Download neuroanatomy - NC State Veterinary Medicine

NEUROANATOMY
2016
NEUROANATOMY ©
LC Hudson, DVM, PhD
Professor Emerita of Anatomy
North Carolina State University
College of Veterinary Medicine
Raleigh NC 27607
[email protected]
I.
CENTRAL NERVOUS SYSTEM
All 5 divisions of the brain are involved with the function of the eyes and/or adnexa - in either
conscious/response pathways or reflex pathways. The divisions are the telencephalon (cerebral
hemispheres), diencephalon (thalamus, hypothalamus, metathalamus (lateral geniculate nuclei)),
mesencephalon (midbrain (pretectal, oculomotor, trochlear nuclei, parasym. nucleus of CN III)),
metencephalon (cerebellum and pons (vestibular nuclei, spinal tract of CN V)) and myelencephalon
(medulla oblongata (abducens, facial, vestibular nuclei)).
The more cranial spinal cord is also involved with function of the eyelids through sensory
innervation into the cervical spinal cord, and through sympathetic autonomic function (T 1 -T 3
segments). The spinal cord caudal to T 3 is not involved with eye functions.
II.
PERIPHERAL NERVOUS SYSTEM
The majority of the 12 pr. of cranial nerves have some function with the eyes/adnexa- CNs II, III,
IV, V (ophthalmic and maxillary branches), VI, VII, and VIII including appropriate sensory and/or
autonomic ganglia of these cranial nerves.
Retrograde tracing studies in cats showed that some cervical spinal nerves have sensory projections
from the eyelids even though apparently physically distant. Autonomic sympathetic fibers are
projected via T l -T 3 spinal nerves into the sympathetic trunk, traveling through the neck in the
vagosympathetic trunk and synapsing in the cranial cervical ganglion. Postganglionic fibers then
travel along blood vessels and with other cranial nerves to reach the globe.
III.
NOMENCLATURE
Veterinary ophthalmologists tend to use human/zoological nomenclature including eponyms such
as Meibomian gland, Descemet's membrane, and canal of Schlemm; probably because of the
intense use of human literature.
Veterinary anatomists try to follow the Nomina Anatomica Veterinaria (NAV) <http://www.wavaamav.org/Downloads/nav_2012.pdf> (then click on organa sensuum on the left side list for the
eye), which specifically forbids eponyms. Except for the eye, the use of directional terms superior,
inferior, anterior and posterior are also not used, but with the globe and adnexa such directional
terms are acceptable.
Human/zoo
Superior
Inferior
Anterior
Posterior
Alternate term for Dorsal
Ventral
Rostral
Caudal
eye - NAV
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2016
TELENCEPHALON = cerebrum = cerebral hemispheres ≅ cerebral cortex.
The cerebrum functions in perception and integration of vision as well as voluntary control of
eye/eyelid movements. The occipital lobes and the motor cortex of the frontal/parietal lobes are the
primary regions involved.
The occipital lobe occupies the caudal aspect of the cerebral hemispheres. It borders with the
parietal lobe dorsorostrally and the temporal lobe laterally. Medially, the left and right occipital
lobes meet across the longitudinal fissure between the hemispheres. Caudally the occipital lobes
lie against the osseous tentorium and the tentorium cerebelli which lie in the transverse fissure
(between the cerebrum and cerebellum). Unlike some borders between lobes, there is not a definite
landmark to demarcate all the borders of the occipital lobe and different neuroanatomists will
include greater or lesser areas. However, despite this dichotomy, everyone includes the visual
cortex in the occipital lobe.
Gyri included in the human occipital lobe laterally are the cuneus, lateral occipital gyrus, superior
and inferior occipital gyri, descending gyrus, superior and inferior polar gyri. Medially, more of
the cuneus, parts of the lingual gyrus, and medial occipitotemporal gyrus are included. For
domestic animals (dog), the occipital lobe includes parts of the marginal, ectomarginal, caudal
suprasylvian. It also includes the splenial, occipital, and caudal composite gyri ventrally and
medially.
Within the visual cortex is the so-called striate cortex. It has the stripe (or line) of Gennaricytoarchitectural layer of myelinated fibers that can be seen on unstained and stained sections.
