the visual system or chapter 20 of fundamental neuroscience
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The Visual system or Chapter 20 of Fundamental Neuroscience. Genevieve Legault AHD Wednesday April 1 st 2008. Plan of presentation. Brief overview of eye anatomy Retina anatomy Different photoreceptors Transduction of signal Retinal cells Visual pathway - PowerPoint PPT PresentationTRANSCRIPT
The Visual systemThe Visual systemor Chapter 20 of Fundamental Neuroscienceor Chapter 20 of Fundamental Neuroscience
Genevieve LegaultAHD
Wednesday April 1st 2008
Plan of presentationPlan of presentation
• Brief overview of eye anatomy • Retina anatomy• Different photoreceptors• Transduction of signal• Retinal cells• Visual pathway• Visual cortex and its organization• Summary
Objectives of presentationObjectives of presentation
• Understand the mechanism of photoreceptors
• Recognize the differences between rods and cones
• Know the visual pathway, and their associated VF defects if interrupted
• Learn the organization of the visual cortex
Brief overview of eye anatomy:Brief overview of eye anatomy:
Anatomy quizz → GUESS WHO ….
Anatomy quizzAnatomy quizz
• I’m a transparent protective coating for the optic structures.
Brief overview of eye anatomyBrief overview of eye anatomy
Anatomy quizzAnatomy quizz
• I’m a transparent protective coating for the optic structures.
• Its lateral margin are continuous with which structure?
• Conjunctiva!
Anatomy quizzAnatomy quizz
• Circumferentially organized muscle
Brief overview of eye anatomyBrief overview of eye anatomy
Anatomy quizzAnatomy quizz
• Iris sphincter is paraΣ:– Begins with preganglionic
neurons with cell bodies in EW nucleus
– Axons end in the ciliary ganglion
– Postganglionic end as neuromuscular synapse (ACh)
Anatomy quizzAnatomy quizz
• Iris sphincter is paraΣ:– Begins with preganglionic
neurons with cell bodies in EW nucleus
– Axons end in the ciliary ganglion
– Postganglionic end as neuromuscular synapse (ACh)
Anatomy quizzAnatomy quizz
• Epithelium covering which structure produces the fluid filling the anterior chamber?
Brief overview of eye anatomyBrief overview of eye anatomy
Anatomy quizzAnatomy quizz
• Epithelium covering which structure produces the fluid filling the anterior chamber?
• This fluid then drains into me.
Brief overview of eye anatomyBrief overview of eye anatomy
Anatomy quizzAnatomy quizz
• Epithelium covering which structure produces the fluid filling the anterior chamber?
• This fluid then drains into me.
• If the outflow is obstructed, we then get me.
Glaucoma capsuleGlaucoma capsule
• Damage from the periphery toward the center• ~ 90% open-angle or normal angle
– Idiopathic ↑ in pressure• ~ 5% angle abnormally acute (closed-angle)
– Obstruction of normal flow of fluid• Remaining: canals blocked by debris
(infection, DM, hemorrhage into anterior chamber)
Glaucoma: which one is true?Glaucoma: which one is true?
• Vision is typically:– 1) blurred and dimmed– 2) not blurred but dimmed– 3) blurred but not dimmed– 4) normal
Anatomy quizzAnatomy quizz
• Radially arranged muscle
Brief overview of eye anatomyBrief overview of eye anatomy
Anatomy quizzAnatomy quizz
• Iris dilator is Σ:– Preganglionic begins in the intermediolateral
cell column of the spinal cord, in the upper thoracic region
– Axons end in superior cervical ganglion– Postganglionic end in neuromuscular synapse
(NE)
Iris is smart!Iris is smart!
• With light, ACh is released on both muscarinic sphincter muscles (contraction) and dilator muscles (presynaptic inhibition of NE release, therefore blocking dilator contraction)
Anatomy quizzAnatomy quizz
• Uvea is formed by which 3 structures?
