See the 2,456 pharmacies on the National E-Pharmacy Platform
Physiology of the visual pathway & cerebral integration
1. By Dr. Henok Samuel (R1)
Moderator: Dr.Tiliksew Teshome ( Associate Professor, Consultant
Ophthalmologist and Vitreoretinal surgeon)
PHYSIOLOGY OF THE VISUAL PATHWAY
& CEREBRAL INTEGRATION
2. Contents
Introduction
Recap on anatomy of visual pathway
Axonal conduction
Role of myelin in axonal conduction
Axonal transport
Optic nerve injury and regeneration
Retinotopic Organization and visual field defect
Summary
References
2
3. Introduction
The role of the central visual pathways is to process and integrate visual
information that travels to the brain by means of the optic nerves.
Visual processing poses an enormous computational challenge for the brain,
which has evolved highly organized and efficient neural systems to meet these
demands.
In primates, approximately 55% of the cortex is specialized for visual processing
(compared to 3% for auditory processing and 11% for somatosensory processing)
3
4. Introduction
When light reaches the retina, its energy is converted by retinal photoreceptors
into an electrochemical signal that is then relayed by neurons.
4
5. Retina Ganglion cells
RGC axons begin at the RGC bodies in the
inner layer of the retina.
The RGCs receive input from bipolar cells and
amacrine cells, and their axons form nerve
fiber layer.
5
6. Retina Ganglion cells
10 types of RGC found
(two of them are well studied)
Eighty percent of ganglion cells are Midget cells, 10% are Parasol cells, and 10%
are other types.
6
7. Retina Ganglion cells
7
LARGE PARASOL RGC
- Project to the magnocellular / M cell system in LGN.
-for low spatial resolution, fast temporal resolution, stereopsis, contrast
sensitivity.
- high receptive field & more for peripheral vision.
SMALL MIDGET RGC
-Project to the parvocellular / p cell system in LGN.
-for high spatial resolution but has low contrast sensitivity
-many in number & more for macular vision
“K” (koniocellular) pathway
- more recently identified, small bistratified ganglion cells
8. Retina Ganglion cells
At any given retinal eccentricity, parasol cells have a larger receptive field and
lower spatial resolution than midget cells due to their broad dendritic
arborization.
Parasol ganglion cells have spatially opponent center surround organization,
allowing edge detection, but they lack spectrally opponent organization; in
essence, these cells are color-blind.
The anatomical features of parasol cells underlie their specialization for low
spatial resolution, motion detection, and coarse stereopsis.
8
9. Cont…
Foveal ganglion cells send axons directly to
the temporal aspect of the optic disc in the
papillomacular bundle.
The remaining temporal ganglion cell nerve
axons are arranged on either side of the
horizontal raphe and form arcuate bundles
that course above and below the fovea, and
finally enter the superior and inferior
portions of the optic nerve.
Finally, axons originating nasal to the disc
enter the nasal portion of the optic nerve.
9
10. Optic Nerve Head
The ganglion cell axons form the optic nerve after they
leave the eye. And The optic disc or optic nerve head is
the point of exit for ganglion cell axons leaving the eye.
There are no rods or cones overlying the optic disc, it
corresponds to a small physiological blind spot in each
eye.
The optic disc in a normal human eye carries from 1 to 1.2
million neurons from the eye towards the brain
10
11. Optic Nerve
Each optic nerve is comprised of
approximately 1.2 million retinal ganglion cell
axons (in constrast to the acoustic nerve, for
example, which has only 31 000 axons).
The intraocular segment of the optic nerve
head (the optic disc) is typically located 3–4
mm nasal to the fovea and is 1 mm thick.
11
13. Retinotopic Organization of Optic Nerve
• In the proximal third of the optic nerve the
positions of ganglion cell axons are rearranged.
• Macular ganglion cell axons which initially lie
temporally move to the nerve’s center.
• Peripheral temporal fibers become positioned
temporally, both superior and inferior to the
macular fibers.
• Finally, nasal fibers remain in the nasal portion
of the optic nerve. 13
14. Optic Chiasm
Is the site of decussation for axons from
the optic nerve.
It lies in the subarachnoid space of the
suprasellar cistern surrounded by
meningeal sheaths and cerebrospinal
fluid
10 mm above the pituitary, which rests in
the sella turcica within the sphenoid bone
The chiasm lies within the circle of Willis. 14
16. Relations with neighboring structures
• The exact location of the chiasm with respect to the sella is variable.
