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Neuroanatomy of the Visual Pathways

Neuroanatomy of the Visual Pathways

NEUROANATOMY OF THE VISUAL PATHWAYS

Magrane Basic Science Course

June 2018 Central

 All 5 divisions involved in ocular function and/or adnexa  Telencephalon  Cerebral hemispheres  and  Mesencephalon  and Divisions (midsagittal plane)

Telencephalon ()

Cerebellum (dorsal metencephalon)

Diencephalon Mesencephalon Myelencephalon (midbrain) Pons (medulla oblongata) (ventral metencephalon) Brain Divisions on MRI (midsagittal) Telencephalon (cerebrum)

Cerebellum (dorsal metencephalon)

Foramen magnum – Diencephalon junction of brain and Mesencephalon Myelencephalon (midbrain) Pons (medulla oblongata) (ventral metencephalon) Telencephalon

 Site of , initiation of voluntary movements and stimuli  Functions in perception and integration of vision as well as voluntary control of movements Cerebral Lobes Audition

Sensory Processing Behavior Somatosensory

Memory Parietal Learning

Behavior

Intellect Sensory Processing Vision Motor function

Piriform lobe

Sensory Processing Olfaction Occipital Lobe

 Caudal part of  Not well delineated  Contains visual Occipital Lobe

 Occipital lobe occupies the caudal 1/3 of cerebral hemispheres  Right and left occipital lobes meet between the cerebral hemispheres across the  There is no definite line or to demarcate the borders of the occipital lobe  Dog: includes parts of the marginal, ectomarginal, caudal suprasylvian, caudal composite, splenial and .  Dog: endomarginal and ectomarginal gyri and sulci are part of the (rostral 2/3) and the occipital lobe (caudal 1/3). 15. ectomarginal 16. marginal 17. endomarginal 18. caudal suprasylvian 19. occipital Occipital Lobe

ectomarginal, marginal, endomarginal, caudal suprasylvian Occipital Lobe

ectomarginal, marginal, endomarginal, caudal suprasylvian, caudal composite Occipital Lobe

Splenial, occipital Occipital Lobe

Splenial, occipital Occipital Lobe

 Brodmann’s area 17 = striate = visual I  Lies in the posterior pole of the occipital lobe  In the cat, it occupies the posteromedial portion of the cortex, extending from the crown of the lateral on the dorsal surface to the superior bank of the splenial sulcus on the medial surface  In the dog, it is located at the junction of the marginal and endomarginal gyri  Brodmann’s area 18 = parastriate = visual II  Brodmann’s area 19 = peristriate = visual III  Area 18 & 19 – may be referred to as or as visual association area Occipital Lobe

 extent of visual resolution in a reflected in  the surface area of the striate cortex  the magnification factor (a term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis)  the number of visually responsive areas hedgehog: 3; mouse: 4; cat: > 12; nonhuman primates: > 30 Occipital Lobe

 point of central vision  is located in the striate cortex  varies in position between species and perhaps between breeds  important when recording visual evoked potentials  stereotaxic coordinates of the area centralis in the feline cortex are P3-L5 (3 mm posterior to the interaural plane and 5 mm lateral to the midline). Anatomically, it is located on the crown of the lateral gyrus, near the junction of the lateral and posterior lateral gyri. Another source listed this area in the cat as the junction of marginal and endomarginal gyri.  on average, the projection of the canine area centralis is 13.4 mm anterior to the interaural plane and 8.4 mm lateral to the midline. Beagle: 11.3 mm rostral to the interaural plane and 8.3 mm lateral to the midline; Greyhound: 15.6 mm rostral to the interaural plane and 8.5 mm lateral to the midline. Visual areas 17, 18, and 19 of the cat brain

Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; and Physiology of Vision by Ron Ofri Occipital Lobe

 cells of the primary , visual I, are arranged in 6 layers  layer 4 is heavily myelinated; in this layer the incoming LGN with cortical  magnocellular (, movement, directionality and contrast sensitivity) projections synapse in layer 4Ca  parvocellular (spatial resolution and sensitivity) projections synapse in layer 4Cb  most of the neurons are GABAnergic, inhibitory neurons that do not project outside of visual I and are devoted to processing of the signal in the striate cortex  a minority of the cells are excitatory, spiny (stellate or pyramidal) neurons that project outside of area 17 Occipital Lobe

