THE INNERVATION OF THE HEAD AND NECK The head and the neck serve as origins but also as passing ways for a number of important nerves of the body. Obviously, some of these nerves are destined to provide the proper innervation of the cephalic extremity. The innervation of the head and neck is alike to the allover innervation of the human body, namely general sensitive, special sensorial, motor and vegetative. The general sensitive innervation acts for collecting the superficial sensitive (nociceptive, thermal, tactile) information from the tegument and lining mucosae, the profound stretching and pressure (proprioceptive) stimuli from some deeply situated somatic structures like muscles, ligaments and articular capsules and the sensitive-vegetative innervation (visceral pain and plenitude) of visceral and glandular organs. The first sensitive neuron (protoneuron) is situated outside the neuraxis, grouped in the sensitive ganglia. The peripheral sensitive fibers are more-or-less myelinated dendrites, originating from the peripheral sensitive ganglia. The sensory (or special sensitive) afferent innervation is developed for some particular types of stimuli, one single for each of the specialized organs of the senses: the olfaction for odors, the sight for the light, the hearing for mechanic waves, the vestibular for the position and movements of the head and the taste for soluble chemicals. The special sensitive afferents can be of somatic (vision, hearing, equilibrium) or of visceral nature (olfaction, taste). The motor innervation targets the generic somatic effector organs, the striated muscles. According to their ontogenetic origin, the striated muscles and their motor innervation can be somitic or branchial. Branchially derived muscles can be encountered exclusively on the head and on the neck. It is to be mentioned, that some branchial striated muscles are located in various visceral organs, thus they deserve visceral functions. The motor innervation of the striated muscles or, at least the initiation of the movements is typically deliberate and well sensed by the proprioceptive sensitive system. Movements are subject to highly effective, feedback-based regulation mechanisms. The vegetative innervation is particular to the vegetative effectors, smooth muscles and glands. The vegetative motor and excretory functions are largely autonomous with almost no sensitive feedback and with certainly no purposeful control of the conscience. Unlike the somatic nerves, in addition to the intra-neuraxial centers, the autonomic nerves possess extra- neuraxial relay stations, the vegetative ganglia (ganglion, Ggl.). The vegetative nerve fibers (axons) originating from the centers emerge from the neuraxis as pre-ganglionar fibers, than they synapse in a vegetative ganglion with the post-ganglionar (in fact: ganglionar) neuron, wherefrom the post-ganglionar fiber (axon) originates. According to their neuromediators and their final physiological action, the vegetative nerves can be of two types, sympathetic (orthosympathetic) or parasympathetic. A vegetative effector organ, by its receptor apparatus usually is responsive to only one of these two types of stimuli. The peripheral nervous system (Systema nervosum periphericum) by the means of the cranial nerves (Nn. craniales seu encephalici), a number of spinal nerves (Nervi spinales) and the autonomous nervous system (Systema nervosum vegetativum seu autonomicum) serves as the anatomical basis of all these types of innervation. THE CRANIAL NERVES The cranial nerves in fact are nerves originating from the brain stem, and accordingly, their correct name would be encephalic nerves. Ancient anatomic studies described 12 pairs of cranial nerves, and despite it is not the case, this numbering prevails. Actually, the first pair is not a unitary cranial nerve and the second pair is not a cranial nerve, but an outgrown pathway (tract) of the neuraxis. Moreover, there are two other pairs of peripheral nerves which could
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Transcript
THE INNERVATION OF THE HEAD AND NECK
The head and the neck serve as origins but also as passing ways for a number of important
nerves of the body. Obviously, some of these nerves are destined to provide the proper
innervation of the cephalic extremity. The innervation of the head and neck is alike to the
allover innervation of the human body, namely general sensitive, special sensorial, motor and
vegetative.
The general sensitive innervation acts for collecting the superficial sensitive (nociceptive,
thermal, tactile) information from the tegument and lining mucosae, the profound stretching
and pressure (proprioceptive) stimuli from some deeply situated somatic structures like
muscles, ligaments and articular capsules and the sensitive-vegetative innervation (visceral
pain and plenitude) of visceral and glandular organs. The first sensitive neuron (protoneuron)
is situated outside the neuraxis, grouped in the sensitive ganglia. The peripheral sensitive
fibers are more-or-less myelinated dendrites, originating from the peripheral sensitive ganglia.
The sensory (or special sensitive) afferent innervation is developed for some particular types
of stimuli, one single for each of the specialized organs of the senses: the olfaction for odors,
the sight for the light, the hearing for mechanic waves, the vestibular for the position and
movements of the head and the taste for soluble chemicals. The special sensitive afferents can
be of somatic (vision, hearing, equilibrium) or of visceral nature (olfaction, taste).