•
16
17. Arrangement of Nerve fibres in Optic chiasm
The lower (inferior) fibers (subserving the
superior visual field) traverse the chiasma
low and anteriorly - are first to cross.
The upper nasal fibers traverse the chiasma
high and posteriorly.
macular fibers tend to cross decussate in the
posterior most part of chiasm which is
related to supraoptic recess.
17
18. Optic Tract
Flattened cylindrical band that
travel posteriolaterally from
angle of chiasma.
Between tuber cinereum and
anterior perforated substance
upto lateral geniculate body.
Each tract contains
uncrossed temporal fibres and
crossed nasal fibres .
18
20. Lateral Geniculate Body
(LGB)
Elevation produced by lateral
geniculate nucleus in which most optic
tract fibers end.
Axons of ganglion cells of retina
synapse with dendrites of LGB cells -
3rd order neurons begins
Primary termination of OT fibers
Each LGN receives input from the
contralateral visual field.
20
21. Lateral Geniculate Body
Dorsal nucleus
Ventral nucleus (rudimentary)
6 laminae (alternating grey & white matter)
Axons from the ipsilateral eye –2, 3, 5
Axons from the contralateral eye - 1, 4,6
21
22. Lateral Geniculate Body
Large magnocellular neurons (M
cells) - 1 and 2 layer-Y ganglion
cells - perception of movement, gross
depth, and small differences in
brightness
Small parvocellular neurons (P cells)
3,4,5,6 layer- X ganglion cells Colour perception,
texture shape &fine depth
Koniocellular cells (K cells or
interlaminar cells)
Short-wavelength "blue" cones
22
23. Retinotopic arrangement of LGB
• Macular fibres – posterior 2/3 of LGB
• Upper retinal fibres – medial half of
anterior 1/3 of LGB
• Lower retinal fibres – lateral half of
anterior 1/3 of LGB
23
24. Geniculocalcarine Tract (optic radiations)
Axons of LGN neurons travel to primary visual cortex
(Area 17) via the geniculocalcarine tract located in the
retrolenticular and sublenticular portions of the
internal capsule.
Axons from upper visual fields take a looping course
into the temporal lobe on the way to visual cortex.
(=Meyer’s loop)
Axons from lower visual fields take a more direct route
to visual cortex.
Macular fibers are in an intermediate location in the
optic radiation
24
25. Superior colliculus
Pretectum
Pulvinar
Hypothalamic nuclei
AOS
25
Extra geniculate targets of the retinal projections
26. Superior Colliculus
26
Approximately 10 percent of all retinal ganglion cells project to the SC.
Cells in SC have ill-defined "On" "Off" regions, therefore will respond to most
visual stimulus regardless of shape, color, or orientation.
SC cells seem most interested in location of visual stimulus, not identity of visual
stimulus, therefore, SC has often been referred to as the "where" system, rather
than the "what" system.
SC appears to have important role in directing ballistic eye movements toward a
visual stimulus in the periphery
Most of the fibers projecting to SC are from the M type of RGC, therefore,
messages often get to SC more quickly than to LGN.
27. 27
A group of small midbrain nuclei, is
just rostral to the SC.
Involved with the control of the
pupillary light reflex by means of a
projection to the Edinger–Westphal
nucleus of the oculomotor complex.
The pretectal complex
28. The pulvinar is the largest nuclear mass in the primate thalamus and receives a
projection from the small-caliber fibers from the optic nerve and the SC.
It projects to several visual cortical areas, including V1, and extrastriate,
parietal areas.
Pathway that can bypass the LGN to get to V1 and may play a role in
processing form vision.
Integrates neural signals associated with eye, hand and arm movements and
may receive signals associated with saccadic eye movements.
28
Pulvinar nucleus of the thalamus
29. The suprachiasmatic nucleus receives a sparse projection from fibers that leave
the dorsal surface of the optic chiasma and has been implicated in the
synchronization of circadian rhythms.
The paraventricular and supraoptic nuclei are likely also involved with the
regulation of the light–dark cycle for neuroendocrine functions
The AOS plays an important role in optokinetic nystagmus (OKN) in which slow
compensatory and pursuit type eye movements alternate with fast saccadic-type
eye movements in response to viewing prolonged large field motion
29
Other targets RGC
30. Primary Visual Cortex (Area 17)
Located on either side of & within the
calcarine fissure.
Upper fields project to the lingual gyrus.
Lower fields project to the cuneus.
Macular representation is most caudal in
Area 17.
Peripheral field representation is in the
rostral 2/3rds of Area 17.
Lesions of Area 17 result in blindness in
the contralateral visual field.