Adjacent to the striate cortex is the parastriate cortex and then next is the peristriate cortex. The
point of central vision is located in the striate cortex. The same general region may be referred to
as Brodman's area 17 for different species (Brodman's areas are based on cytoarchitecture)
area 17 = striate = visual I This has 6 neuronal layers; layer IV has subgroups and is the
termination of fibers from the lateral geniculate body.
area 18 = parastriate = visual II
area 19 = peristriate = visual III
Areas 18 and 19 together may be referred to as the extrastriate cortex or as visual
association area.
The point of central vision in striate cortex varies in position between species and perhaps
between breeds.
dog Beagles: 11.3 mm rostral to interaural line and 8.3 mm lateral to midline;
Greyhound: 15.6 mm rostral and 8.5 mm lateral
cat junction of marginal and endomarginal gyri
other domestic species: unable to find specific information
Afferent connections to visual areas:
From the lateral geniculate body via optic radiation of internal capsule (main white matter
connection between hemisphere and rest of brain). [This may also be referred to as the
geniculocalcarine tract as it originates in the lateral geniculate body and projects to the cortex
surrounding the calcarine sulcus of human brains.] It is located in the caudal aspect of internal
capsule, and the fibers sweep rostrally, then laterally, then caudally in the hemisphere.
There are also reciprocal connections with other lobes and with parastriate/peristriate areas via
association fibers.
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Efferent connections from visual areas:
1.
association areas- long and short association fibers connect visual cortex with other
lobes of the same hemisphere such as motor cortex at frontal/parietal lobe.
2.
opposite hemisphere via corpus callosum
3.
brain stem- to lateral geniculate body and rostral colliculus, pontine nuclei and
reticular formation.
V.
DIENCEPHALON
Structures of the diencephalon involved with vision are the optic chiasm, optic tract, lateral
geniculate body, internal capsule, hypothalamus, and thalamus.
The optic chiasm is located at the rostroventral surface of the brain stem and demarcates the rostral
level of the diencephalon. It is closely associated with the 3rd ventricle, hypothalamus, and to
some extent the pituitary gland.
The percentage of optic nerve fibers crossing the midline in different species varies widely.
most birds and fish: 100%
cat: ~65%
dog: ~75%
large animals: ~80-90%
It is the existence of uncrossed fibers that give the ability for binocular vision.
[There is a letter in Nature from several years ago about an inherited, congenital lack of an optic
chiasm in Belgian sheep dogs. In these animals all fibers project ipsilaterally, but project to the
lateral geniculate body and to the correct layers of the body.]
The optic tract is located lateral to internal capsule of diencephalon; it begins ventrally, and then
travels laterally, caudally, and finally, dorsally. In doing so, it stays on the surface of the
diencephalon. If you follow the optic tract it will lead you to the lateral geniculate body (LGB)
where most of the fibers terminate. In the whole brain, most of the optic tract is covered by the
overlying cerebrum.
There is conflicting evidence about segregation of fiber size within the tract although there is no
doubt that different sizes exist, W, X, Y. There is a difference in conduction speeds and projections
of those different classes of axons - in general, fastest fibers (Y) project to the lateral geniculate
body, intermediate (X) to the pretectum, and slowest (W) to the rostral colliculus. In the lateral
geniculate body, larger fibers synapse in the dorsal laminae while smaller fibers synapse in the
ventral laminae.
More specifically there are connections from the optic tract to:
1.
lateral geniculate body (actually more dorsal in position in domestic animals). It is
estimated that 80% of optic tract fibers terminate in the LGB.
2.
via brachium of rostral colliculus to pretectal nuclei
3.
via brachium of rostral colliculus to rostral colliculus
4.
hypothalamus (controversial for carnivores)
5.
accessory nucleus of optic tract
Lateral Geniculate Body- This structure has topographic (retinotopic) organization- dorsal or main
nucleus has become more complex with advent of binocular vision. The ventral nucleus may also
be called the pregeniculate nucleus.
The LGB is part of the diencephalon which has grown caudally to overhang the mesencephalon.
Its major function is as a complex relay nucleus on the conscious vision pathway.
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The dorsal nucleus (pars dorsalis) has a sigmoid curved laminar structure - number of cell layers
varies in species - 3 in cat and 6-7 in primates. The optic tract fibers enter on the concave surface.
