The 12 Cranial Nerves
Most of the nerves that provide motor and sensory innervation throughout the body arise from the spinal cord. However, there are a few – twelve pairs, to be exact – that have intracranial origins. Although these cranial nerves originate within the cranial vault, they have far-reaching impact throughout the body.
There are two generally acceptable nomenclatures for cranial nerves. They can either be addressed by their specific names, or by the abbreviation CN followed by their representative Roman numeral.
|CN I||Olfactory nerve (sensory) - Oh|
|CN II||Optic nerve (sensory) - Oh|
|CN III||Oculomotor nerve (motor) - Oh|
|CN IV||Trochlear nerve (motor) - To|
|CN V||Trigeminal nerve (mixed) - Touch|
|CN VI||Abducens nerve (motor) - And|
|CN VII||Facial nerve (mixed) - Feel|
|CN VIII||Vestibulocochlear nerve (sensory) - Very|
|CN IX||Glossopharyngeal nerve (mixed) - Good|
|CN X||Vagus nerve (mixed) - Velvet|
|CN XI||Spinal Accessory nerve (motor) - Such A|
|CN XII||Hypoglossal nerve (motor) - Heaven|
|Mnemonic||Oh, oh, oh, to touch and feel very good velvet, such a heaven|
Each pair of cranial nerve can provide either purely sensory (i.e. CN I, II and VIII), or purely motor (CN III, IV, VI, XI, XII), while others provide a mixture of both sensory and motor supply (CN V, VII, IX, X).
The goal of this article is to review the cranial nerves while briefly addressing their anatomical origins, course and eventual supply. More details about the fiber composition and specific pathology related to each cranial nerve can be found in their respective articles.
- Embryology of the Cranial Nerves
- Cranial Nerve I
- Cranial Nerve II
- Cranial Nerve III
- Cranial Nerve IV
- Cranial Nerve V
- Cranial Nerve VI
- Cranial Nerve VII
- Cranial Nerve VIII
- Cranial Nerve IX
- Cranial Nerve X
- Cranial Nerve XI
- Cranial Nerve XII
- Summary & Mnemonics
- Related Atlas Images
Embryology of the Cranial Nerves
All the cranial nerves, except the olfactory, optic, and oculomotor nerves, are derivatives of the rhombencephalon (hindbrain). The olfactory, optic, and oculomotor nerves are telencephalic, diencephalic, and mesencephalic structures, respectively. The formation of the brainstem cranial nerves (i.e. all except CN I and II) are related to the dorsal and ventral thickened areas on either side of the cranial end of the developing neural tube. These areas are the alar (dorsal area) and basal (ventral area) plates. They develop in response to continuous migration of neuroblasts within the mantle layer of the primitive brain. Neuroblasts can be identified by their dark-staining nucleolus enclosed within a large, dark nucleus in the pale nucleoplasm.
The alar and basal plates in the primitive brain are arranged as caudocranial columns of cells that are continuous with the same structures observed within the developing spinal cord.
As the primitive brain develops and folds, the columns lose their continuity as the respective nuclei become more defined. The basal and alar plates travel cranially, and are situated around the central canal of the spinal cord. At the level of the medulla, the central canal is still relatively small and as a result the alar plates assume a posterior relationship to the central canal, while the basal plates are anteriorly located to the same structure.
The central canal is continuous with the expanded, rhomboid cavity known as the fourth ventricle. At the caudal apex (the obex), the alar plates migrate laterally, while the basal plates persist in the anterior position, along the midline of the structure. As the fourth ventricle tapers off cranially, into the cerebral aqueduct of Sylvius, the alar plates revert to their posterior position, while the basal plates continue anteriorly. The two plates – and their adult derivatives – are separated by the sulcus limitans.
The cranial nerves that develop from the basal plates provide motor supply to the respective end organs; giving rise to the somatic efferent, special visceral efferent and general visceral efferent supply. The motor component of cranial nerve nuclei (i.e. CN IV, V, VI, VII, IX, X, and XII) arise from eight corresponding rhombomeres, which are segmented neuroepithelial centres formed along the hindbrain.