The motor innervation targets the generic somatic effector organs, the striated muscles.
According to their ontogenetic origin, the striated muscles and their motor innervation can be
somitic or branchial. Branchially derived muscles can be encountered exclusively on the head
and on the neck. It is to be mentioned, that some branchial striated muscles are located in
various visceral organs, thus they deserve visceral functions. The motor innervation of the
striated muscles or, at least the initiation of the movements is typically deliberate and well
sensed by the proprioceptive sensitive system. Movements are subject to highly effective,
feedback-based regulation mechanisms.
The vegetative innervation is particular to the vegetative effectors, smooth muscles and
glands. The vegetative motor and excretory functions are largely autonomous with almost no
sensitive feedback and with certainly no purposeful control of the conscience. Unlike the
somatic nerves, in addition to the intra-neuraxial centers, the autonomic nerves possess extra-
neuraxial relay stations, the vegetative ganglia (ganglion, Ggl.). The vegetative nerve fibers
(axons) originating from the centers emerge from the neuraxis as pre-ganglionar fibers, than
they synapse in a vegetative ganglion with the post-ganglionar (in fact: ganglionar) neuron,
wherefrom the post-ganglionar fiber (axon) originates. According to their neuromediators and
their final physiological action, the vegetative nerves can be of two types, sympathetic
(orthosympathetic) or parasympathetic. A vegetative effector organ, by its receptor apparatus
usually is responsive to only one of these two types of stimuli.
The peripheral nervous system (Systema nervosum periphericum) by the means of the
cranial nerves (Nn. craniales seu encephalici), a number of spinal nerves (Nervi spinales) and
the autonomous nervous system (Systema nervosum vegetativum seu autonomicum) serves as
the anatomical basis of all these types of innervation.
THE CRANIAL NERVES
The cranial nerves in fact are nerves originating from the brain stem, and accordingly, their
correct name would be encephalic nerves. Ancient anatomic studies described 12 pairs of
cranial nerves, and despite it is not the case, this numbering prevails. Actually, the first pair is
not a unitary cranial nerve and the second pair is not a cranial nerve, but an outgrown pathway
(tract) of the neuraxis. Moreover, there are two other pairs of peripheral nerves which could
be regarded as individual cranial nerves too: the terminal nerve (N. terminalis) and the
vomeronasal nerve (N.vomeronasalis), but they are not included in this group. According to
the tradition, the cranial nerves are numbered from above to below in the order they leave the
surface of the neuraxis.
Descriptively, a cranial nerve has its origin in a (or some) nucleus (nucleus, nuclei, Nc., Ncc.)
situated in the brain stem. This is the so-called proper origin of the cranial nerve, that in some
cases ( N.IV., N. VI., XII.) is unique, but in other cases (N.III., N.V., N. VII., N.VIII., N. IX., N.
X., N. XI.) is multiple. Following the proper origin, the cranial nerve emerges on the surface of
the brain, at the site of its apparent origin. The upper cranial nerves appear as unitary trunks,
but the N.V., N. VII., N. VIII., N. IX., N. X., N. XI., XII.) rise under the form of convergent
roots (Radix) or multiple radicular filaments (Fila radicularia) which merge in a trunk at a
distance of several mm from the surface of the medulla. The cranial nerves are paired and
symmetrical on the midline and they emit collateral branches and endbranches. Exception
from under this rule of symmetry is the N. X.
Topographically, the real encephalic nerves have their origin on the brain stem, in the
posterior fossa of the neurocranium (Fossa cranii post.). After their apparent origin, the
cranial nerves travel various distances on the inner skull base (Basis cranii interna) until they
reach the corresponding exit orifice of the neurocranium. On their intracranial way, the cranial
nerves are ensheathed by the leptomeninges, the pia mater and the arachnoidea. They also
establish clinically important topographic relationships (syntopy) with other intracranial
structures. When leaving the cranial cavity, they might exit straight away or they course quite
for a distance inside the cranial base until eventually showing up extracranially. Than the
cranial nerves continue their way to the target organs and structures, branch exhaustively and
develop elaborated syntopy. The branches can be collaterals (sidebranches) or terminal
(endbranches).
Systematically, a nucleus of a cranial nerve can be a nucleus of origin (Nc. originis) for
somatic motor and vegetative nerves, or nucleus of termination (Nc. terminationis) for
sensitive or sensory nerves.
Functionally, a cranial nerve can be purely motor, purely sensitive or mixed. Branchiomotor
and somitic motor functions never mix. There is no pure vegetative cranial nerve.
