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Neuroscience Exam 3: Visual System
Terms in this set (68)
3 Layers of the eye
-Consists of tough white fibrous tissue
-The anterior portion is called the cornea (transparent and lets light enter)
-Continuous with the iris and ciliary body
-Pigmented part of the eye
-Central opening of iris is the pupil
-Pupil is controlled by circular and radial muscle of the iris
-Controls amount of light that enters the eye
-Constricts to prevent damage tot he retina by bright light
-Dilation of pupil increases sensitivity of eye
-Helps bring images into focus by changing the depth of field
-Optic nerve exits the retina at a region called ht optic disc
-Blood vessels of the eye enter at the optic disc
-There are no photoreceptors in the optic disc thats why it is called the blind spot
-Circular portion in the lateral region of the retina
-Important for central vision
-Has a depression called the fovea, contain cones
-Fovea centralis is area of retina w maximum visual acuity
-Space btw the lens and cornea
-Filled w aqueous humor
-Produced by epithelial cells of the ciliary processes that make up the vascular component of the ciliary body
-Flows into the anterior chamber through the pupil and provided nutrients tot he lens and cornea
-Reabsorbed into canal of Schlemm (located at junction of iris and cornea)
Aqueous Humor Disorders
-Normally production and uptake are in equilibrium
-Accumulation of fluid causes pressure exerted on the anterior and posterior chambers to inc and thus not he entire eye
-Increased intraocular pressure reduces blood flow tot he eye -> leading to damage of retina causing glaucoma (major cause of permanent blindness)
-Starts in the periphery and can affect central vision
1. Primary Open-angle glaucoma
2. Closed-angel glaucoma
3. Normal-tension glaucoma
Primary Open-Angle Glaucoma
-Most common form of glaucoma
-Occurs when drainage canals become blocked
-Removal of aqueous humor is limited due to blockage in canal of Sclemm
-Treated with prostaglandin analog (inc outflow from anterior chamber) and beta-adrenergic receptor blockers (decrease secretion)
-Eye pressure rises quickly
-Drainage canals get blocked or covered
-Anterior chamber narrows by forward movement of the iris
-Treated w drugs or removal of the iris
-Optic nerve is damaged even though the pressure in the eye is not very high
-High risk: ppl w family history of NTG, ppl of Japanese ancestry, ppl w a history of systemic heart disease, such as irregular rhythm
-Gelatinous material btw lens and retina
-Contain phagocytes that remove debris and blood int he eye
Order that light passes through the eye
-Light passes through the cornea, lens and anterior and posterior chambers and reach the photoreceptors in the retina.
-Focusing of images on the photoreceptors depends on the refraction of light rays as they pass through the cornea and lens
-The change in refractive power of the lens
-Attaches to ciliary muscles and hold the lens in place
-Forms a ring
-When it contracts, the zonule fibers relax, the tension on the lens is reduced and its shape becomes rounder (natural shape) and thicker (great for near sightedness)
-When the ciliary muscle is relaxed, the zonule fibers are stretched to exert tension on the lens and its shape is thin and flat. Good for distant vision
-With age the lens may not completely round up bc of a decrease in elasticity
Layers of the Retina
1. Pigment layer
2. Photoreceptor layer
3. Outer nuclear layer
4. Outer plexiform layer
5. Inner nuclear layer
6. Inner plexiform layer
7. Ganglion cell layer
8. Nerve fiber layer.
-Outermost layer of the retina.
-Cell provide nutrition (glucose and essential ions) to photoreceptors
-The black pigment (melanin) absorbs any light not captured by the retina and prevents it from reflecting back to the retina. Protects photoreceptors from damaging levels of light
Layers of Rods and Cones
There are more rods that cones in the retina except in the fovea (area of greatest visual acuity) which contains only cones
Outer Plexiform Layer
-Contains axonal processes of rods and cones, horizontal cells and dendrites of bipolar cells
Inner Plexiform Layer
-Contains axons of bipolar cells, processes of amacrine cells, dendrites of ganglion
-Synaptic interaction between retinal cells
-Have 3 segments
1. Outer segments: located towards the outer surface of the retina and involved in phototransduction
-Contains a stack of discs that have light absorbing pigments
2. Inner segments - contains the nucleus and most of the biosynthetic mechanism
-Connected to the outer segment by a stalk or cilium that contains microtubules
3. Synaptic terminal - make connection with other cells
-Responsible for day vision and concentrated in the fovea.
-Loss of cones leads to blindness. Vision mediated by cones has higher acuity than rods
-Cones mediates color vision and have a faster response with shorter integration time
-Their conical shape makes them more sensitive to direct axial rays
-Are very sensitive and detect dim light (specialized for night)
-Loss of rods leads to night blindness and peripheral vision
-Contain more photosensitive pigment responsible for the ability of rods to capture more light
-Rods respond slowly to light with graded change in membrane potential.
