Neuro-opthalmology and Vision Pathway

The globe and adnexa are innervated by what nerves?

-cranial nerves II-VIII and X

-sympathetic and parasympathetic

Cranial nerve II

optic nerve

optic nerve (CN II)

made up of ganglion cell axons from the retina; contains both visual and pupillary motor fibers

Cranial nerve III

oculomotor nerve

oculomotor nerve (CN III)

-provides parasympathetic motor fibers to the pupil

-innervates levator palpebrae superiors

-innervates dorsal rectus, ventral rectus, medial rectus, and inferior/ventral oblique muscles

Cranial nerve IV

trochlear nerve

trochlear nerve (CN IV)

motor innervation to superior/dorsal oblique muscle

Cranial nerve V

trigeminal nerve

trigeminal nerve (CN V)

has three branches:

1.) ophthalmic

2.) maxillary

3.) mandibular

ophthalmic branch of trigeminal nerve

sensory fibers from cornea, conjunctiva, and superior eyelid as well as afferent arm of oculocardiac reflex

maxillary branch of trigeminal nerve

sensory fibers to skin of lower eyelid

mandibular branch of trigeminal nerve

motor fibers to muscles of mastication

Cranial Nerve VI

abducens nerve

abducens nerve (CN VI)

motor fibers to lateral rectus and retractor bulbi muscle

Cranial nerve VII

facial nerve

facial nerve (CN VII)

motor fibers to muscles of facial expression; closure of eyelids (orbicularis oris muscle); carries parasympathetic fibers to lacrimal gland

Cranial nerve VIII

vestibulocochlear nerve

vestibulocochlear nerve (CN VIII)

afferent component of ocular position; controls extra ocular position so eyes can remain fixed on an object while head turns

Cranial nerve X

vagus nerve

vagus nerve (CN X)

efferent arm of oculocardiac reflux; mediated by trigeminal nerve

oculocardiac reflux

pressure of the extra ocular muscles leads to sudden decrease in heart rate; causes bradycardia when eye is being operated on

Which cranial nerves are efferent?

oculomotor nerve (CN III), trochlear nerve (CN IV), and abducens nerve (CN VI)

Which cranial nerves are afferent?

optic nerve (CN II) and vestibulocochlear nerve (CN VIII)

Which cranial nerves are both efferent and afferent?

Trigeminal Nerve (CN V), Facial Nerve (CN VII), and Vagus Nerve (CN X)

Sympathetic innervation pathway

a three neuron pathway that causes a sympathetic, or fight or flight, response in the eye

Three neurons of the sympathetic innervation pathway

1.) first neuron

2.) second neuron

3.) third neuron

first neuron of sympathetic innervation pathway

midbrain to T1-T3

second neuron of sympathetic innervation pathway

thorax to the neck

third neuron of sympathetic innervation pathway

cranial cervical ganglion to the eye

Pathway of the sympathetic innervation pathway

1.) sympathetic nerve originates in midbrain, and travels along the lateral aspect of spinal cord to T1-T3

2.) branches of the sympathetic nerve exit from T1-T3 at ventral nerve roots and join the thoracic sympathetic trunk

3.) then travel as second order neurons to the cranial central ganglion where they synapse high in the neck

4.) then third order neurons travel up to the eye

Four muscles sympathetic nerves innervate

1.) dilator muscle of pupil

2.) mueller's muscle

3.) smooth muscle of periorbital fascia

4.) smooth muscle of blood vessels

dilator muscle of pupil

causes mydriasis (pupil dilation)

mueller's muscle

elevation of superior eyelid

smooth muscle of periorbital fascia

pushes globe position forward

smooth muscle of blood vessels

constricts blood vessels

Overall response of sympathetic nervous system

eyes wide open, pupils dilated, globe forward, vasoconstriction

Parasympathetic innervation pathway

causes pupil constriction and tear production

Parasympathetic pupillary fibers

originate in the parasympathetic nucleus of CN III and travel with CN III to ciliary ganglion

Parasympathetic fibers travel with...

CN VII and provide efferent arm of lacrimation

Horner's syndrome

caused by damage along the sympathetic nerve pathway; characterized by droopy eyelids, globe sunk back in, small pupil, and third eyelid elevated

What will be the difference between Horner's syndrome and an inflamed eye?

Horner's syndrome will have no ocular pain!

Palpebral/blink reflex

tap upper and lower eyelids and medial and lateral canthus

What cranial nerves does the Palpebral/blink reflex test?

-CN V (ophthalmic and maaxiallry branches)

-CN VII (orbicularis oris muscle)

Corneal reflex

touch wisp of cotton from a swap to the cornea and gradually increase pressure

What cranial nerves does the Corneal reflex test?