Endbranches of mixed cranial nerves are pure. It is important to understand that regardless to
their functional composition at their origin, on the periphery, in their tree, cranial nerves
frequently exchange branches, completing eachother’s effect. Usually some post-ganglionar
parasympathetic axons are swapped, and the typical accepting nerve is the N. V.
As for listing the cranial nerves, the anatomy uses roman numerals and/or proper names.
These, with the above-mentioned reserves are:
the N. I. or the olfactory nerve (Nn. olfactorii),
the N. II. or the optic nerve (N. opticus),
the N. III. or the oculomotor nerve (N. oculomotorius),
the N. IV. or the trochlear nerve (N. trochlearis),
the N. V. or the trigeminal (N. trigeminus),
the N. VI. or the abducent nerve (N. abducens),
the N. VII. or the intermediofacial nerve (N. intermediofacialis),
the N. VIII. or the vestibulocochlear nerve (N. vestibulocochlearis seu N. statoacusticus),
the N. IX. or the glossopharyngeal nerve (N. glossopharyngeus),
the N. X. or the vagal nerve (N. vagus),
the N. XI. or the accessory nerve (N. accessorius) and
the N. XII. or the hypoglossal nerve (N. hypoglossus).
The N. I. or the olfactory nerve (nerves) (Nn. olfactorii)
Despite from the view of the systematical anatomy they can be regarded as one single nerve
pair, descriptively the plural should be used. Systematically descirptively, the olfactory nerve
consists of approximately 20 slim bundles of axons penetrating into the cranial cavity. It is a
special visceral afferent nerve, as it conveys the olfactory information elicited by odoriferous
chemical compounds. The olfactory nerve in many ways is an exception from the usual
anatomy of the cranial nerves.
The proper origin is the first-order olfactory neuron (protoneuron), located inside the
olfactory region of the nasal mucosa (Tunica mucosa nasalis, Pars olfactoria). This neuron is
bipolar, and as an exception, with no intervening specialized receptor cell, it is directly
stimulated by the odoriferous chemicals. Thus, contrary to other visceral afferents, the fibers
of this nerve are not dendritic processes of sensitive ganglia, but unmyelinated axons.
The course of the nerves extracranially is hidden inside the olfacory mucosa, and then upon
approaching the cribriform plate of the ethmoid bone (Lamina cribrosa, Os ethmoidale) they
join each other in bundles of some 100 fibers each. About 20 such bundles are formed in this
way, and descriptively these are regarded as the proper olfactory nerves.
The olfactory nerves enter the neurocranium’s anterior fossa through the Foramina cribrosa.
As passing the cribriform foramina, each bundle gains its own internal pial and external dural
envelope, extensions of the intracranial meninges. These layers form a tight seal around the
axon bundles and fix them to the bony foramina, isolating the subdural, CSF-filled spaces
from the nasal cavity. When entered the anterior fossa of the neurocranium (Fossa cranii
ant.), the dural envelope enlarges and it is detached from the bundles, whereas the pial sheath
remains in a continuous solidarity until reaching the nerve’s apparent origin. The nerve fibers
then enter the olfactory bulb (Bulbus olfactorius). The olfactory nerve does not form a trunk,
nor does emit branches.
The apparent origin is the olfactory bulb. This lies just on the cribriform plate and it is
linked to the basal surface of the cerebral hemisphere by the means of a slender fascicle, the
olfactory tract (Tractus olfactorius). This latter, being an exposed neuraxial tract, is not to be
confused with the olfactory nerve.
The N. II. or the optic nerve (N. opticus)
By no means a nerve, it should be regarded an outdrawn tract of the diencephalon. It contains
the circa one million axons originating in the eyeball. However, given its position and course,
it is traditionally treated with the cranial nerves. Descriptively, the optic nerve is considered
that part of the optic pathways which is situated between the posterior surface of the eyeball
(Bulbus oculi) and the optic chiasm (Chiasma opticum) of the diencephalon. The tissue of the
optic nerve is built up by myelinated axons and it is populated by neuroglia, so that it equals
to the cerebral white matter. Thus, the term of optic nerve is purely descriptive.
The proper origin is in the eyeball, in the multipolar neurons of the nuclear layer of the
retina. Their axons converge toward the papilla of the optic nerve (Papilla nervi optici), pierce
the sclera and leave the eyeball.
The apparent origin is the optic disc, situated embedded in the sclera, somewhat medial to
the posterior pole of the eyeball.
The course is divided into four parts: intraocular, orbital, intracanalicular and intracranial.