The laminae are usually referred to as A, AI, B or 1-6 (layer 1 is the deepest). These have
intervening fiber layers. The dorsal nucleus receives large diameter optic tract fibers. Specifically
the A and B layers or layers 1, 4, 6 receive crossed fibers whereas the AI layer or layers 2,3,5
receives uncrossed fibers. Layers 1 and 2 are also called the magnocellular subdivision; 3-6 are
parvocellular subdivision. Only the dorsal nucleus of the LGB projects to the visual cortex.
Ventral nucleus or pars ventralis is wedged between entering branches of the optic tract and
receives small diameter optic tract fibers. Some authorities also recognize the ventral (anterior) and
dorsal (posterior) pregeniculate nucleus.
The afferent connections to the LGB: mostly optic tract neurons but also some cortical fibers and
rostral colliculus fibers. The efferent connections from the LGB: internal capsule via optic radiation
to layer IV of visual cortex and rostral colliculus via brachium of rostral colliculus.
Hypothalamus
In addition to endocrine functions with the pituitary, the hypothalamus also is a UMN source for
the autonomic nervous system. In general the rostromedial nuclei are associated with the
parasympathetic system, and the caudolateral nuclei with the sympathetic system.
Thalamus
Part of the pathway for noxious stimuli. Painful eye impulses (1st order neurons) would travel via
CN V into the caudal brain. 2nd order neurons to ventral caudal medial nucleus of the thalamus and
synapse, then 3rd order neurons to the parietal lobe for perception of pain.
VI.
MESENCEPHALON
The mesencephalon or midbrain is grossly divided into the tectum and tegmentum by the line of the
sulcus limitans of the mesencephalic aqueduct (aqueduct of Sylvius). In general the tectum is
associated with sensory function and integration of vision and widespread body movement, and the
tegmentum with motor function- control of extraocular muscles and parasympathetic control of
bulbar smooth muscle. The pretectum is located at the dien/mesencephalic border slightly rostral
to the mound of the rostral colliculus and is associated with pupillary light reflex.
The pretectal area includes the pretectal nuclei and the caudal commissure. Pathways of the
pupillary light reflex are associated. Intermediate sized afferent fibers in the optic tract bypass the
LGB, travel into the initial part of the brachium of the rostral colliculus and enter the pretectal
nuclei.
Projections from the pretectal nuclei include a large bundle crossing in the caudal commissure to
the opposite parasympathetic nucleus of CN III (older papers/texts refer to this as nucleus of
Edinger-Westphal or accessory oculomotor nucleus. Since late 70s/early 80s anteromedian nucleus
extending rostral to E-W is identified as the parasympathetic source). There are some uncrossed
fibers between ipsilateral pretectal and parasympathetic nuclei. Several authorities write of
commissural connections of the pretectal nuclei in the caudal commissure and of crossing
projections as part of the PLR pathway.
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Tectum
The rostral (superior) colliculi (part of corpora quadrigemina) are two mounds of neural tissue
lying close to one another on the dorsal brain stem. The rostral colliculi are layered and have a
topographic organization. The general function is visuomotor coordination. Each rostral colliculus
has connections from both crossed and uncrossed optic tract fibers via the brachium of the rostral
colliculus.
The rostral colliculus in mammals has 7 layers - 3 cellular (strata griseum superficiale,
intermediate, and profundus) alternating with 4 of fibers (strata zonale, opticum, album
intermediale, and album profundum). Retinotectal fibers generally pass through the stratum
opticum, and enter the superficial and intermediate gray layers. Corticotectal fibers enter via the
stratum album profundum and pass to the deep gray layer and the stratum zonale. Other afferents
to the rostral colliculus are from the spinotectal tract, trigeminal lemniscus (sensory impulses from
the head), caudal colliculus, and reticular formation.
Efferent connections from the rostral colliculus consist mostly of a large bundle to the ipsilateral
pons (and then to cerebellum), and crossed bundles to the spinal cord (tectospinal tract). In general,
projections from the superficial layers ascend in the brain and projections from the deep layer
descend in the brain.
The tectum is the location of UMN neurons involved in dilation of the pupils. The
tectotegmentospinal tract may actually be a misnomer in the usual scheme of naming tracts. [title
usually names the location of nerve cell bodies and the location of the synapse and may include a
directional term (lateral, ventral) if there is more than one tract with similar beginning and ending.
