Cranial Nerves: Olfactory Nerve (I)

• There are 12 pairs of cranial nerves that convey general somatic and visceral efferent/afferent fibres to and from the periphery
• Certain cranial nerves also contain special visceral efferent fibres for the muscles of mastication, and special somatic/visceral afferent fibres for transmission of the special senses (hearing, vision, taste, smell, balance)
• Nuclei for both sensory and motor nerves originate from specific regions in the brain and brainstem
• Cranial nerves exit the skull through paired foramina in the skull to reach their effector organs
• Cranial nerves do not emerge from the spinal cord
• They are allotted specific roman numerals between 1 and 12 based on the location of their attachments along the brain and brain stem, from anterior to posterior

Olfactory Nerve (I)

• Sensory nerve containing special visceral afferent fibers for the special sense of smell
• The system is made up of primary olfactory neurons embedded in the olfactory epithelium lining the roof of the nasal cavity, nasal septum, and medial wall of superior nasal conchae
• Dendrites project into the nasal cavity, specialized to synapse after detecting a specific chemical aerosol
• Axons collect into approximately 20 bundles of olfactory nerve fibers, which project superiorly through the cribriform plate and pierce the dura/arachnoid mater to enter the olfactory bulb
• The olfactory bulb contains the cell bodies for mitral cells
• Synapse between primary olfactory neurons and mitral cells occurs in the olfactory bulb
• Axons of mitral cells project posteriorly, forming the olfactory tract, which divides posteriorly into lateral and medial olfactory striae
• Lateral olfactory stria terminate in the anterior portion of the temporal lobe, this is where processing of olfactory stimuli begins
• Medial olfactory stria pass through the anterior commissure to contralateral olfactory centers and also send a small number of efferent fibers from olfactory centers to contralateral olfactory bulbs, working to down-regulate the firing pattern of constitutively activated olfactory nerve fibers (phasic stimulus)

Optic Nerve (II)

• Sensory nerve containing special somatic afferent fibers for the special sense of vision
• The nerve is continuous with the eye anteriorly and lies just posterior to the optic disk (blind spot)
• The optic nerve begins where the nerve axons pierce the sclera posteriorly
• The retina is the light-sensitive region along the posterior surface of the eye, made up of photosensitive rod and cone cells
• Rods are primarily sensitive to the intensity of light, helping to decipher shades for vision under dark conditions
• Cones are more sensitive to varying frequencies of photoemissions, determining differences in colors under well-lit conditions
• Cell bodies lie along the deep surface of the retina, axons from photosensitive neurons start superficial, and then run medially towards the optic disk
• At the optic disk, axons continue posterior, then pass the sclera of the eye to enter the optic nerve
• The paired optic nerves run posteromedially to enter the optic canal, then continue to run posteromedially to fuse at the optic chiasm in the middle cranial fossa, and continue posteriorly past the optic chiasm as the left and right optic tracts
• At the optic chiasm, optical information from the nasal half of the visual field on either side decussates (crosses over to the contralateral side)
• Information from the left visual field is carried to visual centers along the right optic tract and vice versa
• Decussation is essential for proper binocular vision and depth perception
• Optic tracts terminate in paired lateral geniculate bodies in the thalamus, then synapse with nuclei which continue to the visual centers in the occipital lobes of the brain

Oculomotor Nerve (III)

• Contains somatic motor (for extraocular musculature) and visceral motor (parasympathetic control of pupil and lens) fibers
• The major nerve of eye motion and control, along with cranial nerves IV and VI, responsible for all of the motions of the eye
• Motoneurons arise from two collections of cell nuclei found in the gray matter of the midbrain region
• The somatic motor nucleus sends α-motoneurons to extraocular musculature, then axons emerge and exit the cranial fossa through the superior orbital fissure to enter the orbit, and then splits into superior and inferior divisions
• The superior division innervates the superior rectus muscle (draws pupils superiorly) and the levator palpebrae superioris muscle (responsible for opening the upper eyelid)
• The inferior division provides innervation to the medial rectus muscle (cross-eye muscles), the inferior rectus muscle (draws pupils inferiorly), and the inferior oblique muscle (draws pupils superiorly)
• Visceral presynaptic motoneurons arise from the accessory oculomotor (Edinger-Westphal) nucleus, located superior and posterior to the somatic oculomotor nucleus
• Presynaptic axons run along with α-motoneurons through the inferior division of the oculomotor nerve to the ciliary ganglion located adjacent to the optic nerve
• Postsynaptic axons exit the ciliary ganglion anteriorly, then run through the short ciliary nerves to the eye and provides innervation to the iris (constriction of the pupil) and ciliary muscle of the lens (adjustment of focus)
• The nerve also conveys a minimal amount of efferent proprioceptive feedback (fine-tuning for smooth eye movement)