Anatomy quizzAnatomy quizz
• Uvea is formed by which 3 structures?– Iris– Ciliary body– Choroid
• Highly vascularized• Pigmented tissue layer between retina and sclera
Brief overview of eye anatomyBrief overview of eye anatomy
Retina = neural retina + Retina = neural retina + retinal retinal pigment epitheliumpigment epithelium
→ Continuous sheet of cuboidal cells bound by tight junctions
Functions:-nutrition supply-protection of photoreceptors-phagocytosis
Neural retina = photoreceptors and Neural retina = photoreceptors and associated neuronsassociated neurons
• Photoreceptors absorb quanta of light (photons) and convert it into electrical signal
• Ganglion cells send axons as the optic nerve
7 layers of neural retina7 layers of neural retina
• **** Pathway of light and neural outflow are inverted
****
Blood supplyBlood supply
• Internal carotid artery↓
ophthalmic artery → Posterior ciliary artery→Central retinal artery - external portion
-inner retina of the optic nerve (neural retina) - choroidal -pial arteries to circulation optic nerve -outer retina
PhotoreceptorsPhotoreceptors
• Detection and transduction of light in outer segment pointing toward the pigment epithelium
• Narrow stalk (cilium) connects to the inner segment (containing mitochondria)
• Then outer plexiform layer where synapse
Pathway of light - CiliumPathway of light - Cilium
2 types of photoreceptors2 types of photoreceptors
• Rods • Cones
TransductionTransduction
• Conformational change
• ↓cGMP• Closing Na+
current• Hyperpolariz
ation• Passive
propagation
TransductionTransduction
• Photoreceptors are the only sensory neurons that hyperpolarize in response to the relevant stimulus
TransductionTransduction
• At rest:– ↑ cGMP level– ↑ Na+ current– Resting potential -40
mV– Constant glutamate
release
• Light:– ↓ cGMP level– Blocks Na+ current– Hyperpolarize: -60 mV
– ↓ in tonic glutamate release
ConesCones
• 3 types, each tuned to a different wavelength– L-cones = long wavelengths (red cones)– M-cones = medium wavelengths (green cones)– S-cones = short wavelengths (blue cones)
• Each color = unique combination
Color confusionColor confusion
• Genetic defect in one of the opsin (one type of cone)– L and M opsins are located on Chromo X,
therefore more frequent in ♂
– Inability to perceive red = protanopia– Inability to perceive green = deuteranopia
FoveaFovea• Light reaches the
macula• Center = fovea
FoveaFovea• Light reaches the
macula• Center = fovea• Thinner inner retina,
to allow max of light (outer nuclear and photoreceptor outer segment only in the center)
• Only cones
Receptive fieldsReceptive fields
• Definition:– Sum of the areas in which the stimulus affects
the activity of that neuron• Roughly circular
– Center – Doughnut-shaped outer rim with usually the
opposite response
Retinal synapsesRetinal synapses
• Only ganglion cells have voltage-gated Na+ channels therefore only one using action potentials
• Other cells use graded potentials
Retinal synapsesRetinal synapses
• Outer plexiform layer– One photoreceptor– 1 centrally placed bipolar cell triad– 2 laterally placed horizontal cells
• Inner plexiform layer– Bipolar cells with on or off ganglion cells– Amacrine cells with ganglion cells, other
amacrine cells, and bipolar cells
Horizontal cellsHorizontal cells• Course parallel to the retina• Glutaminergic input from photoreceptor• GABAergic output
to adjacent photoreceptors → inhibiting surround to sharpen receptive field
Bipolar cellsBipolar cells
• Between photoreceptor cells and ganglion cells
• 1st cells to exhibit the center-surround receptive field organization
• 2 types– On: depolarizing, sign-inverting– Off: hyperpolarizing, sign-conserving
Bipolar cellsBipolar cells
Amacrine cellsAmacrine cells
• May contain different neurotransmitters
• Sense change in change (variation in speed)
Ganglion cellsGanglion cells
• Output cells of the retina: axons converge on the optic disc to form the optic nerve
• Also have center-surround receptive fields (like bipolar cells)
Ganglion cells: different typesGanglion cells: different types
• Alpha• Largest• Periphery (input mainly
from rods)• Y cells• Participate little in color,
larger receptive field• M cells = magnocellular
layers in LGN
• Beta• Medium-sized• Central retina (input
mainly from cones)• X cells• Color, small receptive
fields• P cells = parvocellular
layers in LGN
Ganglion cells: different typesGanglion cells: different types
• Other types: gamma, delta, epsilon• W cells• Smaller cell bodies• Variety of receptive field sizes and
physiologic responses
Retinal projectionsRetinal projections
Retinal projectionsRetinal projections
Retinogeniculate projections
Optic disc and nerveOptic disc and nerve
• No photoreceptors in the optic disc (only ganglion cell axons): blind spot