 basic cortical unit that processes an incoming signal is termed a column, which descends through all six layers of the cortex  vertical penetration through six cortical layers of the column will result in passage through cells with approximately identical receptive fields  therefore, adjacent retinal receptive fields project onto adjacent columns in visual I  no difference in size between columns serving central and peripheral  rather, more columns are used to process visual input from the central retina Occipital Lobe

 area visual I receives input from the DLGN  input consists of the entire contralateral visual hemifield as projected on both  this visual hemifield is mapped on the surface of the cortex in a retinotopic manner (adjacent loci of the contralateral visual hemifield are projected onto adjacent loci of the cortex in a simple, point to point manner)  each visual hemifield projects onto the cortical surface  vertical meridian is a vertical line of demarcation that passes through the area centralis (or fovea) and divides the retina into a nasal hemifield (projected to the contralateral cortex) and a temporal hemifield (projected to the ipsilateral cortex) Visual hemifield on the cat cortex

Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri Occipital Lobe

 Large cortical areas are devoted to processing signals originating from the area centralis  magnification factor: term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis  In the retina, each degree of the is projected onto a similar-sized retinal area, regardless of whether it is peripheral or central  increased resolution and processing achieved by the central retina is obtained by increasing the density of cell or photoreceptor population  In the cortex, the density of neurons serving the peripheral or central fields is identical  increased cortical visual discrimination from the area centralis is a result of the increased cortical area devoted to representing the area centralis  larger area results in magnification of its representation Occipital Lobe

 Hedgehog: the surface area of the striate cortex is 20 mm2 and ½ of this area is devoted to representing the central 35 degrees of the contralateral visual hemifield  Cat: the surface area is 380 mm2 and ½ of this area is devoted to the central 20 degrees of the visual field. In other words, ½ of visual I is devoted to the central 20 degrees of the visual field and ½ is devoted to the rest of the visual field Occipital Lobe

 Afferent connections to visual areas  come from the LGN via of (main connection between hemisphere and rest of brain)  there are also reciprocal connections with other lobes and with parastriate/peristriate areas  Efferent connections from visual areas  include the association areas (long and short association fibers connect visual cortex with other lobes of the same hemisphere such as at frontal/parietal lobe)  the opposite hemisphere via  the brain stem (to LGN and rostral colliculus, and ) Telencephalon

Structure of the telencephalon involved with vision is the occipital cortex

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri Diencephalon

 thalamus, hypothalamus  thalamus receives, processes and relays to the information from other regions of the brain and most sensory information. Only the olfactory information does not pass through the thalamic relay  Structures of the diencephalon involved with vision are the optic , , lateral geniculate body (LGB)/lateral geniculate (LGN), internal capsule and hypothalamus Diencephalon Diencephalon Landmarks - Ventral View

 Mamillary bodies  Pituitary (hypophysis) Optic chiasm  Usually detached Hypothalamus  Infundibulum  Opening communicates with  Hypothalamus surrounds infundibulum Diencephalon – Visual Pathway

 Optic (CN II)  Optic chiasm  Majority of optic n. fibers cross midline Optic chiasm  Optic tract Optic tract  Projection from chiasm toward brain  Lateral to remainder of diencephalon  Travels in caudodorsal direction to lateral geniculate nucleus Lateral  Lateral geniculate nucleus geniculate sends input to occipital nucleus cortex in telencephalon Optic tract Optic chiasm Diencephalon

Structures of the diencephalon involved with vision are the optic chiasm, optic tract, lateral geniculate nucleus (LGN), internal capsule and hypothalamus

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri Diencephalon - chiasm

 optic chiasm  is at the base of the hypothalamus  it is specifically located at the rostroventral surface of the  demarcates the rostral level of the diencephalon  is closely associated with the 3rd ventricle, hypothalamus, and to some extent the depending on the species Diencephalon – optic nerve/chiasm

 arrangement of retinal ganglion cells within the optic nerve is not random  fibers are arranged in a retinotopic manner (precise spatial arrangement of the retina is maintained within the nerve)  fibers from the superior retina form the superior half of the optic disk and fibers from the inferior retina form the inferior optic disk  fibers from the central retina are in the center of the optic nerve, while those in the retinal periphery are in the periphery of the optic nerve  this precise arrangement is needed for subsequent accurate projection of the visual field in both the DLGN and the visual cortex Diencephalon - chiasm