Trochlear Nerve (IV)

• Contains somatic efferent fibers for extraocular musculature
• Responsible for innervation of a single muscle associated with eye motion, the superior oblique muscle
• The superior oblique tendon hooks along the trochlea (pulley) anteriorly, which is where the nerve derives its name
• The nucleus for the trochlear nerve is located in the gray matter of the midbrain inferior to the nuclei for the oculomotor nerve
• Axons project dorsally from the midbrain (only cranial nerve to do so) and run anteriorly to exit the cranial cavity through the superior orbital fissure
• Axons travel along the superior margin of the orbit to the superior oblique muscle
• The nerve also conveys a minimal amount of efferent proprioceptive feedback (fine-tuning for smooth eye movement)

Trigeminal Nerve (V)

• Contains somatic afferent fibers, which serve as the principal general sensory nerve for the anterior portion of the head (face, teeth, mouth, nasal cavity, and dura)
• Also contains special visceral efferent fibers for the muscles of mastication and other muscles located in the neck
• Cell bodies for somatic afferent fibers are located in the paired trigeminal ganglion (analogous to the dorsal root ganglion of spinal nerves) located anterior to the pons of the brainstem
• The trigeminal nerve can be divided into three separate sensory nerve branches
• Ophthalmic division (V1) conveys sensory information from the forehead:
  • Supraorbital, supratrochlear, and infratrochlear nerves supply sensory information from the scalp and forehead
  • Nasal branches of the anterior ethmoidal nerve supply sensation to the bridge of the nose
  • Anterior and posterior ethmoidal nerves contain sensory information from the eye and orbit
  • Nerves enter the cranial cavity through the superior orbital fissure
• Maxillary division (V2) conveys sensation from the upper jaw:
  • Zygomatic and infraorbital nerves supply the skin around the maxilla
  • Additional branches of the infraorbital nerve supply sensation to the maxillary (upper) teeth and palate
  • The division enters the cranial cavity via the foramen rotundum, deep to the zygomatic bone
• Mandibular division (V3) conveys sensory information from the lower jaw and temporal region of the head:
  • The inferior alveolar nerve supplies the mandibular (lower) teeth
  • The mental nerve branches from the inferior alveolar nerve and reaches the subcutaneous tissue around the lower jaw via the mental foramen in the mandible
  • The lingual nerve relays somatic sensory information from the mucosa of the mouth and the anterior two-thirds of the tongue
  • The auriculotemporal nerve runs posteriorly, inferior to the temporomandibular joint, then curves superiorly to supply sensation to the temporal region anterior to the ear
  • The mandibular division enters the cranial cavity through the foramen ovale of the skull
• Sensory fibers from the trigeminal ganglion continue into the pons of the midbrain, then synapse with the mesencephalic, principal sensory nucleus, and spinal tract nucleus, which sends information to the sensory cortex
• Special visceral motor fibers arise from the motor nucleus superior to the sensory nuclei in the pons and reach their effector muscles through branches off the mandibular division
• The tensor tympani and tensor veli palatini branches arise from the proximal portion of V3
• The mylohyoid and anterior digastric belly are supplied by a branch off the inferior alveolar nerve

Abducens (VI)

• Somatic motor nerve supplying a single muscle in the orbital region
• The nucleus is found in the pons, inferior to that for trigeminal
• Axons exit the brainstem between the pons and medulla oblongata
• Fibers run anteriorly to exit the cranial cavity through the superior orbital fissure and supplies the lateral rectus muscle of the eye
• The muscle functions in abduction of the eye

Facial Nerve (VII)

