The retina Flashcards
This lecture will continue from the end of lecture 1 to discuss the circuits in the retina that underlie vision. We will discuss:
- Phototransduction
- Rod and cone circuitry
- The concept of the receptive field
- ON and OFF bipolar cells
- ON and OFF ganglion cells
- Adaptation
- Magno and Parvo cells in the retina
- Colour vision
process of phototransduction
Transduction of light into electrical signals occurs in the outer segment of rods and cones. Here we will focus on the process in rods (but it is similar in cones).
Rhodopsin molecules in the outer-segment discs absorb photons. This then closes cyclic guanosine monophosphate (cGMP)-gated Na+ channels, hyperpolarizing the membrane and reducing the rate of glutamate release
Visual pigment in rods
Rhodopsin is the covalent complex of two components, consisting of opsin, a large protein with 348 amino acids that loops seven times across membrane of the rod disc, and retinal, a small light-absorbing compound.
Absorption of light by 11-cis retinal causes a rotation around the double bond. As retinal adopts the more stable all-trans configuration, it causes a conformational change in the opsin to an activated state called metarhodopsin II. Metarhodopsin II is unstable and splits within minutes, yielding opsin and free all-trans retinal. The all-trans retinal is then transported from rods to pigment epithelial cells, where it is reduced to all-trans retinol (vitamin A), the precursor of 11-cis retinal, which is subsequently transported back to rods.
All-trans retinal is thus a crucial compound in the visual system. Its precursors, such as vitamin A, cannot be synthesized by humans and so must be a regular part of the diet.
Opsins in rods and cones are similar, particularly in L and M cones (96% identity in amino acid sequences).
Visual pigment in rods
Mechanisms for phototranduction
Rhodopsin is the covalent complex of two components, consisting of opsin, a large protein with 348 amino acids that loops seven times across membrane of the rod disc, and retinal, a small light-absorbing compound.
Absorption of light by 11-cis retinal causes a rotation around the double bond. As retinal adopts the more stable all-trans configuration, it causes a conformational change in the opsin to an activated state called metarhodopsin II. Metarhodopsin II is unstable and splits within minutes, yielding opsin and free all-trans retinal. The all-trans retinal is then transported from rods to pigment epithelial cells, where it is reduced to all-trans retinol (vitamin A), the precursor of 11-cis retinal, which is subsequently transported back to rods.
All-trans retinal is thus a crucial compound in the visual system. Its precursors, such as vitamin A, cannot be synthesized by humans and so must be a regular part of the diet.
Opsins in rods and cones are similar, particularly in L and M cones (96% identity in amino acid sequences).
Molecular processes in phototransduction
Remember the overalll target is to hyperpolarize the photoreceptors through closing of the sodium channels.
When the light shines on to the photoreceptors (cones and rods). The rhodopsin inside it receives the light, the cis-retinal as one component of the rhodopsin is activated with its rotated double bond, as the cis-retinal is changing to the all-trans retinal, it activates the opsin as another component of rhodopsin to undergo a conformation change to an activated state called metarrhodopsin II.
This unstable compound quickly converted to opsin and all-trans retinal (through phosphorylation by a rhodopsin kinase followed by binding of protein arrestin, which blocks the interaction with transducin).
The all-trans retinal is then transported from rods to pigment epithelium and converted to all-trans retinol (vitamin A), the precursor of 11-cis retinal, which is subsequenctly transported back into rpd.
This entire process allows the activation cycle of rhodopsin.
Light->1-cis retinal doble bond rotated-> cis-retinal converting to all trans-retinal-> opsin underconformation change-> activated state as metarhodopsin II ->opsin+ all-trans retinal-> transported from rods to photo receptor epithelium -> all-trans retinol (viamin A)
The light shines on the photoreceptors activates rhodopsin which closes the cGMP-gated Na+ channels by activating the transducin, which inturn activates the PDE to reduce cGMP level, hyperpolarizing the membrane and reducing the rate of glutamate release.
cGMP is produced by a guanylate cyclase (GC) from guanosine triphosphate (GTP) and hydrolyzed by a phosphodiesterase (PDE).
