NEUROANATOMY OF THE VISUAL PATHWAYS Magrane Basic Science Course June 2018

**It is important for me to acknowledge the extensive input from Dr. Lola Hudson and Dr. Karen Munana to both these notes and the Power Point presentation.**

CENTRAL NERVOUS SYSTEM

All 5 divisions of the brain are involved with ocular function and/or adnexa in either conscious pathways or reflex pathways. The 5 divisions are the telencephalon (cerebral hemispheres), diencephalon (thalamus, hypothalamus), mesencephalon (midbrain), metencephalon (cerebellum and pons) and myelencephalon (medulla oblongata). The telencephalon and diencephalon together can be referred to as the prosencephalon (forebrain), and the metencephalon and myelencephalon can be referred to as the rhombencephalon (hindbrain). These 3 larger divisions (prosencephalon, mesencephalon and rhombencephalon) relate to the embryology of the brain from three original neural tube vesicles.

The more cranial spinal cord is also involved with ocular function through sensory innervation into the cervical spinal cord and through sympathetic autonomic function particularly in the cranial thoracic spinal cord.

PERIPHERAL NERVOUS SYSTEM

The majority of the 12 pairs of cranial (CN) have some involvement with ocular function: CNs II, III, IV, V (ophthalmic and maxillary branches), VI, VII and VIII including appropriate sensory and autonomic ganglia of these CNs.

TELENCEPHALON (cerebrum=cerebral hemispheres=cerebral cortex)

Telencephalon is the site of awareness, initiation of voluntary movements and perception of stimuli. 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 in ocular and eyelid function.

The occipital lobe occupies the caudal one-third of the cerebral hemispheres. It borders the parietal lobe dorsorostrally and the temporal lobe laterally. Medially, the right and left occipital lobes meet between the cerebral hemispheres across the longitudinal fissure. Caudally, the occipital lobes are adjacent to the osseous tentorium and the tentorium cerebelli, which both lie in the transverse fissure (between the cerebrum and cerebellum). Unlike the border between frontal and parietal lobes, there is not a definite line or sulcus to demarcate the borders of the occipital lobe and different sources will include greater or lesser areas. However, everyone includes the visual cortex in the occipital lobe.

In domestic animals (dog), the occipital lobe includes parts of the marginal, ectomarginal, caudal suprasylvian, and caudal composite gyri. It also includes the splenial and occipital gyri medially. Endomarginal and ectomarginal gyri and sulci are part of the parietal lobe (rostral 2/3s) and the occipital lobe (caudal 1/3).

The primary visual area, the main recipient of dorsal lateral geniculate nucleus (DLGN) output, occupies the region known as Brodmann’s area 17. This area is a histologically defined region in the occipital lobe. In all mammalian species mapped so far, this area 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 gyrus 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. This area has also been called the striate cortex. Adjacent to the striate cortex is the parastriate cortex and then next is the peristriate cortex.

Brodmann’s area 17 = striate = visual I Brodmann’s area 18 = parastriate = visual II Brodmann’s area 19 = peristriate = visual III

Areas 18 and 19 together may be referred to as the extrastriate cortex or as visual association area.

The extent of visual resolution in a species is reflected not only in the surface area of the striate cortex and the magnification factor (a term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis) but also in the number of visually responsive areas. Multiple visual areas have been discovered in almost every species studied. Three cortical areas have been identified in the hedgehog (a primitive insectivore) and four such areas in the mouse. In the cat, there are over a dozen visual areas. Three of them, visual I, visual II, and visual III (corresponding to Brodmann’s 17, 18 and 19) occupy most of the feline occipital lobe. Each of these contains one representation of the visual hemifield. More than 30 such areas have been identified in nonhuman primates on the basis of behavioral studies. It is assumed that these extraoccipital areas deal with “higher” visual processing such as shape and location discrimination as well as facial recognition.

The point of central vision is located in the striate cortex. The point of central vision in the striate cortex varies in position between species and perhaps between breeds. The stereotaxic coordinates for the representation 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 line 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. Knowledge of these coordinates is important when recording visual evoked potentials.