• Components include branchial motor fibers, somatic sensory fibers, special sensory fibers, and autonomic fibers
• Branchial motor fibers supply the muscles of facial expression and some of the muscles of mastication
• Somatic sensory fibers supply a small region of skin around the external acoustic meatus
• Special sensory fibers are responsible for taste
• Autonomic fibers supply the salivary and lacrimal glands of the face and mouth
• The nerve emerges from the junction of the pons and medulla lateral to the abducens nerve in two branches
• The motor root exits the cranial cavity through the internal acoustic meatus, then runs through the temporal bone along the facial canal and emerges through the stylomastoid foramen into the parotid gland, and supplies six branches along the head:
  • The posterior auricular branch supplies branches to auricular muscles in the inner ear and continues posterior and superior as the occipital branch to innervate the occipitalis muscle
  • The temporal branch is the most superior of the five anterior branches, innervating the orbicularis oculi and frontalis muscles
  • The zygomatic branch runs along the zygomatic arch, supplying elevators of the upper lip and superior portion of the orbicularis oris, and also supplies muscles responsible for nostril movements
  • The buccal branch runs directly anterior under the zygomatic arch, supplying depressors of the upper lip and the inferior portion of the orbicularis oris
  • The mandibular branch runs along the superficial surface of the lower jaws, along with the mental nerve (from V3), and supplies depressors of the lower lip
  • The cervical branch runs inferior to the angle of the mandible and supplies the platysma muscle
• Additional branches carry parasympathetic motor fibers
• Presynaptic fibers arise from the superior salivatory nucleus in the pons and branch off the facial nerve in the facial canal as the greater and lesser petrosal nerves, which run anteriorly to the pterygopalatine and otic ganglion, respectively
• Additional presynaptic fibers run with the sensory fibers down the lingual nerve to the submandibular ganglion
• Following synapse, postsynaptic fibers emerge and course to the lacrimal gland in the orbit and salivary glands in the oral cavity
• They are joined by postsynaptic sympathetic fibers, which control the rate of glandular secretions (parasympathetic fibers increase secretions, sympathetic fibers decrease secretions)

Vestibulocochlear Nerve (VIII)

• Nerve of the inner ear (bony labyrinth)
• Composed entirely of special sensory fibers responsible for both hearing and equilibrium
• Nuclei for CN VIII are located in the medulla oblongata, adjacent to the inferior cerebellar peduncle (communicates with cerebellum):
  • Four vestibular nuclei are located medial to peduncle
  • Two cochlear nuclei are located lateral to peduncle
• Nerves arise as separate branches from the junction between the pons and medulla, in close proximity to the facial nerve, and travel a short distance before entering the labyrinth of the temporal bone through the external acoustic meatus
• The cochlear nerve runs to the spiral-shaped organ called the cochlea
• Cell bodies for these sensory nerves are located in a collection of spiral ganglia located throughout the middle portion of the cochlea
• The cochlea receives vibratory transmissions from the external environment through the external acoustic meatus
• Stimulation of nerve endings from the cochlear branch of CN VIII are sent back to the cochlear nuclei in the medulla and perceived as sound
• Fibers spread out in a spiral fashion to different regions of the cochlea, and the location of nerve endings differentiates between pitches (high frequencies absorbed along the base of the cochlea, lower frequency fibers absorbed closer to the apex (helicotrema)
• The degree of stimulation of specific nerve endings differentiates the intensity of sound (higher intensity generates greater vibrations in the cochlea, increased nerve conduction)
• Vestibular fibers continue along to the vestibular apparatus
• Cell bodies for these sensory nerves are located in a single vestibular ganglion found in the facial canal
• Fibers extend to the saccule and utricle, which are specialized regions of the vestibular apparatus responsible for sensation of vertical and horizontal acceleration, respectively
• Fibers terminate in a gelatinous covering covered with otolith crystals, and sudden accelerations of the head in vertical and horizontal directions cause acceleration of hair cells with “dragging” of the otolithic layer
• Bending is relayed to terminal cilia in the nerve, resulting in depolarization and sensation of acceleration transmitted to the vestibular nuclei in the medulla
• Additional fibers extend to the ampullae of superior, lateral, and posterior semicircular canals and are responsible for the sensation of rotation in different planes
• Fibers terminate in the membranous cupula, which extends into the semicircular duct
• During rotational movement, semicircular ducts rotate with the head, while fluid remains motionless, then fluid pushes against the cupula, which causes depolarization of vestibular nerve endings, and the message is interpreted by the vestibular nuclei as rotational motion