-CN V (opthalmic branch)

-CN VI (retractor bulbi muscle)

-CN VII (orbicularis oris muscle)

Pupillary light reflex

stimulate an eye with pen light and examine both eyes for a response, then repeat on the other side

What cranial nerves does the Pupillary light reflex test?

-parasympathetic fibers of Edinger-Westphal nucleus with CN III

direct PLR

eye being stimulated constricts

consensual PLR

contralateral pupil constricts

Swinging flashlight test

perform PLR on one eye and swing light rapidly to opposite eye to assess consensual response

Dazzle reflex

uses bright light source to quickly illuminate eye; animal should blink

The dazzle reflex indicates _______ not ___________

light perception; not vision!

Medial visual field

lateral retina and optic nerve fibers; things you see on the medial eyeball are projected onto the lateral retinal and optic nerve fibers

Does the lateral optic nerve desscuate?

no!

Lateral visual field

medial retina and optic nerve fibers; things you see on the lateral eyeball are projected onto the medial retinal and optic nerve fibers

Does the medial optic nerve desscuate?

yes!

What type of vision is the medial visual field?

binocular vision

What type of vision is the lateral visual field?

peripheral vision

Four tests of vision

1.) menace response

2.) cottonball test

3.) maze test

4.) visual placing

menace response

cover one eye, and draw a closed fist to the open eye; animal should blink and might retract head

What cranial nerves does the menace response test?

CN II and CN VII

cottonball test

cover one eye and drop cottontails in front of opposite eye; watch for tracking or evidence they saw cotton ball

maze test

set up obstacle course and asses animal's navigation in both bright and dim light

visual placing

hold animal in space supported under their chest, then approach a flat surface; animal should raise and extend leg in preparation to stand on surface

Vision pathway

the neural route that transmits visual information from the retina of the eye to the visual cortex of the brain, where images are processed

Three parts of the vision pathway

1.) optical

2.) photochemical

3.) neurologic transmission/processing in brain

Optical portion of vision pathway

light --> cornea --> lens --> retina

light is refracted on cornea, then further refracted after hitting lens onto the retina

The image is flipped ____________ on the retina

upside down

Phototransduction of vision pathway

in the photoreceptor layer of the retina, cells convert photons of light to electrical energy

cones

day vision and acuity

rods

night vision and motion detection

signal transmission of photoreceptors to optic nerve

photoreceptors communicate with bipolar cells, to retinal ganglion cells, to ganglion cell axons, to optic nerve

neurologic transmission/processing in brain of visual pathway

from optic nerve, signal of image is sent to the visual cortex where the image is processed

optic nerve

contains both visual and pupillary motor fibers; therefore, involved in both visual and PLR pathways

The ____________ axons of the optic nerve will decussate

medial

After axons decussate, where do they travel?

travel as the optic tract to the lateral geniculate nucleus

Binocular overlap

field of view seen by both eyes; vision from the medial field

Monocular overlap

peripheral vision seen by one eye; vision from lateral visual field

What is the purpose of decussation?

allows integration of images from each eye

Predators have _________ binocular overlap

Prey animals have __________ binocular overlap

large

small

Complete Visual pathway

retina/optic nerve --> optic chiasm --> optic tracts --> lateral geniculate nucleus --> optic radiations --> visual cortex

Pupillary light reflex

normal constriction of pupils when bright light shines on retina

Two arms of pupillary light reflex

1.) afferent

2.) efferent

afferent arm of Pupillary light reflex

retina/optic nerve --> optic chiasm --> decussation --> optic tract --> pretectal nucleus ---> pretectal nucleus fibers--> central decussation --> synapse with EWN

efferent arm of pupillary light reflex

parasympathetic fibers of EWN travel with CN III to synapse at the ciliary ganglion --> short cilliary nerves --> iris sphincter muscle --> pupil constriction

The afferent arm of the PLR pathway and visual pathway are the same until...

after decussation at the optic chiasm; the PLR pathway will go to the pretectal nucleus, while the visual pathway goes to lateral geniculate body

The efferent arm is or isn't associated with the optic nerve?

is not!

Central decussation

decussation of fibers of PLR pathway after synapsing on pretectal nucleus; why if light is shined on one eye, both eyes will dilate

Lesions of the afferent arm of PLR cause...

absent PLR and blindness

Lesions of from the LGN to visual cortex of visual pathway cause...

normal PLR and blindness

Lesions of efferent arm of PLR cause...

absent PLR but no blindness

When light is shone in eyes during an afferent deficit

affected eye:

unaffected eye:

affected eye: light will not cause PLR in either eye

unaffected eye: light will cause PLR in both eyes

When light is shone in eyes during an efferent deficit

affected eye:

unaffected eye:

affected eye: light will cause opposite eye to constrict

unaffected eye: light will cause only that eye to constrict

With lesions of LGN, optic radiations, or visual cortex, what is the result of PLR testing?

normal PLR but blind