Cells of the primary visual cortex, visual I, are arranged in 6 layers, which are defined on the basis of cytoarchitecture and myelination patterns of the cells. Layer 4 is heavily myelinated. It is in this layer that incoming LGN axons synapse with cortical neurons. Magnocellular (stereopsis, movement, directionality and contrast sensitivity) projections synapse in layer 4Ca and parvocellular (spatial resolution and color sensitivity) projections synapse in layer 4Cb (maintains the segregation and dual processing of visual information that has characterized the visual system up to this point). Layers 2 and 3 contain excitatory neurons that project to other cortical areas, while layers 1 and 2 receive feedback input from these same extrastriate visual areas. Similar descending feedback loops project from layers 5 and 6 back to the DLGN. Most of the neurons are GABAnergic, inhibitory neurons that do not project outside of V I and are devoted to processing of the signal in the striate cortex. Only a minority of the cells are excitatory, spiny (stellate or pyramidal) neurons that project outside of area 17.

The basic cortical unit that processes an incoming signal is termed a column, which descends through all six layers of the cortex. As in the DLGN, 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. There is no difference in size between columns serving central and peripheral retina; rather, more columns are used to process visual input from the central retina.

Area visual I receives input from the DLGN. This input consists of the entire contralateral visual hemifield as projected on both retinas. Again, 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. The 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). Medial movement (away from the lateral gyrus) on the surface of the striate cortex represents peripheral movement (away from the area centralis) in the visual field.

Large cortical areas are devoted to processing signals originating from the area centralis. Magnification factor is a 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 visual field is projected onto a similar-sized retinal area, regardless of whether it is peripheral or central. The increased resolution and processing achieved by the central retina is obtained by increasing the density of ganglion 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. This larger area results in magnification of its representation. In the 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. In the 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.

Afferent connections to visual areas come from the lateral geniculate nucleus via optic radiation of internal capsule (main white matter 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 motor cortex at frontal/parietal lobe), the opposite hemisphere via corpus callosum and the brain stem (to lateral geniculate nucleus and rostral colliculus, pontine nuclei and reticular formation).

DIENCEPHALON (thalamus, hypothalamus)

The thalamus receives, processes and relays to the cerebral cortex 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 chiasm, optic tract, lateral geniculate body (LGB)/lateral geniculate nucleus (LGN), internal capsule and hypothalamus.

The optic chiasm is at the base of the hypothalamus. It is specifically located at the rostroventral surface of the brainstem and demarcates the rostral level of the diencephalon. It is closely associated with the 3rd ventricle, hypothalamus, and to some extent the pituitary gland depending on the species.

The arrangement of retinal ganglion cells within the optic 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 ON, while those in the retinal periphery are in the periphery of the ON. This precise arrangement is needed for subsequent accurate projection of the visual field in both the dorsal lateral geniculate nucleus (DLGN) and the visual cortex.

The 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 eyes and no overlap between the visual fields of the two eyes exhibit complete decussation 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.

most birds, many amphibians and reptiles, 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 human beings: 50% crossover

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 humans (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. 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 binocular vision.

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 cerebral peduncle. 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. Before reaching the LGN, 20% of the fibers leave the tracts and enter the pretectal area. 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 neuron in the visual system which then passes without further synapse to the visual cortex. The DLGN receives input from the contralateral visual hemifield of both eyes and outputs to the visual cortex. In most species with a ventral lateral geniculate nucleus (VLGN) and a DLGN, the optic tract terminates in the DLGN. Primates do not have a VLGN, so the optic tract synapses in the lateral geniculate nucleus. 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.

There is conflicting evidence about segregation of fiber size within the tract although 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.

More specifically there are connections from the optic tract to: 1. hypothalamus (controversial for carnivores) 2. LGN (actually more dorsal in position in domestic animals). It is estimated that 80% of optic tract fibers terminate in the LGN. 3. pretectal nuclei via brachium of rostral colliculus and then pass to pretectum 4. rostral colliculus via brachium of rostral colliculus 5. accessory nucleus of optic tract

The major function of the LGN is as a complex relay nucleus on the conscious vision pathway. The 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.

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 receptive field in both eyes. This provides the basis for the merging of visual information that occurs in the cortex.