In the dark, phosphodiesterase activity low, cGMP concentration high, and cGMP-gated channels are open, allowing influx of Na+ and Ca2+.
In the light, rhodopsin (R) is excited by absorption of photons, then activates transducin (T), which in turn activates the PDE; the cGMP level drops, membrane channels close, and less Na + and Ca2+ enter the cell.
There are then multiple mechanisms that shut off the cascade.
a) Metarhodopsin II is inactivated through phosphorylation by a rhodopsin kinase followed by binding of the protein arrestin, which blocks the interaction with transducin.
b) Active transducin (Tα-GTP) has an intrinsic GTPase activity, which eventually converts bound GTP to GDP. Tα-GDP then releases phosphodiesterase and recombines with Tβγ, ready again for excitation by rhodopsin. Once the phosphodiesterase has been inactivated, the cGMP concentration is restored by a guanylate cyclase that produces cGMP from GTP.
c) Calcium ions have negative feedback role. Light leads to closure of cGMP-gated channels → drop in intracellular concentration of Ca 2+.
Ca2+ modulates function of at least 3 components of cascade (rhodopsin, GC, and cGMP-gated channel): drop in Ca 2+ counteracts the excitation caused by light
c1) Rhodopsin phosphorylation is accelerated through the action of the calcium-binding protein recoverin on rhodopsin kinase, thus reducing activation of transducin.
c2) The activity of guanylyl cyclase is accelerated by calcium-dependent guanylyl cyclase–activating proteins.
c3) the affinity of the cGMP-gated channel for cGMP is increased through the action of Ca 2+ -calmodulin. All these effects promote the return of the photoreceptor to the dark state.
There are then multiple mechanisms that shut off the cascade.
Recovery
There are 4 mechanisms leading to the recovery of photoreceptor, throguh action on metarhodopsin II, transducin and all-trans retinal and Ca2+ concentration which acts as a neagtive feedback.
The metarhiodopsin II is inactivated through phosphorylation by the rhodopsin kinase, following by the binding of arrestin which competes for the active site against transducin thus reduce the transducin activation.
The active transducin contains a intrinsic GTPase function, which will convert the GTP to GDP, this will deactivate the PDE by releasing it and recombines to Tβγ, while transducin recovered for further activation from the metarhodopsin. Once the PDE has been inactivated, the cGMP level is restored throguh the guanylate cyclase producing cGMP from GTP.
The reduction of intracellular Ca2+ concentration due to the inactivation of cGMP-gated channel acts as a negative feedback. With the decrease of Ca2+ concentration, it increases activity rehrodopsin kinase, and affinity of cGMP-gated Na+ channel towards Na+ and GC.
The rhodopsin phosphorylation is accelerated by the reduction of [Ca2+] with calcium-binding protein recoverin on rhodopsin kinase, thus reduce activation of transducin.
The activity of guanulate cyclase is aceelerated by calcium-dependent guanylate cyclase-activating proteins. The reduced level of [Ca2+] leads to hihger activity of GC.
The affinity of the cGMP-gated channel for cGMP is increased throguh the action of Ca2+-calmodulin.
The all trans-retinal is converted back into 11-cis retinal through a step to all-trans retinol. It is a slow process because the all-trans retinal needs to be transported into the pigmented epithelium. The all-trans retinol is also called vitamin-A. Then the 11-cis retinal is ready to be activated by light again,
Photoreceptors are hyperpolarized by Light
For dim flashes, response amplitude increases linearly with intensity. At high intensities, the receptor saturates and remains hyperpolarized steadily for some time after the flash; this leads to the afterimages that we perceive after a bright flash.
The response peaks earlier for brighter flashes, and cones generally respond faster than rods.
Concept of the receptive field (RF)
- The receptive field is the location on the retina where the cell is responsive to light.
- The receptive fields also refers to the matching location in the visual field (because of the correspondence to the matching location on the retina). In this case the units are visual degrees.
Retinal circuit
Cone photoreceptors synapse onto bipolar cells
Bipolar cells synapse onto ganglion cells (two major types of ganglion cells are ON and OFF cells)
Axons of ganglion cells exit the eye via the optic nerve
Horizontal and amacrine cells make lateral connections in the retina
Rod photoreceptors synapse onto rod bipolar cells
Rod bipolar cells do not directly synapse onto ganglion cells but take a detour via cone bipolar cells, then onto ganglion cells.
