CRANIAL NERVES

Vishnu Karunakaran

Neuroscience & Neurological Disorders

Medical Faculty Udayana University

Denpasar

Introduction

Cranial nerves are nerves that emerge directly from the brain stem, in contrast

to spinal nerves which emerge from segments of the spinal cord.1 There are twelve

pairs of cranial nerves; they are attached to the brain and are transmitted through

foramina in the base of the cranium.2-9 They are:

I. Olfactory

II. Optic

III. Oculomotor

IV. Trochlear

V. Trigeminal

VI. Abducens

VII. Facial

VIII. Vestibulocochlear

IX. Glossopharyngeal

X. Vagus

XI. Accessory

XII. Hypoglossal

The first two cranial nerves attach directly to the forebrain while the rest

attach to the brain stem.3,7 Human cranial nerves are nerves 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 totaling

Figure 1: Inferior view of the brain and brain stem showing cranial nerves.

twelve pairs. These characters are synapomorphies (traits that are shared by two or

more taxa and their last common ancestor) for their respective clades. In some

primitive cartilaginous fishes, such as the spiny dogfish or mud shark (Squalus

acanthias), there is a terminal nerve numbered zero (as it exits the brain before the

traditionally designated first cranial nerve).1

The function of the cranial nerves is for the most part similar to the spinal

nerves, the nerves that are associated with the spinal cord. The motor components of

the cranial nerves are derived from cells that are located in the brain. These cells

send their axons (bundles of axons outside the brain is a nerve) out of the cranium

where they will ultimately control muscle (e.g., eye movements), glandular tissue

(e.g., salivary glands) or specialized muscle (e.g., heart or stomach). The sensory

components of cranial nerves originate from collections of cells that are located

outside the brain. These collections of nerve cells bodies are called sensory ganglia.

They are essentially the same functionally and anatomically as the dorsal root ganglia

which are associated with the spinal cord. In general, sensory ganglia of the cranial

nerves send out a branch that divides into two branches: a branch that enters the brain

and one that is connected to a sensory organ. Examples of sensory organs are pressure

or pain sensors in the skin and more specialized ones such as taste receptors of the

tongue. Electrical impulses are transmitted from the sensory organ through the

ganglia and into the brain via the sensory branch that enter the brain. There are two

exceptions to this rule that should be noted when the special senses of smell and

vision are discussed. In summary, the motor components of cranial nerves transmit

nerve impulses from the brain to target tissue outside of the brain. Sensory

components transmit nerve impulses from sensory organs to the brain.6-8

Cranial Nerve Nuclei

A cranial nerve nucleus is a collection of neurons (gray matter) in the brain

stem that is associated with one or more cranial nerves. Axons carrying information

to and from the cranial nerves form a synapse first at these nuclei. Lesions occurring

at these nuclei can lead to effects resembling those seen by the severing of nerve(s)

they are associated with. All the nuclei excepting that of the IV nerve supply nerves

of the same side of the body.1,5

Motor and sensory nuclei of the cranial nerves are the relay stations for

impulses from higher nerve centers to effectors and from peripheral impulses to nerve

centers that are higher in the central nerve system. The motor or efferent cranial

nerves arise within the brain from groups of nerve cells which constitute their nuclei

of origin. The sensory or afferent cranial nerves arise from groups of nerve cells

outside the brain; these nerve cells may be grouped to form ganglia on the trunks of

the nerves or may be situated in peripheral sensory organs such as the nose and eye.

The central processes of these cells run into the brain, and there end by arborizing

around nerve cells, which are grouped to form nuclei of termination. The nuclei of

origin of the motor nerves and the nuclei of termination of the sensory nerves are

brought into relationship with the cerebral cortex, the former through the geniculate

fibers of the internal capsule, the latter through the lemniscus. The geniculate fibers

arise from the cells of the motor area of the cortex, and, after crossing the middle line,

Figure 2: Locations of each cranial nerve nuclei

end by arborizing around the cells of the nuclei of origin of the motor cranial nerves.

On the other hand, fibers arise from the cells of the nuclei of termination of the

sensory nerves, and after crossing to the opposite side, join the lemniscus, and thus

connect these nuclei, directly or indirectly, with the cerebral cortex.2-5

Afferent Nuclei

Nerve fibers which carry general sensory information such as touch, pressure,

pain and temperature from the head, enter the brain through the trigeminal nerve at

the pons and terminate in the trigeminal sensory nucleus. The somatic afferent nuclei

are the most laterally placed of the cranial nerve nuclei. These are the trigeminal

nucleus (V), the cochlear nuclei (VIII), and the vestibular nuclei (VIII). This is a large

nucleus that extends throughout the whole length of the brainstem and caudally into

the cervical spinal cord. Fibers that convey the unique senses of hearing and

motion/positional sense run in the vestibulocochlear nerve. The general

somatosensory nuclei are represented by the three sensory nuclei of the trigeminal

nerve: nucleus mesencephalicus, nucleus pontinus (nucleus sensibilis principalis) and

nucleus spinalis.1-3,7

The term special sensory has been used to describe the olfactory and optic

nerves, because of their origins in the specialized sensory organs, namely the nasal

epithelium and the eye, respectively, and because of their forebrain origins, as

opposed to the brainstem origins of the other cranial nerve nuclei. The special

somatosensory nuclei are located in the recessus lateralis ventriculi quarti region,

which are the vestibular nuclei. The cochlear nuclei, which is situated more laterally,

has close topographic connections with the pedunculus cerebellaris caudalis.1-3,7

Visceral afferent nuclei are the most medially placed compared to all the

sensory neuron groups. Visceral afferents, which include taste fibers, terminate in the

nucleus solitarius of the medulla.2,7

Efferent Nuclei

The nuclei of the somatic efferent cell column lie near the midline and

mediate eye movements (III, IV, and VI) and tongue movements (XII). Somatic

efferent (SE) nerves innervate the skeletal muscles derived from the somites. A part

of the somatomotor nuclei is located at the base of the fossa rhomboidea and another

part is at the base of aquaductus mesencephali (cerebrum). The oculomotor nucleus

lies in the ventral apex of the periaqueductal grey of the midbrain at the level of the

superior colliculus. Its efferent fibers run in the oculomotor nerve to innervate the

levator palpebrae superioris and all of the extraocular muscles, except the superior

oblique and lateral rectus. The trochlear nucleus also lies in the midbrain, at the

ventral border of the periaqueductal grey, but at the inferior colliculus level. Fibers

leave in the trochlear nerve to innervate the superior oblique muscle of the eye. The

abducens nucleus is located in the caudal pons beneath the floor of the fourth

ventricle. Its efferents run in the abducens nerve and innervate the lateral rectus

muscle. In the medulla, the hypoglossal nucleus innervates the intrinsic and extrinsic

muscles of the tongue via the hypoglossal nerve.1-3,7

Visceral efferents (VE) are preganglionic cranioparasympathetic fibers that

innervate smooth muscles of the inner eye (III), the lacrimal and salivary glands (VII,

IX), and bowel, heart, and lung muscles that mediate secretions and movement (XI).

