II,III,IV,V,VI CRANIAL NERVES
The optic nerve (II cranial nerve)
Special somatic afferent (SSA)
The optic nerve is the nerve of sight. It consists of the ganglion cell axons of the retina. The optic nerve runs through the optic canal together with the internal ophthalmic artery. A percentage of the optic fibers cross behind the jugum sphenoidale, giving rise to the optic chiasm. After the chiasm, the optic nerve fibers (decussated and non decussated) form the optic tract that reaches the lateral geniculate body of the thalamus.
Oculomotor nerve (III cranial nerve)
General somatic efferent (GSE) and general visceral efferent (GVE)
The ocumolotor nerve leaves the brain stem between the cerebral peduncles, and runs rostrally in relation to the cavernous sinus and the trochlear, abducent and ophthalmic nerves. It exits the cranial cavity through the orbital fissure, dividing itself in a dorsal and a ventral branch.
The dorsal branch innervates the dorsal rectus and levator palpebrae superioris muscles. The ventral branch innervates the medial rectus, ventral rectus, and ventral oblique muscles. The GVE component is formed by fibers that reach the ciliary ganglion located on the ventral branch. The parasympathetic postganglionic fibers form the short ciliary nerves, that join the optic nerve to enter the eye, to innervate the ciliary muscle and the sphincter pupillary muscle.
Proprioceptive fibers from the extra-ocular muscles travel in the III, IV and VI cranial nerves. They connect with the ophthalmic nerve at the level of the cavernous sinus. The soma of these neurons is located in the trigeminal ganglion.
Trochlear nerve (IV cranial nerve)
General somatic efferent (GSE)
The trochlear nerve is the only cranial nerve that leaves the brain stem dorsolaterally. The left and right throclear nerves decussate at the level of the rostral medullary velum. It travels, included in the dura mater, through the lateral portion of the tentorium cerebelli and of the cavernous sinus, to exit the cranial cavity through the orbital fissure. Outside of the skull, it is located between the oculomotor nerve and the ophthalmic branch of the trigeminal nerve. It innervates the dorsal oblique muscle.
Trigeminal nerve (V cranial nerve)
Special visceral efferent (SVE) and general somatic afferent (GSA)
The trigeminal nerve leaves the brain stem at the caudal level of the pons, rostral to the trapezoid body. It passes through the trigeminal canal (located on the medial aspect of the petrosal bone) and divides into three nerves: ophthalmic, maxillary and mandibular. These nerves leave through foramina in the wing of the sphenoid bone. The ophthalmic nerve leaves the cranial cavity through the orbital fissure, together with the oculomotor, trochlear and abducent nerves. The maxillary nerve enters the round foramen, that opens into the alar canal, and exits through the rostral alar foramen. The mandibular nerve leaves through the oval foramen. The three nerves have a sensory component (GSA) but only the mandibular has a motor component (SVE).
The sensory neurons have their bodies located in the trigeminal ganglion (also called Gasser's ganglion or semilunar ganglion). This is located in a trigeminal cavity of the dura mater in the rostral opening of the trigeminal canal, in the rostromedial surface of the petrous temporal bone. There is an exception in the case of the proprioceptive neurons of the mandibular nerve, whose fibers form the mesencephalic tract of the trigeminal nerve inside the brain stem. The soma of these neurons is located in the nucleus of the mesencephalic tract of the trigeminal nerve, lateral to the periaqueductal gray matter of the mesencephalon.
. The ophthalmic nerve
General somatic afferent (GSA)
Before going through the orbital fissure, the ophthalmic nerve gives off a sensory meningeal branch. Just after leaving the cranial cavity, it enters the periorbita and divides into the following nerves: frontal, lacrimal and nasociliary.
. The frontal nerve is sensory (GSA) to the upper eyelid.
. The lacrimal nerve is sensory to the upper eyelid, conjunctiva and lacrimal gland. It carries postganglionic parasympathetic fibers from the pterogopalatine ganglion to the lacrimal gland. These are GVE fibers from the facial nerve.
. The nasociliary nerve branches off the ethmoidal and infratrochlear nerves. The ethmoidal nerve passes through the ethmoid foramen into the cranial cavity. It reaches the cribriform plate and enters the nasal cavity to innervate the nasal mucosa and the skin of the nasal vestibule. The infratrochlear nerve is located medial to the orbit. It is sensory to the skin of the medial canthus of the eye (GSA), and to the third eyelid and associated lacrimal gland. The nasociliary nerve sends a branch to the ciliary ganglion that (without synapsing) forms part of the short ciliary nerves (GVE). The rostral continuation of the nasociliary nerve forms the long ciliary nerves. These are sensory (GSA) to the eyeball and carry postganglionic sympathetic fibers (from the cranial cervical ganglion) to the dilator muscle of the pupil and to the smooth muscle fibers of the periorbita.
. The maxillary nerve
General somatic afferent (GSA)
After sending out a meningeal branch, the maxillary nerve leaves the cranial cavity through the round foramen into the alar canal and, then, through the rostral alar foramen. It divides into the zygomatic, pterygopalatine and infraorbital nerves.
. The zygomatic nerve leaves the maxillary nerve at the level of the round foramen. After exiting through the rostral alar foramen, it enters the periorbita and divides into the zygomaticotemporal and zygomaticofacial nerves. The zygomaticotemporal nerve runs over the orbital ligament. It is sensory to the skin dorsal to the zygomatic arch and the lateral canthus of the eye. It carries postganglionic parasympathetic fibers from the pterygopalatine ganglion to the lacrimal gland (these are GVE fibers from the facial nerve). The zygomaticofacial nerve runs under the orbital ligament. It is sensory to the lower eyelid and lateral canthus of the eye.