During its run the optic nerve establishes important and complicated topographic
relationships. Meningeal layers, as extensions of the cerebral meninges envelope the nerve: an
outer dural sheath (Vagina externa nervi optici), inserting anterior on the sclera with whom it
merges, arachnoidea in the middle and pia mater on the neural surface (Vagina interna nervi
optici). The subdural space (Spatium intervaginale), filled with CSF is continuous along the
optic nerve up to the nerve’s origin from the sclera. The dural sheath tightly surrounds the
margins of the optic disc, thus the intracranial pressure is directly transmitted to the optic disc
and the intraocular papilla of the optic nerve.
The intraocular part (Pars intraocularis) of the nerve is the shortest, consisting of the axons
just transiting the layers of the eyeball. Here the axons form the optic disc, and this is
continuous with the optic nerve. The optic disc is pierced from posterior to anterior by the
central artery of the retina (A. centralis retinae). Beginning from the optic disc, the axons
acquire myelin sheath provided by oligodendroglia. The intraocular part is further divided in a
prelaminar (Pars prelaminaris), an intralaminar (Pars intralaminaris) and a postlaminar (Pars
postlaminaris) segments.
The orbital part (Pars orbitalis) of the nerve is the longest, of circa 25 mm. The nerve is
sinuous, being curved at first medially, then laterally. These windings provide an extra length
of the nerve, mandatory for avoiding its stretching during the rotational movements of the
eyeball. The nerve stretches towards the deepest point of the bony orbit (Apex orbitae), where
the optic canal (Canalis opticus) begins. The nerve and its sheaths are surrounded by the four
rectus muscles of the eyeball and the orbital fat pad (Corpus adiposum orbitae). 1 cm
posterior to the eyeball a branch of the ophthalmic artery (A. ophtalmica), the A. nervi optici
approaches of the nerve, perforates its sheaths, enters the neural white matter and travels in
the center of the nerve. Then, inside the optic disc this artery is continuous with the A.
centralis retinae.
The optic nerve enters the neurocranium’s middle fossa through the optic canal.
The intracanalicular part (Pars intracanalicularis) of the nerve is short, occurring as it passes
through the narrow optic canal (Canalis opticus) and leads from the orbit into the middle
cranial fossa (Fossa cranii media). The canal imposes the nerve a postero-medial direction.
Here the dural sheath is fixed to the upper and medial bony wall of the canal. At the posterior
end of the canal the ophthalmic artery (A. ophtalmica) lies infero-lateral to the nerve.
Advancing forward, the artery makes a quarter of turn around the nerve, going inferiorly and
then medially. As the canal can accommodate the nerve and the artery only, there is no
reserve space, thus the nerve is prone to damage already at the least of compression or space-
occupying lesion.
The intracranial part (Pars intracranialis) is of 10 mm in its length. It begins at the cranial
(posterior) opening of the optic canal and ends at the optic chiasm (Chiasma opticum). Here
the dural sheath suddenly enlarges and continues in the dural lining of the middle cranial
fossa. The nerve turns even more medially, approaches from lateral the prechiasmatic furrow
(Sulcus prechiasmatis) and ends by joining the diencephalon. There is no clear-cut limit
between the optic nerve and the chiasma.
Naturally, in the case of the optic nerve there is no question of forming a nervous trunk or
emitting branches.
The N. III. or the oculomotor nerve (N. oculomotorius)
Functionally, this is a mixed somitic motor and vegetative parasympathetic efferent
encephalic nerve with some proprioceptive sensitive component hosted in its endbranches. It
is the thickest of the optomotor nerves (oculomotor, trochlear, abducent).
Its somitic somatomotor branches are axons which innervate some of the extrinsic muscles
of the eye and the elevator muscle of the upper eyelid, as follows:
the medial rectus (M. rectus medialis bulbi),
the superior rectus (M. rectus superior bulbi),
the inferior rectus (M. rectus inferior bulbi),
the inferior oblique (M. obliquus inferior bulbi) and
the elevator of the upper eyelid (M. levator palpebrae superioris)
Its autonomous division consists of vegetative motor parasympathetic preganglionar axons.
This component leaves the nerve in the orbit and joins some of the branches of the trigeminal
nerve, and via the trigeminal branches innervates some of the intrinsic muscles of the eyeball,
as follows:
the constrictor of the iris (M. sphincter pupillae seu M. constrictor pupillae) and
the circular muscles of the ciliary body (Fibrae circulares, M. ciliaris)
The proprioceptive component is only present at the very periphery of the axonal tree. It is
represented by a few dendritic fibers, which soon pass as communicating branches where they
truly belong, into the trigeminal system.