The visceromotor nuclei retain their primitive position near the ventricular

composition of the brain. It encompasses nucleus salivatorius cranialis and caudalis,

and nucleus dorsalis nervi vagi.1-3,7

The nuclei of the branchiomotor (special somatomotor nuclei) cell column,

which innervate striated muscles derived from the branchial arches, is situated on the

lateral part of the somatomotor nuclei columns. Due to neurobiotaxis, these nuclei

move towards the ventrolateral part of the brainstem but still retain their

craniocaudally oriented cell columns. In the tegmentum of the mid-pons is located the

trigeminal motor nucleus, which supplies fibers to the trigeminal nerve and innervates

the muscles of mastication, tensor tympani, tensor veli palitini, mylohyoid and the

anterior belly of the digastrics muscle. In the caudal pontine tegmentum lies the facial

motor nucleus. This innervates the muscles of facial expression and the stapedius

muscle via the facial nerve. Within the medulla lies the nucleus ambiguus. This long

nucleus sends motor fibers in the glossopharyngeal, vagus and cranial part of the

accessory nerve to innervate muscles of the pharynx and larynx.1-3,7

Lateral to the branchiomotor nuclei are the parasympathetic visceral efferent

nuclei. The parasympathetic cell column consists of preganglionic parasympathetic

neurons that send axons into the III, VII, IX, and X cranial nerves. These are the

Edinger-Westphal oculomotor nucleus (III); which lies in the midbrain periaqueductal

grey matter adjacent to the oculomotor nucleus, the superior (VII) and inferior (IX)

salivatory nuclei; which lie in the pontine tegmentum, and the dorsal motor nucleus

of the vagus (X); which lies in the medulla. The superior salivatory nuclei supply

preganglionic fibers to the facial nerve, which terminate in the pterygopalatine and

submandibular ganglia. Postganglionic fibers from the pterygopalatine ganglion

innervate the lacrimal gland, nasal and oral mucous membranes, while those from the

submandibular ganglion innervate the submandibular and sublingual salivary glands.

The inferior salivatory nucleus sends preganglionic fibers into the glossopharyngeal

nerve, which terminate in the otic ganglion. This ganglion sends postganglionic axons

to the parotid salivary gland. The rostral position of the dorsal motor nucleus of the

vagus nerve lies immediately beneath the floor of the fourth ventricle, lateral to the

hypoglossal nucleus. Fibers leave in the vagus nerve and are widely distributed to

thoracic and abdominal viscera.3,7

Cranial Nerve I: Olfactory nerve

Figure 3: Olfactory nerve; supplies the chemoreceptor cells of the nose

The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It

is instrumental in the sense of smell. This is a pure sensory nerve fiber. The cell

bodies of the olfactory nerve are in the nasal mucosa. Their axons form the olfactory

nerves which ascend through the cribriform plate to synapse in the olfactory bulb of

the brain. Olfaction is less developed in humans than in other mammals such as

rodents. As a chemical sensor, the olfactory system detects food and influences social

and sexual behavior. The specialized olfactory epithelial cells characterize the only

group of neurons capable of regeneration. Humans are able to detect many different

airborne chemicals at low concentrations. Olfaction and taste work together to

achieve the sensation referred to as taste; if for any reason olfaction is impaired, the

patient complains that food cannot be properly tasted. In contrast to the taste system,

which distinguishes relatively few modalities of sour, sweet, bitter, and salt, the

olfactory system can distinguish very many different odorants, which contribute to

the subtle modality of smell.1,3,10-13

The specialized olfactory receptor neurons of the olfactory nerve are located

in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves do

not form two trunks like the remaining cranial nerves, but consist of a collection of

many sensory nerve fibers that extend from the olfactory epithelium to the olfactory

bulb, passing through the many openings of the Cribriform plate of the Ethmoid

bone; a sieve-like structure. Olfactory receptor neurons continue to be born

throughout life and extend new axons to the olfactory bulb. These olfactory receptors

are actually bipolar cells, with short, tubular dendrites leading towards the surface

and the tips contain smooth cilia, and axons that exit the inside of the unmyelinated

cell-body. Olfactory ensheathing glias wrap bundles of these axons and are thought to

facilitate their passage into the central nervous system.1,2,11,13

Figure 4: Anatomy of the olfactory nerve

The olfactory system is completely neural, since the receptors are modified

neurons that transduce and transmit olfactory inputs to the brain via the olfactory

bulb, the lateral olfactory tract, and from there to the olfactory cortex. The sense of

smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by

small molecules of different spatial, chemical, and electrical properties that pass over

the nasal epithelium in the nasal cavity during inhalation. These interactions are

transduced into electrical activity in the olfactory bulb which then transmits the

electrical activity to other parts of the olfactory system and the rest of the central

nervous system via the olfactory tract. The olfactory system is unique among the

senses, in that receptors project directly to cortex; the other senses relay through the

thalamus. The olfactory bulb is part of the forebrain, situated on its ventral surface in

the olfactory sulcus (the cranial roof of nasal cavity), and attached to it by the

olfactory tract. The olfactory tract branches out into a trifurcation posteriorly: stria

olfactoria lateralis (largest) which will reach and terminate in the anterior part of the

uncus (primary olfactoric cortex area); stria olfactoria medialis which reaches the

medial cerebral hemispherium surface and ends in the cerebral cortex right next to the

anterior lamina terminalis in Brodmann 25 area; stria olfactoria intermedia which

terminates at the cortex in the substantia perforate anterior region.1-3,5,7-9,11-13

