NROC64H3 Chapter Notes - Chapter 10: Ocular Dominance, Optic Radiation, Ganglion Cell

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17 Mar 2012
Chapter 10 - The Central Visual System 341-373
- Retina extract info from different parts of visual image 100 million photoreceptors but only 1 million axons
leaving w information
- Pathways serving conscious visual perception includes lateral geniculate nucleus/LGN of thalamus and 1ary
visual cortex/area 17, V1, striate cortex
- Geniculocortical pathway for analysis of different stimulus attribute --? Striate cortex feeds info to >24 cortical
areas in temporal + parietal lobes
- Studies from cat, rhesus monkey, Macaca mulatta, and macaque monkey test done to understand
The Retinofugal Projection
- Retinofugal Projection: Neural pathway leaving eye,
beginning w/ optic nerve (Latin- fugal = to flee)
The Optic nerve, Optic Chiasm and Optic Tract
- Ganglion cell axons flee retina through 3 structures before
forming synapses w/ brain stem
- Optic Nerves: Exit left and right eyes at optic disks,
travel through fatty tissue behind eyes in bony orbits,
then pass through holes in the floor of skull
- Optic Chiasms: Nerves form this, (names for X shape of
Greek-chi), lying at base of brain, anterior to pituitary
glands. Axons from nasal retinas cross form one side to
eh other
- Decussation: Crossing of fiber form one side to another
- Optic Tracts: Axons of retinofugal projections form this,
run just under pia long lateral surfaces of diencephalon
Right and Left Visual Hemifields
- Visual field is entire region of space that
can be seen w/ both eyes
- Left Visual Hemifield: Objects
appearing left of midline
- Right Visual Hemifield: Objects
appearing to right of midline
- Binocular Visual Field: Region of
space of both visual hemifield viewed
by both retinas
- Left visual hemifield is viewed by right
hemisphere and v.v.
Targets of Optic Tract
- Lateral Geniculate Nucleus/LGN:
Neurons in LGN give rise to axons that
project to 1ary visual cortex
- Optic Radiation: Projection from LGN
to cortex
- Retinofugal projection form eye LGN
mediates conscious visual projection
- Impairments: Transection of left optic nerve renders person blind in left eye, transaction of left optic tract would
lead to blindness in right visual field through either eye. Regions of visual field viewed in nasal portion would
mean blindness in nasal retina (peripheral visual fields on both sites
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- Nonthalamic Target sof Optic Tract: Some retinal ganglion
cells sends axons to innervate structures other than LGN =
direct projections play important role in synchronizing
biological rhythms (sleep and wakefulness w/ daily
dark=light cycle)
- Direct projections to midbrain pretectum control the
size of pupil and eye movement
Superior colliculus (Latin-little hill) 10% of ganglion cells
in retina project to midbrain tectum
-Optic Tectum: Tectum of midbrain, in vertebrate
=superior colliculus= retina-SC= retinotectal projection
- In SC, patch of neurons activated by point of light,
commanding eye and head movements to bring image
Of Special: David and Goliath PG 347
The Lateral Geniculate Nucleus
- Right and left lateral geniculate nuclei,
located in dorsal thalamus, are major
targets of 2 optic tracts
- Shape explains name geniculate (Latin-
geniculatus-like a little knee)
The Segregation of Input by Eye and by
Ganglion Cell Type
- LGN receives synaptic input from retinal
ganglion cells, most neurons project axon
to 1ary visual cortex via optic radiation
- Segregation of neurons suggest different types
of retinal information kept separate at
SYNAPTIC RELAY Axons from M-type, P-type
and nonM-nonP ganglion cells in retinas
synapses on cells of different LGN layers
Right LGN Info about LEFT visual field
(viewed LEFT nasal left retina and temporal
RIGHT retina)
- Ventral layers magnocellular LGN layers,
M0type ganglion cells here
- Dorsal Layers parvocelular LGN layers, p-
type ganglion cell here
- Koniocellular Layers (Greek-dust)
Receive input from nonM-nonP types of
retinal ganglion cells to visual cortex
Receptive Fields
- Visual receptive fields in LGN neurons are almost same as ganglion cells that feed them (i.e. M LGN neurons have
large center-surround receptive fields and respond to stimulation in big burst of AP, insensitive to difference in
wavelength, v.v. in P=LGN, the neurons are activated by 1 eye monocular and ON=center and OFF-center cells are
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Nonretinal Inputs to LGN
- Retina is NOT main source of synaptic input
- MAJOR INPUT s 80% comes from primary visual cortex corticofugal
feedback alter qualities of visual responses in LGN
- Also receives input form neurons in brain stem (related to alertness and
attentiveness) can modular magnitive of LGN response to visual stimuli
Anatomy of the Striate Cortex
- Primary visual cortex/Brodmann’s’ Area 17/VI/STRIATE CORTEX
(striate-VI has dense stiple of myelinated axons running parallel to
surface, appears white) Located in OCCIPITAL LOBE, lies on MEDIAL
- Retinotropy: Organization where neighbouring cells I retina feed
information to neighbouring places in target structures (LGN and
striate cortex), 2D surface of retina is “mapped”
- 3 important points:
1. Mapping of visual field to retinoptically organized structure is
distorted visual space is not sampled uniformly by cells in retina,
usually in striate cortex
2. Discrete point of light can activate MANY CELLS in retina (overlap in
receptive fields)
3. don’t’ be misled by “MAP” – there is a mapping b/w retina & VI,
perception is based on brains’ interpretation of pattern/activity
Lamination of Striate Cortex
-Have ~ 6 layers, the cell layers are named VI, V, IV, III, II, AND II. Layer 1
has little neurons and have AXONS and DENDRITES fo cell in other layers
- However, there are >9 layers of neurons, neuroanatomists combine 3
sublayers into layer IV (IVA, IVB, IVC-2 tiers)
- The Cells of Different Layers: 2 types of neuronal shapes:
1. Spiny stellate cells: Small, spine-covered dendrites that radiate out of
cell body
2. Pyramidal Cells: Spine covered, 1 thick apical dendrite that branches
as ascending toward pia mater, multiple basal dendrite that extend
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