The proper origin lies in the midbrain (Mesencephalon) and it is multiple:
the Nc. n.-i oculomotorii - somatomotor for the extrinsic ocular muscles,
the Nc. centralis (of PERLIA) - somatomotor for the elevator of the upper eyelid and
the Nc. accessorius (of EDINGER – WESTPHAL) - vegetative parasympathetic, origin for the
preganglionar vegetative axons
The apparent origin is the on the ventral surface of the neuraxis, on the mesencephalon, in
the interpeduncular fossa (Fossa interpeduncularis), lateral to the posterior perforated
substance (Substantia perforata post.), medial to the cerebral peduncles (Pedunculus cerebri).
The course of the nerve on the inner skull base is quite lengthy. It passes through the
interpeduncular cistern (Cisterna interpeduncularis) enveloped by its own pial sheath,
between the superior cerebellar artery (A. cerebellaris sup.) and the posterior cerebral artery
(A. cerebri post.). The nerve then pierces the dura lateral to the dorsum sellae and below the
posterior clinoid process (Processus clinoideus post.) and enters the cavernous sinus (Sinus
cavernosus). In the cavernous sinus the nerve runs under the internal carotid artery (A. carotis
int.), and on descent crosses the vessel’s lateral surface.
The oculomotor nerve exits the neurocranium’s middle fossa through the superior orbital
fissure (Fissura orbitalis sup.).
Further, it stretches medial and then below to the ophthalmic (N. ophtalmicus), trochlear and
abducent nerves. Inside the cavernous sinus the oculomotor nerve emits its two terminal
branches.
Branches: the nerve gives rise to no collaterals, instead in the anterior part of the cavernous
sinus splits into two endbranches. These are the superior and the inferior branch, as follows:
The superior branch (R. superior) is the weaker one. Exits the cavernous sinus and becomes
evident under the lesser wing of the sphenoid (Ala minor ossis sphenoidalis). It transits the
superior orbital fissure (Fissura orbitalis sup.), enters the orbit. Further, it passes through the
tendinous ring (Anulus tendineus) of the extrinsic ocular muscles, lateral and then superior to
the optic nerve and the ophthalmic artery, arriving on the lower surface of the superior rectus
muscle. There it branches exhaustively for the M. rectus superior and the M. levator
palpebrae superioris.
The inferior branch (R. inferior) is thicker. Likewise, it leaves the cavernous sinus,
penetrates the orbit through the superior orbital fissure. The nerve passes the inferior part of
the tendinous ring, medial to the abducens nerve. Gaining on the upper surface of the M.
rectus inf. it gives off two short branches for this one and for the M. rectus med. A longer
terminal branch stretches further inferior to innervate the M. obliquus inf., but this is the one
carrying the vegetative component too. The vegetative motor fibers leave this branch and
enter the ciliary ganglion (Ggl. ciliare) as the preganglionar root of this ganglion (Radix
parasympathetica).
Delicate, sparse connections emerge from the above described motor branches and unite with
twigs of the ophtalmic nerve. These are proprioceptive sensitive dendrites, proper of the
trigeminal system, dispatched from here via the motor nerves into the extrinsic ocular
muscles.
The ciliary ganglion (Ggl. ciliare) is a small peripheral nervous structure situated in the orbit,
behind the eyeball, lateral to the optic nerve, medial to the M. rectus lat. bulbi. This ganglion
houses the ganglionar („postganglionar”) neurons of the vegetative parasympathetic division
of the oculomotor nerve. It is morphologically linked by the means of roots and branches to
the oculomotor, the trigeminal and the sympathetic nervous trees, but functionally and
systematically belongs to the oculomotor system only. According to ARNOLD’s rule, from the
point of view of the systematic anatomy, as the cephalic parasympathetic ganglia generically,
the ciliary ganglion has three roots and one set of branches. Out of the three roots one is real,
with functional value, the other two are spurious.
The parasympathetic root (Radix parasympathetica seu oculomotoria), conveying the
preganglionar axons, is the only proper root of the ganglion, originating in the Nc. accessorius
(of EDINGER – WESTPHAL). This emerges from the inferior branch of the oculomotor nerve.
The preganglionar axons synapse on the ganglionar cells, which emit postganglionar axons
representing the output of the ciliary ganglion.
The sympathetic root (Radix sympathetica) only apparently belongs to the ganglion, because,
in fact, these axons merely transit it. Their origin resides in the superior cervical ganglion
(Ggl. cervicale sup.) of the cervical sympathetic chain, and they approach this ganglion via
the nervous plexus surrounding the internal carotid artery (Plexus caroticus internus) and its
continuation, the Plexus ophtalmicus. The fibers pass the ciliary ganglion without making any
functional connection with it and join the vegetative branches emerging from here, the short
ciliary nerves (Nn. ciliares breves).