Olfactory bulb

Olfactory tract

Cribriform plate ofethmoid bone

Fascicles ofolfactory nerve (I)

Nasal mucosa

Figure 5: Anatomy of the olfactory nerve

The olfactory nerve is the shortest of the twelve cranial nerves and only one of

two cranial nerves (the other being the optic nerve) that do not join with the

brainstem. The effect of damaged olfactory nerve is impaired sense of smell. Clinical

test which can diagnose the impairment is by determining whether subject can smell

(not necessarily identify) aromatic substances such as coffee, vanilla, clove oil, or

soap. Anosmia follows damaged olfactory nerves. There is loss not only of the sense

of smell but also of the flavor, other than the basic tastes, of foods. Anosmia usually

occurs due to head trauma and can happen when meningiomas invade olfactory

nerves.1,4,7

Cranial Nerve II: Optic nerve

The optic nerve, also called cranial nerve II (nerve of sight), transmits visual

information from the retina to the brain. The eye and optic nerve develop as an

outgrowth of the embryonic brain and the nerve is therefore enveloped in meninges.

The optic nerve is composed of axons of the ganglion cells in the eye. This is a pure

sensory nerve fiber. This nerve travels posteromedially from the eye, exiting the orbit

at the optic canal in the lesser wing of the sphenoid bone. The optic nerves join each

Figure 6: Optic nerve; supplies the photoreceptor cells of the retina

other in the middle cranial fossa to form the optic chiasm. The cell bodies are in the

retina and the axons pass back in the optic nerve to the optic chiasma where the axons

from the nasal halves of the retina cross over but those from the temporal side

continue on the same side. They then form the optic tract on each side.1,3-5,10,12,13

The optic nerve is the second of twelve paired cranial nerves but is considered

to be part of the central nervous system as it is derived from an outpouching of the

diencephalon during embryonic development. All of the optic nerve fibers arise from

the ganglionic cells in the retina and in between these fibers is a matrix of neuroglial

network. Consequently, the fibres are covered with myelin produced by

oligodendrocytes rather than the Schwann cells of the peripheral nervous system and

are encased within the meninges. Therefore the distinction of nerve is technically a

misnomer, as the optic system lies within the central nervous system and nerves exist,

by definition, within the peripheral nervous system. The optic nerve is ensheathed in

all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium,

perineurium, and endoneurium found in peripheral nerves. Fibre tracks of the

mammalian central nervous system (as opposed to the peripheral nervous system) are

incapable of regeneration and hence optic nerve damage produces irreversible

blindness. In the posterior part where the optic nerve exits the optic bulb, the optic

nerve is penetrated by the arteria centralis retinae, which is a branch of the opthalmic

artery. It is followed by the vein and are directed ventrally towards the center of the

optic nerve disc.1,2,5

The optic nerve is composed of retinal ganglion cell axons and Portort cells. It

leaves the orbit (eye) via the optic canal, running posteromedially (the optic canal is

located superomedially from the superior orbital fissure) towards the optic chiasm

where there is a partial decussation (crossing) of fibres from the temporal visual fields

of both eyes. The fibers that form the decussation are those that arise from the nasal

half/medial retina, which continue to the optic tract on the contralateral side. The

fibers from the temporal half/lateral retina do not form the decussation and continue

to the optic tract on the ipsilateral side. Most of the axons of the optic nerve terminate

in the lateral geniculate nucleus from where information is relayed to the visual

cortex, while other axons terminate in the pretectal nucleus and are involved in

Figure 7: Central connections of the optic nerve

reflexive eye movements and other axons terminate in the suprachiasmatic nucleus

and are involved in regulating the sleep-wake cycle. Most of these fibres terminate in

the lateral geniculate body.1-3,5,7,8,13

From the lateral geniculate body, all the cells reach out their axons to

Brodmann 17 area and fibrae geniculocalcarinae, and form the optic radiation, which

terminates in the primary visual cortex of the occipital lobe. The optic radiation fibers

circulate around the cornu inferius and posterius ventriculi lateralis before

terminating at area striata. Fibers carrying information from the contralateral superior

visual field traverse the temporal lobe (Meyer's loop) to terminate in the lingual gyrus

below the calcarine fissure in the occipital lobe, and fibers carrying information from

the contralateral inferior visual field terminate in the visual cortex above the calcarine

fissure.1-3,5,7,13

Cranial Nerve III: Oculomotor nerve

The oculomotor nerve is the third of twelve paired cranial nerves. It controls

most of the eye's movement, constriction of the pupil, and maintains an open eyelid.

The main aim of eye movement is to focus external objects on to the fovea of the eye,

and to keep the focus on the fovea. The eye has to be stabilized even when the head

moves. Each eye has six extraocular muscles, and each eye has five movements,

which are governed by three bilateral groups of brain stem oculomotor nuclei. The

Figure 8: Oculomotor nerve; supply muscles of the eyeball and eyelid

Oculomotor nerve (III)

Superior branch

Inferior branch

Ciliary ganglion

Superior orbital fissure

oculomotor nerve supplies somatic motor fibers to all the ocular muscles, except the

Obliquus superior and Rectus lateralis; it also controls, through its connections with

the ciliary ganglion, smooth muscle within the eye using parasympathetic

neurons.1,3,5,7,10,12,13

The oculomotor nerve contains two types of fibers: somatic efferent fibers

(derived from oculomotor nucleus) and visceral efferent fibers (derived from

Edinger-Westphal nucleus). The motor neurons serving the extraocular muscles have

their cell bodies in the oculomotor nucleus, which lies at the base of the

periaqueductal grey of the midbrain at the superior colliculus level. The oculomotor

nucleus, which consists of mutipolar neurons resides at the ventral area of the

substantia grisea centralis. Preganglionic parasympathetic neurons are derived from

the Edinger-Westphal nucleus, which consists of small, ovoid neurons that resemble

the dorsal nucleus of vagus nerve, and supplies the two intraocular muscles:

constrictor pupillae and ciliary muscle. It is located at the tip and dorsomedial part of

the cranial oculomotor nucleus.1,2,5,7,8

Figure 9: Nerves of the orbit

On emerging from the brain, the nerve is invested with a sheath of pia mater,

and enclosed in a prolongation from the arachnoid. It passes between the superior

