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Cranial Nerves
CN 0 - Cranial nerve zero
CN I - Olfactory
CN II - Optic
CN III - Oculomotor
CN IV - Trochlear
CN V - Trigeminal
CN VI - Abducens
CN VII - Facial
CN VIII - Vestibulocochlear
CN IX - Glossopharyngeal
CN X - Vagus
CN XI - Accessory
CN XII - Hypoglossal

Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in humans fit this description, twelve are conventionally recognized. The nerves from the third onward arise from the brain stem. Except for the tenth and the eleventh nerve, they primarily serve the motor and sensory systems of the head and neck region. However, unlike peripheral nerves which are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories.

Names of Nerves

The 12 pairs of cranial nerves are traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nuclei lie in the brain stem, e.g. Cranial Nerve III (the Oculomotor nerve) leaves the brainstem at a higher position than Cranial nerve XII, whose origin is located more caudally (lower) than the other cranial nerves.

Number Name Sensory,
motor,
or both
Origin Nuclei Function
I Olfactory Purely sensory Telencephalon Anterior olfactory nucleus Transmits the sense of smell from the nasal cavity.[1] Located in olfactory foramina in the cribriform plate of ethmoid.
II Optic Purely sensory Retinal ganglion cells [Lateral geniculate nucleus][2] Transmits visual signals from the retina of the eye to the brain.[3] Located in the optic canal.
III Oculomotor Mainly motor Anterior aspect of Midbrain Oculomotor nucleus, Edinger-Westphal nucleus Innervates the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique, which collectively perform most eye movements. Also innervates the sphincter pupillae and the muscles of the ciliary body. Located in the superior orbital fissure.
IV Trochlear motor Dorsal aspect of Midbrain Trochlear nucleus Innervates the superior oblique muscle, which depresses, rotates laterally, and intorts the eyeball. Located in the superior orbital fissure.
V Trigeminal Both sensory and motor Pons Principal sensory trigeminal nucleus, Spinal trigeminal nucleus, Mesencephalic trigeminal nucleus, Trigeminal motor nucleus Receives sensation from the face and innervates the muscles of mastication. Located in the superior orbital fissure (ophthalmic nerve - V1), foramen rotundum (maxillary nerve - V2), and foramen ovale (mandibular nerve - V3).
VI Abducens Mainly motor Nuclei lying under the floor of the fourth ventricle Pons Abducens nucleus Innervates the lateral rectus, which abducts the eye. Located in the superior orbital fissure.
VII Facial Both sensory and motor Pons (cerebellopontine angle) above olive Facial nucleus, Solitary nucleus, Superior salivary nucleus Provides motor innervation to the muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, and stapedius muscle. Also receives the special sense of taste from the anterior 2/3 of the tongue and provides secretomotor innervation to the salivary glands (except parotid) and the lacrimal gland. Located in and runs through the internal acoustic canal to the facial canal and exits at the stylomastoid foramen.
VIII Acoustic or Vestibulocochlear (or auditory-vestibular nerve or acoustic nerve) Mostly sensory Lateral to CN VII (cerebellopontine angle) Vestibular nuclei, Cochlear nuclei Senses sound, rotation, and gravity (essential for balance and movement). More specifically, the vestibular branch carries impulses for equilibrium and the cochlear branch carries impulses for hearing. Located in the internal acoustic canal.
IX Glossopharyngeal Both sensory and motor Medulla Nucleus ambiguus, Inferior salivary nucleus, Solitary nucleus Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid gland, and provides motor innervation to the stylopharyngeus. Some sensation is also relayed to the brain from the palatine tonsils. Located in the jugular foramen.
X Vagus Both sensory and motor Posterolateral sulcus of Medulla Nucleus ambiguus, Dorsal motor vagal nucleus, Solitary nucleus Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles (except the stylopharyngeus, which is innervated by the glossopharyngeal). Also provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure. Receives the special sense of taste from the epiglottis. A major function: controls muscles for voice and resonance and the soft palate. Symptoms of damage: dysphagia (swallowing problems), velopharyngeal insufficiency. Located in the jugular foramen.
XI Accessory or spinal-accessory (or cranial accessory nerve or spinal accessory nerve) Mainly motor Cranial and Spinal Roots Nucleus ambiguus, Spinal accessory nucleus Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus nerve (CN X). Symptoms of damage: inability to shrug, weak head movement. Located in the jugular foramen.
XII Hypoglossal Mainly motor Medulla Hypoglossal nucleus Provides motor innervation to the muscles of the tongue (except for the palatoglossus, which is innervated by the vagus nerve) and other glossal muscles. Important for swallowing (bolus formation) and speech articulation. Located in the hypoglossal canal.
File:Parasympathetic head ganglia.jpg

