BIOL 2P94 Lecture Notes - Papez Circuit, Trailing Edge, Lamina Terminalis

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The Retina
0. Introduction
- Images from the retina are analyzed for form/movement/color. For each point in the
outside world, there is a corresponding image point on the retina in each of the two eyes. These
images from the two eyes are then brought together and compared at higher stations in the visual
pathway for extration of information about depth.
- Development of retina: The prosencephalon protrudes laterally and enlarges to form
primary optic vesicles. These later invaginate to give double-walled optic cups, with a largely
obliterated ventricular space between the two walls. The outer wall becomes the pigment
epithelium, while the inner wall becomes the neural retina. The two are separated by the
subretinal space, the residue of the ventricular cavity.
I. Structure of retina
- The retina is composed five main types of neurons, segregated in to three main layers:
1) Outer nuclear layer: photoreceptors, no direct blood supply. Instead, they
receive nourishment from the choroidal circulation filtered thru the pigment epithelium.
2) Inner nuclear layer: bipolar (second order visual neurons), horizontal (lateral
association neurons), amacrine (ditto), Mueller glial cell bodies
3) Ganglion cell layer: retinal ganglion cells (third order visual neurons, who
axons constitute the optic nerve).
- The outer and inner plexiform layers are sites of synapses between layers
II. Photoreceptors
- There are two types of photoreceptors
1) rods: most sensitive in dim light, only one type
2) cones: less sensitive to light, three types mediate color vision (peaks at blue,
green and yellow).
- Distribution of photoreceptors is not uniform: fovea has high density of cones and
few rods, periphery has more rods. Blue sensitive cones are also absent from the
center of the fovea.
- The fovea has the highest visual acuity, or spatial resolution of the visual space. This
is achieved by having thinner cones and more of them per unit space than elsewhere
in the retina; with the layers of cells anterior to the receptors being pushed aside.
This clearing minimizes light scattering which tends to degrade the quality of the
image.
- The structure of the photoreceptors:
oOuter segment: where light is absorbed and a neural signal is generated. It
contains an orderly stack of membranes in which is embedded the light
sensitive visual pigment (rhodopsin in rods, and one of three cone pigments in
cones). In cones these membranes are continuous with the plasma membrane
to form a highly convoluted surface membrane. In rods, the membranes are
completely internalized to form a stack of flattened membranous discs. The
high density of pigment provides a high probability of absorption of an
incident photon.
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oInner segment: contains the metabolic machinery of the cell, while the
synaptic terminal forms chemical synaptic connections with bipolar and
horizontal cells.
oGlu is the NT released by both rods and cones.
- Pigment epithelial cells serve to (1) absorb light not absorbed by the rods/cones using
melanin, (2) phagocytose shedded fragments of rods/cones, (3) regenerate pigment by
supplying frest chromophore to the bleached pigment in the outer segment.
III. Visual pigments
- Pigment is composed of a chromophore covalently bound to an opsin 7TM protein that
is different in different pigments. The two parts are joined together by a protonated Schiff base.
- Humans have 4 pigments (1 rod, 3 cones) each maximally sensitive at different
wavelengths
- Upon photon absorption, the chromophore undergoes several configuration changes; the
configuration that triggers vision takes several ms to reach; the ultimate result is the release of
chromophore, and binding of fresh chromophore with opsin
IV. Phototransduction
- In darkness, rods and cones have a membrane potential of about –30 to –40 mV.
- Rods/cones hyperpolarize in response to light in a graded fashion with respect to
intensity. Cones require more light and their responses rise and fall more rapidly than
rod responses. Their briefer responses permit cones to have a better time resolution of
light stimuli.
- Phototransduction mechanism: In darkness the surface membrane of the outer segment
has a higher permeability to cations, as a result of which there is a steady influx of sodium (and
Ca and Mg) ions into the outer segment, being driven by their inward-directed electrochemical
gradients. This steady influx of + charge in darkness (the dark current) maintains the cell in a
partially depolarized state and, consequently, a steady release of NT from the cell’s synaptic
terminal onto second-order neurons. In light, the ionic permeability of the outer segment is
reduced, thus decreasing the influx of cations and producing a membrane hyperpolarization,
which spreads passively to the synaptic terminal where it reduces the rate of transmitter release
from the receptor. The permeability reflects the opening of a cGMP-activated conductance,
which by itself has no intrinsic light sensitivity. Light, however, closes the conductance by
activating an enzyme cascade that leads to the lowering of the cGMP level in the outer segment.
