent -Inferior: nodose, general and special afferent -Vagal: parasympathetic, visceral efferent	-Hindbrain NCCs -Hindbrain NCCs ; 3rd and 4th epibranchial placodes -Hindbrain NCCs
CNXI – accessory	No ganglion *	Hindbrain ; NCCs

Abbreviations: CN, cranial nerve; m, purely motor nerve; mix, mixed nerve ; NC, neural crest; PA, pharyngeal arch; r, rhombomere; s, purely sensory nerve. * There is no known ganglion of the accessory nerve. The cranial part of the accessory nerve sends occasional branches to the superior ganglion of the vagus nerve.