The Visual system or Chapter 20 of Fundamental Neuroscience

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


• I’m a transparent protective coating for the optic structures.

• Its lateral margin are continuous with which structure?

• Conjunctiva!


• Circumferentially organized muscle



• 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)


• 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)


• Epithelium covering which structure produces the fluid filling the anterior chamber?




• This fluid then drains into me.




• 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


• Radially arranged muscle



• 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)


• Uvea is formed by which 3 structures?


• Uvea is formed by which 3 structures?– Iris– Ciliary body– Choroid

• Highly vascularized• Pigmented tissue layer between retina and sclera


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


• Photoreceptors are the only sensory neurons that hyperpolarize in response to the relevant stimulus


• 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


• 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



• Rods– Rhodopsin– Smaller

spherule– More in

periphery

• Cones– Opsin– Larger

pedicle– More

central


• 2 types of photoreceptors• 2 types of ganglion cells


• 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


• 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



pathway produce typical VF defects• Fovea has stronger VA, therefore thinner

inner retinal layers

FoveaFovea



pathway produce typical VF defects• Fovea has stronger VA, therefore thinner

inner retinal layers– mainly cones

Fovea againFovea again


• Photoreceptors are the only sensory neurons that hyperpolarizein response to the relevant stimulus

• Which cell(s) can detect speed change?



• Which cell(s) can detect speed change?– Amacrine cells




• Which retinal cell(s) have center-surround receptive field?




• Which retinal cell(s) have center-surround receptive field?– Bipolar and ganglion cells


• Primary visual cortex have – Orientation columns– Ocular dominance columns

THANK YOU!THANK YOU!

QUESTIONS???

Documents

The Visual system or Chapter 20 of Fundamental Neuroscience