On the other hand, the cranial nerve nuclei arising from the alar plates receive sensory input from their target organs; namely, somatic afferent, special afferent, and general visceral afferent nuclei. The sensory ganglia of the cranial nerves, however, are derivatives of neural crest cells and ectodermal placodes (nasal, otic, and epibranchial placodes). Specific details about the origin of each cranial nerve are discussed below.
|CN I||Sensory - nose|
|CN II||Sensory - eye|
|CN III||Motor - extrinsic eye muscles except lateral rectus and superior oblique|
|CN IV||Motor - superior oblique muscle|
|CN V||Sensory - face, sinuses, teeth, etc.
Motor - muscles of mastication
|CN VI||Motor - external rectus muscle|
|CN VII||Sensory - anterior part of tongue and soft palate
Motor - facial muscles
|CN VIII||Sensory - inner ear|
|CN IX||Sensory - posterior part of tongue, tonsil, pharynx
Motor - pharyngeal muscles
|CN X||Sensory - heart, lungs, bronchi, trachea, larynx, pharynx, GI tract, external ear
Motor - heart, lungs, bronchi, GI tract
|CN XI||Motor - sternocleidomastoid and trapezius muscles|
|CN XII||Motor - muscles of the tongue|
|Mnemonic||Some say money matters but my brother says big brains matter more (S - sensory, M - motor, B - both)|
Cranial Nerve I
The first of 12 cranial nerves is a special sensory nerve that is responsible for olfaction. The olfactory nerve neurons arise from the epithelium of the primordial nasal sac. The bipolar neurons have axons that are clustered into bundles of 20 axons. The cribriform plate of the ethmoid bone grows and ossifies around these nerves. There are three major regions of the olfactory nerve:
- The olfactory receptor cells,
- The olfactory bulb, and
- The olfactory tract.
The actual bipolar neurons of CN I are olfactory receptor cells; each have their cell bodies in the respiratory mucosa located around the roof of the nasal canal. They are suspended in a matrix with other supporting cells beneath the mucus layer. Distally, they are equipped with olfactory hairs (cilia) that are bathed in mucus produced by the ciliated columnar cells with goblet cells of the respiratory epithelium. They then extend proximally through the cribriform plate of the ethmoid bone.
As the olfactory nerve fibres ascend, they form numerous synapses within the olfactory bulb. The olfactory bulb is an ovoid entity that sits on the cribriform plate of the ethmoid bone; there is one bulb on either side of the crista galli. It functions as a relay station that receives olfactory impulses from both the ipsilateral and contralateral olfactory nerves fibres.
The cell bodies within the olfactory bulb leave the structure in a thin strip of white matter known as the olfactory tract. The tract travels along the ventral surface of the frontal lobe. It has several important areas where it terminates in order to integrate olfaction into daily function and creation of memories.
Cranial Nerve II
During early embryological development, a part of the wall of the forebrain invaginates and migrates towards the optic placode, where it is incorporated into the eye. This area becomes the primordial retina that is rich in neuroblasts. The neuroblasts then grow hundreds of thousands of nerve fibres into the developing forebrain, to form the optic nerves. This nerve is also a special sensory afferent tract that is responsible for visual perception.
Recall that the retina has ten histological layers. The ganglionic layer contains the cell bodies of the fibres of the optic nerves. On funduscopic examination of the retina, a circular, orange-red disc can be appreciated towards the nasal region of the retina. This area is the optic disc, which marks the point of convergence of all the axons to form the optic nerve. The nerve then travels through the optic foramen of the sphenoid bone (medial to the base of the superior orbital fissure). It then passes through the optic canal (along with the ophthalmic artery) to gain access to the prechiasmatic groove within the middle cranial fossa. Of note, the intact optic chiasm rests above the pituitary gland, in the suprasellar space. The neurons of CN II are myelinated by oligodendrocytes (as opposed to Schwann cells of the peripheral nervous system). Additionally, the nerve is enclosed by all three meningeal layers.
Each optic nerve unites above the pituitary gland, forming the optic chiasma. Two resulting nerve tracts – i.e. the optic tracts – then arise from the chiasma.