Glossopharyngeal Nerve (IX)

• The name is based on its innervation pattern and is responsible for innervation of the posterior two-thirds of the tongue and the pharynx
• Nerves play a variety of roles including general somatic, as well as general and special visceral sensory fibers and general and special visceral motor fibers (carries parasympathetic fibers)
• Fibers arise from/run to four separate nuclei found in the medulla oblongata, with fibers associated with each nuclei coming together and projecting off the superolateral aspect of the medulla as the glossopharyngeal nerve
• The majority of fibers exit the cranial cavity through the jugular foramen
• The solitary tract nucleus and spinal tract nucleus contain cell bodies which synapse with sensory cells in the periphery
• The lingual branch runs from the posterior two-thirds of the tongue and contains sensory nerves sensitive to touch and special sensory innervation responsible for taste sensations along the posterior part of the tongue
• The pharyngeal and tonsillar branches of the hypoglossal nerve relay somatic innervation from the mucosal and muscular layers of the pharynx (back of the mouth and throat)
• Tympanic nerve branches prior to its exit through the jugular foramen and relays sensory information from the mucosa of the tympanic cavity (middle ear)
• The carotid sinus branch runs from the carotid sinus and body (at the bifurcation of the external and internal carotid arteries) and relays visceral sensory information regarding blood pressure
• Cell bodies for all sensory fibers are located in one of two ganglia (superior and inferior ganglia of the glossopharyngeal nerve) found prior to the nerve’s exit through the jugular foramen
• The nucleus ambiguus contains cell bodies for branchial motor innervation specifically to the stylopharyngeus muscle found posterior to the pharynx
• The inferior salivatory nucleus contains preganglionic parasympathetic cell bodies, with axons that travel through the proximal portion of the glossopharyngeal nerve, then follow the course of the tympanic nerve to the tympanic cavity, then exit the tympanic cavity anteriorly through the lesser petrosal nerve and run to the otic ganglion, where they synapse with the postganglionic cell bodies
• Postganglionic axons follow the course of the mandibular branch of the trigeminal nerve to the parotid gland anterior to the external ear, and neural outflow stimulates salivation in the parotid gland

Vagus Nerve (X)

• Means “wanderer” and is named for its extensive branching, and neural supply to structures throughout most of the axial skeleton (pharynx, larynx, heart lungs, most of the abdominal organs up to the splenic flexure)
• Contains a variety of different nerve fibers for a number of different functions
• Somatic and visceral sensory fibers:
  • Somatic sensory fibers generally cover the pharynx and larynx
  • Visceral sensory fibers supply the heart, lungs, and abdominal organs
• Special sensory fibers convey taste sensation from the root of the tongue, glottis
• Branchial motor fibers supply muscles of the pharynx and muscles controlling the larynx (phonation)
• Parasympathetic fibers course to numerous structures throughout the thorax and abdomen and function to slow heart rate and increase blood flow to abdominal viscera
• Fibers arise from four major nuclei located in the medulla (nuclei also supply fibers to glossopharyngeal, and accessory nerve)
• Fibers collect together as the vagal trunk and branches from the lateral aspect of the medulla, just inferior to the glossopharyngeal nerve, and it travels with the glossopharyngeal nerve through the jugular foramen
• The nucleus ambiguus contains cell bodies for motor branches for pharyngeal and laryngeal muscles and reaches their effector organs through the pharyngeal branch of the vagus nerve, superior and recurrent vagus nerves
• The solitary tract and spinal tract nuclei contain cell bodies for communication with peripheral sensory nerves which run through the vagus
• The spinal tract nucleus receives somatic sensory information sent back from the external ear by the auricular branch and from the meninges of the brain through a recurrent running meningeal branch of the vagus nerve
• The solitary tract receives visceral afferents from the pharynx, which share the pharyngeal branch of the vagus nerve with motor fibers supplying this region, and also receives special afferents conveying the sensation of taste from the root of the tongue and glottis
• Cell bodies for neurons running to the spinal tract are primarily located in the superior ganglion of the vagus nerve located near the jugular foramen
• Cell bodies for neurons running to the solitary tract are primarily located in the inferior ganglion just inferior to the jugular foramen
• The dorsal vagal nucleus contains a mixture of preganglionic cell bodies for parasympathetic outflow and cell bodies which communicate with peripheral visceral afferent fibers returning from the thorax and abdomen
• The same collection of nerve branches sends preganglionic axons and collects sympathetic fibers from the periphery
  • Three cardiac branches run to the region in and surrounding the SA node of the heart (pacemaker), with parasympathetic motor branches helping to control heart rate and strength of contraction, and visceral afferents conveying messages back to the brain on the status of the heart:
  • The superior cervical cardiac branch passes medial to the common carotid artery
  • The inferior cervical cardiac branch passes lateral to the common carotid artery
  • The thoracic cardiac branch diverges from and lies lateral to the inferior cervical cardiac branch
  • The left recurrent laryngeal branch supplies fibers to the trachea and superior portion of the esophagus before entering the larynx
  • Inferior portions of the left and right vagal trunks run along the anterior and posterior surfaces of the esophagus, respectively, to the abdominal cavity and provides parasympathetic innervation to all the accessory organs and the majority of the GI tract, excluding the descending and sigmoid colons, the rectum, and the anus
  • For visceral afferents returning from the periphery, cell bodies are primarily located in the inferior ganglion of the vagus nerve