The sensitive root (Radix sensitiva) is a trigeminal branch, passing through, but functionally
not linked to the ganglion. It originates from the nasociliary nerve (N. nasociliaris), a branch
of the ophthalmic nerve (N. ophtalmicus), which in turn is a primary branch of the trigeminal
nerve (N. trigeminus). Up to the ciliary ganglion this nerve is purely sensitive, built up by
some peripheral dendrites dispatched from the trigeminal sensitive ganglion (Ggl.
trigeminale). Obviously, these fibers do not synapse in the ciliary ganglion; instead they
directly enroll into its vegetative output.
The emerging branches of the ciliary ganglion are the short ciliary nerves (Nn. ciliares
breves), in a number of 8-10. They contain the postganglionar parasympathetic axons
originating in the ciliary ganglion, other postganglionar sympathetic axons just transiting it
and general somatic sensitive dendrites from the trigeminal system. These thin nerves then
penetrate the eyeball through its posterior pole and provide the motor innervation of the
intrinsic, smooth intraocular muscles.
The N. IV. or the trochlear nerve (N. trochlearis)
The trochlear nerve is a somitic somatomotor efferent encephalic nerve with the appearance of
a slender bundle. It is unique among the encephalic nerves by the facts that contains
intraneuraxially crossed fibers and arises on the posterior surface of the neuraxis.
It only innervates the superior oblique muscle of the eye (M. obliquus superior bulbi).
The proper origin lies in the mesencephalon, in the trochlear nucleus (Nc. nervi trochlearis).
From their origin, the axons turn posterior and decussate through the midline in the white
matter of the Velum medullare superius, part of the mesencephalic tectum (Tectum
mesencephali).
The apparent origin is to be found on the Velum medullare sup., below the inferior colliculi
(Colliculus inf.), nearby to the midline.
The course of the intracranial segment of the nerve begins with a half a turn around the
cerebral peduncles (Pedunculus cerebri), right above the superior border of the pons. As the
nerve encircles the posterior, then lateral surface of the mesencephalon, it passes the Cisterna
ambiens and then the pontine cistern (Cisterna pontis). On the lateral surface of the brain the
nerve stretches between the superior cerebellar artery (A. cerebellaris sup.) and the posterior
cerebral artery (A. cerebri post.) and pierces the dura mater near the medial, free margin of the
tentorium (Tentorium cerebelli), little above the trigeminal nerve, and penetrates the
cavernous sinus (Sinus cavernosus). Here the nerve runs anteriorly along the ophthalmic
nerve, above the abducent nerve. Then it leaves the cavernous sinus, shows up in the middle
cranial fossa, then disappears under he lesser wing (Ala minor) of the sphenoid bone.
The trochlear nerve exits the neurocranium’s middle fossa by passing through the superior
orbital fissure (Fissura orbitalis sup.).
In the orbit the nerve is placed above the origin of the levator palpebrae superioris muscle and
stretches forward and medially to meet from above its target, the superior oblique muscle of
the eye (M. obliquus sup. bulbi).
The trochlear nerve does not emit branches.
The N. V. or the trigeminal nerve (N. trigeminus)
The trigeminal nerve is a branchial somatomotor efferent and general somatic sensitive
afferent encephalic nerve. Its endbranches are joined by twigs of other cranial nerves with
different functions. It is unique among the encephalic nerves by the fact that it does not form a
nervous trunk, instead its primary branches emerge directly from its sensitive ganglion.
Therefore, the concept of „trigeminal nerve” is a systematic and functional one, rather than
descriptive or topographic. Despite that the very distal branches of the nerve gather with
vegetative axons of different provenances, in its central and proximal parts the trigeminal
nerve has no vegetative component.
The branchial somatomotor division consists of axons from the motor nucleus and provides
the innervation of the striated muscles derived from the I. branchial arch (Arcus branchialis
I.). These are the gnathomotor muscles and some smaller muscles too, as follows:
the temporal muscle (M. temporalis),
the lateral pterygoid muscle (M. pterygoideus lat.),
the medial pterygoid muscle (M. pterygoideus med.),
the masseter muscle (M. masseter),
the anterior belly of the digastric muscle (M. digastricus, Venter ant.),
the mylohyoid muscle (M. mylohyoideus),
the tensor tympani muscle (M. tensor tympani) and
the tensor veli palatini muscle (M. tensor veli palatini).
The general sensitive function is fulfilled by means of the peripheral dendrites of the nervous
tree with their origin in the sensitive ganglion (Ggl. trigeminale seu semilunare) (of GASSER)
and the sensitive root of the nerve (Radix sensitiva seu Portio major).
