Download AHD Legault Visual system Apr 1

The Visual system
or Chapter 20 of Fundamental Neuroscience
Genevieve Legault
AHD
Wednesday April 1st 2008
Plan of presentation
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Brief overview of eye anatomy
Retina anatomy
Different photoreceptors
Transduction of signal
Retinal cells
Visual pathway
Visual cortex and its organization
Summary
Objectives 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:
Anatomy quizz → GUESS WHO ….
Anatomy quizz
• I’m a transparent protective coating for the
optic structures.
Brief overview of eye anatomy
Anatomy quizz
• I’m a transparent protective coating for the
optic structures.
• Its lateral margin are continuous with which
structure?
• Conjunctiva!
Anatomy quizz
• Circumferentially organized muscle
Brief overview of eye anatomy
Anatomy 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 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 quizz
• Epithelium covering which structure
produces the fluid filling the anterior
chamber?
Brief overview of eye anatomy
Anatomy quizz
• Epithelium covering which structure
produces the fluid filling the anterior
chamber?
• This fluid then drains into me.
Brief overview of eye anatomy
Anatomy 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 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?
• Vision is typically:
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1) blurred and dimmed
2) not blurred but dimmed
3) blurred but not dimmed
4) normal
Anatomy quizz
• Radially arranged muscle
Brief overview of eye anatomy
Anatomy 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!
• With light, ACh is released on both
muscarinic sphincter muscles (contraction)
and dilator muscles (presynaptic inhibition
of NE release, therefore blocking dilator
contraction)
Anatomy quizz
• Uvea is formed by which 3 structures?
Anatomy 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 anatomy
Retina = neural retina + retinal
pigment epithelium
→ Continuous sheet
of cuboidal cells
bound by tight
junctions
Functions:
-nutrition supply
-protection of
photoreceptors
-phagocytosis
Neural retina = photoreceptors and
associated 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 retina
• ****
Pathway of
light and
neural
outflow are
inverted
****
Blood 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
Photoreceptors
• 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 - Cilium
2 types of photoreceptors
• Rods
• Cones
Transduction
• Conformatio
nal change
• ↓cGMP
• Closing Na+
current
• Hyperpolariz
ation
• Passive
propagation
Transduction
• Photoreceptors are the only
sensory neurons that
hyperpolarize in response to
the relevant stimulus
Transduction
• 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
Cones
• 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 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
Fovea
• Light reaches the
macula
• Center = fovea
Fovea
• 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 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 synapses
• Only ganglion cells have voltage-gated Na+
channels therefore only one using action
potentials
• Other cells use graded potentials
Retinal synapses
• Outer plexiform layer
– One photoreceptor
– 1 centrally placed bipolar cell
– 2 laterally placed horizontal cells
triad
• Inner plexiform layer
– Bipolar cells with on or off ganglion cells
– Amacrine cells with ganglion cells, other
amacrine cells, and bipolar cells
Horizontal cells
• Course parallel to the retina
• Glutaminergic input from photoreceptor
• GABAergic output
to adjacent
photoreceptors →
inhibiting surround
to sharpen receptive
field
Bipolar 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 cells
Amacrine cells
• May contain different neurotransmitters
• Sense change in change (variation in speed)
Ganglion 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 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 types
•
•
•
•
Other types: gamma, delta, epsilon
W cells
Smaller cell bodies
Variety of receptive field sizes and
physiologic responses
Retinal projections
Retinal projections
Retinogeniculate
projections
Optic 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 Nucleus
• Layers 1 to 6,
ventral to dorsal
• 1 and 2: M type
– Rapid
– Larger field
– Sensitive to
moving stimuli
• 3-6: P type
Temporal retina (Nasal VF): layers 2,3 and 5
Nasal retina (Temporal VF): layers 1,4 and 6
– Slower
– Smaller field
– Tonic response to stationnary stimuli
Optic radiations
• Geniculostriate or
geniculocalcarine
pathway
Through retrolenticular limb of
internal capsule
Lingual gyrus
Cuneus
Blood supply
Thalamogeniculate
artery, branch of
Ant
PCAchoroidal artery
Anteromedial
branches of
Branches
Acom andofA1
ophthalmic
artery
Branches
of MCA
and PCA
Primary 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
neurons
• 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 organization
• Orientation columns
– Perpendicular to surface:
• Random # of simple, complex, hypercomplex
• same optimal stimulus orientation
• Ocular dominance columns
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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 organization
Other visual cortical areas
In 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 Messages
• 2 types of photoreceptors
2 types of photoreceptors
2 types of photoreceptors
• Rods
– Rhodopsin
– Smaller
spherule
– More in
periphery
• Cones
– Opsin
– Larger
pedicle
– More
central
Take Home Messages
• 2 types of photoreceptors
• 2 types of ganglion cells
Ganglion 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 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 Pathway
Take 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
Fovea
Take 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 again
Take Home Messages
• Photoreceptors are the only
sensory neurons that hyperpolarize
in response to the relevant stimulus
• Which cell(s) can detect speed change?
Take Home Messages
• Photoreceptors are the only
sensory neurons that hyperpolarize
in response to the relevant stimulus
• Which cell(s) can detect speed change?
– Amacrine cells
Take Home Messages
• Photoreceptors are the only
sensory neurons that hyperpolarize
in 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 Messages
• Photoreceptors are the only
sensory neurons that hyperpolarize
in 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 Messages
• Primary visual cortex have
– Orientation columns
– Ocular dominance columns
THANK YOU!
QUESTIONS???