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VS142 Visual Neuroscience
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VS142 Visual Neuroscience Neural Retina: Basic Pathways Televisions, video monitors and digital cameras use regular arrays of red, green, and blue phospors/sensors to ... – PowerPoint PPT presentation
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Title: VS142 Visual Neuroscience
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VS142 Visual Neuroscience
- Neural Retina
- Basic Pathways
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Televisions, video monitors and digital cameras
use regular arrays of red, green, and blue
phospors/sensors to record or reproduce an image.
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But not the retina!Blue cones are sparse, mostly
red and green, but they are located in patches
not in regular repeating arrays.
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For form vision the red and green cones appear
to be used almost exclusively (and perhaps
interchangeably similar wavelength
sensitivity).The blue cones are rare and
sparsely localized absent in the fovea! used
mostly only for color vision.
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- Synaptic Contacts of Mammalian Photoreceptors
- Pedicles terminals of cones. Foot-shaped.
- Spherules terminals of rods. Ball-shaped.
- -gt In the mammalian system, any given bipolar
cell contacts either rods or cones but not both.
- Triad dendrite of a cone bipolar cell
invaginates the cone pedicle and is flanked by
two other invaginating processes from horizontal
cells. - Invaginating Bipolar Cell bipolar cell part of a
triad. - Flat Bipolar Cells contact cone pedicels without
invaginating. - Midget Bipolar Cells in primates, both
invaginating and flat, can be postsynaptic to a
single cone in the central retina, provide
exclusive bipolar input to a single midget
ganglion cell.
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- Synaptic Contacts of Mammalian Photoreceptors
(cont) - Rod bipolar cells of all mammals are strictly
invaginating, form triads with rod spherules. - Bipolar cells do not generate action potentials
all graded potentials. - -gt Light depolarizes invaginating bipolar cells,
thus increasing their neurotransmitte release.
On bipolar cell. - -gt Light hyperpolarizes flat bipolar cells, thus
decreasing their neurotransmitter release. Off
bipolar cell. - Invaginating On
- Flat Off
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- -gt All photoreceptors are hyperpolarized by light
and decrease their transmitter (glutamate)
release in light. - -gt So the on- and off- classes of bipolars are
created by different kinds of glutamate
receptors. - Sign-inverting synapse cone/rod and on-bipolar
cell. Cone hyperpolarizes and bipolar cell
depolarizes. Transmitter affects ion channels
indirectly through a second messenger (cGMP)
system. - Sign-conserving synapse cone only (never rod)
and off-bipolar cell. Transmitter affects ion
channels directly. - This splitting up into on- and off- pathways will
continue throughout much of the visual system.
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- RECEPTIVE FIELD (of a neuron) (sound effects)
- The region of the retina (or the visual field
visual space) where a stimulus must be placed
for the neuron to be affected. For all cells
other than the photoreceptors, the receptive
field (RF) is a funnction of its functional
connections to the photoreceptors.
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- Receptive fields of bipolar cells
- On-bipolar cells are depolarized by a spot of
light in the center, but hyperpolarized by light
in the surround. This is an on-center,
off-surround receptive field. - Off-bipolar cells have the opposite effects, are
said to have off-center, on-surround receptive
fields. - Effects thought to be due to horizontal cells.
Lateral inhibition. - Effects of center and surround tend to cancel
with uniform illumination, though typically some
imbalance.
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- Synaptic terminals of bipolar cells make
characteristic dyad junctions in the inner
plexiform layer. One contact is usually an
amacrine cell, the other may be either an
amacrine or a ganglion cell. Can have a
reciprocal synapse between amacrine and bipolar.
Feedback circuit? - BT bipolar terminal
- A amacrine cell
- G ganglion cell
- Large open arrow reciprocal synapse from
amacrine cell to bipolar cell terminal.
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- Cartoon of Dyad Junction
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- Amacrine cells an enormous variety of types.
Function(s) still not very well understood. An
area of intensive research. - -gt some amacrine cells can generate action
potentials. - -gt many different neurotransmitters
- Interplexiform cells similar in some ways to
amacrine cells, but also send processes into the
outer plexiform layer where they contact
horizontal cells. Not clear what the point of
this is.
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- Ganglion Cells the final total output of the
retina! - -gt The action potentials of ganglion cells are
the ONLY thing that make it to the rest of the
brain. Retinal activity is only important for
visual perception to the extent that it affects
the pattern of firing of action potentials for
ganglion cells. - -gt Transmission is one-way. The brain does not
send signals to the retina. (About the only part
of the visual system that is a one-way street). - -gt Usually have center-surround receptive fields,
both on- and off-types.
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- Neurotransmitters and Neuromodulators in the
retina - Rapid signaling of information in the retina is
conveyed by classical small-molecule
neurotransmitters (like glutamate and GABA-) and
also by electrical synapses. - Slower processes are mediated by peptides and
also by dopamine. We dont really know what is
being regulated by these slower mechanisms, so
you wont have to memorize them (yet!).
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- GABA Gabara Gamera
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- Light and dark adaptation the human visual
system can operate over an incredibly large
dynamic range. The difference in light energy
between a dark and stormy night and a sunny day
at the beach can be over 10 billion to 1. - -gt Iris/pupil size changes
- -gt Photoreceptor adaptation (slower). Non-linear
effects of bleached photopigment? - -gt Network adaptation (faster). Switching from
rods to cones (AII amacine cells)
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- Scotopic vision dark-adapted vision
- Photopic vision light-adapted vision
- Purkinje shift shift of visual sensitivity
towards longer wavelengths with light adaptation,
because the balance of cones are responsive to
longer wavelengths than are rods.
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- Midget Ganglion cells both on- and off-types.
About 80 of the ganglion cells in primates.
Connect to midget bipolar cells. Four basic
types - Red-ON
- Red-OFF
- Green-ON
- Green-OFF
- -gt Remember, blue cones not really used for
detailed pattern vision, therefore blue does not
(mostly, maybe) need a direct line to the brain. - -gt In central retina midget bipolars get their
input from just one cone. A little more diffuse
(I.e., more than two cones/midget) in the
periphery but not by much.
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- -gt There are lots of different types of ganglion
cells, for example bistratified (connections in
both on and off layers). We are not sure of
the functional role of many of these types of
ganglion cells. - -gt Directly photosensitive melanopsin-containing
ganglion cells! (more later on) - -gt SMALL RELATIVE NUMBERS OF NEURONS DOES NOT
MEAN THAT THEIR FUNCTIONS ARE NOT IMPORTANT!!!! - Technical Jargon
- P-Cell (parvo) midget ganglion cell
- M-Cell (magno) parasol ganglion cell
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