Vision Flashcards
What are the layers of the eye that are involved in refraction, and which is strongest? Which is the most variable? Which becomes opaque in cataracts?
The cornea and the lens are involved in refraction. The cornea is actually the strongest amount of refraction, but the lens provides variable refraction due to changing shape. This change in shape becomes less with age (presbyopia). The lens also can become opaque in cataracts.
How is the retina organized?
the retina has an inner neural layer and an outer layer of pigmented epithelium. The neural layer has several layers of cells and connections. The innermost layer (closest to the center of the eyeball) is the ganglion cell layer. The middle layer is the bipolar cell layer, and the outer layer is the layer of rods and cones that are the photoreceptors. In between these three sets of cells are layers with many connections (plexiform layers) which include horizontal cells near the photoreceptors and amacrine cells near the ganglion cells.
What is the structure of the photoreceptive cells (rods and cones)?
The rods and cones each have three sections. They have a synaptic region, an inner segment (that includes the nucleus and most of the organelles), and an outer segment. The outer segment includes stacks of infolded membrane, termed disk membranes. Photopigment is embedded in these disk membranes. Thus, light has to pass through all layers of the neural retina as well as the entirety of the rod or cone to reach the site where they can interact with their signaling pathways.
What is “dark current” in photoreceptive cells?
In the dark, cyclic GMP binds to and opens cGMP gated ion channels that are permeable to sodium and calcium. This results in positive ions entering the cell (“dark current”) and maintains a state of partial depolarization of the photoreceptor. This, in turn, results in constant release of neurotransmitter (glutamate) onto bipolar cells.
How does a phtoreceptor cell respond to light?
The particular light sensitivity of a photoreceptive cell is based on the specific photopigment and its concentration in disk membranes. Photopigment consists of a protein (opsin) which is a GPCR, and a molecule of cis-retinal (derivative of vitamin A). Light striking the photopigment converts the cis-retinal to trans-retinal. This change in confirmation permits the opsin to activate transducin. This, in turn, activates a phosphodiesterase that converts cGMP to GMP. This, in turn, closes the cGMP gated ion channels, resulting in hyperpolarization of the photoreceptor cell and terminating its release of glutamate onto bipolar cells. Since this glutamate is inhibitory to “on” bipolar cells, these bipolar cells will be excited (“disinhibited”). The associated ganglion cell will be excited by release of glutamate from the “on” bipolar cell.
What is color blindness?
Colorblindness results from lack of one or more of the three photoreceptive pigments contained in cones. Thus, colorblind individuals cannot discriminate certain colors (most often red from green).
What are the different types of retinal ganglion cells and their roles in visual processing?
Magnocellular (M-type) have large receptive field and appear to be involved in motion detection. Parvocellular (P-type) have smaller receptive fields and are involved in color vision. W-type ganglion cells do not have center/surround organization and respond to the overall illumination of their receptive field. At least some of these cells are directly photosensitive (they contain melanopsin). They are most sensitive to blue light and are important for circadian rhythms and pupillary reflexes.
What is the physiological blind spot?
The physiologic blind spot is the part of the retina at the beginning of the optic nerve (where ganglion cell axons leave the retina). This is the optic disc on the medial side of the retina (lateral side of the visual world). There are no photoreceptive cells in this area.
What is the distribution of rods and cones in the retina?
he fovea centralis is the region right in the middle of the retina, where central vision is most acute. There are only cone photoreceptors in this region. The more peripheral in the retina, the more rods are present (meaning greater sensitivity to light in this peripheral vision at night, but no color vision).
What is the projeciton pathway from the retina to the visual cortex?
Ganglion cell axons comprise the optic nerve. Those axons arising from the temporal visual field remain ipsilateral, while those from the nasal retinal field decussate. The optic tract continues posteriorward from the optic chiasm to reach the lateral geniculate nucleus of the thalamus. This nucleus gives rise to optic radiations to the cortex. Those optic radiations extending through the white matter under the parietal lobe go to the part of the visual cortex above the calcarine sulcus; while those looping through the temporal lobe (Meyers’ loop) terminate below the calcarine sulcus.
What other areas do retinal ganglion cells project to and what do these regions do with this information?
etinal ganglion cells project to the suprachiasmatic nucleus to entrain circadian rhythms; to the pretectal area for the pupillary light reflex (consensual pupil constriction to light exposure); and to the superior colliculus for reflex head and eye movements to novel visual stimuli.
Where does visual information first terminate in the cerebral cortex? Where is the lower visual world represented? The upper visual world? The central region of vision?
Visual information first reaches the contralateral primary visual cortex, which is located on the medial occipital lobe just above and below the calcarine sulcus. This region is called the calcarine or striate cortex. The gyrus above the sulcus processes information from the contralateral lower visual world; the gyrus below the sulcus processes information from the contralateral upper visual world. Central vision is represented at the occipital pole of the brain (most posterior).