 percentage of optic nerve fibers crossing the midline in different species varies widely  as the chiasm is approached, the fibers from the temporal retina remain on the ipsilateral side of the brain and fibers from the nasal retina cross over to the contralateral side  animals with laterally directed and no overlap between the visual fields of the two eyes exhibit complete at the chiasm and the information from the right or left visual field is processed entirely by the opposite visual cortex  as the eyes become more frontally placed, an object becomes more visible from both visual fields  an object on the animal’s right visual field (right side) falls on the nasal area of the right retina and the temporal area of the left retina  in order for the same side of the brain (left side in this example) to process the information from the right visual field, some optic nerve fibers must remain ipsilateral and not decussate at the chiasm Diencephalon - chiasm

 percentage of optic nerve fibers crossing the midline in different species varies widely  most , many and , and fish: 100% crossover  in other amphibians and reptiles: 95% crossover  in rodents: 97-98% crossover  large animals: 80-90% crossover  horses: 83-87% crossover  dog: 75% crossover  cat: 65-67% crossover  beings: 50% crossover Diencephalon - chiasm

 Because of the decussation, fibers of the optic tract conduct information from the opposite visual hemifield of both eyes  In animals where a greater percentage of fibers cross over, the left occipital cortex will input a greater proportion of the right visual field of the right eye and a smaller proportion of the right visual field of the left eye  In (with 50% cross over), a lesion in the left optic radiation or occipital cortex will cause a loss of the right visual hemifield with symmetric deficits in both eyes (homonymous hemianopia)  In animals, however, the same lesion (left optic radiation or occipital cortex) will cause greater visual deficits in the visual field of the right eye than those of the left eye Diencephalon - chiasm  In animals where a greater percentage of fibers cross over, the left occipital cortex will input a greater proportion of the right visual field of the right eye and a smaller proportion of the right visual field of the left eye  In the dog with 25% of fibers on the ipsilateral side and 75% crossed over in the chiasm, a unilateral lesion will cause deficits of 25% in the visual field of the ipsilateral side and of 75% in the visual field of the contralateral side  More decussation (horses and cattle) will lead to an increased tendency to walk into objects on the side of the visual deficit (contralateral to the lesion)  Theoretically, these deficits can be tested with medial and lateral visual field tests, but these are unreliable  It is the existence of uncrossed fibers that give the ability for Visual Fields

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 1; Structure and Function of the Eye by Paul Miller Projection of visual field onto retina, LGN and visual cortex

Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri Diencephalon – optic tract

 From the optic chiasm, fibers enter the optic tracts which then pass laterally from the chiasm anterior to the hypophysis and beneath the ventral surface of the  The tracts then curve dorsally and posteriorly between the cerebral peduncle and the pyriform lobe to the LGN  The optic tract is located lateral to the internal capsule of diencephalon. It begins ventrally, and then travels laterally, caudally and, finally, dorsally. This pathway keeps the optic tract on the surface of the diencephalon. In the brain, most of the optic tract is covered by the overlying cerebrum. Diencephalon – optic tract/LGN

 Before reaching the LGN, 20% of the fibers leave the tracts and enter the . Some of these fibers pass to the rostral (superior) colliculus directly and others pass via the tracts and LGN to the colliculus indirectly  The majority of fibers entering the LGN synapse there with the 3rd ascending in the which then passes without further synapse to the visual cortex Diencephalon – optic tract/LGN

 DLGN receives input from the contralateral visual hemifield of both eyes and outputs to the visual cortex  In most species with a ventral LGN and a DLGN, the optic tract terminates in the DLGN  Primates do not have a VLGN, so the optic tract in the LGN  The DLGN of the cat and the LGN of the macaque have been studied extensively (DLGN of cat has 4 layers and the LGN of the macaque has 6) but are relatively similar Diencephalon – optic tract

 conflicting evidence about segregation of fiber size within the tract  there is no doubt that different sizes exist: X, Y and W.  there is a difference in conduction speeds and projections of those different classes of axons.  in general, fastest fibers (Y) project to the LGN, intermediate (X) to the pretectum, and slowest (W) to the rostral colliculus.  in the LGN, larger fibers synapse in the dorsal laminae while smaller fibers synapse in the ventral laminae Diencephalon – optic tract

 there are connections from the optic tract to:  hypothalamus (controversial for carnivores)  LGN (actually more dorsal in position in domestic animals). It is estimated that 80% of optic tract fibers terminate in the LGN  pretectal nuclei via brachium of rostral colliculus and then pass to pretectum  rostral colliculus via brachium of rostral colliculus  accessory nucleus of optic tract Diencephalon - LGN

