Biol 6102 / Neur 6010: Neurobiology

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Nov 16, 2013 (3 years and 6 months ago)

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Biol 6102

/ Neur 6010
:


Neurobiology




updated

9/
2
5
/1
1

Vision

Chapter
9
, 10




Properties of light

o

Electromagnetic radiation



Anatomy of the Eye


o

Blind spot due to optic disk
-

axons exit eye and blood vessels enter.



Focal Properties

o

Cornea does most of the f
ocusing

o

Lens is used for accom
m
odation

o

Corrective lenses change the focal distance



Visual Field



the space seen by an eye.



Retinal circuitry

o

Bipolar cells

form vertical pathway

o

Horizontal cells

and
Amacrine cells

form lateral pathways

o

Retinal Ganglion
cells



Convey information through optic nerve to the Lateral Geniculate Nucleus



Organization of
photoreceptor
s


o

Outer segment



Contains visual pigments in disks



New disks are constantly generated and old disks are shed into pigment
epithelium

o

Inner segme
nt

o

Synaptic terminal

o

Rods



More sensitive to light



Mediate night vision (scoptic)



More prevalent in periphery of retina



Involved in circuits that convey motion information

o

Cones



More prevalent in fovea



Mediate high acuity vision



Three types of cones

each sensitive to different wavelengths


trichromatic color
vision



Fovea

is specialized for high acuity vision

o

very small portion of the visual field

o

Foveola

is an area of fovea with the highest density of cones and lacks overlying cells
and blood vess
els



Phototransduction process


o

Light activates visual pigments



Rhodopsin

i
n Rods (Color Opsins
in Cones)



membrane protein in the disks of the outer segment



7 transmembrane spanning domains (G protein


coupled receptor)



11
-
cis retinal

is bound to rhodops
in (synthesized from Vitamin A)



Light changes the conformation of 11
-
cis retinal to

All
-
trans retinal




This produces a conformational change in rhodopsin, activating a G
-
protein
(
Transducin
)

o

The pigments stimulate
cGMP phosphodiesterase




This reduces th
e concentration of cGMP



cGMP
-
gated sodium channels are open in dark



The reduction of [cGMP] closes the cGMP
-
gated sodium channels

p.
2

o

This
hyperpolarizes

the cell

and stops transmitter release



In the dark, photoreceptors are depolarized and release neurot
ransmitter



Neurotransmitter is released as a function of membrane potential


Non
-
spiking,
graded release

o

Same basic logic as G
-
protein coupled receptors in synapses



Color Vision

o

There are three types of cones, tuned to different best wavelengths.

o

Color v
ision arises out of a
comparison

of the activity of the different cones
.

o

Rods also have spectral sensitivity.

o

Color blindness arises from alteration in visual pigments

o

Trichromatic color vision arose after split of old world and new world monkeys



Properti
es of
Retinal Ganglion Cells

(RGCs)

o

The receptive field of an RGC is the area of the visual field that a RGC responds to.



This corresponds to an area on the retina that receives the light from that part of
the visual field.

o

RGCs have a "
Center
-
Surround
" r
eceptive field organization



Receptive center



Antagonistic surround

o

Two basic types of RGC



On
-
Center



Off
-
Center

o

Reports contrast, not ab
solute light levels

o

Formation of RGC responses



Two typ
es of bipolar cells



On
-
center bipolar


inhibited by gluta
mate



Off
-
center bipolar


excited by glutamate



Horizontal cells inhibit release of neurotransmitter from photoreceptors



Causes antagonistic surround

o

M + P Cells
: Two additional classifications of RGCs



M
-
type (magnocellular) RGCs



larger size and larger r
eceptive field



Sensitive to low c
ontrast



Transie
n
t responses
-

moving stimuli



P
-
type (parvocellular) RGCs



Make up 90% of RGCs in primate retina



small receptive fields (high acuity)



Sustained responses



Color sensitive



through color opponent center
-
surroun
d organization.

o

Color opponency

is the basis for color contrast



However, new experiments suggest that it’s not so simple



The brain extracts information about the world.

o

Visual illusions show that this information is not always accurate.



The Central proj
ections of the Retina

o

The
optic nerves

leave the retina.

o

They join at the
optic chiasm


o

After the chiasm, the axons are referred to as the
optic tracts


o

The retina projects to four areas of the brain



Hypothalamus


Suprachiasmatic nucleus




Circadian rhyt
hm



The
Pretectal Area




Used for pupillary responses



The
Superior Colliculus




Used for generating saccadic eye movements and orienting to stimuli



Contains retinotopic map



The
Lateral Geniculate Nucleus

(LGN) of Thalamus



Involved in visual perception



Orga
nization of Visual Space

o

Binocular field


central area seen by both eyes

o

Monocular fields


lateral areas seen by one eye only



Approximately half of the Retinal Axons cross at the optic chiasm

p.
3

o

Only axons from the nasal retina cross

o

Axons from the tempor
al retina remain ipsilateral

o

Thus, left visual field is represented in right optic tract

o

Lesions in the visual pathway produce specific visual defects.



The LGN maintains an orderly map of the retina.

o

Retinal Ganglion axons segregate into 6 eye
-
specific lay
ers



Input from each eye remains segregated in different layers



Each layer has a complete
retinotopic

map



Layers 1 and 2 are the Magnocellular layers



Layers 4
-
6 are the Parvocellular layer



Koniocellular layers located in between main layers



All retinal i
nput to cortex passes through the LGN



But only 20% of the synapses are from RGCs, the rest are feedback
connections

o

Neurons in LGN have receptive fields with properties similar to those of the retinal
ganglion cells that they receive input from.



Axons of

LGN neurons form the optic radiation to
Primary Visual Cortex


o

This cortical area is also known as V1, area 17, Striate Cortex



Structure of Striate Cortex

o

Six cell layers

o

pyramidal cells are projection neurons, spiny stellate cells are local interneurons.

o

Different layers have different functions

o

Cells in the same column get the same thalamic input

o

Magnocellular input stays segregated from parvocellular input.

o

Inputs from each eye project to different columns



ocular dominance columns



binocular
neurons com
bine input from both eyes



Some neurons in Primary Visual cortex respond to visual disparity


Basis for
Random Dot Stereograms



Receptive field properties of Neurons in primary visual cortex

o

Orientation Selective

o

Direction Selective

o

Simple
cells

elongated
receptive field with antagonistic surround

o

Complex cells

respond to bars of a particular length and orientation, anywhere in
receptive field.

o

Receptive field properties are thought to arise from a summation of the receptive field
properties of input neuron
s



Cortex has a
columnar organization


o

Neurons of similar orientation selectivity are aligned in vertical columns

o

Blobs

are areas of non
-
orientation selective, but color selective cells.

o

Each area of visual space is multiply represented by different orient
ation columns

o

Orientation columns form a repeated pinwheel structure



determined with optical imaging



Extrastriate cortex

o

Visual processing continues in other areas of cortex

o

Two pathways



Dorsal


“Where” pathway


object location



Area MT



Medial
-
tempora
l cortex


involved in motion perception



Our brains can distinguish the difference between movement of our
eyes and movement of a target



Movement is perceived the same regardless of whether the image is
stable on the retina or moves across it.



Ventral


“W
hat” pathway


color and object recognition



Input from parvocellular pathway



V4
-

Inferotemporal cortex



Face recognition cells