Within the chiasma, there is decussation of the fibres of the optic nerve. The crossing over results in the left optic tract containing fibres originating from the temporal side (nasal field) of the left eye, and the nasal side (temporal field) of the right eye. Similarly, the right optic tract will contain fibres arising from the temporal side (nasal field) of the right eye, and the nasal side (temporal field) of the left eye. Each tract eventually terminates at several points that allow adequate interpretation of visual input.
Cranial Nerve III
A part of the fate of the prechordal mesoderm (found at the cranial pole of the embryo) is to form three preoptic myotomes. These primitive muscle structures will subsequently form myoblasts that go on to form the extrinsic muscles of the eye. The first preoptic myotome gives rise to all the extrinsic eye muscles, except the lateral rectus and superior oblique muscles. Neuroblasts of the somatic efferent column of the basal plate will grow and form the axons of the oculomotor nerve , which will innervate these muscles.
CN III is a pure motor nerve that is served by two nuclei:
- In the periaqueductal grey matter of the midbrain, the main oculomotor nucleus is located anteriorly to the cerebral aqueduct of Sylvius at the level of the superior colliculi. This nucleus is also regulated by corticonuclear fibres arising bilaterally from each cerebral hemisphere. It also has connections with the visual cortex (i.e. information relayed by CN II) via the tectobulbar (arising from the tectum or roof of the midbrain and ending in the cranial nerve) tract. There is also inter-cranial nerve communication among CN IV, VI and VIII with the CN III nucleus through the medial longitudinal fasciculus pathway.
- The Edinger-Westphal nucleus acts as an accessory nucleus to CN III; providing parasympathetic impulses to the main nucleus. These preganglionic fibres travel with CN III to the orbit. However, it forms synapses with cells of the ciliary ganglion. The short ciliary fibres arising from the ganglion mitigate accommodation, and direct and consensual reflexes to the ciliary muscles of the iris under the influence of corticonuclear and pretectal fibres, respectively.
As the nerve fibres leave the nucleus, they penetrate the red nucleus and continue anteriorly to exit the midbrain in the interpeduncular fossa. They maintain an anterior course as they travel between the superior cerebellar and posterior cerebral arteries. In the middle cranial fossa, CN III enters the cavernous sinus. At this level, the nerve bifurcates into its superior and inferior rami, which then pass through the superior orbital fissure to enter the orbit. In addition to supplying the superior, medial, and inferior recti, CN III also innervates the inferior oblique and levator palpebrae superioris (elevates the upper eyelid) muscles.
Cranial Nerve IV
The second preoptic myotome gives rise to the superior oblique muscle of the eye. The innervating fibres originate from the somatic efferent column of the basal plate in the midbrain that forms the trochlear nerve. Like CN III, the nucleus of CN IV is located in the periaqueductal grey matter. It has a caudal relationship to CN III, at the level of the inferior colliculi. Another similarity shared between CN III and IV is that CN IV also receives bilateral corticonuclear supply from the cerebral hemispheres, tectobulbar innervation from the visual cortex via the superior colliculi, and CN II, VI and VIII communication via the medial longitudinal fasciculus.
CN IV has one of the longest intracranial courses of the cranial nerves. It exits the brainstem at the apex of the fourth ventricle (lateral to the superior medullary velum) and decussates with the contralateral CN IV. It travels around the base of the crus cerebri to gain access to the middle cranial fossa. Here it enters the lateral wall of the cavernous sinus and travels to the orbit through the superior orbital fissure; where it innervates the superior oblique muscle.
Cranial Nerve V
Most of the sensory modalities of the head are mitigated by the trigeminal nerve. Although CN V functions mainly as a sensory nerve, it also has motor components as well. It is the largest of the cranial nerves; being served by four nuclei – the main sensory, mesencephalic, motor, and spinal nuclei. CN V can be subdivided into three parts:
- the ophthalmic division (CN V1 or Va)
- the maxillary division (CN V2 or Vb)
- the mandibular division (CN V3 or Vc)
All divisions, except CN Va, arise from and innervate derivatives of, the first pharyngeal arch.