 major function of the LGN is as a complex relay nucleus on the conscious vision pathway  LGN has topographic (retinotopic) organization. Each layer of the LGN constitutes a precise map of the contralateral visual hemifield  In the cat, uncrossed fibers from the temporal hemifield of the right eye terminate in layers A1 and C1 of the right dorsal LGN; fibers crossing over from the nasal hemifield of the left eye terminate in layers A and C of the right dorsal LGN. Furthermore, the 4 resulting maps are in register, meaning that if an electrode vertically penetrated all 4 layers, the cells it would pass through all represent the same point in the visual field of both retinas Diencephalon - LGN

 The maps of the DLGN preserve the topography of the visual field and also reflect the physiologic processing of the signal that has occurred in the retina  Layers A and A1 of the cat DLGN input from X-type ganglion cells and layers C and C1 input from Y-type ganglion cells  Therefore, the 4 cells through which the above- mentioned line of projection passes receive both X- type and Y-type data about an identical in both eyes. This provides the basis for the merging of visual information that occurs in the cortex Projection of visual field onto retina, LGN and visual cortex

Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri Diencephalon – LGN/optic radiations

 The actual synapses between the ganglion cell axons and the of the thalamic cells take place in a structure termed the synaptic  The thalamic cells may be of 2 types: (provide for some signal processing) or projecting cells  Axons of the projecting cells exit the DLGN and form the optic radiations  The axons relay the visual signal from the DLGN to the primary visual cortex, where they synapse  In some species, these radiations also contain axons that descend from the visual cortex to the DLGN and rostral colliculus  The afferent connections to the LGN: mostly optic tract neurons but there are also some cortical fibers and rostral colliculus fibers.  The efferent connections from the LGN: internal capsule via optic radiation to layer IV of visual cortex and rostral colliculus via brachium of rostral colliculus. Diencephalon - hypothalamus

 located on the ventral aspect of the diencephalon  divided into 3 nuclear areas: rostral (supraoptic), intermediate (tuberal) and caudal (mamillary) hypothalamic areas  in addition to endocrine functions with the pituitary, the hypothalamus also is the upper (UMN) source for the  in relation to vision, the hypothalamus receives afferent fibers from the optic tract and the matter (mesencephalon)  efferent fibers from the hypothalamus include projections to the periaqueductal gray matter and the tectum Mesencephalon - midbrain

 participates in the control of motor functions, in coordination of visual and auditory and in the processing of sensory information (auditory)  grossly divided into the tectum and the by the line of the of the mesencephalic aqueduct (aqueduct of Sylvius)  the tectum is associated with sensory function and integration of vision and widespread body movement  the tegmentum is associated with motor function: control of extraocular muscles and parasympathetic control of bulbar smooth muscle  the pretectum is located at the dien-mesencepahlic border and is associated with the pupillary light . Pretectal nucleus is in the mesencephalon Mesencephalon Mesencephalon – Cranial

(CN III) exits the brain on the ventral surface of the mesencephalon

(CN IV) exits on the dorsal surface of the Metencephalon mesencephalon Visual and Auditory Pathways

Mesencephalon (reflex)

 Rostral colliculus (visual)

 Caudal colliculus (auditory)

Diencephalon (perception)

 Medial geniculate nucleus (auditory)

 Lateral geniculate nucleus (visual)

• Note proximity of areas in brain Mesencephalon

Tectum

Tegmentum Mesencephalon - tectum

 The rostral colliculi are two mounds of neural tissue lying close to one another on the dorsal brain stem. These nuclei are located at the dorsal aspect of the midbrain (tectum)  The rostral colliculi are laminated structures and have a topographic organization. They have seven layers: three cellular alternating with four layers of fibers  Their general function is visuomotor coordination. For example, they control saccadic eye movements  Each rostral colliculus has connections with both crossed and uncrossed optic tract fibers via the brachium of the rostral colliculus Mesencephalon - tectum

 Rostral colliculi receive axons from the optic nerve, the cerebral cortex (especially the visual cortex) and the spinal cord (via the spinotectal tract)  Axons of cells in the rostral colliculus project to the tegmentum of the midbrain and medulla, outputting to both left and right motor nuclei of CN III, IV, and VI  Motor fibers exiting a nucleus of the rostral colliculus decussate as they exit and descend through the brainstem and into the spinal cord  Other neurons project from the rostral colliculus to the ascending reticular formation  In the brainstem, the ascending reticular formation receives projections from special including vision

