Vision Flashcards
*Identify the major parts of the eye?
[cornea: transparent cells, does most of the refraction (bending) of light to bring image in focus on fovea; lens, transparent cells, does some refraction, and flexes to increase refraction, bringing close objects into focus; aqueous and vitreous humor, fluids of the respective anterior and posterior chambers; retina, CNS tissue containing rods, cones and ganglion cells, where transduction occurs; choroid, dark pigmented layer that absorbs light, part of inner choroid includes retinal epithelium layer; sclera: tough, outer covering of eye, white, and not present anteriorally (where cornea is)]
*Identify the layers of the retina from choroid?
[receptor layer (rods/cones), outer nuclear layer (cell bodies of rods/cones), outer plexiform (synapses with receptors and bipolars), inner nuclear (bipolars, amacrines), inner plexiform (synapse with bipolars, ganglion cells), ganglion cell layer]
Name 4 nuclei that receive input from retina?
[lateral geniculate nucleus (thalamus, conscious vision), suprachiasmatic hypothalamus (diurnal rhythms), superior colliculus (mesencephalon, eye/head movements), pretectal nucleus (mesencephalon, mediates light response reflex)]
What is the visual field?
[The area over which we can see at any given time… ~180 deg across, 110 deg up/down, it is divided into a left and right visual field… the right visual field is represented in the left visual cortex, the left visual field is represented in the right visual cortex]
What is the binocular visual field, and how big is it?
[The visual field observed by both eyes; in humans, the binocular field covers ~120 deg] To clarify the two above, the visual field includes the entire visual field seen by the left eye AND/OR right eye. The binocular field is the field seen by the right eye AND the left eye (some parts of our visual field can only be seen by the right eye, some can only be seen by the left eye(temporal monocular crescents).
What is a right homonymous hemianopsia?
[Loss of vision in the right visual field, most likely attributable to loss of left primary visual cortex. The left/right distinction is referenced to the visual field. Homonymous refers to the fact that it’s the same loss in each eye…in this case the left half (temporal side) of the left eye, the left half (nasal side) of the right eye. Heteronymous would mean it’s different between the two eyes… the classic being a pituitary tumor which pushes up and splits the chiasm…. crossing fibers (nasal retinae) are lost, but it would be the right half of the left eye and the left half of the right eye… referred to as a heteronymous hemianopsia (hemianopia). Hemianopia references a half visual field loss (cf quadranopsia … usually loss of upper (superior) or lower (inferior) quarter of visual field
What do you call loss of vision in the left eye?
[anopia of the left eye]
What is the retina?
[where transduction and initial visual processing occur; a layer of tissue at the back of the eyeball that is about the size of postage stamp and just as thick (160 um) that contains the rods, cones, bipolar cells and ganglion cells necessary for vision]
Name the major cell types of the retina?
[rods, cones, horizontals, amacrines and retinal ganglion cells]
What is the fovea?
[a specialization of the retina that exists as a ‘pit’, and is where the optic axis focuses light. It is pure cone (no rods), and these cones are tightly packed, ~10 um between cones. In the most central 2o , there are no blue cones, just red and green.. to minimize chromatic aberration].
Does the human fovea have any rods?
[no]
What is the range of wavelengths within the electromagnetic spectrum that constitutes the ‘visual’ spectrum?
[~400-700 nm]
*What color is at the bottom of a rainbow?
[blue, violet… because short wavelength bend more]
Is it true light travels through the ganglion cell layer of the retina to reach the rods and cones?
[yes… it probably seems stupid, but I wouldn’t 2nd guess evolution]
What vitamin is associated with rods/cones?
[vitamin A, an essential vitamin. Essential means you can’t make it, or you can’t make enough of it, so you need it in your diet; vitamins are organics that catalyze reactions. Retinal is a form of vitamin A that combined with opsin (a protein) constitutes rhodopsin]
Does light hyperpolarize or depolarize a rod/cone?
[hyperpolarize, decreasing the ‘dark current’, which controls the ‘drip’ rate of glutamate at the synapse]
*What happens when a photon hits a rod?
[The vitamin A (retinal) part of the rhodopsin molecule flips from cis to the more stable trans conformation. This form activates 100’s of transducin molecules which activate 100’s of phosphodiesterase molecules. The phosphodiesterase hydrolyzes cGMP which closes the dark current (decreased Na+ conductance), hyperpolarizing the rod and decreasing glutamate drip onto the bipolar cell]
What is the ‘dark current’?
[an inward (depolarizing) current attributable to Na+ conductance when the rod/cone is in the dark]
In myopia (nearsightedness), the image is focused in front of the fovea?
[true, somewhere in the vitreous]
In presbyopia, the lens has lost its flexibility?
[probably true. This happens with age… older people (> 40) need convex (positive) lens to increase refraction bringing close objects into focus]
At the optic chiasm, the axons crossing are from the _______ retina?
[nasal]
Humans have 3 cone systems but can identify _____ hues?
[> 300].
- What are hues?
[separable colors; we can do this because the opsins associated with each cone system have broad absorption spectra and peak at different wavelengths… it is the relative activation of each of the 3 cone systems that allow us to name tens of hues]
*Are there tetrachromats?
[Among animals, goldfish and some finches. The 4th opsin is in the ultraviolet range. It is possible there are humans (certain women, there’s a genetic link) that are true tetrachromats. Among mammals, UV sensitivity is greatly diminished by the fact that the cornea absorbs most of UV light]
What is ‘color blind’?
[a defect in color discrimination. It is almost always a genetic defect, much more common in males. The most common, red-green, is interesting because although they still have 3 cone systems, the absorption spectra of red and green nearly perfectly overlap, thus the ratios of activation are nearly identical, and there’s no discrimination on the basis of wavelength…. In reality, most of these individuals can do a good job of color discrimination because different colors reflect light at different intensities… allowing discrimination on the basis of intensity/wavelength]
*What is a detached retina?
[a portion of the retina is displaced from the choroid. It is an emergency situation as it compromises the blood supply to the retina… it can be fixed by effectively ‘spot welding’ the retina back onto the eyeball with a laser. Because of the shape of the eyeball, very myopic people are more vulnerable than others]
What are cataracts?
[clouding of the lens (most common) or cornea that degrade vision. Risk factors are age, time exposed to sunlight, diabetes]
*What is the leading cause of blindness in the US?
[diabetic retinopathy (glaucoma is 2nd)]
*What causes blindness in diabetes?
[chronic high glucose levels in blood encourage breaking of capillaries and blood to pool on retina, disturbing vision… over time, this leads to poor vascularization of retina itself (in addition to the fact that the pooling of blood detracts from the optics)]
*What is optic neuritis?
[inflammation of optic nerve, associated with visual loss (compression on fibers) and pain associated with eye movement (the sheath of the optic nerve is innervated by the ophthalmic division of V). Optic nerve is CNS, and optic neuritis is the most common 1st episode of multiple sclerosis]
