Lecture 19: Visual System

Outermost connective tissue layer of the eye

- cornea - transparent and avascular

- sclera - opaque and tough

Avascular, biconvex structure posterior to the iris

lens

What controls lens thickness?

ciliary muscles

How does lens shape change for near versus far vision?

- near = fat/thick

- far = flat

Vascular middle layer of the eye

uvea

layer of the uvea containing a specialized network of large capillaries

choroid

component of the uvea containing muscles for accommodation

ciliary body

Pigmented disc containing a central aperture and smooth muscle fibers that regulate light entering the eye

iris

Central aperture of the iris

pupil

What are the 2 smooth muscles of the iris?

sphincter pupillae and dilator pupillae

Ring-shaped muscle surrounding the pupillary margin

sphincter pupillae

(parasympathetic innervation)

Radially arranged smooth muscle fibers of the iris

dilator pupillae

(sympathtic innervation)

Delicate, transparent tissue covering the inner surface of the posterior eye

retina

Single layer of cells in the neural retina responsible for absorption of excess light, maintenance of nutritional requirements, etc.

retinal pigment epithelium (RPE)

What are the 2 types of photoreceptors?

rods and cones

Photoreceptors responsible for low-light and night vision (scotopic)

rods

(out-number cones)

Photoreceptors specialized for visual acuity and color vision at the expense of sensitivity (phototopic)

cones

What are the 3 visual pigments found in cones?

blue, green, and red

What results from equal stimulation of all 3 cone types?

white light

Retinal neurons that receive input from rods and cones and synapse with ganglion cells

bipolar cells

Retinal association neurons that enhance borders, contours, and contrast

horizontal and amacrine cells

Retinal cells extending throughout all portions of the retina and providing metabolic support

Muller cells

Final retinal neurons whose axons form the optic nerve and is in contact with the vitreous

ganglion cells

What cells provide synaptic input to retinal ganglion cells?

bipolar cells

what is the distribution of rods and cones in the central fovea?

rods are absent and it is rich in cones

How does rod distribution change toward the peripheral retina?

rods increase in number

How does cone distribution change toward the peripheral retina?

decreases rapidly in all peripheral directions

Small, pale yellow circular region at the posterior pole of the eye that represents the area of central vision

macula lutea

Shallow depression at the center of the macula lutea with the sharpest vision and most acute color discrimination

fovea centralis

Structure where the optic nerve fibers exit and lacks photoreceptors

optic disc (papilla)

Why does the optic disc have a pale pink color?

ophthalmic blood vessels enter and exit the eye there

What meningeal layers surround the optic nerve as it exits the globe?

all 3 meningeal layers

Swelling of the optic disc due to increased intracranial pressure

papilledema

What is the typical laterality and time course of papilledema?

usually bilateral, developing over hours to weeks

What are treatment options for papilledema?

diuretics, steroids, and CSF removal

What fills the large posterior compartment of the eye?

gelatinous vitreous humor

(not replenished)

Is vitreous humor continually replenished?

no

What fills the anterior compartment of the eye?

watery aqueous humor

(continually replenished)

Projections carrying visual information from the eyes to internal structures

retinofugal projections

Everything visible when the eyes are fixed on an object directly ahead

visual field

Why is the visual field somewhat oblong?

limited by supraorbital ridges and cheekbones

Why is vision predominantly binocular with a small monocular peripheral segment?

visual fields overlap considerably but not completely because of the nose

Center of the visual field when looking straight ahead, projecting to the foveae of both retinas

fixation point

What divides the visual field into nasal and temporal halves?

imaginary vertical line through fixation point

What divides the visual field into upper and lower quadrants?

imaginary horizontal line through fixation point

How is the visual field projected onto the retina?

inverted and reversed

(ex. right visual field = left temporal retina and right nasal retina)

Are visual deficits described in terms of the retinal field or visual field lost?

visual field lost

Where do optic nerves unite and partially decussate?

optic chiasm

Which retinal fibers cross at the optic chiasm?

nasal retinal fibers carrying temporal visual field information

Which retinal fibers remain ipsilateral at the optic chiasm?

temporal retinal fibers

Where does the entire left visual field project?

right side of brain

Where does the entire right visual field project?

left side of brain

What structure is formed by visual fibers after passing the optic chiasm?

optic tract

What percentage of optic tract fibers synapse in the LGN?

