Eye Anatomy and Vision

Overview

This lecture covers the anatomy of the eye, the basic processes behind light detection, and explains key concepts related to vision including common vision disorders.

Muscles Controlling Eye Movement

• Extrinsic muscles move the eyeball in the socket (lateral, medial, elevation, depression, and rotational movements).
• Intrinsic muscles within the eyeball include ciliary muscles (change lens shape) and iris muscles (control pupil size).
• The sphincter pupillae constrict the pupil; the dilator pupillae dilate the pupil.

Layers and Structures of the Eye

• The eye wall has three layers: fibrous (sclera, cornea), vascular/uvea (choroid, ciliary body, iris), and sensory (retina).
• The sclera provides protection and structure; the cornea allows light to enter.
• The choroid is vascular and pigment-rich, supplying nutrients and oxygen.
• The ciliary body contains muscles controlling lens shape; the iris controls pupil size.
• The retina contains the cells that detect light (rods, cones).

Retina and Light Detection

• The retina has an outer pigmented layer and an inner neural layer.
• Light-detecting cells include rods (dim light, peripheral vision, no color) and cones (color, sharp vision, bright light).
• Photoreceptor signals are relayed via bipolar and ganglion cells to the optic nerve.
• The optic disk is the blind spot where the optic nerve exits.
• The fovea centralis in the macula lutea is the area of sharpest focus.

Vision Disorders and Conditions

• Macular degeneration is the loss of central vision, caused by wet (leaky vessels) or dry (unknown cause) forms.
• Cataracts are clouding of the lens, treatable by lens replacement.
• Myopia (near-sightedness) is due to a long eyeball; corrected with concave lenses.
• Hyperopia (far-sightedness) is due to a short eyeball; corrected with convex lenses.
• Presbyopia is age-related lens hardening, leading to difficulty focusing on nearby objects.

Accommodation Reflex

• Adjustment of the eye for near or distant objects involves both pupil constriction/dilation and lens shape changes via ciliary muscles.
• For near vision: ciliary muscles contract (round lens), pupils constrict.
• For distance vision: ciliary muscles relax (flat lens), pupils dilate.

Eye Humors

• Aqueous humor (fluid) occupies the anterior chamber, nourishes cornea and lens.
• Vitreous humor (gel-like) fills the posterior chamber, provides structural support.
• Glaucoma is increased intraocular pressure, typically from excess aqueous humor, potentially damaging the optic nerve.

Photoreceptors and Vision Pigments

• Rods use rhodopsin (retinal + opsin), accumulate in dark, break down in light ("bleaching"), affecting cation channels and membrane potential.
• Cones have similar pigments with different opsins, allowing color vision (blue, green, red cones).
• Color vision results from combinations of cone activation; color blindness is due to missing red or green cones.

Key Terms & Definitions

• Extrinsic Muscles — Eye muscles controlling eyeball movement in the socket.
• Intrinsic Muscles — Eye muscles within the eyeball (ciliary and iris muscles).
• Fibrous Layer — Outer eye layer; includes sclera and cornea.
• Vascular Layer (Uvea) — Middle eye layer; includes choroid, ciliary body, iris.
• Retina — Inner sensory layer; contains photoreceptors (rods, cones).
• Rods — Photoreceptors for dim light and peripheral vision, not color-sensitive.
• Cones — Photoreceptors for color and sharp vision in bright light.
• Fovea Centralis — Spot of sharpest vision in the retina.
• Accommodation Reflex — Eye adjustments for focusing on near/distant objects.
• Cataract — Clouding of the lens.
• Myopia — Near-sightedness due to a long eyeball.
• Hyperopia — Far-sightedness due to a short eyeball.
• Presbyopia — Age-related loss of lens flexibility.
• Aqueous Humor — Fluid in the eye's anterior chamber.
• Vitreous Humor — Gel in the posterior chamber providing eye structure.
• Glaucoma — Increased eye pressure damaging the optic nerve.
• Rhodopsin — Light-detecting pigment in rods (retinal + opsin).

Action Items / Next Steps

• Review the phototransduction process in the next video.
• Be able to explain eye anatomy, vision disorders, the accommodation reflex, differences between rods and cones, and the role of rhodopsin.