Accessory Nerve (XI)

• Named because a number of its fibers originate inferior to the brainstem (as far down as the gray matter for spinal nerves C5 and C6)
• Strictly somatic motor fiber (plus proprioception) for innervation of skeletal muscle fibers
• The accessory nerve originates from two nuclei in the spinal cord:
  • The nucleus ambiguus in the medulla, which is also the origin of glossopharyngeal and vagus fibers
  • The spinal nucleus, which is a continuation of the spinal tract nucleus
• Branches of the spinal nucleus run in a recurrent fashion between ventral and dorsal cervical roots in the vertebral canal to reach the cranial fossa through the foramen magnum and merge briefly with the roots from the nucleus ambiguus to form the accessory nerve proper, which exits the cranial cavity through the jugular foramen
• Fibers from the nucleus ambiguus branch from those originating from the spinal nucleus to fuse with the vagal trunk at the level of the inferior vagal ganglion and continues along vagal branches (pharyngeal and laryngeal branches) to provide somatic motor innervation to these structures
• Spinal nucleus fibers continue inferiorly along the internal carotid artery and split into two terminal branches, which run to the sternocleidomastoid and through the posterior triangle of the neck to reach the trapezius muscle
• Branches from C2 through C4 provide the sensory innervation and additional motor innervation to this region

Hypoglossal Nerve (XII)

• Named for its region of innervation (inferior to the base of the tongue)
• Strictly a somatic motor nerve (with proprioceptive feedback) to both intrinsic (superior and inferior longitudinal, transverse, vertical) and extrinsic (styloglossus, hyoglossus, genioglossus) muscles of the tongue
• Cell bodies for the α-motoneurons originate in the hypoglossal nucleus in the medulla oblongata, and fibers come off the anterior surface of the medulla as the hypoglossal nerve and exit the cranial cavity through the hypoglossal canal located just superior and anterior to the foramen magnum
• Fibers run inferiorly, then anteriorly to the base of the tongue to supply the aforementioned muscle groups
• The descending portion of the hypoglossal nerve anastomoses with the ventral rami of the C1 nerve
• Motor fibers from the C1 branch away from the hypoglossal nerve to supply the hyoid muscles
  • The inferior portion of the C1 branch loops up in a recurrent fashion to fuse with C2, C3 ventral rami, creating the ansa cervicalis
• Sensory fibers from C1 travel in the opposite direction to motor fibers, back up the hypoglossal nerve into the cranial cavity, and branch from the hypoglossal nerve immediately after passing through the hypoglossal canal, and give off the meningeal branch of the hypoglossal nerve (sensory to meningeal layers)