The Eye

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  • Topic: Eye, Retina, Photoreceptor cell
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  • Published : April 4, 2013
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Chapter 9: The Eye

* Light is electromagnetic energy that is emitted in form of waves; waves crash into objects and are absorbed, reflected, scattered, and bent * Half of human cerebral cortex is involved with analyzing visual world * MAMMALIAN VISUAL SYSTEM: begins with eye, back of eye is retina (contains photoreceptors specialized to convert light energy to neural activity) * Eyes have features to track moving objects and keep transparent surface clean (i.e. by tears) * Each eye has two overlapping retinas: one specialized for low light levels and one for higher light levels and detection of colour * Retina specialized to detect differences in the intensity of light * Axons of retinal neurons bundled into optic nerves, which distributes visual information (in form of AP) to several brain structures that perform difference functions * First synaptic relay in pathway that serves visual perception occurs in a cell group of dorsal thalamus called the lateral geniculate nucleus (LGN) * Visual information from LGN -> cerebral cortex (where it is interpreted and remembered)

PROPERTIES OF LIGHT:
* Light: electromagnetic radiation that is visible to our eyes * Electromagnetic radiation has wavelength (distance between peaks), frequency (# of waves/second), and amplitude (difference between wave peak) * Energy content of electromagnetic radiation is proportional to its frequency (radiation emitted at high frequency – shorter wavelength – has highest energy content, vice versa) * Visible light = wavelength consist of 400 – 700 nm

* Optics
* Wave of electromagnetic radiation travels in straight line = rays. Rays travel until they interact with atoms and molecules; interactions include reflection, absorption, and refraction. [study of light rays and their interactions is called optics] 1. Reflection: bouncing of light rays off a surface.

2. Absorption: transfer of light energy to a particle or surface. 3. Refraction: forms images in the eye, bending of light rays that can occur when they travel from one transparent medium to another.

THE STRUCTURE OF EYE:
* Eye: organ specialized for detection, localization, and analysis of light. * Gross Anatomy of the Eye:
* Pupil – opening that allows light to enter the eye and reach the retina; surrounded by light-absorbing pigments in retina (appears black) * Iris – surrounds pupil, pigmentation provides eye’s colour. Contains 2 muscles that can vary size of pupil; one makes it smaller when it contracts, other makes it larger. * Cornea – covers pupil and iris, continuous with sclera (“white of the eye”) * Sclera – 3 pairs of extraocular muscles, which move the eyeball in orbit (muscles behind conjunctiva – membrane that folds back from the inside of the eyelids and attaches to the sclera) * Optic nerve - carrying axons from retina, exits the back of eye, passes through orbit, reaches base of brain near pituitary gland.

* Ophthalmoscopic Appearance of the Eye:
* Ophthalmoscopic – device that allows one to peer into the eye through pupil to the retina * Optic disk – where retinal vessels originate and where the optic nerve fibers exit the retina * Sensation of light cannot occur at the optic disk because no photoreceptors present * Macula – middle of each retina – part of retina for central vision, absence of large blood vessels from this region improves the quality of central vision * Fovea – dark spot 2 mm, retina thinner in fovea because it marks center of retina. * Part of retina that lies closer to the nose than the fovea is called nasal, near the temple is called temporal, above fovea is called superior and below it is called inferior. * Cross-Sectional Anatomy of the Eye: shows path taken by light cornea -> retina * Cornea lacks blood vessels and nourished by the fluid behind it, aqueous humor * Transparent lens behind iris is suspended by zonule...
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