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MIT 9 01 - RECITATION #4

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MIT OpenCourseWare http://ocw.mit.edu 9.01 Introduction to Neuroscience Fall 2007 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.9.01 Recitation (R02) RECITATION #4: Tuesday, October 16th Review of Lecture: 9, 10 Reading: Chapter 9 and 10 of Neuroscience: Exploring the Brain (3rd edition) Outline of Recitation: I. Questions on the Exam? II. Review of Material: VISION a. The Eye and Retina • Structure of the eye • Anatomy of Retina • Phototranduction III. Practice Exam Questions • Receptive Fields • Color Perception • Visual Pathway THE EYE AND THE RETINA: Structure of the eye: (p283) • pupil: opening that allows light to enter the eye • iris: surrounds pupil; pigmented • sclera: “white of the eye” • extraocular muscles: moves eyeball • conjunctiva: membrane that covers extraocular muscles • aqueous humor: nourishing fluid behind cornea • lens: transparent; adjusts focus • cilliary muscles: controls shape of lens • zonule fibers: ligaments that suspend lens • vitreous humor: lies between lens and retina; keeps eyeball spherical View of the Retina (ophthalmoscope): • optic disk: where optic nerve fibers exit retina and retinal blood vessels enter. No photo receptors (blind spot). • macula: part of the retina for central vision; absence of blood vessels • fovea: marks the center of retina 1 Diagram removed due to copyright restrictions.Photo removed due to copyright restrictions.Focusing light onto the retina: (p283-288) Refraction by cornea: Accommodation by lens: • Near point: • Far point: Pupillary light reflex: Anatomy of retina: (p288-292) Two important points: (1) Photoreceptors are the only light-sensitive cells in the retina. (2) Ganglion cells are the source of output from the retina to the brain via the optic nerve. Only cells fire AP. Two different types of photoreceptors: (1) Rods: Found mainly in the: *Outnumber cones 20:1 (2) Cones: Found mainly in the: Phototransduction: (p292-298) Photoreceptors convert or transducer light energy into changes in membrane potential. Dark: depolarize Light: hyperpolarize w/ light stimulus (only OFF response) Vm time 2Receptive Fields and Retinal Processing: (p298-306) Receptive Field: • the region of visual space that, when stimulated, causes a retinal neuron to respond • the region of a sensory surface (retina) that, when stimulated, changes the membrane potential of a neuron • point-to-point mapping • locality of receptive fields a result of structure of cells in retina… - short dendrites (doesn’t spread across retina) - center-surround receptive fields Bipolar Cell Receptive Fields: ON bipolar cells: OFF bipolar cells: Center-surround receptive fields: Ganglion Cell Receptive Fields: • ON and OFF ganglion cells like bipolar cells • Center-surround receptive fields Ganglion cells are mainly responsive to differences in illumination that occur within their receptive fields. • Light vs. reality (examples: illusions) Adaptation: (box 4.6) - visual system detects reflection, not luminance - incident light x reflectance - based on context Vm time 3Color perception: (p296, 304) Young-Helmholtz trichromacy theory Color after-effect: (example: pink/green dot illusion) Color-opponent ganglion cells: • Center-surround receptive fields Pathway for “conscious” visual perception: (Chapter 10) Retinofugal projection: -------------> ------------> Via the Cortical cell receptive fields: • elongated ON and OFF sub regions • orientation selectivity - preferred orientation - null orientation PRACTICE EXAM QUESTIONS: 17. A strange disease, Lentifical Pancakium, results in a b) is due to the inward flow of Na+ caused by PDE binding to permanent flattening of the lenses of your eyes. This results the Na+ channels in which of the following while you are looking at a picture in c) is due to the inward flow of Na+ caused by transducin your Neuro 1 textbook? binding to the Na+ channels a) Images of very close objects would be focused in front of d) is a result of the activation of a Rhodopsin molecule the retina. e) none of the above b) Images of very close objects would be focused behind the retina. 21. Phosphodiesterase activation in a photoreceptor leads to c) Light entering your eye would not be refracted (directly and indirectly): d) More than one of the above are correct. a) the activation of transducin b) a break down of cGMP into GMP 18. Which of the following would cause the lens of the eye to c) Na+ channel closing become more round? d) Na+ channel opening a) focusing on your pencil while bringing it from your nose to e) B and C one arm-length away. f) B and D b) zonule fiber tightening c) ciliary muscle contraction 22. Some snakes can see infrared electromagnetic radiation d) more than one of the above (wavelengths above 700nm), why can't we? e) none of the above a) This energy is not present in our environment. b) Our photopigments are not sensitive to these 19. In general, which of the following are true about the wavelengths. retina and its organization: c) We do not have a cortical area that can process this a) light passes through ganglion cells before photoreceptors information. b) only photoreceptors and horizontal cells are sensitive to d) It would turn us red and drive us insane. light (contain photopigment) c) only ganglion and amicrine cells project out of the eye 23. Under prolonged scotopic light conditions: d) there are no cones in the peripheral retina a) your rods saturate and can't hyperpolarize anymore e) more than one of the above b) your cones saturate and can't depolarize anymore c) rod hyperpolarization correlates with the light intensity 20. Dark Current d) you will be forced to tear up this exam and eat it a) refers to a resting membrane potential that is more hyperpolarized than that of a typical neuron Answers: b, c, a, e, e, b, c


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