• Whereas greatest visual acuity is at the fovea
• Subarachnoid space extends along the ON: ↑ICP can block axoplasmic flow and lead to stasis and papilledema
Lateral Geniculate NucleusLateral Geniculate Nucleus• Layers 1 to 6,
ventral to dorsal• 1 and 2: M type
– Rapid– Larger field– Sensitive to
moving stimuli• 3-6: P type
– Slower– Smaller field– Tonic response to stationnary stimuli
Temporal retina (Nasal VF): layers 2,3 and 5
Nasal retina (Temporal VF): layers 1,4 and 6
Optic radiationsOptic radiations
• Geniculostriate or geniculocalcarine pathway
Through retrolenticular limb of internal capsule
Lingual gyrusCuneus
Blood supplyBlood supply
Anteromedial branches of Acom and A1Branches of
ophthalmic artery
Ant choroidal artery
Thalamogeniculate artery, branch of PCA
Branches of MCA and PCA
Primary visual cortexPrimary visual cortex
• Striate cortex, area 17• Cortex with wide layer IV, with an extra
band of myelinated fibers: stria of Gennari, which give its name to the cortex
• Macular sparing: collateral of MCA to caudal parts of visual cortex
Cortical Cortical neuronsneurons
• Concentric, as retinal ganglion cells and LGN cells
• Elongated receptive fields:– Simple: anywhere, but max when entirely fills– Complex: sensitive to position and angle– Hypercomplex: also sensitive to lenght of
stimulus (if extends into inhibitory zone, will ↓ the response)
Columnar organizationColumnar organization
• Orientation columns– Perpendicular to surface:
• Random # of simple, complex, hypercomplex• same optimal stimulus orientation
• Ocular dominance columns– Stronger response when stimulus from one eye– Adjacent ocular dominant column: other eye– 1 ipsi + 1 contralat = hypercolumn– Critical to stereopsis (depth perception): requires proper
stimulation from both eyes
Columnar organizationColumnar organization
Other visual cortical areasOther visual cortical areas
In briefIn brief
• Ganglion cells are the ouput cells of the retina• Damage to optic radiations may result in
homonymous quadrantanopia• Lesions in the visual cortex may result in macular
sparing• Interruption of visual input from 1 eye during
critical period may result in loss of stereopsis• Lesions of association cortices result in various
types of agnosia
Take Home MessagesTake Home Messages
• 2 types of photoreceptors
2 types of photoreceptors2 types of photoreceptors
2 types of photoreceptors2 types of photoreceptors
• Rods– Rhodopsin– Smaller
spherule– More in
periphery
• Cones– Opsin– Larger
pedicle– More
central
Take Home MessagesTake Home Messages
• 2 types of photoreceptors• 2 types of ganglion cells
Ganglion cells: different typesGanglion cells: different types
• Alpha• Largest• Periphery (input mainly
from rods)• Y cells• Participate little in color,
larger receptive field• M cells = magnocellular
layers in LGN (layers 1 and 2): localization
• Beta• Medium-sized• Central retina (input
mainly from cones)• X cells• Color, small receptive
fields• P cells = parvocellular
layers in LGN (layers 3 through 6): recognition
Take Home MessagesTake Home Messages
• 2 types of photoreceptors• 2 types of ganglion cells• Lesions at different places in visual
pathway produce typical VF defects: know your anatomy!!
Visual PathwayVisual Pathway
Take Home MessagesTake Home Messages
• 2 types of photoreceptors• 2 types of ganglion cells• Lesions at different places in visual
pathway produce typical VF defects• Fovea has stronger VA, therefore thinner
inner retinal layers
FoveaFovea
Take Home MessagesTake Home Messages
• 2 types of photoreceptors• 2 types of ganglion cells• Lesions at different places in visual
pathway produce typical VF defects• Fovea has stronger VA, therefore thinner
inner retinal layers– mainly cones
Fovea againFovea again
Take Home MessagesTake Home Messages
• Photoreceptors are the only sensory neurons that hyperpolarizein response to the relevant stimulus
• Which cell(s) can detect speed change?
Take Home MessagesTake Home Messages
• Photoreceptors are the only sensory neurons that hyperpolarizein response to the relevant stimulus
• Which cell(s) can detect speed change?– Amacrine cells
Take Home MessagesTake Home Messages
• Photoreceptors are the only sensory neurons that hyperpolarizein response to the relevant stimulus
• Which cell(s) can detect speed change?– Amacrine cells
• Which retinal cell(s) have center-surround receptive field?
Take Home MessagesTake Home Messages
• Photoreceptors are the only sensory neurons that hyperpolarizein response to the relevant stimulus
• Which cell(s) can detect speed change?– Amacrine cells
• Which retinal cell(s) have center-surround receptive field?– Bipolar and ganglion cells
Take Home MessagesTake Home Messages
• Primary visual cortex have – Orientation columns– Ocular dominance columns
THANK YOU!THANK YOU!
QUESTIONS???