cerebellar (below) and posterior cerebral arteries (above), and then pierces the dura

mater anterior and lateral to the posterior clinoid process, passing between the free

and attached borders of the tentorium cerebelli. It runs along the lateral wall of the

cavernous sinus, above the other orbital nerves, receiving in its course one or two

filaments from the cavernous plexus of the sympathetic, and a communicating branch

from the ophthalmic division of the trigeminal. It then divides into two branches,

which enter the orbit through the superior orbital fissure, between the two heads of

the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal

and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed

between its two rami.1,2,5,7

In the orbit, the visceral efferent fibers run towards the ciliary ganglion on the

lateral side of the optic nerve. The preganglionic parasympathetic fibers form a

synapsis at this ganglion, while the postganglionic parasympathetic fibers from ciliary

ganglion enter the bulbus oculi to serve the ciliary muscle and sphincter papillae

muscle. These visceral efferent fibers are also the efferent part from arc of the pupil

light reflex and accommodation-convergence reflex.2

Pupillary Light Reflex

The size of the pupil regulates the amount of light that enters the eye. The

direct light reflex happens when the constriction of the pupil is caused by illumination

of the retina through contraction of the sphincter pupillae muscle, thus reducing the

amount of light reaching the retina. Although only one retina is illuminated, both

eyes’ pupils constrict, causing the constriction of the non-illuminated eye to be called

as consensual light reflex. Lesion of the oculomotor nerve on either side can cause

mydriasis (dilated pupil) and iridoplegia (loss of pupillary light reflex.2,4,7-9,13

Accommodation Reflex

The eyes converge when objects move closer, and diverge when objects

move away. This is controlled by the vergence system. The lens of the eye must also

accommodate for moving objects through contractions of the ciliary muscle. Fixation

upon a nearby object, by convergence of the optic axes, involves concomitant

contraction of the ciliary muscles to increase the convexity of the lens, thus focusing

the image. Pupillary constriction also accompanies this phenomenon, which involves

the visual cortex with the corticobulbar fibers activating the parasympathetic neurons

of the Edinger-Westphal nuclei bilaterally. Lesion to the oculomotor nerve with

respect to accommodation reflex is loss of convergence reflex and accommodation

(cycloplegia).2-4,7-9,13

Cranial Nerve IV: Trochlear nerve

Figure 10: Anatomic overview of the trochlear nerve

The trochlear nerve is a motor nerve (a somatic efferent nerve) that innervates

a single muscle: the superior oblique muscle of the eye. The trochlear nerve is purely

a motor nerve. It arises from a nucleus situated in the floor of the cerebral aqueduct,

opposite the upper part of the inferior colliculus and next to the dorsal fasciculus

longitudinalis medialis. The trochlear nerve is unique among the cranial nerves in

several respects. It is the smallest nerve in terms of the number of axons it contains. It

has the greatest intracranial length. Along with the optic nerve (cranial nerve II), it is

the only cranial nerve that decussates before innervating its target. Finally, it is the

only cranial nerve that exits from the dorsal aspect of the brainstem.1,2,4,5,7,12,13

The nerve is directed across the superior cerebellar peduncle, and then winds

forward around the cerebral peduncle, instantly above the pons, pierces the dura

mater in the free border of the tentorium cerebelli, just behind, and lateral to, the

posterior clinoid process, and passes forward in the lateral wall of the cavernous

sinus, between the oculomotor nerve and the ophthalmic division of the trigeminal. It

crosses the oculomotor nerve, and enters the orbit through the superior orbital fissure.

It now becomes the highest of all the nerves, and lies medial to the frontal nerve. In

the orbit it passes medially, above the origin of the levator palpebra superioris, and

finally enters the orbital surface of the obliquus superior and innervates the superior

oblique muscle.1,5,6,10

Figure 11: The cavernous sinus

In the lateral wall of the cavernous sinus the trochlear nerve forms

communications with the ophthalmic division of the trigeminal and with the

cavernous plexus of the sympathetic. In the superior orbital fissure it occasionally

gives off a branch to the lacrimal nerve. It gives off a recurrent branch which passes

backward between the layers of the tentorium cerebelli and divides into two or three

filaments which may be traced as far as the wall of the transverse sinus.5

Cranial Nerve V: Trigeminal nerve

The trigeminal nerve is responsible for sensation in the face, forehead, nasal

cavity, dura mater, major intracranial blood vessels, tongue, gums and teeth (touch,

and temperature). It contains branchiomotor and general somatic afferent fibers

(which are extroceptive and proprioceptive). Sensory information from the face and

body is processed by parallel pathways in the central nervous system. The trigeminal

nerve is primarily a sensory nerve, but it also has certain motor functions (biting,

chewing, and swallowing). The trigeminal nerve is the largest cranial nerve and is the

great sensory nerve of the head and face, and the motor nerve of the muscles of

mastication.1-5,7-9,12,13

Figure 12: Trigeminal nerve; supplies the forehead, cheek, and masticator muscles

The trigeminal nerve arises from the brain at the side of the pons by a motor

and a sensory root. The sensory root carries the trigeminal ganglion which consists of

the cell bodies of the sensory axons and lies in a depression on the petrous temporal

bone. They pass backward below the superior petrosal sinus and tentorium cerebelli,

and, entering the pons, divide into upper and lower roots. The upper root ends partly

in a nucleus which is situated in the pons lateral to the lower motor nucleus, and

partly in the locus caeruleus; the lower root descends through the pons and medulla

oblongata, and ends in the upper part of the substantia gelatinosa of Rolando. This

lower root is sometimes named the spinal root of the nerve. It then divides into

ophthalmic, maxillary and mandibular divisions. The motor root forms part of the

mandibular division. The fibers of the motor root arise from two nuclei, a superior

and an inferior. The superior nucleus consists of a strand of cells occupying the whole

length of the lateral portion of the gray substance of the cerebral aqueduct. The

inferior or chief nucleus is situated in the upper part of the pons, close to its dorsal

surface, and along the line of the lateral margin of the rhomboid fossa. The fibers

from the superior nucleus constitute the mesencephalic root: they descend through the

mid-brain, and, entering the pons, join with the fibers from the lower nucleus, and the