30
35. Visual Cortex
•
35
What/Where Pathways
Evidence from Neuropsychology
Visual agnosia:
Inability to identify objects and/or people
Caused by damage to inferior (lower) temporal lobe
Disruption of the “what” pathway
Visual neglect:
Inability to see objects in the left visual field
Caused by damage to right parietal lobe
Disruption of the “where” pathway
36. Deficits in Motion Perception:
Akinetopsia
•
36
Deficits in Color Perception -
Achromatopsia
37. Visual Cortex
Like RGCs and cells of LGN, cells of the PVC have
receptive fields which monitor a restricted zone in
the retina.
Cortical magnification: the disproportionally large
amount of cortical tissue dedicated to processing
the foveal retina compared to the peripheral
retina.
Number of RGCs connected to photoreceptors in
fovea vs. RGCs connected to photoreceptors in
periphery. Remember degree of convergence?
37
38. 38
Orientation specificity: cells in PVC
differentially respond to stimuli of
varying orientations.
Simple cells: Well defined excitatory
region; location important.
Complex cells: less well defined
excitatory region; location less
important.
Hyper-complex: length of contour
important Oblique effect
Visual Cortex
39. 39
Direction specificity: cells in PVC also differentially respond to movement in
different directions. Simple cells: slower movement. Complex cells: faster
movement
Visual Cortex
40. • Binocular cells: Two receptive fields, one for each eye. Stronger response to one eye
(ocular dominance).
• Cells give a most vigorous response when a stimulus of the same size, shape, and
orientation is located in a particular position in 3-d space across the two retinae.
• Cells are important for the visual system's use of the depth cue called retinal disparity.
40
Visual Cortex
41. 41
Cell columns: in the PVC cells (s,
c, hc) are arranged in layers by
their preferred orientation.
Cell hyper columns: the layers of
orientation specific cells are
arranged such that an orderly
incrementing of orientation
change exists across layers until a
complete cycle has been achieve.
Include both eyes.
Layer 4: input layer, monocular
cells.
Blobs: non-orientation specific
cells for color processing.
Visual Cortex
42. Axonal conduction
Retinal ganglion cell electrophysiology and synaptic
transmission
RGCs receive synaptic input from bipolar and amacrine cells,
which primarily use glutamate as the major excitatory
neurotransmitter.
Axonal conduction
Action potentials
RGC axons transmit information via action potentials, which are
all-or-nothing spikes of electrical activity. This is in contrast to
most intercellular communication within the retina, where graded
potentials are used to transmit information
42
43. 43
Mechanism of axonal conduction
AT REST(Non depolarization state)
Negative voltage in the axon ( negative resting potential)
Due to flow of K down its concentration gradient ( from inside to outside) which
create a leak current giving negative potential
Leak current continue until concentration gradient becomes equal which forms
K equilibrium potential
At rest flow of Na down its concentration gradient (from outside to inside)
occur giving Na equilibrium potential but its much more smaller than K flow
44. 44
DEPOLARIZATION
Occurs when voltage sensitive Na channels activated More Na enter the cell so
axon becomes positive (depolarized)
REPOLARIZATION
Its the return of membrane potential to resting state & its necessary for another
AP to be transmitted down the axon
Due to closure of voltage gated Na channels & transient opening of voltage gated
K channels
Mechanism of axonal conduction
45. Role of myelin in AP conduction
It’s a multilaminated fatty structured produced by oligodendrocytes which
insulates each axon
It increase the speed & efficacy of AP conduction by these mechanisms;
DECREASE CAPACITANCE (ion needed to change voltage)
- less Na needed to depolarize axons
INCREASE RESISTANCE (the resistance for ion leakage)
- less leakage of charge across the membrane
The above two decrease amount of ion flux needed to change voltage across the
membrane
45
46. Node of Ranvier
Its myelin free space in the axon containing many Na channels
so AP jump from one node to the other speeding up AP conduction
Such type of conduction is called salutatory conduction
46
Role of myelin in AP conduction
47. Axonal transport
Axonal transport occurs in two directions, orthograde (away from the cell
body and towards the brain), and retrograde (towards the cell body and
away from the brain).
Orthograde Axonal Transport
Transport away from the cell body &
toward the brain
Has two types;
47
48. 1. Slow axonal transport; two classes
Slower class ( 0.2 – 1 mm/day )
- for cytoskeletal proteins (tubulin & neuro filament)
More rapid class (2 – 8 mm / day)
- for actin , myosin , enzymes
2. Fast axonal transport ( 90 – 350 mm/day)
- for neurotransmitters to axon terminal
- continue after transection of the cell body from the axon
48
Axonal transport…
49. Introduction
Retrograde Axonal Transport
Transport toward the cell body and away from the brain
- Occur as half velocity of the fast axonal transport
- For neurotrophic molecules & growth factors which induce
transcription & translation of gens for axonal survival & growth.