Visual projections from rostral colliculus

Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 29; Neuro-ophthalmic Anatomy by Merav Shamir and Ron Ofri Mesencephalon - tectum

 location of UMN sympathetic neurons involved in constriction of the  tectotegmentospinal tract may actually be a misnomer. Some references use the term lateral 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  Tectospinal tract = medial tectospinal tract: orientation of the head  Spinotectal tract: movements of (head and eyes) towards movements Mesencephalon – pretectal area

 located on the dorsal diencephalic/mesencephalic border, slightly rostral to the mound of the rostral colliculus. This area includes the pretectal nuclei and the caudal commisure  pathways of the PLR & reflex are associated with this area  intermediate sized afferent fibers in the optic tract bypass the LGN, travel into the initial brachium of the rostral colliculus and enter the pretectal nuclei  projections from the pretectal nuclei include a large bundle crossing in the caudal commisure to the parasympathetic nucleus of CN III (Edinger-Westphal or accessory )  there are some uncrossed fibers between ipsilateral pretectal and parasympathetic nuclei Mesencephalon - tegmentum

 the ventral mesencephalon  some definitions exclude the crus cerebri  this area is the location of several (LMN) nuclei associated with ocular functions  in addition, there are pathways passing through this region, such as the tectotegmentospinal tract Mesencephalon - tegmentum

 locations of the cell bodies, which in turn project via certain , have an organization scheme around the of the brain stem  schemes often refer to these nuclei as General Somatic Efferent (GSE), General Visceral Efferent (GVE), and Special Visceral Efferent (SVE)  GVE (autonomic nuclei) nuclei are located ventral to the ventricular system with GSE nuclei slightly ventrolateral 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 brain stem

Examples of each type include:  GVE-parasympathetic nucleus of CN III  GSE-oculomotor, trochlear and abducens nuclei  SVE-facial nucleus Mesencephalon - tegmentum

 the LMN nuclei found here are the oculomotor nucleus, , and parasympathetic nucleus of CN III. (Cranial nerves III and IV emerge from the mesencephalon so the motor nuclei giving rise to those axons are also in the mesencephalon)  afferents - via the medial longitudinal fasciculus, cortical projections and reticular formation  efferents - to extraocular muscles Mesencephalon - tegmentum

 parasympathetic nucleus of CN III (Edinger-Westphal or accessory oculomotor nucleus) is the source of preganglionic autonomic fibers in the oculomotor nerve  afferents to this nucleus are primarily crossed fibers of the pretectal nucleus via the caudal commisure but uncrossed fibers are also received  nucleus is located at the ventral 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 on 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 nerve rendering them somewhat more susceptible to , such as compression Mesencephalon - tegmentum

 The oculomotor nucleus is located slightly ventrolateral 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 fibers in the medial longitudinal fasciculus (MLF). These function with conjugate movement of the eyes. Mesencephalon - tegmentum

 trochlear nucleus is also located in the tegmentum  although further caudal than the oculomotor nucleus, it is in the same column line  afferents to the both the oculomotor and trochlear nuclei include mostly ipsilateral vestibular fibers in the medial longitudinal fasiculus Mesencephalon - tegmentum

 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  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  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 had unipolar neurons, is the location of proprioceptive nerve cell bodies 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 . Mesencephalon - tegmentum

 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 (voluntary control of )  the trigeminal (conscious sensory pathway from head including eyes) Mesencephalon

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri Metencephalon – cerebellum and pons

 Dorsal metencephalon (cerebellum) is responsible for controlling the rate, range and force of voluntary movements. It does not initiate movements  Ventral metencephalon (pons) conveys information about movement from the cerebrum to the cerebellum  Ascending sensory and descending motor pathways pass through the pons  The cerebellum is going to provide coordination between vision, vestibular, and input. Metencephalon Metencephalon – cerebellum and pons

 Pons is part of the brain stem  Many structures or pathways which pass through the mesencephalon also pass through the pons:  the principal tract and nucleus of CN V, reticular formation, MLF, , and continuation of crus cerebri as the longitudinal fibers of the pons (corticopontocerebellar tract).  The corticopontocerebellar fibers traveling in longitudinal fibers of the pons synapse in the pontine nuclei surrounding the fibers. The second order neuron cross the midline and enter the middle cerebellar peducle (brachium pontis). This particular pathway is part of the menace response. Metencephalon – cerebellum and pons