80%

Thalamic relay station for visual information

lateral geniculate nucleus (LGN)

What happens to the remaining 20% of optic tract fibers?

enter brachium of superior colliculus and terminate in diencephalic or mesencephalic nuclei

Axons from LGN forming the pathway to primary visual cortex

geniculocalcarine tract (optic radiations)

What type of organization is maintained in optic radiations?

retinotopic organization

What loop of optic radiations carries upper visual field information?

temporal loop (Meyer's loop)

What loop of optic radiations carries lower visual field information?

parietal loop

Refractive errors caused by structural changes to the eye

ametropia

Inability to focus on near objects due to an eyeball that is too short

hyperopia (farsightedness)

Inability to bring distant objects into clear focus due to an overly curved cornea or elongated eyeball

myopia (nearsightedness)

Where do LGN axons terminate in primary visual cortex?

primarily layer IV of the 6-layered striate cortex

Where do upper visual field fibers terminate relative to calcarine sulcus?

inferior to calcarine sulcus

Where do lower visual field fibers terminate relative to calcarine sulcus?

superior to calcarine sulcus

How does cortical representation vary with retinal receptor density?

higher receptor density receives disproportionately greater cortical representation

Why does the fovea occupy a disproportionately large cortical area?

high receptor density

What lobes contain visual association cortices?

occipital, parietal, and temporal lobes

What is the function of the ventral visual association pathway?

high-resolution form vision and object recognition

(temporal lobe)

What is the function of the dorsal visual association pathway?

spatial vision, motion analysis, and positional relationships between objects

(parietal lobe)

Reflex that protects the retina from excessive light by constricting both pupils

pupillary light reflex

What happens when light is shone into one eye during the pupillary light reflex?

bilateral pupillary constriction

where does the afferent limb of the pupillary light reflex pathway terminate?

pretectal area

What is the associational pathway of the pupillary light reflex?

pretectal nucleus → bilateral Edinger-Westphal nuclei

What is the efferent pathway of the pupillary light reflex?

Edinger-Westphal nucleus → CN III → ciliary ganglion → sphincter pupillae → pupillary constriction

What happens when light is shone into an eye with an afferent optic nerve lesion?

neither pupil constricts

What happens when light is shone into the healthy eye in a patient with a unilateral afferent lesion?

both pupils constrict

What happens to the affected pupil with an efferent CN III/parasympathetic lesion?

no direct or consensual constriction regardless of which eye is illuminated

How does the healthy eye respond in a patient with a unilateral efferent lesion?

normal direct and consensual responses

Why is the pupil enlarged in an eye with an efferent CN III lesion?

unopposed action of dilator pupillae

Reflex that brings an out-of-focus near object into focus

near (accommodation-convergence) reflex

What are the 3 responses of the near reflex?

ocular convergence, lens thickening, and pupillary constriction

(requires cerebral cortex)

Small, irregular pupil that does not react to light but constricts during accommodation

Argyll-Robertson pupil

What lesion is associated with Argyll-Robertson pupil?

pretectal lesion

(due to neurosyphilis or tumors)

Loss of one half of the visual field

hemianopsia

Visual field deficit involving the same side of the visual field in both eyes

homonymous

Visual field deficit involving opposite halves of the visual fields in both eyes

heteronymous

Loss of one quarter of the visual field

quadrantanopsia

What deficit results from a complete optic nerve lesion (A)?

complete blindness in ipsilateral eye

What visual deficit results from an optic chiasm lesion (B)?

bitemporal heteronymous hemianopsia

What visual deficit results from a lateral optic chiasm lesion (C)?

ipsilateral nasal hemianopsia

What visual deficit results from an optic tract lesion (D)?

contralateral homonymous hemianopsia

What visual deficit results from complete optic radiations lesion (E)?

contralateral homonymous hemianopsia

What visual deficit results from Meyer's loop lesion (F)?

contralateral superior quadrantanopsia

What visual deficit results from a parietal optic radiations lesion (G)?

contralateral inferior quadrantanopsia

What visual deficit results from a unilateral primary visual cortex lesion (H)?

contralateral homonymous hemianopsia with macular sparing