Anatomy and Physiology of the Eye

How does the eye detect light and how does the brain interpret what you see? Part I: How does image get to retina? 1) Light Quality: intensity and wavelength 2) Lens and light refraction/ focusing: 3) Light refraction in aqueous/vitreous humor: 4) Inverted light (photons) reach the retina after passing through different cell structures! 5) Light reaches rod and cone cells at back 6) Pigmented epithelium of choriod absorbs extra light to prevent reflection back int orods/cones and blurring.

How does the eye detect light and how does the brain interpret what you see? Part II: How does the brain detect image/APs? 1) Light hits one of two pigments: rhodopsin on rods OR iodopsin on cones. 2) Light changes molecular shape and enzyme function. 3) Modified function modifies Action Potential output! 4) Action potentials sent from ganglion cells to optic nerves to brain via optic nerve! 5) Primary visual center in occipital lobe receives modified train of action potentials 6) Association areas interpret action potentials.

Blood vessels and nerves run across the surface, with the photodetector cells (rods and cones) at the back of the retina! Photons of light pass through several structures before reaching the photodetectors. Light modifies the way detector cells send APs to the brain via the optic nerve.

Our eye has to regulate the amount of light reaching the retina for protection. Our eye also has to modify the lens shape to focus the image. Both of these activities are involuntary! Ciliary muscles and suspensory ligaments: modify lens shape and help to focus the inverted image on the retina. VS. Pupillary muscles and iris: modifies pupil diameter and light access to retina to protect in strong light and help in dim light. Color vision vs. Night vision High acuity vs High Sensitivity Fovea centralis vs. Peripheral vision Lots of cones vs. Lots of rods Optic Disc is your blind spot: location of optic nerve exit!

We need a flat lens to bring an inverted image of a distant object into focus on the retina. Lens accomodation allows us to look at objects that are close. The suspensory ligament and ciliary muscles have a natural tension that pulls them tight towards the margin of the retina. Constriction of the ciliary muscles draws these structures away from the edge of the retina, relaxing the suspensory ligaments which lets the elastic lens become round for near vision.

Visual projection can be easily understood by looking at the effects of lesions on images from lateral and nasal fields. Consider cutting the tracts at the above “letter” locations! What would you see? “DARK” means loss of image

Trans-retinol is formed is formed in the light and leads to the breakdown of cyclic GMP. DARK: cGMP opens Na+ channels and DEpolarizes rod/cone membrane and inhibitory glutamate is released to bipolar cell. DARK: Bipolar cell is hyperpolarized by glutamate (turned off) LIGHT: trans-retinol is created and leads to cGMP destruction. Na+ entry stops  HYPERpolarization LIGHT: Bipolar cell not inhibited, so it sends EPSPs to ganglion cell LIGHT: Ganglion cell sends APs into optic nerve.

Light causes the ganglion cells to send APs to the brain. Why may “seeing a blue-green haze” be a Viagra side effect?

Why do rods give better sensitivity? Why do cones give better acuity? Consider the number of rods/cones per ganglion cell (AP output to brain) Why do cones let you see the best fine detail (fovea centralis) and why do rods let you see faint stars in the night with peripheral vision?

Six extrinsic muscles determine where you look. What are the muscles and how are they controlled? Voluntary Ability to track objects with the eye: Lateral Rectus: Abduction- Abducens Nerve VI Medial Rectus: Adduction- Occulomotor Nerve III Superior Rectus: Elevation- Occulomotor Nerve III Inferior Rectus: Depression- Occulomotor Nerve III Superior Oblique: Depression/Rotation-Trochlear Nerve IV Note unique pulley system of trochelear trochlea Inferior Oblique: Elevation/Rotation- Occulomotor Nerve III Remember to contrast these activities with the involuntary activity of Ciliary and Pupillary muscles!

What are some common diseases of the eye? Myopia (nearsightedness)-image IN FRONT OF retina Hyperopia (farsightedness)-image BEHIND retina Astigmatism (non-uniform lens shape)-non uniform image Effect of stroke- effect in tracts and areas Glaucoma-elevated occular pressure and reduced blood supply to retina Cataracts- precipitate occurs inside lens that causes cloudiness: UV light, smoking, and/or diabetes Detached Retina- most retinal blood supply from choroid, detachment causes ischemia/infarct Macular Degeneration- oxidative or UV injury to the macula densa (fovea centralis)

How do eye glasses move the focal point of an image?