What is the dorsal stream of visua information processing the cortex? What is the ventral stream?
The dorsal stream is the “how” and “where” pathway that is mostly focused on movement and location. An example is MT (the posterior part of the Middle Temporal gyrus) that is involved in recognizing movement of objects. The ventral stream, which mostly involves the part of the visual association area in the inferior temporal lobe, is the “what” pathway. This includes object recognition such as the fusiform gyrus, which is involved in recognizing faces (mostly in the nondominant hemisphere).
What kind of deficits should you expect from damage to the optic nerve? The optic chiasm? The optic tract? The optic radiations in the parietal lobe white matter? The optic radiations in the temporal lobe white matter?
Damage to the optic nerve should produce monocular vision problems (blindness or blind spots). Damage to the optic chiasm should produce bitemporal vision problems. Damage to the optic tract should produce contralateral homonymous hemianopia. Damage to the optic radiations in the parietal lobe white matter should produce contralateral homonymous inferior quadrantanopia. Damage to the optic radiations in the temporal lobe white matter should produce contralateral homonymous superior quadrantanopia.
accommodation
Dynamic changes in the lens of the eye that enable the viewer to focus. When viewing distant objects, the lens is made relatively thin and flat; for near vision, the lens becomes thicker and rounder and has more refractive power.
amacrine cells
Retinal neurons that mediate lateral interactions between bipolar cell terminals and the dendrites of ganglion cells.
bipolar cells
Retinal neurons that provide a direct link between photoreceptor terminals and ganglion cell dendrites.
ciliary body
Two-part ring of tissue encircling the lens of the eye. The muscular component is important for adjusting the refractive power of the lens. The vascular component produces the fluid that fills the front of the eye.
cones
Photoreceptor cells specialized for high visual acuity and the perception of color.
cornea
The transparent surface of the eyeball in front of the lens; the major refractive element in the optical pathway.
fovea
Area of the retina specialized for high acuity in the center of the macula; contains a high density of cones and few rods.
foveola
Capillary-free and rod-free zone in the center of the fovea.
glaucoma
Condition in which the eye’s aqueous humor is not adequately drained, resulting in increased intraocular pressure, reduced blood supply to the eye, and eventual damage to the retina.
horizontal cells
Retinal neurons that mediate lateral interactions between photoreceptor terminals and the dendrites of bipolar cells.
macula lutea
The central region of the retina that contains the fovea (the term derives from the yellowish appearance of this region in ophthalmoscopic examination); also, the sensory epithelia of the otolith organs.
mesopic vision
Vision in light levels at which both the rods and cones are active.
opsins
Proteins in photoreceptors that absorb light (in humans, rhodopsin and the three specialized cone opsins).
optic disk
The region of the retina where the axons of retinal ganglion cells exit to form the optic nerve and where the ophthalmic artery and vein enter the eye. Also called the optic papilla.
describe the pupillary eye reflex
This pathway includes bi-lateral projections from the retina to the pretectum and projections from the pretectum to the Edinger–Westphal nucleus. Neurons in the Edinger–West- phal nucleus terminate in the ciliary ganglion, and neurons in the ciliary ganglion innervate the pupillary constrictor muscles. Notice that the afferent axons activate both Edinger–Westphal nuclei via the neurons in the pretectum.
describe the first phase of the retinoid cycle and photoadaptation.
(A) Following photoisomerization, all-trans retinal is converted into all-trans retinol and is transported by the chaperone protein IRBP into the pigment epithelium. There, in a series of steps, it is converted to 11-cis retinal and transported back to the outer segment (again via IRBP), where it recombines with opsin.
(slap til stiv)
(A) Rhodopsin resides in the disk membrane of the photoreceptor outer seg- ment. The seven transmembrane domains of the opsin molecule enclose the light-sensitive retinal molecule. (B) Absorption of a photon of light by retinal leads to a change in configuration from the 11-cis to the all-trans isomer.
(C) The second messenger cascade of phototransduction. The change in the retinal isomer activates transduc- in, which in turn activates a phosphodiesterase (PDE). The PDE then hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of channels in the outer segment membrane.
Cyclic GMP–gated channels and light-induced changes in the electrical activity of photoreceptors.
A
This simplified diagram shows a rod, but the same scheme applies to cones. (A) In the dark, cGMP levels in the outer segment membrane are high; cGMP binds to the Na+-permeable channels in the membrane, keeping them open and allowing sodium and other cations to enter, thus depolarizing the cell.
Cyclic GMP–gated channels and light-induced changes in the electrical activity of photoreceptors.
B
This simplified diagram shows a rod, but the same scheme applies to cones. (B) Absorption of photons leads to a decrease in cGMP levels, closing the cation channels and resulting in receptor hyperpolarization.
What is the state of the lens?
Unaccommodated
What is the state of the lens?
Accommodated