The trigeminal nerve provides nociceptive innervation for the teguments of the head, the oral,
ocular and nasal mucosae, the dental arches, paranasal sinuses, the eyeball and the content of
the orbit, the meninges of the anterior and middle cranial fossae, the muscles of the head and
chin and the TMJ (Art. temporomandibularis).
The trigeminal nerve is protopathic and epicritic tactile by innervating the cephalic teguments,
mucosae and mainly the tongue (Lingua).
The trigeminal nerve supplies proprioceptive innervation for all the muscles located on the
head (gnathomotor, Diaphragma oris, ocular, facial and auricular), the tongue, the dental
arches and the TMJ.
The somatomotor function is exerted by the means of the efferent axons originating in the
pons, which at first run separately, then join a primary sensitive branch of the nerve, the N.
mandibularis, and directly innervate the target muscles.
The proper origin lies in four nuclei.
the Nc. tractus mesencephalici nervi trigemini – proprioceptive, actually a centrally migrated
peripheral ganglion, in the mesencephalon,
the Nc. motorius nervi trigemini (Nc. masticatorius) – origin of the somatomotor efferences,
in the pons,
the Nc. sensitivus principalis – epicritic tactile and conscious proprioceptive, in the pons and
the Nc. tractus spinalis nervi trigemini – nociceptive, in the medulla, descending into the
spinal chord, in the first 2-3 cervical segments.
The axons originating in the motor nucleus form a single bundle and will emerge separately to
the sensitive nerve fibers. The sensitive axons, originating in the trigeminal ganglion enter the
brainstem forming a single bundle with the proprioceptive sensitive dendrites which leave the
brainstem.
The apparent origin is to be found medially on the ventrolateral surface of the middle
cerebellar peduncle (Pedunculus cerebellaris medius), at their junction with the pons in the
form of two emergent white bundles, the two roots of the trigeminal nerve. The medial one is
the thinner motor root (Radix motoria seu Portio minor). The thicker lateral root is the
sensitive (Radix sensitiva seu Portio major). In fact, the motor root, as its structure and
systematic anatomy does not change during its run, could be considered a separate motor
nerve by itself.
The course of the intracranial segment of the nerve is marked by the fact that unlike in the
case of the other encephalic nerves, the two roots, motor and sensitive, do not merge into a
unitary nervous trunk. They have separate pial envelopes but a common arachnoid sheath. The
roots, keeping close, advance and ascend laterally in the pontine cistern (Cisterna pontis).
They then pierce together the basal dura mater just below the insertion of the tentorium. After
a run of 1 cm, the roots penetrate a duplication of the basal dura mater, the Cavum trigeminale
(of MECKEL). The trigeminal cavum is pocket-like, with an upper and a lower wall, and rests
in the Impressio trigeminalis, a shallow pit of the petrous part of the temporal bone (Pars
petrosa ossis temporalis), just lateral to its apex.
Inside the trigeminal cavity the sensitive root meets a flattened, crescent-shaped ganglion and
fuses with its concavity. This is the peripheral sensitive ganglion of the trigeminal nerve (Ggl.
trigeminale seu semilunare) (of GASSER). The lesser motor root passes under the ganglion
with no functional connection with it. Medial to the ganglion lays the cavernous sinus (Sinus
cavernosus) with the internal carotid artery inside, inferior to it stretch medially the two
petrosal nerves, toward the foramen lacerum.
The primary branches of the trigeminal nerve are its endbranches too: the ophthalmic nerve,
the maxillary nerve and the mandibular nerve. These all separate from the convexity of the
trigeminal ganglion. A trigeminal nerve, as a descriptive anatomical entity actually never
forms, because the main branches emerge directly from the sensitive ganglion. The primary
branches are short as they soon separate in their own endbranches. Next, these branches are
treated in the same manner as the individual encephalic nerves are.
The ophthalmic nerve (N. ophtalmicus)(V/1) is the first branch of the trigeminal nerve. It is
the thinnest, medial division. At its origin the nerve is purely somatosensitive, formed by
sensitive dendrites, originating in the trigeminal ganglion (nociceptive and tactile) or in the
Nc. tractus mesencephalici nervi trigemini (proprioceptive). The nerve provides sensitive
innervation for the content of the orbit, the skin of the eyelids (Palpebra) and forehead, for the
meninges in the anterior and the middle cranial fossae, for the nasal mucosa (Mucosa nasalis)
and the skin of the nasal pyramid (Pyramis nasi). However, some of its most distal branches
are parasitized by vegetative postganglionar axons of different provenance. Likewise, the
nerve cedes some proprioceptive dendrites to the nerves innervating the extrinsic ocular
muscles. Apparently, these dendrites are taken over from the motor nerves.