But tectotegmentospinal indicates a NCB in the tectum (OK), a synapse in the tegmentum (not OK)
and descending to the spinal cord (OK). This tract does not synapse in the tegmentum (at least
according to some) but travels ventrally from the tectum into the tegmentum, crosses the midline,
then turns caudally to pass through the brain stem and spinal cord.] Some references use the term
lateral tectospinal tract to name the same structure. If your source uses tectotegmentospinal tract, it
may also mention a different tectospinal tract that is associated with orientation of the head and
body with vision. If your source uses the term lateral tectospinal tract, then it may also mention a
medial tectospinal tract.
tectotegmentospinal tract = (lateral tectospinal tract) UMN sympathetic tract to T 1 -T 3
spinal cord -the caudal hypothalamus projects to part of the tectum, then this tract projects
to the lateral horn of cranial thoracic cord.
medial tectospinal (tectospinal) tract = orientation of the eyes, head, and neck in response to
visual input
spinotectal tract- move neck (head and eyes) towards movements
Tegmentum
The tegmentum is the ventral mesencephalon. Some definitions exclude the crus cerebri. This area
is the location of several LMN nuclei associated with eye functions. In addition, there are
pathways passing through this region, such as the tectotegmentospinal and tectospinal tracts
associated with eye functions, menace response pathway, and pupillary light reflex pathway.
The locations of motor NCBs, which in turn project via certain cranial nerves, have an organization
scheme around the ventricular system of the brain stem. Neurophysiology schemes often refer to
these nuclei as GSE, GVE, and SVE in type. All are located lateral to or ventral to whatever part of
the ventricular system is present. GVE (autonomic nuclei) nuclei are located ventrolateral to the
ventricular system with GSE nuclei slightly ventral to GVE nuclei. [Although GVE and GSE
nuclei can also be found through the spinal cord, SVE nuclei are found only in the brain stem.
These are located ventrolaterally in the stem.]
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Examples of each type include:
GVE-parasympathetic nucleus of CN Ill, of CN VII, of CN IX, of CN X
GSE- oculomotor, trochlear, abducens, and hypoglossal nuclei
SVE- trigeminal nucleus, facial nucleus, nucleus ambiguus
So in the mesencephalon the LMN nuclei are the oculomotor nucleus, trochlear nucleus, and
parasympathetic nucleus of CN III. (Cranial nerves III and IV emerge from the mesencephalon so
these motor nuclei giving rise to those axons are also in the mesencephalon)
afferents - vestibular nuclei via the medial longitudinal fasciculus, cortical
projections, and reticular formation
efferents - to extraocular muscles and smooth muscles of iris and ciliary body
The oculomotor nucleus is located slightly ventromedial to the parasympathetic nucleus, near the
periaqueductal gray matter. It is the source of GSE fibers to various extraocular muscles.
Afferents to this nucleus include vestibular system fibers in the medial longitudinal fasciculus.
These function with conjugate movement of the eyes.
The trochlear nucleus is also located in the tegmentum. Although further caudal than the
oculomotor nucleus, it is in the same column line. Fibers from this nucleus pass dorsally, then
cross the midline - invariably torn when the brain is removed) to reach the dorsal oblique muscle.
Afferents to the both the oculomotor and trochlear nuclei include mostly ipsilateral vestibular fibers
in medial longitudinal fasciculus (MLF). The connections between the vestibular nerve, vestibular
nuclei and the motor nuclei of CN III, IV, and VI are the basis of nystagmus and in some cases of
strabismus.
The parasympathetic nucleus of CN III (anteromedian nucleus (Edinger-Westphal), accessory
oculomotor nucleus) is the source of preganglionic autonomic fibers in the oculomotor n.
Afferents to the nucleus are primarily crossed fibers of the pretectal nucleus via the caudal
commissure-pupillary light reflex pathway. The nucleus is located at the ventrolateral edge of the
periaqueductal gray matter. The radices pass ventrolaterally and join with radices from the
oculomotor nucleus within the substance of the brain. These efferent GVE fibers synapse in the
ciliary ganglion onto the postganglionic neurons. The exact branching of the oculomotor nerve
varies in the different species. The GVE fibers surround the GSE fibers of the oculomotor nucleus
rendering them somewhat more susceptible to injury such as compression.