It helps to introduce the idea of the sign-preserving (electrical and glutamatergic synapses) and sign-inverting connections (GABAergic, glycinergic, or glutamatergic synapses).
Focusing first on the cone signal circuitry: photoreceptors synapse onto two major types of bipolar cells: one that inverts the sign (ON cell) and one that preserves it (OFF cell). Why do they respond in different ways? The answer is that there are two kinds of glutamate receptors
The two major types of bipolar cells are ON and OFF cells
1) OFF bipolar cells have ionotropic glutamate receptors that mediate a classical depolarizing excitatory postsynaptic potential from the influx of Na+. Hyperpolarization of the cone causes less neurotransmitter to be released, resulting in a more hyperpolarized bipolar cell.
2) ON bipolar cells have G-protein-coupled (metabotropic) receptors and respond to glutamate by
hyperpolarizing.
Each bipolar cell receives direct synaptic input from a cluster of photoreceptors
The number of photoreceptors in this cluster ranges from one at the centre of the fovea to thousands in the peripheral retina.
Also note the horizontal cells: they receive input from the photoreceptors and return a reversed signal: effectively providing lateral inhibition.
how the receptive field of the bipolar cells looks like
antagonistic centre-surround receptive field
Let’s now have a look at how the receptive field of the bipolar cells looks like. The central part of the receptive field receives direct input from the cones in that part of the retina. For an ON bipolar, it reverses the signal from the cones such that it increases activity in response to light.
With regards to the surround of the receptive field, horizontal cells integrate activity in the surround which then supresses the signal in the centre (lateral inhibition): thus, the bipolar cell is suppressed by light in the surround.
This type of receptive field is called an antagonistic centre-surround receptive field.
Cone circuit: amacrine and ganglion cells
The next stage in the cone signal circuit is that bipolar cells project onto ganglion cells. Like bipolar cells, the two major types of ganglion cells are ON and OFF cells (receiving inputs from ON and OFF bipolar cells). Like horizontal cells, amacrine cells integrate information across space and contribute to sharpening centre-surround organization using lateral inhibition. In comparison to photoreceptors, horizontal and bipolar cells, amacrine and ganglion cells do not only have graded potentials but display action potentials.
Rod circuit:
In the rod circuit, rod photoreceptors cause depolarization in rod bipolar cells (using a sign reversing connection, similar to ON bipolar cells in the cone circuit). The rod bipolar does not directly synapse onto ganglion cells but instead takes a detour via a special type of amacrine cell that subsequently activates the cone ON bipolar cells (via electric gap junctions) and the cone OFF bipolar cell (via glycinergic inhibitory signals).
Rods also have two other pathways to: they drive neighbouring cones via electrical junctions and make connections with an OFF bipolar cell type that mainly connects to cones. Thus, the rods appear to take advantage of the same circuitry as the cones: this works because of the two different modes of vision that rods and cones contribute to, serving scotopic vs photopic vision (the “duplex retina”).
The two major types of ganglion cells are ON and OFF cells
Hartline (1938, 1940) observed that the optic nerve fiber in the frog could be excited by light on a small circular area of the retina.
In 1953, Kuffler discovered the antagonistic centre-surround organization of RFs of ganglion cells in the cat. Like bipolar cells, ganglion cells are excited by either a light (ON cells) or dark spot (OFF cells) in the centre of the receptive field. We can further distinguish between cells with sustained or transient responses.
If the same spot is presented in the ‘antagonistic’ surround, the effect is opposite: causing a response suppression. Similarly, if homogenous light covering both the center and surround is presented, the response is weaker compared to a spot restricted to the centre.
Finally, the best stimulus (causing the maximum response in terms of action potentials) is when light hits the centre while the surround is dark (ON cell) or when a dark spot hits the centre while the surround is covered by light (OFF cell).
One take-home message is that the center-surround RF (reception field) performs an image processing operation that enhances and helps detect edges in the image (similar types of ‘kernels’ are used in image processing of images to do edge detection). The retina cares about contrast: changes in luminance over space.