motor root, thus formed, passes forward through the pons to its point of

emergence.1,5,10

Figure 13: Anatomy of the trigeminal nerve, and its sensory and motor branches

The ophthalmic branch, or first division of the trigeminal nerve, is a sensory

nerve. It enters the orbit of the eye and carries sensory information from the cornea,

ciliary body and iris, lacrimal gland and conjunctiva, mucous membrane of the nasal

cavity, and skin of the eyelids, eyebrow, forehead, and nose. It is the smallest of the

three divisions of the trigeminal, and arises from the upper part of the semilunar

ganglion as a short, flattened band, which passes forward along the lateral wall of the

cavernous sinus, below the oculomotor and trochlear nerves; just before entering the

orbit, through the superior orbital fissure, it divides into three branches, lacrimal,

frontal, and nasociliary. The ophthalmic branch is joined by filaments from the

cavernous plexus of the sympathetic, and communicates with the oculomotor,

trochlear, and abducent nerves; it gives off a recurrent filament which passes between

the layers of the tentorium.1,3-6,9,13

The maxillary branch, or second division of the trigeminal, is a sensory nerve.

The maxillary branch carries sensory information from the lower eyelid and cheek,

the nares and upper lip, the upper teeth and gums, the posterior region of the nasal

mucosa, the palate and roof of the pharynx, the maxillary, ethmoid and sphenoid

sinuses, and parts of the meninges. It travels forward in a groove on the floor of the

middle cranial fossa, and exits the cranial cavity through the foramen rotundum. It is

intermediate, both in position and size, between the ophthalmic and mandibular. It

begins at the middle of the semilunar ganglion as a flattened plexiform band, and

passing horizontally forward, it leaves the skull through the foramen rotundum,

where it becomes more cylindrical in form, and firmer in texture. It then crosses the

pterygopalatine fossa, inclines laterally on the back of the maxilla, and enters the

orbit through the inferior orbital fissure; it traverses the infraorbital groove and canal

in the floor of the orbit, and appears upon the face at the infraorbital foramen. At its

termination, the nerve lies beneath the Quadratus labii superioris, and divides into a

leash of branches which spread out upon the side of the nose, the lower eyelid, and

the upper lip, joining with filaments of the facial nerve.1,3-6,9,13

The mandibular division is mixed, containing motor and sensory fibers. It

exits the cranium through the foramen ovale. The mandibular branch supplies the

teeth and gums of the mandible, the skin of the temporal region, the auricula, the

lower lip, the lower part of the face, and the muscles of mastication; it also supplies

the mucous membrane of the anterior two-thirds of the tongue (the lingual nerve). It

is the largest of the three divisions of the fifth, and is made up of two roots: a large,

sensory root proceeding from the inferior angle of the semilunar ganglion, and a

small motor root (the motor part of the trigeminal), which passes beneath the

ganglion, and unites with the sensory root, just after its exit through the foramen

ovale. Immediately beneath the base of the skull, the nerve gives off from its medial

side a recurrent branch (nervus spinosus) and the nerve to the Pterygoideus internus,

and then divides into two trunks, an anterior and a posterior.1,3-6,9,10,13

Cranial Nerve VI: Abducens nerve

The abducens nerve or abducent nerve is a somatic efferent nerve that controls

the movement of a single muscle, the lateral rectus muscle of the eye. It is another

Figure 14: Abducens nerve; supplies the lateral rectus muscle

pure motor nerve fiber and originates from the abducens nucleus situated in the upper

part of the rhomboid fossa, close to the middle line and beneath the colliculus facialis.

They pass downward and forward through the pons, and emerge in the furrow

between the lower border of the pons and the upper end of the pyramid of the medulla

oblongata. It has a long intracranial course (so is often the first nerve to be affected in

raised intracranial pressure) to the cavernous sinus, where it is closely applied to the

internal carotid artery, and thence to the orbit via the superior orbital fissure. From

the nucleus, fibers are said to pass through the medial longitudinal fasciculus to the

oculomotor nerve of the opposite side, along which they are carried to the Rectus

medialis. The Rectus lateralis of one eye and the Rectus medialis of the other may

therefore be said to receive their nerves from the same nucleus.1,2,4-10,12,13

Before entering the orbit via superior orbital fissure, the abducens nerve

traverses in the cavernous sinus. In the cavernous sinus, the oculomotor, trochlear,

and ophthalmic nerves are placed in the lateral wall of the sinus, in the order given,

from above downward. The abducent nerve lies at the lateral side of the internal

Figure 15: The path of the abducens nerve

carotid artery. As these nerves pass forward to the superior orbital fissure, the

oculomotor and ophthalmic divide into branches, and the abducent nerve approaches

the others; so that their relative positions are considerably changed.1,-6

Cranial Nerve VII: Facial nerve

The facial nerve emerges from the brainstem between the pons and the

medulla, and controls the muscles of facial expression, and taste to the anterior two-

thirds of the tongue. It also supplies preganglionic parasympathetic fibers to several

head and neck ganglia, the posterior digastric, stapedius, and stylohyoid muscles. The

facial nerve consists of a motor and a sensory part, the latter being frequently

described under the name of the nervus intermedius. The two parts emerge at the

lower border of the pons in the recess between the olive and the inferior peduncle, the

motor part being the more medial, immediately to the lateral side of the sensory part

is the acoustic nerve.1-5,7-10,12,13

The facial nerve leaves the brain near the cerebellum and passes laterally into

the internal auditory meatus. It reaches the medial wall of the middle ear and turns

backwards and downwards to leave the skull via the stylomastoid foramen. It then

traverses the parotid gland, in which it divides into five branches (temporal,

Figure 16: Facial nerve; supplies the facial area and the front of the tongue

zygomatic, buccal, marginal mandibular and cervical) which are distributed to the

muscles of facial expression, the platysma and the posterior belly of the digastric. In

the middle ear it gives off the greater petrosal branch which carries parasympathetic

fibres to the sphenopalatine ganglion and thence to the lacrimal gland. In the middle

ear it also gives off the chorda tympani which joins the lingual nerve and is

distributed with it.1,4-6,10

The motor facial nucleus, which is a multipolar columnar neuron, located in

the lateral tegmentum pontis and directly next to the cranial nucleus ambiguus. This

nucleus is the center for branchiomotor fibers that initially traverse dorsomedially

towards the rhomboid fossa and subsequently encircling the cranial part of nucleus

abducens. The cranial salivatory nucleus is a small group of neurons at the

dorsolateral part of formatio reticularis pontis, next to the cranial part of caudal

salivatory nucleus. This nucleus is the center for visceromotoric, secretomotoric, or

preganglionic parasympathetic fibers. Outside the central nervous system, these fibers

also form the nervus intermedius, and branch out into nervus petrosus superficialis

major and tympanic chord. Facial nerve gustatory fibers carry taste impulses from the

two-third part of ventral dorsum linguae. Facial nerve also contains general somatic

afferent fibers, which are limited in number and non-essential.2,5-8

Lesions of the facial nerve are the most frequent cause of loss of facial

reflexes and facial paralysis. Bell's palsy is one type of idiopathic acute facial nerve