Some of the nuclei have a pontomedullary distribution, such that the spinal nucleus extends throughout the entire length of the medulla oblongata to the second cervical segment of the spinal cord, and is continuous superiorly with the main sensory nucleus located in the posterior pons. The motor nucleus is medial to the main sensory nucleus, while the mesencephalic nucleus resides in the lateral periaqueductal grey matter on the midbrain and extends caudally to the level of the main sensory nucleus. CN Va and Vb are purely sensory nerves; while CN Vc carries both motor and sensory functions.
Sensory Division of Trigeminal Nerve
The territory of each branch of CN V is arranged craniocaudally, such that CN Va supplies the upper third of the face and forehead, CN Vb supplies the middle third, and CN Vc supplies the lower third of the face. Painful, thermal and tactile sensory modalities are transmitted from these areas to the trigeminal (semilunar) sensory ganglion within Meckel’s cave in the middle cranial fossa.
Each trigeminal division has numerous branches that are further discussed in the respective article. However, the tributaries of CN Va unite within the orbit of the skull and gain entrance to the cranial vault by travelling through the superior orbital fissure, in the lateral wall of the cavernous sinus, before joining the ganglion in Meckel’s cave. Similarly, the numerous branches of CN Vb unite and gains access to the cranial vault by way of the foramen rotundum. Shortly thereafter, it passes through the lateral wall of the cavernous sinus (inferior to CN Va), before joining the ganglion in Meckel’s cave. CN Vc eventually joins the ganglion after entering the vault via foramen ovale. However, it does not travel through the cavernous sinus.
The sensory fibres travel medially in the middle cranial fossa, and pierce the lateral wall of the pons. Eventually, the sensory fibres divide, resulting in fibres that transmit tactile and pressure sensation ascending to terminate in the main sensory nucleus. The other fibres that carry painful and thermal stimuli will descend to insert into the spinal nucleus of the trigeminal nerve.
The somatotopic arrangement of the spinal trigeminal nucleus is inverted with respect to the nerve distribution to the face. In other words, CN Va innervates the upper third of the face, but the resulting fibres terminate in the distal third of the nucleus. Similarly, CN Vc innervates the lower third of the face but inserts in the proximal third of the nucleus. CN Vb fibres terminate in the middle of the nucleus. The sensory modalities are subsequently integrated and transmitted to the thalamus via the trigeminal lemniscus.
Motor Division of Trigeminal Nerve
The motor nucleus is highly regulated by bilateral corticonuclear fibres, as well as rubrobulbar (from the red nucleus to the cranial nerves), tectal and medial longitudinal fasciculus fibres. It also participates in a monosynaptic reflex arc with the trigeminal mesencephalic nucleus. The cell bodies for the slender motor branch of CN V are found in the motor nucleus. The nerve exits the pons at the point where the sensory division enters. It then travels with CN Vc to supply the masticators, the anterior part of the digastric, mylohyoid, tensor tympani and tensor veli palatini muscles.
Cranial Nerve VI
The cells and fibres of the abducens (also spelt abducent) nerve is a metencephalic derivative arising from the somatic efferent column of the basal plates. It innervates the lateral rectus muscle, which arises from the third preoptic myotome. The nucleus of CN VI is found in the lower posterior part of the pons, deep to the facial colliculus of the rhomboid fossa (fourth ventricle). The fibres of CN VII, which gives rise to the facial colliculus, wrap around the nucleus of CN VI. Similar to CN III and IV, CN VI also has bilateral corticonuclear regulation, tectobulbar supply via the superior colliculus (coming from the visual cortex) and medial longitudinal fasciculus communication with CN III, IV and VIII.
Each CN VI emerges anteriorly, above the medullary pyramids at the pontomedullary junction. Each has an inferolateral relationship to the basilar groove of the pons. They are also medial to CN VII and its associated intermediate nerve of Wrisberg, as well as CN VIII. As CN VI enters the cavernous sinus in the middle cranial fossa, it travels inferolaterally in relation to the carotid artery. It gains access to the lateral rectus muscle after passing through the superior orbital fissure.
Cranial Nerve VII
While the facial nerve is principally a motor nerve, it also has sensory and parasympathetic modalities as well. The nerve supplies the derivatives of the second pharyngeal arch. The motor nuclei arise from special visceral efferent fibres of the pontine basal plate. The sensory nerves, however, are neural crest derivatives of the cranial segment of the embryo.