 The MLF includes fibers ascending from the vestibular nuclei to the (abducens), oculomotor and trochlear nuclei  The tectospinal tract and tectotegmentospinal tract (mesencephalon) pass through this region of the brain  The spinotectal tract ascending from the spinal cord will pass through this area to the tectum Myelencephalon – medulla oblongata

 the most caudal division of the brain blending gradually with the spinal cord  has centers that regulate many autonomic functions, helps to set the level of , and adjusts for posture and movement  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. Myelencephalon Myelencephalon – medulla oblongata

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  in domestic , 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 trochear nuclei (also a source of GSE fibers)  afferent fibers to the abducens nucleus include crossed medial vestibular nucleus innervation for conjugate eye movements  efferents exit the brain stem ventrolaterally to innervate the and retractor bulbi muscles, if present Myelencephalon – medulla oblongata

 The facial nucleus, particularly the lateral area:  is important for eyelid muscle innervation  afferents to the facial nucleus include cerebellar projections and interneurons from the nucleus of spinal tract of V  is involved as the efferent arm of the corneal and palpebral reflexes and menace response  this is a SVE nucleus located in a ventrolateral position in the rostral myelencephalon at the edge of the reticular formation  as with 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 Myelencephalon – medulla oblongata

 parasympathetic nucleus of CN VII (rostral or superior salivatory nucleus) is associated with autonomic innervation of the lacrimal gland  afferent fibers to this nucleus would include the hypothalamus  efferent fibers project via the “sensory” root of CN VII, then via the and the petrosal nerves to the where the preganglionic fibers synapse  postganglionic fibers travel with the ophthalmic or maxillary branches to the lacrimal gland  remember that the VIIth and VIIIth cranial nerves enter the internal acoustic meatus together before separating to the inner and the Myelencephalon – medulla oblongata

 The following tracts are passing through this area:  Tectospinal tract  Tectotegmentospinal tract  Spinotectal tract Spinal Cord

 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 and then to the cortex Spinal Cord

 the (medial) tectospinal tract is within the MLF located in the ventral funiculus of the cord  its origin is the ipsilateral rostral colliculus  its termination is the ventral horn of the cervical spinal cord  this tract is involved in movements of the head and neck in response to visual stimuli Spinal Cord

 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  its termination is the T1-T3 intermediate horn  this tract is the UMN tract for sympathetic innervation of the eye  the lateral horn (intermediolateral, zona intermedia) of T1-T3 segments is the location of the preganglionic sympathetic nerve cell bodies for innervation of the eye  the preganglionic exits the spinal cord via the ventral root. Cranial Nerves

 cranial nerves exit the brain ventral to lateral in position  they are primarily ipsilateral in peripheral projection  CN IV is an exception to both of these rules  it exits from the dorsal aspect of the brain stem at the mesencephalon/pons border area and is a complete contralateral peripheral projection  as with most cranial nerves, there are extensive anastomoses peripherally Cranial Nerves  Efferent (motor) nerves are divided into General (general somatic efferent) and Visceral (general visceral efferent and special visceral efferent)  GSE – motor to skeletal muscles including extraocular muscles  GVE – motor to muscle, smooth muscle, glands. In the cranial nerves, these impulses are part of the parasympathetic nervous system  SVE – motor to skeletal muscles that develop in branchial arches of the embryo (, , middle ear) Cranial Nerves

 Afferent (sensory) nerves are divided into General (general somatic afferent and special somatic afferent) and Visceral (general visceral afferent and special visceral afferent)  GSA – sensory fibers related to sensations of touch, , temperature, pressure and proprioception  SSA – sensory fibers related to the senses of vision and  GVA – sensory fibers related to viscera and vessels  SVA – sensory fibers related to the senses of smell and Cranial Nerves

Nerve Dog Cat Horse Ruminant Pig

CN I cribiform plate same same same same

CN II 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 (rostral alar same same foramen orbitorotundum foramen orbitorotundum foramen)

CN V – mandibular oval foramen same oval notch of oval foramen oval foramen foramen lacerum

CN VI orbital fissure same same foramen orbitorotundum foramen orbitorotundum Cranial Nerves