The actual synapses between the ganglion cell axons and the dendrites of the thalamic cells take place in a structure termed the synaptic glomerulus. The thalamic cells may be of 2 types: interneurons (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.

Hypothalamus

The hypothalamus is located on the ventral aspect of the diencephalon and is 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 motor neuron (UMN) source for the . In relation to vision, the hypothalamus receives afferent fibers from the optic tract and the periaqueductal gray matter (mesencephalon). Efferent fibers from the hypothalamus include projections to the periaqueductal gray matter and the tectum.

MESENCEPHALON (mid-brain)

The mesencephalon or mid-brain participates in the control of motor functions, in coordination of visual and auditory reflexes and in the processing of sensory information (auditory). The mesencephalon or midbrain is grossly divided into the tectum and the 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. 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 reflex. Pretectal nucleus is in the 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. The rostral colliculi 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. They 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 senses including vision.

The tectum is the location of UMN sympathetic neurons involved in constriction of the pupils. The tectotegmentospinal tract may actually be a misnomer. 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 Tectospinal tract = medial tectospinal tract: orientation of the head Spinotectal tract: movements of neck (head and eyes) towards movements

The pretectal area is 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 pupillary light reflex and accommodation 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 oculomotor nucleus). There are some uncrossed fibers between ipsilateral pretectal and parasympathetic nuclei.

Tegmentum

The tegmentum is the ventral mesencephalon. Some definitions exclude the crus cerebri. This area is the location of several lower motor neuron (LMN) nuclei associated with ocular functions. In addition, there are pathways passing through this region, such as the tectotegmentospinal tract.

The locations of the motor nerve cell bodies, 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 General Somatic Efferent (GSE), General Visceral Efferent (GVE), and Special Visceral Efferent (SVE) in type. All are located ventral to whatever part of the ventricular system is present. 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 stem. Examples of each type include:

GVE-parasympathetic nucleus of CN III GSE-oculomotor, trochlear and abducens nuclei SVE-facial nucleus

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 the 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

The 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. The 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 injury, such as compression.

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 vestibular system fibers in the medial longitudinal fasciculus (MLF). 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. Afferents to the both the oculomotor and trochlear nuclei include mostly ipsilateral vestibular fibers in the MLF.

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 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 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 (voluntary control of eye movement) and the trigeminal lemniscus (conscious sensory pathway from head including eyes).

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 proprioception input.

The pons is part of the brain stem. Many structures or pathways which pass through the mesencephalon also pass through the pons. This includes the principal tract and nucleus of CN V, reticular formation, MLF, trigeminal lemniscus, 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.

The MLF includes fibers ascending from the vestibular nuclei to the (abducens), oculomotor and trochlear nuclei.

The tectospinal tract and tectotegmentospinal tract have already been discussed under mesencephalon and 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 myelencephalon has centers that regulate many autonomic functions, helps to set the level of arousal, and adjusts muscle tone for posture and movement. The myelencephalon is the most caudal division of the brain blending gradually with the 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.

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 trochear nuclei as it is also a source of GSE fibers. In this case, efferents exit the brain stem ventrolaterally to innervate the lateral rectus muscle and retractor bulbi muscles, 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 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.

Remember that the VIIth and VIIIth cranial nerves enter the internal acoustic meatus together before separating to the inner ear and the facial canal.

Afferents to the facial nucleus include cerebellar projections and interneurons from the nucleus of spinal tract of V.

The parasympathetic nucleus of CN VII (rostral or superior salivatory nucleus) is associated with autonomic innervation of the . Afferent fibers to this nucleus would include the hypothalamus. The efferent fibers project via the “sensory” root of CN VII, then via the intermediate nerve and the petrosal nerves to the where the preganglionic fibers synapse. Postganglionic fibers travel with the ophthalmic or maxillary branches to the lacrimal gland.

The following tracts are passing through this area:

Tectospinal tract Tectotegmentospinal tract Spinotectal tract

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 and then to the cortex.

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 T1-T3 intermediate horn. This 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 axon 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.

With most cranial nerves, there are extensive anastomoses peripherally.

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 heart 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 (pharynx, larynx, middle ear).