Sensory Division of Facial Nerve
CN VII is responsible for gustatory input from the anterior two-thirds of the tongue and oral cavity. It receives this information via the chorda tympani nerve. The fibres relaying this information extend to the nucleus of tractus solitarius (NTS; solitary tract nucleus) via the geniculate ganglion. The NTS lies in close proximity to the motor nucleus.
Motor Division of Facial Nerve
The motor nucleus is found in the caudal region of the pons, deep to the vestibular area of the rhomboid fossa. As the motor axons leave the nucleus, they create a prominence in the lower part of the upper half of the fourth ventricle (facial colliculus) by coiling around the nucleus of CN VI. It emerges anteriorly at the pontomedullary junction, above the olive, and medial to CN VIII. It is joined by the nervus intermedius of Wrisberg, which carries sensory and parasympathetic fibres from the NTS and lacrimal nucleus, respectively.
They then exit the cranial vault via the internal acoustic meatus (with CN VIII), after which they form the geniculate ganglion. Subsequently, the nerve leaves the auditory canal via the stylomastoid foramen. The motor fibers make their way to the mandibular ramus, where it passes through the parotid gland. While in the substance of the gland, the nerve branches into its five motor components:
- marginal mandibular
- cervical branches
These nerves innervate the muscles of facial expression and are regulated by corticonuclear fibres. However, the part of the nucleus that innervates the lower set of expressive facial muscles only receive contralateral regulation, while those that supply the upper expressive facial muscles receive bilateral corticonuclear input. Involuntary facial expression, however (sadness or fear), is regulated by the rubrobulbar fibres.
Parasympathetic Division of Facial Nerve
The lacrimal and superior salivatory nuclei are posteriorly and laterally related to the main motor neuron. Descending autonomic fibres from the hypothalamus synapse with the cell bodies of the superior salivatory nucleus to relay gustatory impulses. There are also fibres concerned with emotional response arising from the hypothalamus that communicates with the lacrimal nucleus to pass on emotional cues.
Cranial Nerve VIII
A single cranial nerve is responsible for the special sensory afferent conduction of two end organ structures within the same anatomical location. The vestibulocochlear nerve has both vestibular (balance) and cochlear (hearing) modalities. Around the 4th week of development, there is thickening of the surface ectoderm bilaterally, adjacent to the myelencephalon, under the influence of the notochord and paraxial mesoderm. This otic placode subsequently invaginates into the underlying mesenchyme, where it forms the otic pits. The borders of the otic pits juxtapose and give rise to the otic vesicles, which form the primordium of the membranous labyrinth. The saccular part of the primordium of the membranous labyrinth will give rise to the spiral organ of Corti, while the utricular part produces the semicircular ducts. Cells within the spiral organ of Corti give rise to the cochlear nerve, while the vestibular nerve arises from the semicircular ducts.
The vestibular component of the nerve is concerned with transmitting proprioceptive information with regards to position and movement of the head. The fibres extend from the vestibular ganglia within the utricle and saccule of the semicircular canal, traverse the internal acoustic meatus and eventually pierce the pontomedullary junction to access the vestibular nucleus. The vestibular nucleus has other connections with the cerebellum, which allows it to interpret balance related input from the semicircular units of the inner ear.
The cochlear division of CN VIII focuses on the transmission of auditory impulses. The cochlear ganglia are located in the spiral organ of Corti. From there, their axons travel via the internal acoustic meatus in order to gain access to the pontomedullary junction (adjacent to CN VII), where the nerve enters the brainstem. Within the substance of the pons, the cochlear division of CN VIII bifurcates; giving one branch to the anterior cochlear nucleus, and another to the posterior cochlear nucleus. The subsequent fibres from the cochlear nuclei have an extensive course. They become a part of the lateral lemniscal pathway, before moving on to the inferior colliculus and medial geniculate body. They eventually terminate in the auditory cortex via the acoustic radiation of the internal capsule.
Cranial Nerve IX
The third pharyngeal arch derivatives are supplied by another mixed (parasympathetic, sensory and motor) nerve known as the glossopharyngeal nerve. Both special and general visceral efferent columns originating in the anterior myelencephalic basal plates are responsible for the formation of these cell bodies. The sensory fibres arise from both special (taste to the posterior third of the tongue) and general visceral afferent (to the pharynx) fibres. Along the course of the nerve, there are two ganglia (superior and inferior) located in, and immediately below (respectively), the jugular foramen.
