PHYSIO 303B EXAM 1 Notes—ORGANELLES Photosensitive Cells—These include the rods and cones that are present in the cornea of the eye as well as a very small fraction of retinal ganglion cells, which detect light/dark in a way that cues circadian rhythm (your body clock which is not-reliant on environmental cues, roughly a 24 hour cycle) - Specialized cells control all peripheral vision, central vision and electrical signals. - As light enters into the eye, in moves in the order of—Cornea and tear film  aqueous humor  lens  vitreous humor  retina  specialized cells within the retina that respond to light. Basic Structures of the Eye—- Cornea and Tear Film—The transparent layer forming the front of the eye. (Cataract—Lack of transparency in the retina) - Aqueous Humor—The clear fluid filling the space in the front of the eyeball between the lens and the cornea.- Lens—The transparent structure inside the eye that focuses light rays onto the retina (the nerve layer that lines the back of the eye, senses light and creates impulses that go through the optic nerve to the brain, cells in lens lack a nucleus). o The above three components are all transparent units. - Ciliary Muscles—Rings of striated smooth muscle in the eye's middle layer (vascular layer) that controls accommodation for viewing objects at varying distances; moves the eye around. - Sclera—The outer white layer of the eyeball, continuous with the cornea at the front. - Choroid—The pigmented, vascular layer of the eyeball located between the retina and the sclera. (epithelium) - Vitreous Humor—The transparent, jelly-like tissue filling the eyeball behind the lens.- Retina—A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.o The retina is a very high metabolic tissue that generates and uses a high amount of ATP. The retina generates ATP by both glycolysis and oxidative metabolism (switches back and forth in order to generate the most energy possible) - Optic Nerve—Transmits visual information from the retina to the brain. Visual Pathway Extends into the Brain—Visual information that is transmitted from the retina of the eye, through the optic nerve and into the brain travels along axons that make synapses with the Lateral Geniculate Nucleus (LGN). This information then travels along axons to the posterior part of the brain until they reach the striate cortex (V1). - Retinal signals are relayed to the specific segments of the brain and signals are integrated from both eyes in order to display a clear image. - The optic nerve extends from the eye to the lateral geniculate nucleus (LGN) and the neuronal layers that are present within the LGN correspond to the left and right eyes. - Lack of use of one of these eye pathways will cause the connections to not fully develop properly and they will not be able to be rebuilt later in life. - Binocular Vision—Vision in which both eyes are used together. It is advantageous to have two eyes because it gives a creature a spare eye in case one is damaged and it also gives a wider field of view. It also allows for binocular summation in which the ability to detect faint objects is enhanced and finally, it allows for enhanced depth perception. Retinal Photoreceptors—- Rods—They are very sensitive to light and they enable black and white vision. - Cones—They are fewer in number, they are less sensitive to light, and they allow for color vision to be perceived. - The retina has multiple layers and the photoreceptors that are present within the retina detect the photons, while other neurons process the signal to detect motion, edges, etc. - As light passes through the retina, it passes through the ganglion cell layer (ganglion cells)  Inner synaptic layer  bipolar layer (amacrine cells, bipolar cells, horizontal cells)  outer synaptic layer  photoreceptor layer (rods and cones)  pigment layer of choroid.- Once the photons have passed through the retina, visual data is processed in the OPPOSITE direction until it reaches the optic nerve where the information travels through the brain. Distribution of Rods and Cones—- Rods are far more numerous than cones are in the retina; rods are predominantly located in the peripheral retina whereas cones are predominantly located in the central retina (the fovea). - There is a blind spot present in the eye and a large concentration of cones in the fovea and no rods are present. Retina Terminology—- Macula (Macular Lutea)—The central area of the retina capable of the clearest, most distinct vision. - Fovea—The central part of the macula where all the photoreceptors are cones. - Blind Spot—The optic nerve head where photoreceptors are absent. - Overall, all tissues of the eye are made of cells and these cells are highly specialized. The specialization is different in different tissues and the specializations are apparent at both the molecular and organelle level. Furthermore, for each different tissue, the challenge of homeostasis is different. - Eyes are highly specialized because they have the ability to focus, the ability to turn photons into a cell signal, they can move, and they are transparent. **Having a degree of night blindness as well as defective peripheral vision indicates most likely a rod defect because night vision and peripheral vision are associated with the rod photoreceptor** Cell Theory—It states that cells are the building blocks of animals and plants and that cells arise from pre-existing cells. Cells are the smallest unit having all the functions of living systems and homeostasis of higher levels of biological organization arises from the coordinated activity of cells. Common Cell Structures— Plasma Membrane (cell membrane), nucleus (contains genetic material), and the cytoplasm (everything that is located between the membrane and the nucleus). The cytosol is the aqueous intracellular fluid containing electrolytes and other dissolved solutes and organelles are subcellular structures with specific functions. Membranous Organelles—Nucleus, mitochondrion, endoplasmic reticulum, lysosome and peroxisomeNon-Membranous Organelles—Ribosome, proteosome and cytoskeletonA human cell can be viewed under a light microscope with a clear resolution and has a size of about 10