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Neuroanatomy ©

Neuroanatomy ©

2016

NEUROANATOMY ©

LC Hudson, DVM, PhD Professor Emerita of North Carolina State University College of Veterinary Medicine Raleigh NC 27607 [email protected]

I. CENTRAL All 5 divisions of the are involved with the function of the and/or adnexa - in either conscious/response pathways or pathways. The divisions are the telencephalon (cerebral hemispheres), (, , metathalamus (lateral geniculate nuclei)), mesencephalon ( (pretectal, oculomotor, trochlear nuclei, parasym. of CN III)), ( and (, spinal tract of CN V)) and ( (abducens, facial, vestibular nuclei)).

The more cranial is also involved with function of the through sensory innervation into the cervical spinal cord, and through sympathetic autonomic function (T1-T3 segments). The spinal cord caudal to T3 is not involved with functions.

II. PERIPHERAL NERVOUS SYSTEM The majority of the 12 pr. of cranial 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 .

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 Tl-T3 spinal nerves into the , traveling through the in the vagosympathetic trunk and synapsing in the cranial cervical . Postganglionic fibers then travel along vessels and with other cranial nerves to reach the .

III. NOMENCLATURE Veterinary ophthalmologists tend to use /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) (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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IV. TELENCEPHALON = = cerebral hemispheres ≅ . The cerebrum functions in and integration of vision as well as voluntary control of eye/ movements. The occipital lobes and the of the frontal/parietal lobes are the primary regions involved.

The occupies the caudal aspect of the cerebral hemispheres. It borders with the dorsorostrally and the laterally. Medially, the left and right occipital lobes meet across the 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 in the occipital lobe.

Gyri included in the human occipital lobe laterally are the , lateral occipital , superior and inferior occipital gyri, descending gyrus, superior and inferior polar gyri. Medially, more of the cuneus, parts of the , 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 Gennari- cytoarchitectural 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 of (main 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 of human .] 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, and .

V. DIENCEPHALON Structures of the diencephalon involved with vision are the optic , , lateral geniculate body, internal capsule, hypothalamus, and thalamus.

The 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 .

The percentage of optic fibers crossing the midline in different species varies widely. most and : 100% cat: ~65% dog: ~75% large animals: ~80-90% It is the existence of uncrossed fibers that give the ability for .

[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 - 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 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. NEUROANATOMY 2016

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 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 . 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 .

VI. MESENCEPHALON The mesencephalon or midbrain is grossly divided into the tectum and by the line of the 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 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 .

The 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 . 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 ) are two mounds of neural 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 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 (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 (). 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 . 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 T1-T3 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 of the brain stem. 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,

So in the mesencephalon the LMN nuclei are the oculomotor 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 and

The oculomotor nucleus is located slightly ventromedial to the parasympathetic nucleus, near the matter. It is the source of GSE fibers to various extraocular muscles. Afferents to this nucleus include 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 nuclei and the motor nuclei of CN III, IV, and VI are the basis of and in some cases of .

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 onto the postganglionic neurons. The exact branching of the 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 . 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 .

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 (conscious sensory pathway from head including eyes). NEUROANATOMY 2016

VII. 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 (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 within the MLF of the ventral . 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: 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 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 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 ) resulted in increased size and decreased constriction to light.

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The pathway of palpebral/corneal also involve the pons region: 1st order 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 .

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 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 the transverse fibers of the pons extend enough to cover the . 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 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, 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 and the .

The parasympathetic nucleus of CN VII (rostral (superior) salivatory nucleus) is associated with autonomic innervation of the . 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 and major petrosal nerves to the where the preganglionic fibers synapse. Postganglionic fibers travel with the ophthalmic or branches to the lacrimal gland. This is probably the innervation to the gland of the 3rd eyelid as well. NEUROANATOMY 2016

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 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 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 that they have official names (like ) 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 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 , 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 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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XI. 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 same same same same

CN III orbital fissure same same foramen foramen orbitorotundum orbitorotundum

CN IV orbital fissure same same foramen foramen orbitorotundum orbitorotundum

CN V orbital fissure same same foramen foramen ophthalmic orbitorotundum orbitorotundum

CN V round foramen, then round same foramen foramen maxillary rostral alar foramen foramen orbitorotundum orbitorotundum

CN V oval foramen same oval oval foramen oval foramen mandibu- notch of lar foramen lacerum

CN VI orbital fissure same same foramen foramen orbitorotundum orbitorotundum

CN VII internal acoustic same same same same meatus, facial canal,

CN VIII internal acoustic same same same same meatus

CN IX jugular foramen then same same same same tympanooccipital fissure

CN X jugular foramen then same same same same tympanooccipital fissure

CN XI jugular foramen then same same same same tympanooccipital fissure

CN XII same same same same

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XII. CN II (optic n)- of pupillary light reflex, menace response, and conscious vision pathway. This nerve is technically an extension of the diencephalon, hence the reason that the continue external to the skull along it. Functionally these are SSA fibers with NCBs in of .

XIII. CN III (oculomotor n)- 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 . The maxillary nerve will supply some branches to the orbital region- zygomatic, . 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 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 . This nerve 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.

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The 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. C2-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 for a short distance, then travels in the 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 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 (if the species has one), to join with branches of CN V in the . The exact recombination of short and 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) 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.

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