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, pain, temperature, pressure and proprioception. SSA – sensory fibers related to the senses of vision and hearing. GVA – sensory fibers related to viscera and blood vessels. SVA – sensory fibers related to the senses of smell and taste. OUTLINE OF FORAMINA OF EXIT OF CRANIAL NERVES IN DIFFERENT SPECIES

Nerve Dog Cat Horse Ruminant Pig CN I cribiform plate same same same same CN II optic canal 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 same same foramen foramen maxillary (rostral alar orbitorotundum orbitorotundum foramen) CN V – oval foramen same oval oval foramen oval foramen mandibular notch of foramen lacerum CN VI orbital fissure same same foramen foramen orbitorotundum orbitorotundum CN VII internal acoustic same same same same meatus, facial canal, stylomastoid foramen CN VIII internal acoustic same same same same meatus CN IX jugular foramen same same same same then tympanooccipital fissure CN X jugular foramen same same same same then tympanooccipital fissure CN XI jugular foramen same same same same then tympanooccipital fissure CN XII hypoglossal same same same same canal

CRANIAL NERVE II

Afferent nerve of PLR, menace response and conscious vision pathway. This nerve is technically an extension of the diencephalon. This is the reason that the meninges continue external to the skull 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 iris 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.

CRANIAL NERVE IV

The 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.

The 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 . The will supply some branches to the orbital region: zygomatic and . Parasympathetic fibers from the pterygopalatine ganglion (CN VII) will join the lacrimal nerve.

CRANIAL NERVE VI

This nerve contains GSE fibers from abducens nucleus. In addition to innervating the lateral rectus muscle and the retractor bulbi muscle, in those species which have one, it innervates the striated muscle strip to the 3rd eyelid of the cat.

CRANIAL NERVE VII

This is the efferent nerve of corneal and palpebral reflexes and the menace response. It is a mixed nerve in function: SVA, GVA, GSA, GVE from parasympathetic nucleus of CN VII (rostral salivary 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 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.

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 nerve, dorsal , and ventral buccal nerve. The auriculopalpebral nerve further divides into auricular and palpebral branches. The latter innervates the orbicularis oculi, retractor anguli oculi lateralis and levator anguli oculi muscles.

CRANIAL NERVE VIII

This nerve has indirect influence over ocular movements via its connection to the cerebellum and vestibular nuclei.

CERVICAL 2 through CERVICAL 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).

THORACIC 1 through THORACIC 3 SPINAL NERVES

Autonomic innervation consists of higher centers in hypothalamus, midbrain, pons and medulla. Autonomic innervation is composed of two neurons between the CNS and the organ innervated. The two neurons are labeled as preganglionic and postganglionic. The autonomic innervation also consists of both sympathetic and parasympathetic systems. The sympathetic system (thoracolumbar) has cell bodies of preganglionic neurons in the intermediate gray column of the spinal cord from approximately the first thoracic to the fifth lumbar spinal cord segment. The parasympathetic system (craniosacral) has cell bodies of preganglionic neurons in sacral segments of the sponal cord and in the nuclei of the brainstem associated with cranial nerves III, VII, IX and XI.

The preganglionic axon exits the cord via the ventral root, enters the spinal nerve for a short distance, then travels in the to the thoracic (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. Upon reaching the cranial (superior) cervical ganglion, the preganglionic axon synapses. The postganglionic axon travels through the middle ear cavity with the (if the species has one), to join with branches of CN V in the orbit. 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) 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 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.

Sources of information (large amounts of information have been directly taken from these sources and merged together to make this document).

1. Neuroanatomy of the Visual Pathways; Dr. Hudson; Magrane Basic Science Course, June 2016. 2. Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 4; Optics and Physiology of Vision by Ron Ofri. 3. Veterinary Ophthalmology; Gelatt; 4th Edition; Chapter 29; Neuro-ophthalmic Anatomy by Merav Shamir and Ron Ofri. 4. Slatter’s Fundamentals of Veterinary Ophthalmology; 4th Edition; Chapter 16; Neuroophthalmology by Ron Ofri. 5. Veterinary Neurobiology Notes; Colorado State University