•
49
50. Optic nerve injury
Optic nerve injury from the various forms of optic neuropathy affect the RGC by
the following mechanisms;
1. Blockage of axonal transport
-build up of excess retrograde transported molecules
-lack of neurotrophic factors for cell survival
2. Demyelination
3. Excitotoxicity from excess glutamate
4. free radical formation
If the pathology not treated the final common pathway is RGC death by apoptosis
50
51. Axon repair
It’s a mechanism for protecting RGC & enhancing their survival
after an insult on the axons by the following ways;
Remyelination
-using steroids & immune modulators in demyelinating diseases
Neuroprotection
-Blocking excitotoxicity from glutamate
-Administer neurotrophic factors
-Inhibition of apoptosis at its various stages
51
52. Axonal regeneration
No axonal regeneration after optic nerve injury but there is an
early sprouting called abortive regeneration which latter dies
Mechanisms preventing axon regeneration
1. Glial associated inhibitory signal
- Active molecules from astrocyte & oligodendrocyte prevent
Regeneration( myelin & proteoglycan)
- Glial cells unable to secrete trophic signals after injury
•
52
53. Axonal regeneration…
2. Neuron intrinsic limitation
- Due to gradual increment of gens which limit axonal
growth & gradual decrement of gens important for
rapid growth
•
53
54. Retinotopic Organization & Visual field defect
Visual field
• It is tested monocularly, with the patient looking straight ahead at a fixation point.
•
54
55. Cont…
Inversion and reversal of the field are caused by
the optical system of the eye
The superior field is imaged on the inferior retina
The inferior field on the superior retina;
The nasal field is imaged on the temporal retina
The temporal field on the nasal retina
55
56. Cont…
The nasal part of the field for one eye is the
same as the temporal part of the field seen by
the other eye, with the exception of the far temporal
periphery, which is called the temporal crescent
The temporal crescent is imaged on the nasal
retina of one eye but not on the temporal retina
of the other eye.
Within each temporal field is an absolute scotoma,
the physiologic blind spot.
56
57. Retinal nerve fiber layer
Papillomacular bundle Central /centrocecal/paracentral scotoma
Temporal retinal fiber Arcuate visual field defects
Nasal retinal fiber Temporal wedge shaped defects
Nasal retinal + temporal fiber defect Altitudinal defect
Disorder of rod Ring scotoma,
Disorder of cones Central scotoma
57
58. Introduction
• The role of the central visual pathways is to process and integrate visual
information that travels to the brain by means of the optic nerves
58
60. LESIONS OF OPTIC NERVE :
Causes:
1. Optic atrophy
2. Indirect optic neuropathy
3. Acute optic neuritis
4. Traumatic avulsion of optic nerve.
Characterised by:
- Complete blindness in affected eye
- loss of both direct on ipsilateral &
consensual light reflex on contralateral
side.
60
61. Describe the visual field defect ?
•
61
Bitemporal Heteronymous Hemianopia
Paralysis of pupillaryreflex (usually lead to partial descending optic
atrophy)
MIDDLE CHIASMAL LESIONS:
CENTRAL LESIONS OF CHIASMA
(SAGITTAL)
Causes:
Suprasellar aneurysm
Tumors of pituitary gland
Craniopharyngioma
Suprasellar meningioma &
glioma of 3rd ventricle.
Third ventricular dilatation
due to obstructive hydrocepha
62. Describe the visual field
defect ?
•
62
Junctional scotoma: lesion at junction of optic nerve
and chiasm
ANTERIOR CHIASMAL
LESIONS:
• Ipsilateral optic neuropathy a
central scotoma and a defect
involving the contralateral
superotemporal fieldalso known
as a junctional scotoma
• Abolition of direct light reflex on
affected side & consensual light
reflex on contralateral side.
• Near reflex intact.
63. POSTERIOR CHIASMAL SYNDROME
macular fibers cross more posteriorly in the chiasm
paracentral bitemporal field loss
Posterior lesions may also involve the optic tract and cause a contralateral
homonymous hemianopia.
63
64. Describe the visual field defect ?
•
64
POSTERIOR CHIASMAL SYNDROME
macular fibers cross more
posteriorly in the chiasm
paracentral bitemporal field
loss
Posterior lesions may also
involve the optic tract and
cause a contralateral
homonymous hemianopia.