Temporal

Zygomatic

Buccal

Mandibular

Cervical

Stylomastoid foramen

Figure 17: The five major branches of the facial nerve

paralysis, which is more accurately described as a multiple cranial nerve ganglionitis

that involves the facial nerve, and most likely results from viral infection and also

sometimes as a result of Lyme disease. It represents an acute unilateral inflammatory

lesion of the facial nerve. Iatrogenic Bell's palsy may also be as a result of an

incorrectly placed dental local-anesthetic (Inferior alveolar nerve block).1,3,4,7

Cranial Nerve VIII: Vestibulocochlear nerve

The vestibulocochlear nerve (also known as the auditory or acoustic nerve) is

responsible for transmitting sound and equilibrium (balance) information from the

inner ear to the brain. This nerve is purely a sensory nerve. It consists of two distinct

sets of fibers which differ in their peripheral endings, central connections, functions,

and time of medullation. The vestibulocochlear nerve innervates the hair cell

Figure 18: Vestibulocochlear nerve; supplies the balance and hearing organs

receptors of the inner ear. It carries vestibular information to the brain from the

semicircular canals, utricle, and saccule providing the sense of balance. It also carries

information from the cochlea providing the sense of hearing. This cranial nerve

branches into the vestibular branch (balance) and the cochlear branch (hearing). The

cochlear fibers originate from the spiral ganglion. It is soft in texture and devoid of

neurilemma.1,2,4-10,12,13

The vestibular nerve, the nerve of equilibration, arises from bipolar cells in

the vestibular ganglion, ganglion of Scarpa, which is situated in the upper part of the

outer end of the internal auditory meatus. The vestibular nerve travels from the

vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of

the bipolar neurons and extends processes to five sensory organs, which are the

medial vestibular nucleus, the lateral vestibular (Deiters’) nucleus, the superior

vestibular nucleus, the inferior vestibular nucleus, and the cerebellum. Three of these

are the cristae located in the ampullae of the semicircular canals. Hair cells of the

cristae activate afferent receptors in response to rotational acceleration. The other two

sensory organs supplied by the vestibular neurons are the maculae of the saccule and

utricle. Hair cells of the maculae activate afferent receptors in response to linear

acceleration. The hair cells are oriented in the labyrinth of the ear through the

orientation of the canals and of the otolith organs, the saccule and utricle. The

peripheral fibers divide into three branches: the superior branch passes through the

foramina in the area vestibularis superior and ends in the utricle and in the ampullae

of the superior and lateral semicircular ducts; the fibers of the inferior branch traverse

the foramina in the area vestibularis inferior and end in the saccule; the posterior

branch runs through the foramen singulare and supplies the ampulla of the posterior

semicircular duct.1-5,7,8

The cochlear nerve, the nerve of hearing, arises from bipolar cells in the spiral

ganglion of the cochlea, situated near the inner edge of the osseous spiral lamina. The

peripheral fibers pass to the organ of Corti. It is the inner hair cells of the organ of

Corti that are responsible for activation of afferent receptors in response to pressure

waves reaching the basilar membrane through the transduction of sound. The central

ones pass down the modiolus and then through the foramina of the tractus spiralis

foraminosus or through the foramen centrale into the lateral or outer end of the

internal auditory meatus. The nerve passes along the internal auditory meatus with the

vestibular nerve and across the subarachnoid space, just above the flocculus, almost

directly medially toward the inferior peduncle to terminate in the cochlear nucleus.

The ventral cochlear nucleus is located on the anterolateral surface of pedunculus

cerebellaris caudalis while the dorsal cochlear nucleus is located on the dorsolateral

surface of pedunculus cerebellaris caudalis. All the fibers that exit the cochlear nuclei

move medially towards the border between medulla oblongata and pons, and are

grouped into three categories of stria acusticae. They are stria acustica ventralis, stria

acustica dorsalis, and stria acustica intermedia.1-5,7,13

Figure 19: The course and connections of the vestibulocochlear nerve in the temporal bone

Cranial Nerve IX: Glossopharyngeal nerve

The glossopharyngeal nerve contains both motor and sensory fibers, and is

distributed, as its name implies, to the tongue and pharynx. It exits the brainstem out

from the sides of the upper medulla, just rostral (closer to the nose) to the vagus

nerve. The glossopharyngeal nerve innervates the pharynx (upper part of the throat),

the soft palate and the posterior one-third of the tongue. It carries sensory information

(touch, temperature, and pressure) from the pharynx and soft palate. It carries taste

sensation from the taste buds on the posterior one third of the tongue. It provides

somatic motor innervation to the throat muscles involved in swallowing, salivation,

and gagging. It provides visceral motor innervation to the salivary glands. This

cranial nerve also supplies the carotid sinus and reflex control to the heart. It is

composed of both sensory and motor axons and originates from the nucleus

ambiguous in the reticular formation of the medulla.1,3-5,9,10,12,13

Figure 20: Glossopharyngeal nerve; supplies the back of the tongue, soft palate and reflex control of the heart

There are a number of functions of the glossopharyngeal nerve, which are

receiving general sensory fibers (ventral trigeminothalamic tract) from the tonsils, the

pharynx, the middle ear and the posterior 1/3 of the tongue, receiving special sensory

fibers (taste) from the posterior one-third of the tongue, receiving visceral sensory

fibers (chemoreceptors and baroreceptors) from the carotid bodies, supplying

parasympathetic fibers to the parotid gland via the otic ganglion, supplying motor

fibers to stylopharyngeus muscle, the only motor component of this cranial nerve, and

contributing to the pharyngeal plexus. In the pharyngeal plexus, the fibers of the

glossopharyngeal and vagus nerve are intertwined, making it hard to determine a pure

lesion of the glossopharyngeal nerve.1-3,7,8,10

The sensory fibers arise from the cells of the superior and petrous ganglia,

which are situated on the trunk of the nerve. When traced into the medulla, some of

the sensory fibers, probably sympathetic afferent, end by arborizing around the cells

of the upper part of a nucleus which lies beneath the ala cinerea in the lower part of

the rhomboid fossa. Many of the fibers, probably the taste fibers, contribute to form a

strand, named the fasciculus solitarius, which descends in the medulla oblongata.