Immediately after its origin, the nerve enters the cavernous sinus (Sinus cavernosus). There
the ophthalmic nerve courses anteriorly, at first below and then lateral to the trochlear,
oculomotor and abducent nerves. After exiting the cavernous sinus at its anterior border, the
ophthalmic nerve becomes apparent for a short while in the middle cranial fossa. Here the
nerve emits a collateral branch and after that it splits in three endbranches.
The trifurcation of the nerve happens in the middle cranial fossa, under the lesser sphenoid
wing (Ala minor ossis sphenoidalis), thus in the view of the descriptive anatomy the
ophthalmic nerve does not enter the orbit. Systematically regarded, the nerve is present in the
orbit by its branches, as follows:
R. tentoris (R. meningeus) – the only collateral branch of the ophthalmic nerve and does not
leave the cranial cavity. It turns posterior and medial for providing the sensitive innervation of
the tentorium and the posterior part of the falx (Falx cerebri).
The lacrimal nerve (N. lacrimalis) is the lateral endbranch of the nerve. Its origin is in the
middle cranial fossa, and then passes the superior orbital fissure through its lateralmost part.
Gaining into the orbit it maintains its anterior course, keeping laterally along the upper margin
of the lateral rectus muscle of the eyeball. It pierces the lacrimal gland (Glandula lacrimalis)
and branches exhaustively at the upper eyelid (Palpebra sup.) in its lateral part. These
endbranches serve for the sensitive innervation of the skin of the lateral angle of the eye and
the lateral conjunctiva (Angulus palpebralis lat.). On its way, the lacrimal nerve establishes a
functionally important communication with the zygomatic nerve (N. zygomaticus), the Ramus
communicans cum nervo zygomatico. By this bundle postganglionar parasympathetic axons
are transferred to the lacrimal nerve. These axons arrive from the pterygopalatine ganglion
(Ggl. pterygopalatinum), functionally belong to the system of the intermedius nerve (N.
intermedius) and supply the lacrimal gland (Glandula lacrimalis).
The frontal nerve (N. frontalis) is the strongest, middle endbranch of the ophthalmic nerve. It
also enters the orbit through the middle part of the superior orbital fissure and advances on the
elevator muscle of the upper eyelid under the ceiling of the orbit. It ends without emitting
collateral branches, by bifurcation into the supraorbital nerve, lateral, and the supratochlear
nerve, medial.
The supraorbital nerve (N. supraorbitalis) leaves the orbit through the supraorbital notch or
foramen (Incisura seu Foramen supraorbitale). It splits into two endbranches, the R. med. and
the R. lat. They ascend parallel in the skin of the forehead providing its general sensitive
innervation up to the vertex (Vertex).
The supratrochlear nerve (N. supratrochlearis) exits the orbit above the trochlea of the
superior oblique muscle of the eyeball (Trochlea m.-li obliqui superioris) and fits into the
supratrochlear notch or foramen (Incisura seu Foramen supratrochleare). It runs long in the
skin of the forehead, innervating it, but also provides branches for the skin and conjunctiva of
the interior angle of the eye (Angulus palpebralis med.).
The nasociliary nerve (N. nasociliaris) is the medial endbranch of the ophthalmic nerve. The
nerve passes the superior orbital fissure in its medial part and enters the orbit through the
tendinous ring of the extrinsic ocular muscles, above the optic nerve. It advances below the
superior oblique muscle of the eyeball, and then follows the medial wall of the orbit, joining
the ophthalmic artery up to the anterior ethmoidal foramen (Foramen ethmoidale ant.). There
the nerve bifurcates, but before that gives off several collateral branches. This nerve has
apparent anatomical connections with the ciliary ganglion. (see at the oculomotor nerve)
The R. communicans cum ganglione ciliari is a collateral branch of the nasociliary nerve. It
can be multiple, stretching anterior, lateral to the optic nerve. This nerve is the spurious
sensitive root of the ganglion. It conveys general sensitive dendrites, which merely transit the
parasympathetic ganglion and will join its output only, the Nn. ciliares breves.
The Nn. ciliares longi, 2-3, run parallel with the Nn. ciliares breves and penetrate the eyeball
above the optic nerve. They provide the highly sensitive nociceptive innervation of the cornea
(Cornea).
The posterior ethmoidal nerve (N. ethmoidalis post.) is a laterally oriented collateral branch of
the nasociliary nerve. Immediately after its origin it joins the homonymous artery with which
it enters the Foramen ethmoidale post. for innervating the mucosa of the sphenoid and