65. Describe the visual field defect ?
•
65
Left incongruous homonymous hemianopia
Contralateral hemianopic pupillary reaction( wernicke’s
reaction)
LESIONS OF OPTIC TRACT :
Causes:
1. Syphilitic meningitis/ Gumm
2. Tuberculous meningitis
3. Tumors of optic thalamus
4. Aneurysm of posterior
cerebral arteries.
67. Describe the visual field defect ?
67
Left homonymous hemianopia with macular sparing
68. Describe the visual field defect ?
•
68
Right superior quadrantanopia >> temoporal lobe lesion
LESIONS OF OPTIC RADIATIONS :
Causes:
• Vascular occlusion(Anterior
choroidal artery, middle cerebral
or posterior cerebral artery)
• Primary & secondary tumors
• Trauma
Characterized by : No Optic
Atrophy
Normal pupillary reactions
69. Describe the visual field defect ?
•
69
Left inferior quadrantanopia >> parietal lobe lesion
LESIONS OF OPTIC RADIATIONS :
Causes:
• Vascular occlusion(Anterior
choroidal artery, middle cerebral or
posterior cerebral artery)
• Primary & secondary tumors
• Trauma
Characterized by : No Optic Atrophy
Normal pupillary reactions
70. Describe the visual field defect ?
•
70
Right homonymous hemianopia
Congruous
homonymous macular
defect
Head injury/gun shot
injury leading to
lesions of tip of
occipital cortex
73. Describe the visual field defect ?
•
(A) Posterior choridal artery occlusion leads to homonymous horizontal sectroanopia.
(B) Anterior choridal artery occlusion causes sector-sparing homonymous hemianopia
74. References
Duanes clinical ophtalmology 2012
Anatomy of the eye, R. Snell 2nd edition
BCSC Fundamentals and principles of ophthalmology , AAO 2016-2017
wolff’s anatomy of the eye and orbit, 8th edition
ADELER’S physiology of the eye 11th edition
Clinical Anatomy and physiology 3rd edition
Clinical neuro-ophthalmology – Thomas duane and Edward jaegar
74
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
. The corneal epithelium and stroma are transparent to permit passage of light without distortion (Maurice, 1970).
• The ciliary muscles dynamically adjust the shape of the lens in order to focus light optimally from varying distances upon theretina (accommodation). The total amount of light reaching the retina is controlled by regulation of the pupil aperture.
To arrive at the photoreceptors, light must first pass through transparent inner layers of the neurosensory retina, comprised of the nerve fiber layer, ganglion cells, amacrine cells, and bipolar cells .
• Ultimately, the visual image becomes projected upside-downand backwards on to the retina (Fishman, 1973)
.
project their axon towards the vitreous, whereupon they turn approximately 90° and project towards the optic nerve head in the nerve fiber layer.
There are three main types of ganglion cell, each with specialized functions in thedetection of visual inputs
The different types of ganglion cells comprise separate pathways that are named for their targets in the LGN.
Midget cells have cone-opponent receptive fields, allowing spectral selectivity along the red–green or blue–yellow axes.
These struct arrangts afford midget cells the properties of an extremely small receptive field, with specialization for high spatial acuity, color vision, and fine stereopsis.
What does all this mean? M's are more able to pick up the mare fact that something is present and are faster at communicating that. P's are more able to do fine detailed analysis of what that something is.
The nerve fiber layer is not quite radially arranged around the optic nerve head, as axons near the center of our visual field course away from the fovea, and then towards the optic disk, entering in the superior and inferior portions of the disk .
This interesting anatomy prevents axons from crossing the high-sensitivity fovea, where they might otherwise scatter light and degrade visual acuity.
The axons from more peripheral RGCs are more superficial (vitread) to those arising from less peripheral ganglion cells.
There is strict segregation of those fibers arising from RGCs located superior to the temporal horizontal meridian (raphe) and those fibers arising from RGCs located inferior to the horizontal raphe.
Because of this segregation, visual field defects corresponding to injury to RGC axons typically have stereotyped patterns, e.g. superior orinferior nasal steps, temporal wedges, or arcuate scotomas. These are called nerve fiber bundle defects
The optic disc represents the beginning of the optic nerve and is the point where the axons of retinal ganglion cells come together.
Axons of ganglion cells in the retina of the corresponding eye• Outgrowth of diencephalon, so is a CNS tract & not a „true‟ cranial nerve.• Myelinated by oligodendrocytes The optic disc is also the entry point for the major blood vessels that supply the retina.
