Together the cornea and lens focus incoming light onto the retina The retina

Together the cornea and lens focus incoming light

This preview shows page 179 - 182 out of 215 pages.

Together, the cornea and lens focus incoming light onto the retina The retina contains a thin layer of cells with photoreceptors and several layers of neurons The Retina The retina is composed of three distinct, synapsing cell layers: Photoreceptors Light-sensitive cells (rods, cones), which form a layer at the back of the retina Bipolar Cells An intermediate layer of connecting neurons Ganglion Cells Form the front or innermost layer of the retina and whose axons project to the brain via the optic nerve The retina is composed of three distinct layers Photoreceptors are held in place by pigmented epithelium They synapse with bipolar cells, an intermediate layer of connecting neurons Bipolar cells connect with each other and ganglion cells, which form the innermost layer of the retina Ganglion cell axons project to the brain via the optic nerve Photoreceptors Rods are sensitive to dim light but not color
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Rods dominate most of the retina, but one small spot in the center of the retina (the fovea) has only cones Cones are less sensitive to faint light but are stimulated by different wavelengths (colors) The molecular basis of vision in vertebrates is a shape change in retinal that closes ion channels and decreases the amount of neurotransmitter being released to the sensory neuron Rods and cones are packed with transmembrane proteins that contain the pigment retinal Rods and cones have membrane discs containing thousands of opsin molecules (rhodopsin in rods) Each rhodopsin holds one retinal molecule Retinal changes conformation when it absorbs light A signal transduction pathway connects light absorption to changes in membrane potential In an unstimulated photoreceptor, Na + flows into the cell when light is NOT being received In a stimulated photoreceptor, activation of rhodopsin leads to a reduction in cGMP With less cGMP available, cGMP-gated Na + channels close, and the membrane hyperpolarizes Light shuts down neurotransmitter release Rhodopsin is activated when light causes retinal to change shape Rhodopsin activates the membrane protein transducin, which in turn activates the enzyme phosphodiesterase (PDE) PDE breaks down cyclic guanosine monophosphate (cGMP) to guanosine monophosphate (GMP) As cGMP levels decline, cGMP-gated Na + channels in the plasma membrane of the rod cell close When Na + channels close, Na + entry decreases and the membrane hyperpolarizes, decreasing neurotransmitter release Opsins Although retinal is the light-absorbing molecule in all photoreceptor cells, the different opsins cause each to respond to a different range of light wavelengths Color vision is possible because different opsins absorb different wavelengths of light Each human cone cell contains one of three different types of opsin Each opsin absorbs a different range of wavelengths
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Chemoreception
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