neuroanatomy of the visual pathways€¦ · 05/06/2018 · optic disk and fibers from the inferior...

NEUROANATOMY OF THE VISUAL PATHWAYS

Magrane Basic Science Course

June 2018

Central Nervous System All 5 divisions involved in ocular function

and/or adnexa Telencephalon

Cerebral hemispheres Diencephalon

Thalamus and hypothalamus Mesencephalon

Midbrain Metencephalon

Cerebellum and pons Myelencephalon

Medulla oblongata

Brain Divisions (midsagittal plane)Telencephalon (cerebrum)

Cerebellum (dorsal metencephalon)

Myelencephalon(medulla oblongata)

DiencephalonMesencephalon(midbrain) Pons

(ventral metencephalon)

Brain Divisions on MRI (midsagittal)Telencephalon (cerebrum)

Cerebellum (dorsal metencephalon)

Myelencephalon(medulla oblongata)

Foramen magnum –junction of brain and spinal cord

Diencephalon

Mesencephalon(midbrain) Pons

(ventral metencephalon)

Telencephalon Site of awareness, initiation of voluntary movements and

perception stimuli Functions in perception and integration of vision as well as

voluntary control of eye movements

Cerebral Lobes

Piriform lobe

Sensory Processing Vision

Sensory ProcessingSomatosensory

Sensory ProcessingOlfaction

Sensory ProcessingAudition

Behavior

Behavior

Memory Learning

Intellect

Motor function

Occipital lobe

Temporal lobeParietal lobe

Frontal lobe

Occipital Lobe Caudal part of cerebral hemisphere Not well delineated Contains visual cortex

Visual perception

Occipital Lobe Occipital lobe occupies the caudal 1/3 of cerebral

hemispheres Right and left occipital lobes meet between the

cerebral hemispheres across the longitudinal fissure There is no definite line or sulcus to demarcate the

borders of the occipital lobe Dog: includes parts of the marginal, ectomarginal,

caudal suprasylvian, caudal composite, splenial and occipital gyri.

Dog: endomarginal and ectomarginal gyri and sulci are part of the parietal lobe (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

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 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

Brodmann’s area 18 = parastriate = visual II Brodmann’s area 19 = peristriate = visual III Area 18 & 19 – may be referred to as extrastriate

cortex or as visual association area

Occipital Lobe

extent of visual resolution in a species 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; Optics and Physiology of Vision by Ron Ofri

Occipital Lobe cells of the primary visual cortex, visual I, are arranged in 6

layers layer 4 is heavily myelinated; in this layer the incoming LGN

axons synapse with cortical neurons magnocellular (stereopsis, movement, directionality and

contrast sensitivity) projections synapse in layer 4Ca parvocellular (spatial resolution and color 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 retina 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 retinas 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


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 visual field 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 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 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 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 the brain stem (to LGN and rostral colliculus, pontine nuclei and

reticular formation)

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

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

Diencephalon

Diencephalon Landmarks -Ventral View Optic chiasm Mamillary bodies Pituitary (hypophysis)

Usually detached Infundibulum

Opening communicates with third ventricle

Hypothalamussurrounds infundibulum

Optic chiasm

Hypothalamus

Diencephalon – Visual Pathway Optic nerve (CN II) Optic chiasm

Majority of optic n. fibers cross midline

Optic tract Projection from chiasm

toward brain Lateral to remainder of

diencephalon Travels in caudodorsal

direction to lateral geniculate nucleus

Lateral geniculate nucleus sends input to occipital cortex in telencephalon

Optic nerve

Optic chiasm

Optic tract

Optic tractOptic chiasm

Lateral geniculate nucleus

Diencephalon

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


Diencephalon - chiasm optic chiasm

is at the base of the hypothalamus it is specifically located at the rostroventral surface of the

brainstem demarcates the rostral level of the diencephalon is closely associated with the 3rd ventricle, hypothalamus,

and to some extent the pituitary gland 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 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

Diencephalon - chiasm

percentage of optic nerve fibers crossing the midline in different species varies widely 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

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 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

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 binocular vision

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


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 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.

Diencephalon – optic tract/LGN 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

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 synapses 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 receptive field 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


Diencephalon – LGN/optic radiations 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.

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 motor neuron (UMN) source for the autonomic nervous system

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 - midbrain participates in the control of motor functions, in

coordination of visual and auditory reflexes and in the processing of sensory information (auditory)

grossly divided into the tectum and the tegmentum by the line of the sulcus limitans 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 reflex. Pretectal nucleus is in the mesencephalon

Mesencephalon

Mesencephalon – Cranial Nerves

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

Trochlear nerve (CN IV) exits on the dorsal surface of the mesencephalon

Metencephalon

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 senses 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 pupils 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

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 & 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

Mesencephalon - tegmentum

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

Mesencephalon - tegmentum 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)

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


the LMN nuclei found here 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

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 ciliary ganglion 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.


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 trigeminal nerve

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 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) the trigeminal lemniscus (conscious sensory pathway from

head including eyes)


Mesencephalon

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.

Metencephalon


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, trigeminal lemniscus, and continuation of crus cerebri as the longitudinal fibers of the pons (corticopontocerebellartract).

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 (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 arousal, and adjusts muscle tone 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 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 (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

lateral rectus muscle and retractor bulbi muscles, if present


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


parasympathetic nucleus of CN VII (rostral or superior salivatorynucleus) 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 intermediate nerve and the petrosal nerves to the pterygopalatine ganglion 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 ear and the facial canal


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 lateral funiculus 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 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

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 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)

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, 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

Cranial Nerves

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 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 foramen)

same same foramen orbitorotundum foramen orbitorotundum

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

oval foramen oval foramen

CN VI orbital fissure same same foramen orbitorotundum foramen orbitorotundum

Cranial NervesNerve Dog Cat Horse Ruminant Pig

CN VII internal acoustic meatus, facial canal, stylomastoid foramen

same same same same

CN VIII internal acoustic meatus same same same same

CN IX jugular foramen then tympanooccipital fissure

same same same same

CN X jugular foramen then tympanooccipital fissure

same same same same

CN XI jugular foramen then tympanooccipital fissure

same same same same

CN XII hypoglossal canal same same same same

Orbital Foramina


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 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 short ciliary nerves in

the dog and cat

Parasympathetic (oculomotor) innervation to iris sphincter and ciliary body muscle


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.

ophthalmic nerve 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 nasociliary nerve

maxillary nerve 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 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 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 fourth

ventricle at level of CN VIII

Vestibular nuclei

CN VIII

CN VIII

CN V

Cranial Nerves


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 spinal nerve for a short distance, then travels in the ramus communicans to the thoracic sympathetic trunk (but no synapse yet)

traveling cranially in the sympathetic trunk, the axon(s) pass through the cervicothoracic (stellate) ganglion, the ansa subclavia, the middle cervical ganglion and enter the sympathetic part of the vagosympathetic trunk

upon reaching the cranial (superior) cervical ganglion, the preganglionic axon synapses

the postganglionic axon travels through the middle ear cavity with the internal carotid artery (if the species has one), to join with branches of CN V in the orbit.


Autonomic Innervation

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 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.

Autonomic Innervation


neuroanatomy of the visual pathways€¦ · 05/06/2018 · optic disk and fibers from the inferior...

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