Peripheral fibers deep in Retina superficially in opticnerve
Fibers close to opticnerve head superficial in retina central in optic nerve
The optic nerve travels posteriorly through the lamina cribrosa to exit the back of the globe, where it abruptly increases in diameter from 3 to 4 mm.
In order to accommodate the rotations of the globe, intraorbital segment of the optic - 25 and 30 mm in length, at least 5 mm longer than the distance from the globe to the orbital apex.
Upon passing through the lamina cribrosa, the optic nerve becomes invested with meninges and becomes myelinated.
Upon exiting the orbit, the optic nerve enters the optic canal, within the lesser wing of thesphenoid bone, for approximately 6 mm.
The intracanalicular optic nerve rises at a 45 angle and then exits the optic canal, where it continues in its intracranial portion forapproximately 17 mm before reaching the chiasma.
The chiasm, which has a dumbbell shape when viewed in coronal section,
ANTERIORLY : Anterior cerebral artery and their communicating artery
POSTERIORLY: Tuber cinerium, Hypophyseal stalk, Pituitary body, mamillary body, posterior perforated substance
SUPERIORLY: 3rd ventricle
INFERIORLY: Pituitary gland
LATERALLY: Extracavernous part of ICA Anterior perforated substance
normal….directly superior to sella turcica.
--prefixed….anterior to sella turcica.
--post fixed…..posterior to sella turcica.
Partial crossing of optic nerve axons in the OC is essential to binocular vision
Axons from temporal fields (fibers coming from the nasal retina (approximately 53% of total fibers) cross
So are first affected in the tumor of pituitary producing upper temporal quadrantic field defect – these fibers form convex loops in the terminal part of optic nerve and then cross opposite side and occupy lower quadrant
Why do half the fibers cross?
Nasal half of one eye, and temporal half of other eye, monitor same visual field, therefore, fibers emanating from same areas monitoring same physical space are combined.
Each tract is ~ 5mm long.
The fibers exiting from the chiasm proceed
-circumferentially around the diencephalon
-lateral to the hypothalamus
-in contact with the ambient cistern .
- Represent contralateral & ipsilateral hemi field of
each eye.
Just prior to the LGN, the fibers involved in the pupillary pathways exit.
The ipRGCs also project to the suprachiasmatic nucleus of the hypothalamus
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Function:
1) LGN contains cells with similar receptive field characteristics as RGC, except that "off" portions of cells exert even stronger inhibitory affect. This serves to accentuate to an even greater extent the "edges" and "borders" already identified by RGCs.
2) Parvocellular layers contain cells with receptive fields which respond differentially to color. These are called color opponent cells, most are red/green or blue/yellow color opponent. These cells receive most of their input from the P type of RGC.
3) Magnocellular layers are color blind and receive input mostly from M type of RGCs.
4) Both Magnocellular cells and Parvocellular cells can signal movement, but Magno cells respond to fast movement, Parvo respond to slow movement.
5) LGN may also receive "top-down" visual processing biases due to its interconnections with visual cortex.
6) Interconnections with RAS may serve as “volume control” for visual inputs.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
MEYERS LOOP(inferior retinal fibers)-pass through temporal lobe looping around inferior horn of lateral ventricle
Major targets of the retinal ganglion cell axons
the accessory optic system (AOS) (including the nucleus of the optic tract (NOT) and the dorsal, medial, and terminal nuclei.
Approximately 90 percent of all retinal ganglion cells project to the LGN, which is laminated and displays retinotopic organization.Each LGN layer receives input from a specific eye and class of ganglion cell
SC is the more phylogenetically primitive projection site, in some creatures such as fish and frogs, SC is major site of visual info processing.
SC is a midbrain structure which, in conjunction with the cortical frontal eye fields and the brainstem reticular formation, is involved in the generation of visually guided saccadic eye movements
The majority of retinal axons that terminate in the SC are small caliber, originate from ganglion cells with small dendritic fields, and do not project to other retinal targets.
So what does all this mean?
Ans: it appears that SC is very important for regulating visually guided reflexive behaviours, for example, the SC would quickly signal the presence of an object coming toward the head from the visual periphery. SC only cares that there is something there, LGN will figure later what it was!
Receives signals from a group of small-diameter retinal ganglion cells with large receptive fields.
The pupillary light reflex demonstrates a consensual response primarily resulting from crossed and uncrossed optic nerve fibers that enter each pretectal complex which in turn sends a bilateral projection to the Edinger–Westphal nucleus.