. The pterygopalatine nerve passes over the dorsal surface of the medial pterygoid muscle. It sends out the caudal nasal nerve and the major, accessory and minor palatine nerves. These nerves incorporate postganglionic parasympathetic fibers from the pterygopalatine ganglion (these are GVE and GVA fibers from the facial nerve) to innervate the nasal and palatine glands. The caudal nasal nerve enters the nasal cavity through the sphenopalatine foramen to innervate the ventral aspect of the nasal cavity. The major palatine nerve enters the palatine canal through the caudal palatine foramen and innervates the hard palate. The minor palatine nerve innervates the soft palate. The accessory palatine nerves branch off from the major palatine nerve and run through the accessory palatine foramina on the horizontal portion of the palatine bone to innervate the caudal portion of the hard palate.
. The infraorbital nerve is a continuation of the maxillary nerve. At the level of the pterygopalatine fossa, the infraorbital nerve sends out the caudal superior alveolar branches that enter the alveolar foramina of the maxilla (to supply the caudal upper teeth). Then the infraorbital nerve enters the infraorbital canal via the maxillary foramen. Within the canal, it sends out the middle superior alveolar branches which enter the alveolar canals (to supply the middle upper teeth). Just before leaving the canal, through the infraorbital foramen, the maxillary nerve sends out the rostral superior alveolar branches through the incisivo-maxillary canal (to the upper canine and incisor teeth). Once out of the infraorbital canal, the infraorbital nerve divides into nasal and superior labial branches.
The mandibular nerve
Special visceral efferent (SVE), general somatic afferent (GSA)
After sending out a meningeal branch, the mandibular nerve exits the cranial cavity through the oval foramen and divides into the following nerves: masticator, buccal, lateral pterygoid, medial pterygoid, inferior alveolar, lingual, mylohyoid, tensor tympani, tensor veli palatine and auriculotemporal.
. The masticator divides into the masseteric and deep temporal nerves to the masseter and temporal muscles respectively.
. The buccal nerve is sensory for the mucosa of the cheeks. It is also sensory to the skin located ventrally to the zygomatic arch and dorsocaudally to the commissure of the mouth. It incorporates postganglionic parasympathetic fibers from the otic ganglion to innervate the salivary zygomatic gland (these are GVE nerve fibers from the glossopharyngeal nerve).
. The lateral and medial pterygoid nerves innervate the lateral and medial pterygoid muscles respectively.
. The inferior alveolar nerve enters the mandibular canal through the mandibular foramen to innervate the lower teeth. The rostral, middle and caudal mental nerves are rostral branches of the inferior alveolar nerve that exit through the mental foramina. These nerves are sensory to the lower lip.
. The lingual nerve is sensory for the rostral two thirds of the tongue. The GSA component is formed by trigeminal fibers. The GVE, GVA and SVA (taste) components are facial fibers of the chorda tympani that join the lingual nerve in the dorsomedial surface of the medial pterygoid muscle.
. The mylohyoid nerve is motor to the mylohyoid muscle and rostral belly of the digastricus muscle. It is also sensory to the skin of the intermandibular region and, through a communicating branch with the ventral buccal branch of the facial nerve, to the caudal portion of the lower lip and ventrolateral area of the cheek (caudal to the region innervated by the mental branches).
. The tensor tympani nerve innervates the tensor tympani muscle. When this muscle contracts, the manubrium of the malleus moves medially and tightens the tympanic membrane.
. The tensor veli palatini nerve innervates the tensor veli palatini muscle. When this muscle acts in combination with the levator veli palatini (innervated by the glossopharyngeal and vagus nerves), it opens the orifice of the auditory tube into the pharynx.
. The auriculotemporal nerve leaves the mandibular nerve at the level of the oval foramen and runs caudally to the retroarticular process of the temporal bone. It sends out the external acoustic meatus nerve and a branch to the tympanic membrane. They are sensory to the external acoustic meatus and to the tympanic membrane respectively. The auriculotemporal nerve receives postganglionic parasympathetic fibers from the otic ganglion (GVE fibers from the glossopharyngeal nerve) to innervate the salivary parotid gland. The rostral auricular nerves of the auriculotemporal nerve are sensory to the skin of the ear and the skin that covers the surface of the temporal muscle and the zygomatic arch. The transverse facial branch is formed by auriculotemporal nerve fibers that are sensory to the skin located laterally and ventrally to the zygomatic arch. A communicating ramus between the auriculotemporal nerve and the dorsal buccal branch of the facial nerve, runs dorsal to the masseter muscle. It is sensory to the caudal half of the upper lip. The sensory innervation of the rostral half of the lip is carried by the infraorbital nerve (maxillary nerve).
The fibers of the meningeal ramus enter the cranium with the middle meningeal artery and run together with sympathetic fibers coming from the cranial cervical ganglion
Abducent nerve (VI cranial nerve)
General somatic efferent (GSE)
The abducent nerve leaves the brain stem at the level of the trapezoid body, lateral to the pyramids. It runs rostrally to leave the cranial cavity through the orbital fissure. It innervates the retractor bulbi muscle and the lateral rectus muscle.