The mesencephalon also contains the mesencephalic sensory tract and nucleus (sensory from CN
V, and possibly CNs III, IV, VI), which is the rostral end of the massive input from the trigeminal
nerve. Briefly, sensory fibers of CN V enter further caudally at the pons. Some fibers travel
rostrally in the brain stem to the mesencephalon forming the mesencephalic sensory tract and
nucleus, those at the level of the pons are referred to as the principal (pontine) sensory tract and
nucleus, and those which turn caudally through the myelencephalon and cranial cervical spinal cord
are the spinal tract and nucleus of CN V. It is believed that the mesencephalic nucleus, which has
unipolar-shaped neurons, is the location of proprioceptive NCBs traveling in the trigeminal nerve.
Some authorities also describe this nucleus as the location of proprioceptive neurons from the
extraocular muscles as well as from masticatory and facial muscles.
Other pathways passing through the tegmentum associated with ocular functions but not originating
or terminating in the mesencephalon are the corticopontocerebellar fibers within the crus cerebri
(part of menace response path), and the trigeminal lemniscus (conscious sensory pathway from
head including eyes).
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METENCEPHALON (pons and cerebellum)
The pons is part of the brain stem. Many structures or pathways associated with the eye and that
pass through the mesencephalon also pass through the pons. This includes the principal (pontine)
tract and nucleus of CN V, reticular formation, medial longitudinal fasciculus (MLF), trigeminal
lemniscus, pathway for menace response, etc.
The MLF includes fibers ascending from the vestibular nuclei to the (abducens), oculomotor, and
trochlear nuclei.
The tectospinal tract(s) and tectotegmentospinal tract have already been discussed under
mesencephalon and pass through this region of the brain.
Trigeminal lemniscus (2nd order sensory impulses from the head to the hemispheres) travels
through the pons to the ventral caudomedial thalamic nucleus where they synapse then travel to the
parietal lobe. This is the pathway for conscious perception of pain, touch, and temperature from the
head, including the eye.
The spinotectal tract ascending from the spinal cord will pass through this area to the tectum.
The corticopontocerebellar fibers traveling in longitudinal fibers of the pons synapse in the pontine
nuclei surrounding the fibers. The second order neurons cross the midline in the transverse fibers
of the pons and enter the middle cerebellar peduncle (brachium pontis). This particular pathway is
part of the menace response.
The 4 pairs of vestibular nuclei are located on the border of the pons and myelencephalon lateral to
the 4th ventricle. Projections from the vestibular nuclei both ascend and descend, crossed and
uncrossed especially in the MLF. A descending tract from the medial vestibular nucleus
constitutes the medial vestibulospinal tract within the MLF of the ventral funiculus. The lateral
vestibulospinal tract is also in the ventral funiculus but not in the MLF. This latter tract arises from
projections of the lateral vestibular nucleus.
rostral = superior = Bechterew: ascending projections
lateral = Dieters: ascending tract of Dieters, and lateral vestibulospinal tract
medial = Schwalbe: ascending and descending projections, cerebellum
caudal = inferior = spinal = descending projections, cerebellum
All vestibular nuclei receive afferents of the vestibular n. (vestibulocochlear n = CN VIII). The
flocculonodulus (archicerebellum) and fastigial nucleus of the cerebellum also project to all
vestibular nuclei.
The pontine sensory nucleus and tract (sensory from CN V) is the metencephalic portion of the
sensory tract of V as described previously.
The cerebellum is going to provide coordination between vision, vestibular, and proprioception
input. It is mostly the medial zone (vermis and fastigial nucleus), which are phylogenetically older,
that has the connections. The archicerebellum, flocculonodulus, is involved with vestibular
connections. This portion is tucked on the ventral surface of the cerebellum and is not easily seen
in the intact brain.
It has been noted that lesions of interpositus nucleus (one of the deep cerebellar nuclei) resulted in
increased pupil size and decreased constriction to light.
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The pathway of palpebral/corneal reflexes also involve the pons region: 1st order neuron in the
sensory branches of the trigeminal nerve, enter the pons and pass into the myelencephalon within
the spinal tract of V. 2nd order neurons in the nucleus of spinal tract of V, 3rd order neurons in the
facial nucleus and out the facial nerve.
VII.
MYELENCEPHALON (medulla oblongata)
This is the most caudal division of the brain blending gradually with spinal cord. It contains
several LMN nuclei, including some associated with eye/eyelid function. Additionally, there are
sensory impulses which enter the brain at this level and travel rostrally. As with previous divisions,
there are pathways, ascending and descending, which pass through this area but which do not
synapse in this region. Portions of the vestibular nuclei extend into the rostral myelencephalon: the
functions and connections are as described previously.