Nerve Dog Cat Horse Ruminant Pig

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

CN VIII internal acoustic meatus same same same same

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 Orbital Foramina

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 1; Structure and Function of the Eye by Paul Miller Cranial Nerve II

 afferent nerve of PLR, menace response and conscious vision pathway  technically an extension of the diencephalon. This is the reason that the continue external to the along it  functionally, these are SSA fibers with nerve cell bodies in ganglion cell layer of retina Cranial Nerve III

 efferent nerve of PLR  GSE fibers from oculomotor nucleus to majority of extraocular muscles: dorsal, medial and ventral rectus, ventral oblique and levator palpebrae superioris  GVE fibers from parasympathetic nucleus of CN III to and ciliary muscles  postganglionic parasympathetic nerve cell bodies are located in the ciliary ganglion  there is a difference of formation of in the dog and cat Parasympathetic (oculomotor) innervation to iris sphincter and muscle

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri Cranial Nerve IV

 GSE fibers from trochlear nucleus travel to the contralateral dorsal oblique muscle Cranial Nerve V

 afferent nerve of corneal and palpebral reflexes  each branch has separate foramen of exit in the dog and cat  only the maxillary and ophthalmic branches are associated with ocular functions  although the trigeminal nerve is mixed in function (SVE, GSA), these 2 branches have only GSA fibers. They will be joined by sympathetic and parasympathetic fibers of other sources.  will actually split into further branches prior to entering the periorbital: frontal, nasociliary, long ciliary, and infratrochlear nerves. Sympathetic fibers from the ciliary cervical ganglion will join the will supply some branches to the orbital region: zygomatic and lacrimal nerve. Parasympathetic fibers from the pterygopalatine ganglion (CN VII) will join the lacrimal nerve Cranial Nerve VI

 contains GSE fibers from abducens nucleus  innervates the lateral rectus muscle and the retractor bulbi muscle, in those species which have one  innervates the striated muscle strip to the 3rd eyelid of the cat Cranial Nerve VII

 efferent nerve of corneal & palpebral reflexes and the menace response  a mixed nerve in function  SVA, GVA, GSA, GVE from parasympathetic nucleus of CN VII (rostral salivary nucleus)  SVE fibers from facial nucleus  GVE and SVE fibers are of concern for ocular functions Cranial Nerve VII

 “sensory” root contains GVE fibers from the parasympathetic nucleus of CN VII  these fibers enter the intermediate nerve and join the motor root which passed through the internal acoustic meatus and the facial canal  at the genu of the nerve, the parasympathetic fibers leave in the major (greater) petrosal nerve (petrosal canal). This major petrosal nerve is joined by the deep petrosal nerve and is then called the nerve of the pterygoid canal. This nerve exits the skull through the round foramen and synapses at the pterygopalatine ganglion located ventral to the periorbital and deep to the maxillary nerve  postganglionic fibers then travel to the lacrimal nerve and innervate the lacrimal gland Cranial Nerve VII

 The is sensory in function and as such is not directly involved with ocular function.  The major part of the continues through the facial canal and exits the skull via the stylomastoid foramen and then splits into the auriculopalpebral nerve, dorsal buccal nerve, and ventral buccal nerve.  The auriculopalpebral nerve further divides into auricular and palpebral branches.  The latter (palpebral) innervates the orbicularis oculi, retractor anguli oculi lateralis and levator anguli oculi muscles. Cranial Nerve VIII

 has indirect influence over ocular movements via its connection to the cerebellum and vestibular nuclei. Myelencephalon – Vestibular Nuclei

 Located just lateral to at level of CN VIII

CN VIII Vestibular nuclei

CN V

CN VIII Cranial Nerves

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 1; Structure and Function of the Eye by Paul Miller C 2 through C 4 SPINAL NERVES

 Sensory fibers from area of eyelids travel ipsilaterally to the cervical spinal cord with nerve cell bodies in cervical spinal ganglia (dorsal root ganglia) T 1 through T 3 SPINAL NERVES

 Autonomic innervation  Consists of higher centers in hypothalamus, midbrain, pons and medulla  Autonomic innervation is composed of 2 neurons between the CNS and the organ innervated  Preganglionic and postganglionic  Sympathetic and parasympathetic T 1 through T 3 SPINAL NERVES

 preganglionic axon exits the cord via the ventral root, enters the 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  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 . Autonomic Innervation

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri T 1 through T 3 SPINAL NERVES

 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 just prior to entering the globe. The long ciliary nerves contain 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, short ciliary nerves contain parasympathetic, sympathetic and sensory fibers. Autonomic Innervation

Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri

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