Metarhodopsin II, an intermediate photoproduct of rhodopsin, catalyzes the
activation of GPCR (rod/cone transducin) thru GTP binding G-prot activates cGMP
phosphodiesterase which hydrolyzes cGMP GMP.
Shut off of the cascade involves: 1) phos of the photisomerized rhodopsin,
rendering less effective in activating the G-prot, followed by final capping due to binding of
another protein called arrestin to the phos rhodopsin. 2) deactivation of the active G-prot thru its
intrinsic GTPase activity, which converts the bound GTP to GDP. 3) turn off of the active
phosphodiesterase by rebinding of an inhibitory subunit of the enzyme.
Summary - In dark, cGMP keeps a nonselective cation channel open, and cell is
depolarized. Rhodopsin is coupled to G-protein that activates phosphodiesterase and lowers
[cGMP].
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- Ca influx plays a key negative feedback function and mediates light adaptation.
In dark, there is a circulation of Ca++ at the surface membrane of the outer segment, consisting
of an influx thru the cGMP-gated channels and an efflux thru a transport mechanism involving
an exchange of cations. In the light, the Ca++ influx stops due to closure of the channels, but the
efflux continues, thus resulting in a decrease in the cytosolic Ca++ concentration. This decrease
leads to 1) an increase in guanylate cyclase activity 2) a more effective phosphorylation of the
photoexcited rhodopsin and 3) a higher likelihood of channel opening by cGMP. These effects
all antagonize the action of illumination, and underlie the ability of photoreceptors to adapt to
background lights.
V. Synaptic connections in the retina (see Figure on p. 12)
- The synaptic terminals of rods and cones are morphologically different with rods ending
in spherules (smaller and round) and cones ending in pedicles (larger and with a flat base).
These terminals form connections with horizontal cells and bipolar cells.
- Throughput pathway: photoreceptor bipolar retinal ganglion cell optic nerve
- Lateral associations: between photoreceptors (via gap junctions), and via horizontal and
amacrine cells
- There are distinct synapse morphologies in the retina
- Synaptic triad: photoreceptor with horiztonal-bipolar-horizontal cells, and contains a
synaptic ribbon
- Dyad: proximal bipolar end synapses with two cells
- Primary nxt’s in the retina are Glu (excitatory) and GABA (inhibitory, released by some
horizontal cells). DA is released from interplexiform cells.
VI. Information processing in the retina
- Only amacrine and ganglion cells give all-or-none impulses. The rest of the cells in the
retina have graded responses.
- The receptive field of a single photoreceptor is bigger than itself due to connections
with neighboring photoreceptors; for the same reason, the receptive field of horizontal
cells is bigger than the multiple photoreceptors it contacts
- Since bipolar cells receive synapses from both photoreceptors and horizontal cells, their
receptive field is divided into a center (dominated by photoreceptor) and surrounding
antagonistic ring (dominated by horizontal cells)
- In light, on-bipolars depolarize in response to light in the center of its receptive field
and hyperpolarize in response to light on the surrounding ring; off-bipolars are vice versa.
In terms of synaptic organization, an on-bipolar cell’s receptive field is derived from
sign-inverting (i.e. hyperpolarizing) synapses from the receptors (in the field center) and
sign-preserving (i.e. depolarizing) synapses from the horizontal cells (for the surround).
- The above property of bipolars, propagated to on- and off-center ganglion cells,
probably helps mediate contrast and edge detection
- Ganglion cells have a center-surround receptive field organization like bipolar cells. In
terms of synaptic connections, the on-center ganglion cell probably receives sign-preserving
inputs from on-bipolars in its field center. The on-bipolar cell already has a center-surround
organization, but, in addition, the on-bipolar cells in the surround probably excite amacrine cells,
which in turn have an inhibitory input on the ganglion cell. They can also be classified as X
(slow adapting) and Y (fast adapting) types.
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