A large portion of the sensory fibers from the trigeminal nerve enter at the myelencephalon and
turn caudally spreading over the dorsolateral surface of the brain as the spinal tract of CN V. These
fibers enter the nucleus of the spinal tract of CN V and synapse. The second order neurons cross
the midline and travel rostrally in the trigeminal lemniscus to the thalamus where they synapse.
From the ventral caudomedial nucleus of the thalamus, 3rd order fibers enter the internal capsule to
the somesthetic interpretation center in the parietal lobe. Therefore this pathway of the head is the
parallel to the "pain" pathway (spinothalamic) of the body.
The abducens nucleus is identified by some as being in the pons rather than the myelencephalon.
The reason is a difference in the relative size of the pons. In humans the transverse fibers of the
pons extend enough to cover the trapezoid body. In domestic mammals the trapezoid body can be
seen on the ventral surface of the brain just caudal to the pontine fibers. As the abducens nucleus is
at the same level as the trapezoid body, it may be classified differently as to location. The
abducens nucleus is in the same rostrocaudal column as the oculomotor and trochlear nuclei as it is
also a source of GSE fibers. In this case, efferents exit the brain stem ventrolaterally to innervate
the lateral rectus m., and retractor bulbi m., if present.
Afferent fibers to the abducens nucleus include crossed medial vestibular nucleus innervation for
conjugate eye movements.
The facial nucleus, particularly the lateral area, is important for eyelid muscle innervation and is
involved as the efferent arm of the corneal, palpebral, and dazzle reflexes, and menace response.
This is an SVE nucleus located in a ventrolateral position in the rostral myelencephalon at the edge
of the reticular formation; like the abducens nucleus some anatomists include it in the pons.
Radices emanating from the nucleus pass rostrodorsally, then laterally looping around the medial
side of the abducens nucleus. Afferents to the facial nucleus include cerebellar projections,
interneurons from the nucleus of spinal tract of V.
Remember that the VIIth and VIIIth cranial nerves enter the internal acoustic meatus together
before separating to the inner ear and the facial canal.
The parasympathetic nucleus of CN VII (rostral (superior) salivatory nucleus) is associated with
autonomic innervation of the lacrimal gland. Afferent fibers to the rostral salivatory nucleus would
include the hypothalamus. The efferent fibers project via the "sensory" root of CN VII, then via the
intermediate nerve and major petrosal nerves to the pterygopalatine ganglion where the
preganglionic fibers synapse. Postganglionic fibers travel with the ophthalmic or maxillary nerve
branches to the lacrimal gland. This is probably the innervation to the gland of the 3rd eyelid as
well.
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The following tracts are passing through this area:
tectospinal tract
tectotegmentospinal tract
spinotectal tract
vestibulospinal tract
spinovestibular tract
IX.
SPINAL CORD
The cranial spinal cord also has direct innervation of eye structures as well as several tracts
(tectospinal, spinotectal) which pass through several levels. The spinal tract and nucleus of the
spinal tract of CN V continue caudally from the brain onto the cranial cervical spinal cord
segments.
These cranial cervical segments receive sensory impulses from the eyelids via C2-C4 spinal nerves.
These fibers presumably synapse in the dorsal horn of the spinal cord and 2nd order neurons ascend
to the thalamus, synapse and then go to the cortex.
The lateral horn (intermediolateral, zona intermedia) of Tl-T3 segments is the location of the
preganglionic sympathetic NCBs for innervation of the eye. The preganglionic axon exits the cord
via the ventral root.
The (medial) tectospinal tract is within the MLF located in the ventral funiculus of the cord. Its
origin as described before is the ipsilateral rostral colliculus and its termination is the ventral horn
of the cervical spinal cord. This is involved in movements of the head and neck in response to
visual stimuli.
The tectotegmentospinal (lateral tectospinal) tract travels in the lateral funiculus of the cord. Its
origin is the contralateral rostral colliculus, which in turn received input from the caudal
hypothalamus, and its termination is the Tl-T3 lateral horn. This is an UMN tract for sympathetic
innervation of the eye.
X.
