cranial nerves
TRANSCRIPT
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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