Nerves and nuclei

As well as the visible nerves outside of the brain, most of the cranial nerves have associated nuclei within the brainstem. These nuclei are areas of grey matter, and damage to them can have a similar affect to the severing of an actual nerve. Axons to (and from) cranial nerves synapse first at the nuclei.

Arrangement of the nuclei

Just as grey matter in the ventral (closer to front of a human) spinal cord tends to be efferent (motor) fibers, and the dorsal horn tends to contain sensory neurons, nuclei in the brainstem are arranged in an analogous way.

  • Close to the midline are the somatic efferent nuclei, such as the oculomotor nucleus, which control skeletal muscle. Just lateral to this are the autonomic (or visceral) efferent nuclei (for instance, the Edinger-Westphal nucleus, which controls tears).
  • There is a separation, called the sulcus limitans, and lateral to this are the sensory nuclei. Near the sulcus limitans are the visceral afferent nuclei, namely the solitary tract nucleus.
  • More lateral, but also less posterior, are the general somatic afferent nuclei. This is the trigeminal nucleus. Back at the dorsal surface of the brainstem, and more lateral are the special somatic afferents, this handles sensation such as balance.
  • Another area, not on the dorsum of the brainstem, is where the branchial efferent nuclei reside. These formed from the branchial arches, in the embryo. This area is a bit below the autonomic motor nuclei, and includes the nucleus ambiguus, facial nerve nucleus, as well as the motor part of the trigeminal nerve nucleus.


Cranial nerves in non-human vertebrates

Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. Cranial nerves XI and XII evolved in the common ancestor to amniotes (non-amphibian tetrapods) thus totalling twelve pairs. These characters are synapomorphies for their respective clades. In some primitive cartilagenous fishes, such as the dogfish (Squalos acanthos), there is a terminal nerve numbered zero (as it exits the brain before the first cranial nerve).

Mnemonic devices

As the list is important to keep in mind during the examination of the nervous system, there are many mnemonic devices in circulation to help remember the names and order of the cranial nerves. Because the mind recalls rhymes well, the best mnemonics often use rhyming schemes. The best known example is, "On Old Olympus' Towering Tops A Finn And German Viewed Some Hops,"[4] where And represents auditory vestibular and Some represents spinal accessory. There are numerous mnemonics one example: "Ohh, Ohh, Oh, To Touch And Feel Virgin Girls' Vaginas And Hymens"

Another to help remember the types of information these nerves carry (sensory, motor, or both) is thus:

  • Some Say Money Matters, But My Brother Says Big Boobs Matter More.

For additional memonics, see b:Transwiki:List of mnemonics for the cranial nerves


Thirteen cranial nerves?

Although twelve nerves are classically described in humans, there is a theory that finds favour with some authors, that humans really have thirteen cranial nerves (Andy Lelli , 1999). If the C1 spinal nerve were considered the thirteenth cranial nerve, C2 through C8 would be renamed as the first through seventh cervical nerves (though anatomists might change the term 'cervical' to 'nuchal' or some other term to avoid confusion between the two nomenclatures). This would make the numbering system consistent all the way through the spinal column - every nerve would exit below its corresponding vertebra, and the number of vertebrae would equal the number of spinal nerves.

Alternatively, C1 could be considered the spinal root of the hypoglossal nerve (CN XII). This would, again, make the numbering system consistent.

See also

External links

References

  1. McCracken, Thomas (2000). New Atlas of Human Anatomy, 1–240, China: MetroBooks.
  2. Henry Gray (1821–1865). Anatomy of the Human Body. 1918.
  3. McCracken, Thomass (2000). New Atlas of Human Anatomy, 1–240, China: MetroBooks.
  4. Herlevich NE (1990). Reflecting on old Olympus' towering tops. Journal of ophthalmic nursing & technology 9 (6): 245–6.
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