Sensory Division of Glossopharyngeal Nerve
Peripheral nerve fibres arising from the posterior third of the tongue and the pharyngeal mucosa carry taste and general visceral afferent (respectively) impulses toward the glossopharyngeal ganglia. The postganglionic neurons then travel to the medulla oblongata, where the sensory fibres pierce the anterolateral aspect of the brainstem, lateral to the olive and medial to the inferior cerebellar peduncle. Of note, the superior ganglia receive impulses related to general sensation and relay this information to the spinal trigeminal nucleus via CN IX. However, specialized afferent impulses from the carotid sinus baroreceptors are carried by CN IX to the NTS (as well as the dorsal motor nucleus of CN X), which is the sensory nucleus of CN IX.
Parasympathetic Division of Glossopharyngeal Nerve
The inferior salivatory nucleus receives descending autonomic afferent fibres from the hypothalamus, in addition to olfactory input via the reticular formation. Nucleus tractus solitarius also relays taste to this nucleus. The efferent fibres innervate the parotid gland by way of postganglionic fibres of the otic ganglion.
Motor Division of Glossopharyngeal Nerve
The motor nucleus of CN IX is formed by the cranial pole of the nucleus ambiguus. It is buried in the reticular formation of the medulla oblongata. The nucleus, which supplies stylopharyngeus, receives bilateral corticonuclear regulation.
Cranial Nerve X
Probably the most widely distributed cranial nerve in the human body is the vagus nerve. It supplies structures from the respiratory, digestive, and cardiovascular systems. It is the result of the fusion of the nerves of the fourth and sixth pharyngeal arches. It carries both visceral efferent and afferent modalities to and from the aforementioned systems. Like the glossopharyngeal nerve, the vagus nerve also has a superior and inferior ganglia situated in the jugular foramen.
Parasympathetic Division of Vagus Nerve
The dorsal vagal nucleus results in the prominence of the caudal rhomboid fossa known as the vagal trigone. The nucleus itself is posterolaterally located to the hypoglossal nerve. Descending autonomic fibres from the hypothalamus travels to the nucleus in addition to other afferent fibres from the glossopharyngeal nerve. The resultant efferent fibres from this nucleus assist CN IX in regulating the carotid sinus baroreflex. Furthermore, other efferent fibres act on the involuntary musculature of the respiratory (bronchial smooth muscles), cardiac (sinoatrial and atrioventricular nodes), and gastrointestinal (from the oesophagus to the distal third of the transverse colon) tracts.
Sensory Division of Vagus Nerve
The caudal extent of the NTS forms the sensory nucleus of CN X. The inferior vagal ganglion receives gustatory input from the peripheral nerve fibres and relays them to the NTS. General sensory input travels to the superior vagal ganglion via the vagus nerve, but instead, terminates in the spinal trigeminal nerve.
Motor Division of Vagus Nerve
The motor nucleus of the vagus nerve arises from the nucleus ambiguus. This structure is found in the medulla oblongata and is situated deep within the reticular formation. This nucleus receives bilateral corticonuclear support as it supplies the mesodermal derivatives of the fourth and sixth pharyngeal pouches; namely the intrinsic laryngeal muscles and the pharyngeal constrictors.
The vagus nerve can be seen emerging from the anterolateral surface of the medulla oblongata above CN XII and below CN IX. It leaves the groove between the olive (anteriorly) and the inferior cerebellar peduncle (posteriorly) en route through the posterior cranial fossa, toward the jugular foramen (where its sensory ganglia reside). The nerve travels within the carotid sheath with the internal jugular vein and associated common carotid arteries.
Cranial Nerve XI
The accessory nerve originates from both spinal and cranial roots; therefore it is considered as having cranial and spinal divisions. The spinal roots arise from the spinal nucleus in the anterior grey column of the upper five to six spinal cervical segments. On the other hand, the cranial roots arise from axons of the caudal third of the nucleus ambiguus (located in the reticular formation of the medulla oblongata). Both the spinal and cranial roots receive bilateral corticonuclear regulation from the cerebrum.
