BME++Spec+Topic+Essay+Example+1

BME++Spec+Topic+Essay+Example+1 - EYE AND EAR Though...

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EYE AND EAR Though electromagnetic waves exist in a range of wavelengths, our eyes are sensitive to only a very narrow band (e.g. the visible light spectrum - in between the IR and UV regions. Visible light wavelengths range from ~350 to ~850nm. It is also affectionately known as “ROYGBIV” (i.e. the colors of the spectrum). The ozone layer reflects most of the harmful, very short, UV wavelengths away from the earth, while it transmits more than half of the short to medium visible light towards the earth. In the eye, light first encounters the cornea, a thin membrane that has the dual purpose of protecting the eye and refracting light. The iris is the light sensitive diaphragm that controls the amount of light that goes through the pupil, the opening that allows light into the eye. Light then enters the crystalline lens, which is made of a fibrous, jelly-like material. The lens is attached to the ciliary muscles, which relax and contract to change the shape of the lens (accommodation). The light that passes through the lens is then projected onto the retina. The retina is a seven- layered structure of sensory cells that transduces the light signal to nerve impulses. Photoreceptor cells (rods for black & white; cones for color) change light energy into an electrical signal. There are three kinds of cones, each sensitive to either: red, green, or blue. There are many flat discs of rhodopsin within the outer segment of a photoreceptor cell which upon excitation undergo a photo-isomeric change from rhodopsin (11- cis ) to all- trans retinal. Photoexcited rhodopsin triggers an enzymatic cascade resulting in the hydrolysis of GMP. This in turn closes cation-specific membrane channels which are normally open to Na + influx in the dark, and due to the effect of hyperpolarisation, the inner synatic body sends a nerve signal to other neurons in the retina. Horizontal, bipolar and amacrine cells process the electrical signal;
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BME++Spec+Topic+Essay+Example+1 - EYE AND EAR Though...

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