Lecture #16: Visual System: Eye Functional Anatomy and Vision Pathways

What are the three concentric tissue layers of the eye?

The eye has three concentric layers: the outer fibrous layer (sclera and cornea), the middle vascular layer (choroid, ciliary body, and iris), and the inner neural layer (retina). These layers provide structural support, blood supply, light regulation, and neural processing.

What is the functional role of the sclera and cornea?

The sclera provides rigidity and structural integrity to maintain eye shape, while the cornea is transparent and responsible for most of the eye’s refractive power at the air–water interface.

What structures form the vascular layer of the eye?

The vascular layer consists of the choroid, ciliary body, and iris. This layer supplies blood, produces aqueous humor, and regulates pupil size and lens shape.

What are the two main layers of the retina?

The retina consists of the retinal pigment epithelium (RPE) and the neural retina. The RPE supports photoreceptors metabolically, while the neural retina contains rods, cones, and interneurons for visual processing.

What is retinal detachment and why is it dangerous?

Retinal detachment occurs when the neural retina separates from the RPE, disrupting blood supply to photoreceptors and leading to rapid neural cell death and permanent blindness if untreated.

How does aqueous humor circulate through the eye?

Aqueous humor is secreted by the ciliary body into the posterior chamber, flows through the pupil into the anterior chamber, drains via the trabecular meshwork, and enters the canal of Schlemm to reach venous circulation.

What is the role of intraocular pressure?

Intraocular pressure maintains the shape of the eye, allowing proper focusing of light on the retina. Abnormal pressure can damage ocular structures.

What is glaucoma and how does it develop?

Glaucoma is increased intraocular pressure due to impaired aqueous humor drainage, leading to optic nerve damage and progressive vision loss.

How do open-angle and closed-angle glaucoma differ?

Open-angle glaucoma results from slow trabecular meshwork drainage and is chronic, while closed-angle glaucoma occurs when the iris blocks drainage acutely and is a medical emergency.

Where does most refraction of light occur in the eye?

Most refraction occurs at the corneal air–water interface; the lens fine-tunes focus by changing shape during accommodation.

What is accommodation?

Accommodation is the process by which ciliary muscle contraction relaxes zonular fibers, allowing the lens to become rounder and increase refractive power for near vision.

How does the iris regulate vision quality?

The iris controls pupil size via parasympathetic constriction and sympathetic dilation, balancing depth of field and light entry.

What is the near reflex?

The near reflex includes convergence of the eyes, lens accommodation, and pupillary constriction when focusing on nearby objects.

What is the basic flow of information through the retina?

Visual information flows from photoreceptors to bipolar cells to ganglion cells, whose axons form the optic nerve.

What roles do horizontal and amacrine cells play?

Horizontal cells integrate signals across photoreceptors, while amacrine cells modify bipolar-to-ganglion signaling, enhancing contrast and temporal processing.

How do rods and cones differ functionally?

Rods are highly sensitive to light and mediate night vision but saturate easily, while cones mediate color and high-acuity vision and function best in bright light.

What molecular event initiates phototransduction?

Light converts 11-cis retinal to all-trans retinal, activating opsins, transducin, and phosphodiesterase, reducing cGMP and closing cation channels to hyperpolarize photoreceptors.

Why do photoreceptors hyperpolarize in response to light?

Light reduces cGMP levels, closing cGMP-gated channels, decreasing neurotransmitter release to signal light detection.

What is the fovea and why is it important?

The fovea is the central retinal region with the highest cone density, no blood vessels, and maximal visual acuity and color discrimination.

What is the macula lutea?

The macula lutea is a 5-mm retinal region surrounding the fovea responsible for central vision.

What causes the blind spot?

The blind spot occurs at the optic disc where the optic nerve exits and blood vessels enter, creating a region with no photoreceptors.

Why are we unaware of the blind spot?

The visual system fills in missing information using surrounding visual input, preventing conscious awareness of the blind spot.

What is age-related macular degeneration (AMD)?

AMD damages the macula, causing loss of central vision while sparing peripheral vision, impairing tasks like reading and facial recognition.

What are the three cone types?

Cones are classified as L, M, and S types, each sensitive to different wavelengths and enabling color perception.

How do ganglion cell receptive fields enhance vision?

Ganglion cells have center–surround receptive fields that enhance contrast detection rather than absolute light levels.

What defines the visual field?

The visual field is the region of space seen while the eyes remain fixed without head movement.

How are visual fields represented in the brain?

Visual fields are represented contralaterally in the visual cortex due to partial decussation at the optic chiasm.

What happens at the optic chiasm?

Nasal retinal fibers cross to the contralateral side, while temporal fibers remain ipsilateral, enabling binocular vision.

What visual deficit is caused by pituitary adenoma compression?

Compression of the optic chiasm causes bitemporal hemianopia due to damage of crossing nasal fibers.

What is the function of the lateral geniculate nucleus (LGN)?

The LGN relays visual information from the optic tract to the visual cortex and segregates inputs by eye and function.

How do magnocellular and parvocellular LGN layers differ?

Magnocellular layers process motion and contrast, while parvocellular layers process color and form.

What is Meyer’s loop?

Meyer’s loop is the temporal lobe portion of optic radiation carrying superior visual field information; damage causes contralateral superior quadrantanopia.

How is the primary visual cortex organized?

The primary visual cortex is retinotopically organized, with foveal vision represented posteriorly and peripheral vision anteriorly along the calcarine sulcus.

What is macular sparing and why does it occur?

Macular sparing occurs in posterior cerebral artery infarcts due to dual blood supply to the foveal cortex from middle cerebral artery collaterals.

What is papilledema?

Papilledema is optic disc swelling caused by increased intracranial pressure transmitted through the optic nerve sheath.

What additional brain structures receive optic tract input?

Optic tract fibers also project to the superior colliculus, hypothalamus, and accessory optic nuclei for reflexive eye movements and circadian rhythm regulation.

How is visual information processed beyond V1?

Visual information is processed in parallel streams analyzing motion, color, depth, and form in specialized cortical areas.

What is the dorsal visual stream?

The dorsal stream (“where” pathway) processes spatial location and motion and projects to parietal cortex.

What is the ventral visual stream?

The ventral stream (“what” pathway) processes object form and color and projects to temporal cortex.

What causes motion blindness (akinetopsia)?

Damage to the dorsal stream impairs motion perception while sparing form and color.

What deficits occur with ventral stream lesions?

Ventral stream damage causes deficits such as prosopagnosia (face blindness) and achromatopsia (color blindness).

What is hemispatial neglect?

Hemispatial neglect is a deficit in spatial attention, usually from right posterior parietal cortex damage, causing neglect of the left side of space.

How does hemispatial neglect differ from hemianopia?

Neglect is a perceptual-attentional deficit, not a loss of visual input, and patients fail to attend to stimuli they can technically see.

What is blindsight?

Blindsight is the ability of cortically blind patients to respond to visual stimuli without conscious visual awareness due to preserved subcortical pathways.