VS142 Visual Neuroscience
Neural Retina:
Basic Pathways
Televisions, video monitors and digital cameras use regular
arrays of red, green, and blue phospors/sensors to record or
reproduce an image.
But not the
retina!
Blue cones are
sparse, mostly
red and green,
but they are
located in
patches not in
regular repeating
arrays.
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.
Synaptic Contacts of Mammalian Photoreceptors:
Pedicles: terminals of cones. Foot-shaped.
Spherules: terminals of rods. Ball-shaped.
-> 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.
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.
-> Light depolarizes invaginating bipolar cells, thus increasing
their neurotransmitte release. On bipolar cell.
-> Light hyperpolarizes flat bipolar cells, thus decreasing their
neurotransmitter release. Off bipolar cell.
Invaginating = On
Flat = Off
-> All photoreceptors are hyperpolarized by light and
decrease their transmitter (glutamate) release in light.
-> 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 offbipolar cell. Transmitter affects ion channels directly.
This splitting up into on- and off- pathways will continue
throughout much of the visual system.
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.
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.
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.
Cartoon of Dyad Junction
Amacrine cells: an enormous variety of types. Function(s)
still not very well understood. An area of intensive
research.
-> some amacrine cells can generate action potentials.
-> 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.
Ganglion Cells: the final total output of the retina!
-> 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.
-> 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).
-> Usually have center-surround receptive fields, both onand off-types.
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 don’t really know what is being regulated by
these slower mechanisms, so you won’t have to memorize
them (yet!).
GABA
Gabara
Gamera
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.
-> Iris/pupil size changes
-> Photoreceptor adaptation (slower). Non-linear effects of
bleached photopigment?
-> “Network” adaptation (faster). Switching from rods to
cones (AII amacine cells)
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.
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
-> Remember, blue cones not really used for detailed
pattern vision, therefore blue does not (mostly, maybe)
need a direct line to the brain.
-> 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.
-> 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….
-> Directly photosensitive melanopsin-containing ganglion
cells! (more later on)
-> 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