It has been shown that the pulvinar,which suggests its role is also one of formulating reference frames for hand-eye coordination
Different names for primary visualcortex:• Brodmann’s area 17• V1• primary visual cortex• striate cortex (“striped” cortex)
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
however, the number of cells responding to input from the foveal area of retina far exceeds the number responding to input from more peripheral regions. This is known as cortical magnification.
Cortical magnification stands to reason if you consider the number of RGCs connected to photoreceptors in fovea vs. RGCs connected to photoreceptors in periphery. Remember degree of convergence?
This was not true of LGN cells or RGC cells, both of which contain circular receptive fields insensitive to orientation.
Receptive fields in PVC cells tend to be more oblong shaped.
One cell may respond vigorously if a visual stimulus moves from left to right across the receptive field, but not respond as much if movement is from right to left, other cells do just the opposite (also up/down).
Seeing vs. Acting distinction. Segregation but not complete separation.
RGCs have both NMDA and non-NMDA ionotropic glutamate receptors, as well as metabotropic receptors. Other neurotransmitters modulating RGC activity include GABA, acetylcholine, and aspartate (acting on NMDA receptors), as well as serotonin and dopamine.
Within the retina itself the levels of glutamate are modulated by Müller cells, have glutamate transporters, and which contain the enzymeglutamine synthetase, converting glutamate to the amino acid glutamine.
Conduction down individual axons occurs via the same biophysical mechanisms which occur in any myelinated axon
At rest, the inside of an axon is at a negative voltage with respect to the outside of the axon. This negative restingpotential primarily results from the higher concentration of potassium within the axon
leak current out of the axon results in a negative potential within the axon. Outward flux of potassium continues until the charge separation becomes too great, and can no longer be driven by the concentration difference between inside and outside the axon
During axonal conduction, depolarization of a section of membrane induces opening of voltage sensitive sodium channels located within adjacent membrane. These allow much greater amounts of sodium to enter the axon, and the positive sodium ions cause the axon to become more positive (i.e., depolarized).
Together, these properties decrease the amount of ionic flux needed to achieve changes in voltage across the membrane, saving on the Na+-K+-ATPase activity and thus energy needed to maintain ionic homeostasis after conduction.
Conduction in a myelinated axon, called saltatory conduction, becomes much faster, as depolarization “jumps” from one node to the next. The clustering of sodium channels into nodes, as well as an important developmental switch in sodium channel isoforms, are also induced by oligodendrocytes.
The entire length of the retinal ganglion cell axon must be maintained by transporting proteins and other subcellular constituents.
The rates of transport of specifc moieties may vary as a function of stage of optic nerve development, suggesting a developmental regulation of axonaltransport. Different rates of axonal transport may be related to differences in the motor proteins interacting with microtubules, e.g. kinesin208 for fast transport and dynein for slow transport.
Fast axon transport continues despite transection of the axon distal to the cell body, suggesting that intraaxonal components are suffcient for the process to occur.
Slow axonal transport does not continue after transection of the axon distal to the cell body, unlike fast axonal transport
And is the means by which endocytosed substances from the synapse, such as released neurotransmitter, may be returned to the cell body.
optic neuropathy is damage to the optic nerve from any cause
The optic nerve is commonly involved in disease, resulting in optic neuropathy. Optic neuropathies are major causes of visual loss. The most common optic neuropathy is that associated with glaucoma (i.e, glaucomatous optic neuropathy).
The inflammatory optic neuropathies are most commonly seen in young or middle-aged adults, and the most common is optic neuritis associated with the demyelinating disease multiple sclerosis.
Ischemic optic neuropathy usually affects older adults. The most common type is non arteritic anterior ischemic optic neuropathy, a disorder of unknown cause that results in sudden visual loss associated with disc edema; arteritic anterior ischemic optic neuropathy has a similar clinical presentation but is due to a vasculitic process, usually from giant cell arteritis.
Compressive optic neuropathy, in which the optic nerve is compressed by a mass lesion, commonly a tumor or aneurysm, results in slowly progressive loss of visual function as the mass increases in size. Therefore, most optic neuropathies involve axonal injury.
Death of retinal ganglion cells is the final common pathway underlying virtually all diseases of the optic nerve, including glaucomatous optic neuropathy, anterior ischemic optic neuropathy, optic neuritis, and compressive optic neuropathy.
