Lecture 34: Visual Pathways

-ganglion cell axons exit retina via optic disk

-axons bundle to form optic chiasm

-optic nerve travels posterior to optic chiasm

-bilateral ganglion axons form optic tract

-travels to many different nuclei

Describe the primary visual pathway:

Retinogeniculostriate Pathway

-primary visual pathway

-sends info from the retina to the LGN to the striate cortex

retino-hypothalamic pathway

-pathway that coordinate structures controlled by circadian rhythms

-sends info from the retina to the hypothalamus

Tectal system

-pathway that coordinates head/eye movement towards the visual targets

-sends info from retina to the superior colliculus

pre-tectal system

-system that coordinates pupillary light reflex

-sends info from retina-->neurons in pretectum-->edinger westphal nucleus

nasal visual field

for each eye individually, the part of the visual field on the same side of the eye as the nose

temporal visual field

Part of the visual field closest to the ears

retinotopic organization

-map established in LGN and maintained in projections to striate cortex

•Posterior - foveal/macular regions

•Anterior - peripheral regions

striate cortex organization:

below

upper visual field projects _____ the calcarine fissure of the occipital lobe

above

lower visual field projects ________ the calcarine fissure of occipital lobe

monocular blindness

Common causes include glaucoma, optic neuritis, elevated intracranial pressure

Binasal Hemianopia

-may be due to ICA aneurysms

-lesion at one side of the optic chiasm

bitemporal hemianaopia

-damage to the optic chiasm, typically asymmetrical loss

-common lesions in this area include pituitary adenoma, meningioma, craniopharyngioma, and hypothalamic glioma

homonymous hemianopia

-lesion to the optic tract and Lateral geniculate nucleus (LGN)

-lesions include infarct to anterior choroidal arteries supplying optic tract, demyelination, or tumors; or lesions to the LGN

superior homonymous quadrantanopia

"Pie in the sky" - Damage to inferior optic radiation; Lesions of the optic radiations include infarcts, tumors, demyelination, trauma, and hemorrhage

inferior homonymous quadrantanopia

"Pie on the floor" - Damage to superior optic radiation; Lesions of the optic radiations include infarcts, tumors, demyelination, trauma, and hemorrhage

damage to cortex with macular sparing

what causes this visual deficit

magnocellular layers

-layers 1 and 2 or LGN

-contain large neuron cell bodies

parvocellular layers

-layers 3-6 of LGN

-contain small cell bodies

Y retinal fibers

-fibers (mostly from rods) that terminate in the Magnocellular layers of the LGN

X retinal fibers

-fibers from cones that terminate in the parvocellular layers of the LGN

the ipsilateral temporal retina

layers 2,3, and 5 of the LGN receive input from

the contralateral nasal retinaa

layers 1, 4, and 6 of the LGN recieve input from

monocular

the neurons in the LGN are

binocular

the neurons in the striate cortex are

ocular dominance columns

-axons at the LGN terminate in separate, alternating layers called

stereopsis

-mixing of pathways at striate cortex; improves our ability to have depth perception

retinogeniculate pathway

•Parallel pathways

•Convey distinct types of information to initial stages of cortical processing

ventral , magnocellular layer of LGN

-contain large neurons that carry info from rods

-M-retinal ganglion cells terminate here

magnocellular layer of LGN

lesion here reduces ability to perceive rapidly changing stimuli

dorsal multi-layers (parvocellular layers of LGN)

-contains small neurons that carry information from cones

-P-retinal ganglion cells terminate here

parvocellular layers of LGN

lesion here results in loss of visual acuity and color perception

bilateral consensual closing of eyelids

corneal reflex: tactile stimulation of the cornea should result in

•Ophthalmic division of CN V to spinal trigeminal nucleus

afferent response of corneal reflex

CN VII via facial nucleus

efferent response of corneal reflex

intermediolateral column-->sympathetic trunk-->superior cervical ganglion

pathway of sympathetic innervation to the eye

internal carotid artery

CN V1

sympathetic fibers travel with ______, then hop on ______ to enter orbit

-pupil dilation (iris radial muscle)

-eyelide elevation (superior tarsal muscle)

function of sympathetics to the eye

pretectal neurons-->edinger-westphal nucleus

-fibers travel to ciliary ganglion via CN III

-postganglionic fibers travel to eye via ciliary nerve

pathway of the parasympathetic innervation to the eyes

pupil constriction and lens focus

action of parasympathetics to the eye

-light impinges on retina

-impulses pass from CN II to pretectal nuclei

-secondary impulses pass to bilateral edinger-westphal nucleus

-signals pass back through CN III parasympathetic nerves

-sphincter of both iris/pupil constricts

pupillary light reflex in light

light reflex inhibited, permits pupil dilation

pupillary light reflex in darkness

Marcus Gunn Pupil (Relative Afferent Pupillary Defect)

swinging the flashlight into the R eye causes appropriate consensual response; however, introducing light to the left eye causes relatively large pupils

aniscoria

unequal pupils

horner syndrome

symptoms:

•Pupil constriction (miosis)

•Drooping eyelid (ptosis)

•Lack of sweating (anhidrosis)

Congenital Horner's Syndrome

-horner's syndrome from perinatal damage to sympathetic trunk

central horner's syndrome

horner's syndrome from between hypothalamus and sympathetic axons

peripheral horner's syndrome

horner's syndrome that result from lesion to sympathetic trunk, superior cervical ganglion, or carotid artery

hereditary horner syndrome

autosomally dominant inherited horner syndrome

oculomotor nerve lesion

pupil dilation; eye deviates inferiorly and laterally due to muscle paralysis; resulting in double vision; eyelid droops (ptosis); blurred vision due to loss of accommodation

Argyll Robertson pupil

-small, irregular and asymmetrical pupils that fail to react to light but constrict on accommodation

-accommodation intact

-often seen in CNS syphilis or diabetes

argyll-robertson pupil

potentially caused by lesions to the pretectal nuclei in the midbrain

Adie Tonic Pupil

•Sluggish, segmental pupillary responses to light but constrict on accommodation

•Typically, unilateral and common in females

-accommodation intact

adie-tonic pupil

caused by degeneration of ciliary ganglia and postganglionic parasympathetic

holmes-adie syndrome

adie-tonic pupil is aka