Melanopsin-Containing Retinal Ganglion Cells
-A 3rd type of mammalian photoreceptor
-They utilize a different photopigment (melanopsin), are less sensitive to light, and have less spatial resolution.
-Implicated in photophobia in migraine headache sufferers.
Phototransduction in Darkness
-cGMP-gated Na+ channels are present on the outer segment of the rods and cones
-cGMP binds to cytoplasmic surface of channels allowing Na+ influx.
-In darkness, high levels of cGMP open the channels -> influx Na+ into the outer segment, so the photoreceptor remains depolarized in darkness and K+ flows out of the inner segment through leak channels
-Concentrations of Na+/K+ is maintained by Na+/K+ ATPase
-n rods, the photo pigment rhodopsin is present and it's very sensitive to light
Phototransduction: In the presence of Light
1. Retinal absorbs light resulting in the conformational change of the photoreceptor pigment and a G-protein (transducin in rods) is activated
2. G-protein activates cGMP-phosphodiesterase (PDE)
3. PDE hydrolyzes cGMP and reduces its concentration
4. Reduction in the concentration of cGMP results in closing the cGMP-gated Na+ channels
5. Influx of Na+ is reduced and cell is hyperpolarized
6. Since rods and cones synapse onto bipolar and horizontal cells, the signal is transmitted to their dendrites
-Circular area of the retina when stimulated changes the membrane potential of the cell
-The field has 2 parts: stimulus to these parts are opposite
1. Field center: provides direct input from photoreceptors to bipolar cells
2. Surround center: provides indirect input via horizontal cells
Ganglion Cells: P cells
-Project to the parvocellular layer of the lateral geniculate nucleus
-Response to visual stimuli is sustained
-Can transmit information about color
Ganglion Cells: M Cells
-Project to magnocellular layers of the lateral geniculate nucleus
-Larger cell bodies, dendritic fields and axons
-Response to stimuli is transient
-Can't transmit information about color
-Processing in the visual cortex involves integration of the response of the cones, horizontal cells, ganglion cells and lateral geniculate body cells
Blood Supply of the Retina
-Receives from branches of the ophthalmic artery:
-Central retinal artery enters at the optic disc and supplies inner portion of neural retina
-Ciliary artery penetrates sclera near the exit of the optic nerve and supplies a part of choroid called choroicapillaries
-Region of space that the eye can see looking straight ahead without movement of the head
-The fovea of each retina is aligned with a fixation point
-A vertical line can divide the visual field of each eye into 2 halves: the left half and right field
-Divided into 2 halves by a vertical line drawn through the center of the fovea:
1. A nasal hemiretina that lies medial to the fovea
2. Temporal hemiretina that is lateral to the fovea.
-Opposite and inverted version of the visual field
Pupillary Light Reflex
1. Axons of RGC project to pretectal area via the optic tract
2. Pretectal cells send bilateral projections to preganglionic PS neurons in Edinger-Westphal (EW) nucleus (CN III nuclei)
3. Axons leave the brainstem via the CN III nerve and synapse on postganglionic PS in the ciliary ganglion
4. The postganglionic fibers enter the eye and innervate the circular muscle of the iris and circumferential muscle of the ciliary body
What happens when you shine a bight light in pupil
-Discharged neurons in pretectal area excite preganglionic PS cells in EW.
-Results in the inc of PS innervation to smooth muscles of the pupillary sphincter which contracts the pupil (miosis) and reduce the amount of light that enters the eye.
-Shining a light in one eye normally causes constriction of the pupil (direct pupillary light reflex) and constriction of the pupil of the other eye (consensual pupillary light reflex). This occur because of bilateral pretectal projection to the EW nuclei
-Involved in focusing from a distant object to a near one
-Involved 3 events
Accommodation Reflex: Event 1
-Convergence of the eye: direct or indirect projections of EW activate medial rectus bilaterally
Accommodation Reflex: Event 2
-Pupillary reflex: activate PS pathways from EW nucleus to ciliary ganglion and postganglionic fibers to pupillary muscles
Accommodation Reflex: Event 3
-Fixation of the lens for near vision: activation of parasympathetic component of CN III
-Preganglionic fibers project to ciliary ganglion which then sends postganglionic fibers to circumferential muscles fibers.
-Contraction of these muscles reduces tension on suspensory ligament and allows lens to become more convex
Entire Visual Pathway
-The axons of RGC assemble at the optic disc and pass into the optic nerve, which enters the cranial cavity through the optic canal.
-The two optic nerves converge to form the optic chiasm on the base of the brain
-In the chiasm, axons from the nasal portions of the 2 retina decussate and pass into the contralateral optic tract, while those from the temporal hemiretina remain ipsilateral.