Retinal ganglion cells are particularly susceptible to high levels of glutamate in the extracellular space. This causes cell death mediated via overexcitation, or excitotoxicity. It has been proposed that excitotoxic retinal ganglion cell death may explain pathologic conditions, such as glaucoma or retinal ischemia, in which an excess of retina ganglion cells die, akin to what occurs with cerebral infarct or other diseases
In the case of optic neuritis, where the primary defect in RGC function is attributable to oligodendrocyte demyelination, strategies to enhance remyelination are paramount. Currently, the major approach involves treating patients with steroids, which probably interferes with an ongoing inflammatory insult and allows a faster re-wrapping of optic nerve axons and return to baseline vision.
In the presence of demyelination, oligodendrocytes may reconstitute themselves and remyelinate axons, although the nature of the inducing chemical signals has not been well characterized. similarly, chronic treatment with immune modulators decreases the risk of subsequent demyelinating events
Neuroprotective strategies include blocking retinal excitotoxicity mediated by glutamate (which binds to N-methyl-D-aspartate [NMDA] receptors and induces RGC death); activation of small molecule receptors, which may enhance neuronal resistance to insult; inhibition of nitric oxide synthases, which may prevent axonal injury at the lamina cribrosa; and immunization with certain synthetic polypeptides.
Inhibitors of regeneration includes specific protiens in CNS mylin and other
Plus slower debris clearance in the CNS relative to the PNS
Retinotopic organization …topographic correspondence of RGC fiber in relation to retinal location
Much of the knowledge of retinotopy organization of visual system arise from the visual field defects
Normal extent of field of vision 50o, superior , 60o, nasally , 70o inferiorly, 90o, temporarly
divided into four quadrants by a vertical line and a horizontal line that intersect at the point of fixation
The point of fixation is seen by the fovea and is eccentric because the temporal field is slightly larger than the nasal field.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
==Nerve fiber bundle defects are the following, 1. papillomacular bundle, 2. sup and inf arcuate bundle, 3. nasal bundle
==If temporal retinal fibers are affected, an arcuate defect can be produced that curves around the point of fixation from the blind spot to termination at the horizontal nasal meridian, The abrupt edge (at the horizontal meridian) called nasal step
==a lesion affects a nasal bundle of nerves, producing a wedge-shaped defect arising from the physiologic blind spot into the temporal field.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
LATERAL CHIASMAL LESIONS :Causes:Distension of 3rd ventricle causing pressure on each side of optic chiasmaAtheroma of carotids & posterior communicating artery.
Binasal hemianopia - Parallysis of pupillary reflex(usually lead to partial descending optic atrophy)
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Affects the ipsilateral optic nerve fibers and the contralateral inferonasal fibers• Ipsilateral optic neuropathy manifested as acentral scotoma and a defect involving thecontralateral superotemporal fieldalsoknown as a junctional scotomaAbolition of direct light reflex on affected side& consensual light reflex on contralateral side. Near reflex intact.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Light reflex is absent when light is thrown on the temporal half of the retina of the affected side and nasal half of opposite side; while it is present when the light is thrown on the nasal half of the affected side n temporal half of opposite side
These lesions usually lead to partial descending optic atrophy & may be associated with C/L 3rd nerve paralysis(RAPD) &ipsilateral hemiplegia
Light reflex is absent when light is thrown on the temporal half of the retina of the affected side and nasal half of opposite side; while it is present when the light is thrown on the nasal half of the affected side n temporal half of opposite side
These lesions usually lead to partial descending optic atrophy & may be associated with C/L 3rd nerve paralysis(RAPD) &ipsilateral hemiplegia
Whole segment of Optic tract, Lateral geniculate body is involved
macular sparing homonymous hemianopia occur if tip of posterior pole spared & it have better localizing value.
Homonymous inferior quadrantanopia…upper bank lesion
Homonymous superior quadrantanopia…lower bank lesion
Homonymous hemicentral scotoma…tip of occipital lobe
Homonymous paracentral scotoma…midzone affected
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
macular sparing homonymous hemianopia occur if tip of posterior pole spared & it have better localizing value.
Homonymous inferior quadrantanopia…upper bank lesion
Homonymous superior quadrantanopia…lower bank lesion
Homonymous hemicentral scotoma…tip of occipital lobe
Homonymous paracentral scotoma…midzone affected
The blind spot is a small area which correspond sto optic head where there is no photoreceptor in it. An enlarged blindspot is a feature of optic head enlargement e.g Papilloedema
Congenital anomalies can also causes enlarged blind spot!
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.
Although the eye is responsible for transducing patterns of lightenergy into neuronal signals, it is the brain that is ultimately responsible for visual perception and cognition.