Associated with this strand are numerous nerve cells, and around these the fibers of

the fasciculus end. The somatic sensory fibers, few in number, are said to join the

Glossopharyngeal nerve (IX)

Parotid salivary gland

Parasympathetic fibers

Superior ganglion

Jugular foramen

Inferior ganglion

Otic ganglion

Carotid sinus

Pharyngeal muscles

Figure 21: The glossopharyngeal nerve and associated organs

spinal tract of the trigeminal nerve. The carotid branch of the glossopharyngeal

contains two sets of afferents. One set runs centrally from the baroreceptor stretch

receptors in the wall of the carotid sinus at the beginning of the internal carotid artery.

These receptors respond to changes in the systolic pressure. These afferents synapse

centrally in the medial portion of the solitary nucleus. Another set of afferents run

centrally from the glomus cells of the carotid body. The nerve endings of these

afferents are chemoreceptors, which respond to O2 and CO2 partial pressure changes

in the blood. Their afferents terminate centrally in the dorsal respiratory nucleus.3,5,7,13

The somatic motor fibers spring from the cells of the nucleus ambiguus,

which lies some distance from the surface of the rhomboid fossa in the lateral part of

the medulla, innervates the stylopharyngeus muscles, which takes part in swallowing

and, are continuous below with the anterior gray column of the medulla spinalis.

From this nucleus the fibers are first directed backward, and then they bend forward

and laterally to join the fibers of the sensory root. The nucleus ambiguus gives origin

to the motor branches of the glossopharyngeal and vagus nerves, and to the cranial

part of the accessory nerve. The sympathetic efferent fibers from the nucleus beneath

the ala cinerea, the dorsal nucleus, are probably both preganglionic motor fibers and

preganglionic secretory fibers of the sympathetic system. The secretory fibers pass to

the otic ganglion and from it secondary neurons are distributed to the parotid gland;

the postganglionic nerve innervates the salivary parotid gland.3,5,7,13

Cranial Nerve X: Vagus nerve

The vagus nerve is also called pneumogastric nerve since it innervates both

the lungs and the stomach. It is the longest cranial nerve innervating many structures

in the throat, including the muscles of the vocal cords, thorax and abdominal cavity.

The vagus nerve is the main parasympathetic nerve, having very extensive motor and

sensory components. Upon leaving the medulla between the olivary nucleus and the

inferior cerebellar penduncle, it extends through the jugular foramen, then passing

into the carotid sheath between the internal carotid artery and the internal jugular vein

down below the head, to the neck, chest and abdomen, where it contributes to the

innervation of the viscera. Besides output to the various organs in the body the vagus

Figure 22: Vagus nerve; supplies parts of the abdominal cavity

nerve conveys sensory information about the state of the body's organs to the central

nervous system.1,3-6,12

The vagus nerve encompasses a number of fibers. The nucleus ambiguus,

which is a source of branchiomotor fibers, is a long cell columnnext to the cranial

part of the accessory nucleus and is composed of somatomotor and visceromotor

cells. The nucleus dorsalis nervi vagi, where the vagus nerve’s preganglionic

parasympathetic fibers arise, is a long cell column near the fossa rhomboidea. The

nucleus solitaries, which is located near the tractus solitaries, is divided into two

different functional neuron groups; the nucleus parasolitarius, which is on the

ventrolateral side of tractus solitaries, is a general visceral afferent, and receives

impulse from thoracic and abdominal viscera; while the nucleus gustatorius, which is

a small group of neurons on the dorsomedial side of tractus solitaries, is a special

visceral afferent fiber. The nucleus spinalis nervi trigemini, which receives

extroceptive impulses from a section of the skin bordered around the porus acusticus

externus by the general somatic afferent fibers of the vagus nerve.2

Figure 23: The vagus nerve and associated organs

The vagus is attached by eight or ten filaments to the medulla oblongata in the

groove between the olive and the inferior peduncle, below the glossopharyngeal. The

sensory fibers arise from the cells of the jugular ganglion and ganglion nodosum of

the nerve, and, when traced into the medulla oblongata mostly end by arborizing

around the cells of the inferior part of a nucleus which lies beneath the ala cinerea in

the lower part of the rhomboid fossa. These are the sympathetic afferent fibers. A few

of the sensory fibers of the vagus, probably taste fibers, descend in the fasciculus

solitarius and end around its cells. The somatic sensory fibers from the posterior part

of the external auditory meatus and the back of the ear, probably join the spinal tract

of the trigeminal as it descends in the medulla. General visceral afferent fibers convey

visceral impulses from tunica mucosa centrally. The afferent fibers convey

information from the general sensation receptors in the pharynx, larynx, oesophagus,

tympanic membrane, external auditory meatus and part of the external ear’s concha,

the chemoreceptors in the aortic bodies and baroreceptors in the aortic arch, and the

receptors that are widely distributed throughout the thoracic and abdominal viscera.2,4-