CRANIAL NERVES
CNs generally exit the brain ventral to lateral in position. Some CNs carry a small % of fibers from
contralateral motor nuclei but the peripheral projections are ipsilateral with exception of CN IV.
CN IV is an exception to both these “rules”. It exits from the dorsal aspect of the brain stem at the
mesencephalon/pons border area and is a contralateral peripheral projection.
With most cranial nerves there are extensive anastomoses peripherally; there is not segmental
innervation as seen on the trunk. So many of the "nerves" on the head are actually a
conglomeration of fibers of different sources although we may say that "nerve X is a branch of CN
Y'.
There has not been anatomic identification of cranial nerve plexi in the sense that they have official
names (like brachial plexus) but you can think about them unofficially as superficial, orbital, and
occipital plexi.
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The superficial plexus is a mixture of facial (CN VII) and trigeminal (CN V) nerve fibers located
just deep to the skin and platysma/cutaneus facei muscle. At this level, the facial nerve axons are
motor innervation to muscles of facial expression including the eyelid muscles; the trigeminal
axons are sensory for pain, touch, temperature for the head including the adnexa. Although CN V
is also sensory to the globe those particular branches are deeper and are not part of this superficial
plexus.
The orbital plexus includes axons of CNs III (including parasympathetic fibers), ophthalmic and
maxillary branches of CN V, postganglionic parasympathetic of VII, and postganglionic
sympathetic from cranial cervical ganglion. The interweaving of the fibers occurs internal to the
skull, within the periorbita, and immediately ventral to the periorbita. CNs II, IV, and VI are also
in the immediate area but evidently do not exchange any fibers with branches of other nerves.
The occipital plexus is composed of CNs VII, IX, X, XI, XII, and postganglionic sympathetic from
cranial cervical ganglion. Much of the mixing occurs just external to the tympano-occipital fissure,
additional interweaving occurs in the middle ear cavity.
Note that parts of some CNs are involved in the formation of more than one of these plexi-CN VII,
sympathetics.
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Outline of foramina of exit in different species for CNs
Nerve
Dog
Cat
Horse
Ruminant
Pig
CN I
cribriform plate
same
same
same
same
CN II
optic canal
same
same
same
same
CN III
orbital fissure
same
same
foramen
orbitorotundum
foramen
orbitorotundum
CN IV
orbital fissure
same
same
foramen
orbitorotundum
foramen
orbitorotundum
CN V
ophthalmic
orbital fissure
same
same
foramen
orbitorotundum
foramen
orbitorotundum
CN V
maxillary
round foramen, then
rostral alar foramen
round
foramen
same
foramen
orbitorotundum
foramen
orbitorotundum
CN V
mandibular
oval foramen
same
oval
notch of
foramen
lacerum
oval foramen
oval foramen
CN VI
orbital fissure
same
same
foramen
orbitorotundum
foramen
orbitorotundum
CN VII
internal acoustic
meatus, facial canal,
stylomastoid foramen
same
same
same
same
CN VIII
internal acoustic
meatus
same
same
same
same
CN IX
jugular foramen then
tympanooccipital
fissure
same
same
same
same
CN X
jugular foramen then
tympanooccipital
fissure
same
same
same
same
CN XI
jugular foramen then
tympanooccipital
fissure
same
same
same
same
CN XII
hypoglossal canal
same
same
same
same
NEUROANATOMY
2016
XII.
CN II (optic n)- sensory nerve of pupillary light reflex, menace response, and conscious vision
pathway. This nerve is technically an extension of the diencephalon, hence the reason that the
meninges continue external to the skull along it. Functionally these are SSA fibers with NCBs in
ganglion cell layer of retina.
XIII.
CN III (oculomotor n)- motor nerve of pupillary light reflex. GSE fibers from oculomotor nucleus to
majority of extraocular mm: dorsal, medial, and ventral rectus, ventral oblique, and levator palpebrae
superioris mm.
GVE fibers from parasympathetic nucleus of CN III to iris and ciliary mm.
Postganglionic parasympathetic NCBs are located in the ciliary g. There is a difference of formation
of short ciliary nn. in dog and cat (see XX).
Similarly, the exact point at which other fiber types "hitch a ride" in the orbital plexus can be
different and result in different clinical syndrome.
XIV.
CN IV (trochlear n)- The GSE fibers from trochlear nucleus travel to the contralateral dorsal oblique
muscle.
XV.