-The optic tracts diverge away from the chiasma and pass round the cerebral peduncle to terminate mainly in the LGN
-A small number of fibers leave the optic nerve, before reaching the LGN to terminate in the pretectal area and the superior colliculus. (Pupillary light reflex)
-From the LGN, 3rd order neurons project through the retrolenticular part of the internal capsule and form the optic radiation, which terminates in the primary visual cortex (PVC) of the occipital lobe.
-The PVC is located predominantly on the medial surface of the hemisphere in the region above and below the calcarine sulcus.
-Surrounding this area, the rest of the occipital lobe constitutes the visual association cortex.
-Project to area 17 (striate cortex)
-Fibers from lateral portion of the LGN go over the temporal horn of the lateral ventricle and enter the internal capsule (retrolenticular pathway) and project to the calcarine cortex
-Fibers from medial part of the LGN go dorsally into the lateral capsule and pass through part of the parietal lobe to the calcarine cortex
-Area 17 also project to visual association cortex
Visual Association Cortices
-Include the entire occipital lobe(Brodmann's areas 17, 18, and 19),
-The middle and inferior temporal gyri
-Undersurface of the temporal lobe
-Both superior and medial parietal cortex (Brodmann's area 7)
-The retina has a major projection here
-Controls saccadic eye movements
-Cells in the SC are very sensitive to movement
-SC projects to the pulvinar nucleus in Thalamus
-In the normal eye, light from a distant object is focused on the retina when the ciliary muscle is relaxed.
-To focus on a near object, the ciliary muscle must contract to accommodate for near vision
-Hyperopia; Far Sightedness
-caused by either small eyeball or weak lens so that light can't bend properly and is focused behind retina when the muscle is relaxed
-Corrected w a convex lens, causing light to converge and focus on the retina
-Caused by the increase of the eyeball or increased power of the lens
-When the muscles are relaxed light is focused in front of the retina
-Corrected with a concave lens, which diverge the light and focus it on the retina
-Shape of the cornea is oblong and so the curvature of the cornea is not uniform.
-Seen in infants
-Axis of the eyes are not parallel and diplopia can occur.
-Deficiency in vitamin A can reduce the amount of photosensitive pigments in rods and cones.
-Caused by infarctions that affect CN III
-Caused by lesions in the pretectal area.
-There is paralysis of upper gaze, a large pupil and retracted eyelids
-With tabis dorsalis, the pupils don't contract in response to light but have accommodation reflex
-Prolonged and sluggish constriction of the pupil to light.
-Dilation of the pupil is delayed
-One of the eyes has an optic nerve lesion.
-When light is shone on the normal eye, direct and consensual reflex is seen
-In the eye with the lesion, less signals reach the EW which turn off the PS response to light causing paradoxical dilation of both pupils
Damage to Optic Nerve
Caused by: multiple sclerosis and optic nerve tumors
-Lead to loss of vision in the affected eye (monocular blindness).
Damage to the Optic Chiasm
-Interrupt axons from the nasal hemiretina of both eyes leading to loss of vision to the right half of the right visual field and left half of the left visual field
-Visual Defect: Bitemporal hemianopia
-Compression of the optic chiasm by an adjacent pituitary tumor
Damage to Right Optic Tract
-Axons from the ganglion cells of the temporal retina of the right eye and the nasal retina of the left eye are damaged
-Visual Defect: Contralateral (left) homonymous hemianopia
-Objects in the left side of the visual field of each eye cannot be seen
Damage to the Right Temporal Lobe
-Including damage to Meyer's loop (fibers represent inferior right retinal quadrant)
-Vascular occlusions in the MCA can cause a lesion in the inferior portion of the temporal lobe
-Visual defect: Superior left homonymous quadrantanopia (pie in the sky)
Damage to the Right Parietal Lobe
-Caused by lesions in the MCA.
-Affects geniculocalcarine tract
-Visual Defect: Inferior left (contralateral) homonymous quadrantanopia (pie in the floor)
Damage to Entire Geniculocalcarine Tract
-Damage is similar to right optic tract.
-Visual defect: Contralateral homonymous hemianopia
Lesion of the Inferior Bank of the Calcarine Fissure
-Lesion produces a superior left homonymous quadrantanopia with macula sparing due to the dual vascular supply by branches of both the middle and posterior cerebral arteries
Lesion of the Superior Bank of the Calcarine Fissure
-Receives fibers from the LGN associated with input from the inferior quadrant of the contralateral visual field
-Lesion produces a inferior left homonymous quadrantanopia with macula sparing
Damage to the Entire Right Calcarine Cortex
-Both superior and inferior banks
-Contralateral homonymous hemianopia w macula sparing
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