7,12,13

The motor fibers of the vagus nerve arise from the nucleus ambiguus of the

medulla. They innervate the muscles of the soft palate, pharynx, larynx and upper

part of the oesophagus, and are important in speech and swallowing control. The

sympathetic efferent fibers, distributed probably as preganglionic fibers to the

thoracic and abdominal viscera, i. e., as motor fibers to the bronchial tree, inhibitory

fibers to the heart, motor fibers to the esophagus, stomach, small intestine and gall

passages, and as secretory fibers to the stomach and pancreas, arise from the dorsal

nucleus of the vagus. The filaments of the nerve unite, and form a flat cord, which

passes beneath the flocculus to the jugular foramen, through which it leaves the

cranium. After its exit from the jugular foramen the vagus is joined by the cranial

portion of the accessory nerve, and enlarges into the ganglion nodosum; through this

the fibers of the cranial portion of the accessory are principally distributed to the

pharyngeal and superior laryngeal branches of the vagus, while some of its fibers

descend in the trunk of the vagus to be distributed with the recurrent nerve and the

cardiac nerves. The vagus nerve passes vertically down the neck within the carotid

sheath, lying between the internal jugular vein and internal carotid artery as far as the

upper border of the thyroid cartilage, and then between the same vein and the

common carotid artery to the root of the neck. The further course of the nerve differs

on the two sides of the body.2,4-7,10,12,13

Cranial Nerve XI: Accessory nerve

The accessory nerve is a nerve that controls specific muscles of the neck. It is

pure motor nerve fiber. The accessory nerve originates from neuronal cell bodies

located in the cervical spinal cord and caudal medulla. Most are located in the spinal

cord and ascend through the foramen magnum and exit the cranium through the

jugular foramen. The accessory nerve consists of two parts: a cranial and a spinal.1-7,12

The cranial part (ramus internus; accessory portion) is the smaller of the two.

Its fibers arise from the cells of the nucleus ambiguus and emerge as four or five

delicate rootlets from the side of the medulla oblongata, below the roots of the vagus.

Figure 24: Accessory nerve; supplies the head, neck column and associated structures

It runs laterally towards the jugular foramen, where it interchanges fibers with the

spinal portion or becomes united to it for a short distance; here it is also connected by

one or two filaments with the jugular ganglion of the vagus. It then passes through the

jugular foramen, separates from the spinal portion and is continued over the surface

of the ganglion nodosum of the vagus, to the surface of which it is adherent, and is

distributed principally to the pharyngeal and superior laryngeal branches of the vagus.

Through the pharyngeal branch it probably supplies the musculus uvulae and levator

veli palatini. The cranial branch provides somatic motor innervation to some of the

muscles in the throat involved in swallowing. This cranial branch is accessory to

vagus nerve, with the fibers of the cranial root traveling the same extracranial path as

the branchial motor component of the vagus nerve.2,3,5-8,12

Figure 25: The accessory nerve and associated parts

Spinal Accessory Nerve ( )

The spinal part (ramus externus; spinal portion) is firm in texture, and its

fibers arise from the motor cells in the lateral part of the anterior column of the gray

substance, projecting from the five most rostral segments of the spinal cord. The

spinal accessory nerve provides motor innervation from the central nervous system to

two muscles of the neck: the sternocleidomastoid muscle and the trapezius muscle.

The sternocleidomastoid muscle tilts and rotates the head, while the trapezius muscle

has several actions on the scapula, including shoulder elevation and adduction of the

scapula. Passing through the lateral funiculus of the medulla spinalis, they emerge on

its surface and unite to form a single trunk, which ascends between the ligamentum

denticulatum and the posterior roots of the spinal nerves; enters the skull through the

foramen magnum, and is then directed to the jugular foramen, through which it

passes, lying in the same sheath of dura mater as the vagus, but separated from it by a

fold of the arachnoid. In the jugular foramen, it receives one or two filaments from

the cranial part of the nerve, or else joins it for a short distance and then separates

from it again. The nerve then descends obliquely behind the digastricus and

stylohyoideus to the upper part of the sternocleidomastoideus; it pierces this muscle,

and courses obliquely across the posterior triangle of the neck, to end in the deep

surface of the trapezius. As it traverses the sternocleidomastoideus it gives several

filaments to the muscle, and joins with branches from the second cervical nerve. In

the posterior triangle it unites with the second and third cervical nerves, while

beneath the trapezius it forms a plexus with the third and fourth cervical nerves, and

from this plexus fibers are distributed to the muscle.1-3,5,7,8,10,12,13

Cranial Nerve XII: Hypoglossal nerve

The hypoglossal nerve is the motor nerve of the tongue. It provides somatic

motor innervation to the muscles of the tongue. This pure motor nerve originates

from the hypoglossal nucleus located in the tegmentum of the medulla oblongata in

the preolivary sulcus separating the olive and the pyramid. The hypoglossal nucleus

receives afferents from the solitary nucleus and trigeminal sensory nucleus, which are

involved in chewing, sucking and swallowing. It also receives corticobulbar fibers

from the contralateral motor cortex, which serve in tongue movements such as

speech. It is also used as a sensory neuron to taste bitter.1-3,5,7,8,12,13

It passes through the hypoglossal canal. On emerging from the hypoglossal

canal, it gives off a small meningeal branch (the descendens hypoglossi) and picks up

a branch from the anterior ramus of C1. This joins the descendens cervicalis, derived

from C2 and 3, to form the ansa cervicalis. From this, branches arise to supply the

Figure 26: The hypoglossal nerve and associated parts

‘strap muscles’, i.e. sternothyroid, sternohyoid, thyrohyoid and omohyoid. It spirals

behind the vagus nerve and passes between the internal carotid artery and internal

jugular vein lying on the carotid sheath. After passing deep to the posterior belly of

the digastric muscle, it passes to the submandibular region to enter the tongue. It

supplies motor fibres to all of the muscles of the tongue, except the palatoglossus

muscle which is innervated by the vagus nerve via the pharyngeal plexus. The

innervations of the tongue are ipsilateral.1,2.4-6,10

Summary

In conclusion, the cranial nerves play an extremely important role in our

body, thus affecting our life. Whether it is sensory or motor, somatic or visceral,

general or special, they all play a vital role in determining the quality of life in an

individual.

Lesions and degenerations to the cranial nerves profoundly inhibit an

individual from going about his/her everyday life. For example, the motor neuron

disease is a chronic degenerative disorder which is seen in those aged over 50 years.

The corticobulbar tracts projecting to the nucleus ambiguus and hypoglossal nucleus

degenerate, leading to dysphonia, dysphagia, dysarthria, and weakness and spasticity

of the tongue. One of the probable causes of this disorder is due to damage by

compression caused by tumors in the nerve areas.

Therefore, it is essential that concern towards cranial nerves should be

emphasized from a very young age. Early detections may provide a better outcome

for most individuals.

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