CN V (trigeminal n)- sensory nerve of corneal and palpebral reflexes, each major branch has separate
foramen of exit in the dog and cat. Only the maxillary and ophthalmic branches are directly
associated with ocular functions. Although the trigeminal nerve as a whole is mixed in function
(SVE, GSA), these 2 branches have only GSA-sensory- fibers. They will be joined by sympathetic
and parasympathetic fibers of other sources, peripherally. The trigeminal (=Gasserian= semilunar)
ganglion is the location of sensory NCBs.
The ophthalmic n. will actually split into further branches just prior to entering the periorbita-frontal,
nasociliary, long ciliary, and infratrochlear nerves. Sympathetic fibers from the cranial cervical
ganglion will join the nasociliary nerve. The maxillary nerve will supply some branches to the orbital
region- zygomatic, lacrimal nerve. Parasympathetic fibers from pterygopalatine ganglion (CN VII)
will join the lacrimal nerve.
XVI.
CN VI (abducens n)- This nerve contains motor (GSE) fibers from abducens nucleus. In addition to
innervating the lateral rectus muscle and the retractor bulbi muscle, it innervates the striated muscle
strip to the 3rd eyelid of the cat.
XVII.
CN VII (facial n)- This is the motor nerve of corneal, palpebral reflexes and menace response. It is a
mixed nerve in function- SVA, GVA, GSA, GVE from parasympathetic nucleus of CN VII (rostral
salivatory nucleus), SVE fibers from facial nucleus. The GVE and SVE fibers are of concern for
ocular functions.
The 'sensory' root contains GVE fibers from the parasympathetic nucleus of CN VII. These fibers
enter the intermediate n. and join the motor root which passes through the internal acoustic meatus
and the facial canal. At the genu of the nerve, the parasympathetic fibers leave in the major (greater)
petrosal n (petrosal canal). This major petrosal nerve is joined by the deep petrosal n and is then
called the nerve of the pterygoid canal. This nerve synapses at the pterygopalatine ganglion located
ventral to the periorbita and deep to the maxillary nerve. Postganglionic fibers then travel to the
lacrimal nerve and innervate the lacrimal gland.
NEUROANATOMY
2016
The geniculate ganglion is sensory in function and as such is not directly involved with ocular
function. The major part of the facial nerve continues through the facial canal and exits the skull via
the stylomastoid foramen and then splits into the auriculopalpebral n., dorsal buccal n., and ventral
buccal n. The auriculopalpebral nerve further divides into auricular and palpebral branches. The
latter, innervates the orbicularis oculi, retractor anguli oculi lateralis, and levator anguli oculi medialis
muscles of the eyelid and brow area.
XVIII.
CN VIII (vestibulocochlear n)- This nerve has indirect influence over ocular movements via its
connections to the cerebellum and vestibular nuclei.
XIX.
C 2 -C4 spinal nerves
Sensory fibers from area of eyelids travel ipsilaterally to the cervical spinal cord with NCBS in
cervical spinal ganglia (dorsal root ganglia).
XX.
Tl-T3 spinal nerves
The preganglionic axon exits the cord via the ventral root, enters the spinal nerve for a short distance,
then travels in the ramus communicans to the thoracic sympathetic trunk (but no synapse yet).
Traveling cranially in the sympathetic trunk, the axon(s) pass through the cervicothoracic (stellate)
ganglion, the ansa subclavia, the middle cervical ganglion and enter the sympathetic part of the
vagosympathetic trunk (still no synapse). Upon reaching the cranial (superior) cervical ganglion, the
preganglionic axon synapses. The postganglionic axon travels through the middle ear cavity with the
internal carotid artery (if the species has one), to join with branches of CN V in the orbit. The exact
recombination of short and long ciliary nerves in each species varies and lesions at different spots can
give rise to very different clinical pictures.
In cats: There are 2 (medial/lateral: nasal/malar) short ciliary nerves which arise from the ciliary
ganglion and carry parasympathetic fibers at this level. These will be joined by the long ciliary
nerves just prior to entering the globe. The long ciliary nerves contain the sensory and sympathetic
fibers.
In dogs: There are 5-8 short ciliary nerves which arise from the ciliary ganglion. As sympathetic
innervation travels through the ciliary ganglion of dogs (no synapse), short ciliary nerves contain
parasympathetic, sympathetic, and sensory fibers.