The spinal roots leave the anterior surface of the proximal spinal cord between the anterior and posterior roots of the other spinal nerves. They then unite and the common trunk travels cranially through the foramen magnum, to meet with its cranial counterpart.
The cranial fibres also leave the anterior surface of the medulla oblongata as several rootlets lateral to the olive and medial to the inferior cerebellar peduncle. After these efferent fibres unite, they merge with the spinal root fibres and travel laterally in the cranial fossa toward the jugular foramen.
The united spinal accessory nerve then leaves the cranial fossa via this foramen along with CN X. Soon after the nerves enter the foramen, they bifurcate. The cranial roots merge with the vagus nerve and contribute to the motor innervation of the laryngeal, pharyngeal and soft palate musculature via the pharyngeal and recurrent laryngeal divisions of the vagus nerve. The spinal roots continue inferolaterally toward the sternocleidomastoid muscle. In addition to supplying this muscle, it also travels in the posterior triangle of the neck towards the trapezius muscle, which it also innervates.
Cranial Nerve XII
The last of the four cranial nerves arising from the somatic efferent column of the basal plates is the hypoglossal nerve. While the nerve is formed from the fusion of the ventral roots of occipital nerves, it lacks the sensory component derived from the dorsal roots of the accompanying spinal nerves. The nerve is responsible for innervating the intrinsic tongue muscles (which arise from the occipital myotome) as well as some extrinsic tongue muscles (genioglossus, hyoglossus and styloglossus). The motor cells that form the hypoglossal nucleus are found deep to the hypoglossal trigone, which is cranial to the vagal trigone but caudal to the stria medullaris in the inferior half of the fourth ventricle. While most of the cells of the CN XII nucleus receive bilateral corticonuclear regulation, the cells that supply the genioglossus are innervated only by contralateral corticonuclear fibres.
Separate fibres emerge lateral to the pyramid and medial to the olive on the anterior surface of the medulla oblongata. Shortly after they emerge, they unite to form the common trunk of CN XII. It then travels towards the hypoglossal canals (which pass through the occipital condyles) in the posterior cranial fossa; adjacent to the foramen magnum.
In the neck, the nerve courses between the jugular vein and internal carotid artery. It changes course at the inferior border of the posterior belly of the digastric muscle and travels over the loop of the lingual, and both internal and external carotid, arteries. It reaches the hyoglossus muscle after travelling under the posterior border of the mylohyoid. Eventually, the nerve arborizes, thus giving branches to the aforementioned target organs.
Summary & Mnemonics
Mnemonics for the Cranial Nerves
It may be overwhelming to recall all the cranial nerves, whether or not they convey sensory or motor modalities, or even their target organs. There are several mnemonics that can assist in this process:
- Remembering all cranial nerves - A quick way to recall all the cranial nerves is “Oh, oh, oh, to touch and feel very good velvet, such heaven.”
- Oh – olfactory
- Oh – optic
- Oh – oculomotor
- To – trochlear
- Touch – trigeminal
- And – abducens
- Feel – facial
- Very – vestibulocochlear
- Good – glossopharyngeal
- Velvet – vagus
- Such – spinal accessory
- Heaven – hypoglossal
- To recall whether or not the nerves are sensory (S), motor (M) or both (B), try remembering “Some say money matters but my brother says big brains matter more”:
- S – olfactory
- S – optic
- M – oculomotor
- M – trochlear
- B – trigeminal
- M – abducens
- B – facial
- S – vestibulocochlear
- B – glossopharyngeal
- B – vagus
- M – spinal accessory
- M – hypoglossal
Mnemonics for the Extraocular Muscles
- All the muscles of the eyes are innervated by CN III, except the lateral rectus (CN VI) and superior oblique (CN IV).
- Therefore, a short way to remember it is SO4 (superior oblique to CN IV), LR6 (lateral rectus to CN VI) and AO3 (all others to CN III).
Remembering the Course of CN IV
There is a medical student theory that CN IV went through the back of the brain stem because it heard that’s where it could find sex. But it turned out to be six (i.e. CN VI). So it went back to the normal course.