Visual System

The primary visual pathway

In the optic chiasm fibers from the temporal hemiretinas [blank_start]proceed to the ipsilateral hemisphere[blank_end] and fibers from the nasal hemiretinas [blank_start]cross to the contralateral hemisphere[blank_end].

Visible spetrum: Wavelengths from [blank_start]370[blank_end] to [blank_start]770[blank_end] nm.

The optic disc is [blank_start]nasal[blank_end] to the fovea, so signals from the fovea are processed in the [blank_start]ipsilateral[blank_end] hemisphere of V1.

Temporal to the binocular zone are the [blank_start]monocular crescents[blank_end].

From V1 a ventral pathway goes to the [blank_start]temporal[blank_end] lobe with information about [blank_start]what[blank_end] the stimulus is and its [blank_start]color[blank_end]. A dorsal pathway goes to the [blank_start]parietal[blank_end] lobe with information about [blank_start]where[blank_end] the stimulus is and its [blank_start]motion[blank_end].

Visuotopic maps are more precisly organized at [blank_start]early[blank_end] levels of the pathway.

Retinal ganglion encode information about [blank_start]contrast[blank_end] in the visual field.

Light is refracted by the [blank_start]cornea[blank_end] and the [blank_start]lens[blank_end] and focused onto the [blank_start]retina[blank_end]. (Alphabetical order)

In most vertebrates the [blank_start]cornea[blank_end] is fixed and dynamic focusing is achieved by a flexible [blank_start]lens[blank_end] and the [blank_start]ciliary[blank_end] muscle.

The fovea is free of blood-vessels.

In the foveola the proximal neurons of the retina are shifted aside so light has direct access to the photoreceptors.

The mammalian retina. Put layers left and cell types right

The vertical pathway down the retina is [blank_start]excitatory and inhibitory[blank_end] and goes from the [blank_start]photoreceptors[blank_end] via [blank_start]bipolar cells[blank_end] to the [blank_start]retinal ganglion cells[blank_end]. Horizontal pathways [blank_start]can be excitatory or inhibitory[blank_end] and include [blank_start]horizontal cells[blank_end] and [blank_start]amacrine cells[blank_end].

Retinal Pigmental [blank_start]Epithelium[blank_end] (RPE) is located between the light-sensitive outer segments of the [blank_start]photoreceptors[blank_end] and the blood vessels of the [blank_start]choroid[blank_end].

Bruch's membrane is the [blank_start]innermost[blank_end] layer of the choroid and acts as a [blank_start]diffusion barrier[blank_end] between [blank_start]Retinal Pigmental Epithelium (RTE)[blank_end] and the [blank_start]blood vessels[blank_end].

There are approximately [blank_start]100 million[blank_end] rods and [blank_start]6 million[blank_end] cones. There is only 1 type of [blank_start]rods[blank_end] but 3 types of [blank_start]cones[blank_end]. The fovea contains no [blank_start]rods[blank_end] but is densely packed with small [blank_start]cones[blank_end]. A few millimeters outside the fovea [blank_start]rods greatly outnumber cones[blank_end].

[blank_start]Scotopic[blank_end] vision: vision under low-light levels. [blank_start]Mesopic[blank_end] vision: vision under intermediate lighting conditions. [blank_start]Photopic[blank_end] vision: vision under well-lit conditions.

Rods are responsible for [blank_start]scotopic and mesopic[blank_end] vision. Cones are responsible for [blank_start]photopic[blank_end] vision.

Which one is true?

The central part of the fovea is called foveola or foveal [blank_start]pit[blank_end]. The adjacent region is called foveal [blank_start]slope[blank_end].

The fovea contains mostly [blank_start]M-[blank_end] and [blank_start]L-[blank_end]cones but only very few [blank_start]S-[blank_end]cones.

Receptive field of a visual neuron: The area [blank_start]in visual space[blank_end] where changes in light intensity (or composition, layout, ...) trigger a change in the neuronal response. This change can be excitatory or/and inhibitory.

The mitochondria of photoreceptors are contained in their [blank_start]inner segment[blank_end].

Outer and inner segments of rods and cones are connected by the [blank_start]central cilium[blank_end] (cc).

Absorption of a photon [blank_start]hyperpolarizes[blank_end] photoreceptors and generates [blank_start]OFF[blank_end]-responses.

Phototransduction Rhodopsin is densely packed in the [blank_start]disk membranes[blank_end] of rods. It consists of the protein opsin and the light-absorbing part [blank_start]retinal[blank_end]. In the dark retinal is in the [blank_start]11-cis[blank_end] configuration but absorption of a photon causes a flip to the [blank_start]all-trans[blank_end] configuration. This causes a change in the opsin to an [blank_start]activated[blank_end] state called [blank_start]metarhodopsin II[blank_end]. Metarhodopsin II can then activate [blank_start]hundreds of molecules[blank_end] called transducin. Transducin in turn [blank_start]activates[blank_end] phosphodiesterase which hydrolizes cGMP and leads to [blank_start]a decrease[blank_end] of the cGMP level. PDE hydrolizes [blank_start]more than 1000 cGMP[blank_end] per second. This causes the Na+ and Ca2+ channels to [blank_start]close[blank_end] and a sharp [blank_start]decrease[blank_end] in glutamate release. The metarhodopsin II splits within minutes to opsin and free [blank_start]all-trans retinal[blank_end]. This is either directly transformed back to 11-cis retinal of first reduced to all-trans-retinol, then to 11-cis retinol and then back to 11-cis retinal. This happens in [blank_start]the retinal pigment epithelium[blank_end].

Metarhodopsin II can activate hundreds of transducin molecules because

Termination of the phototransduction cascade (1) Metarhodopsin is first phosphorylated and its interaction with [blank_start]transducin[blank_end] is blocked by the protein [blank_start]arrestin[blank_end]. (2) Active transducin has [blank_start]GTPase[blank_end] activity which leads to [blank_start]inactivation[blank_end] of [blank_start]phosphodiesterase[blank_end]. (3) A negative feedback by [blank_start]decrease[blank_end] of [blank_start]Ca2+[blank_end] influx leads to synthesis of new [blank_start]cGMP[blank_end].

cGMP synthesis is 5-10 times higher in [blank_start]cones[blank_end] than in [blank_start]rods[blank_end].

Adaptation to light is modulated by changes in the influx of [blank_start]Ca2+[blank_end].

Photoreceptors tonically fire action potentials and shut down as a reaction to a light stimulus.

Photoreceptors transmit their signals to bipolar and horizontal cells via [blank_start]ribbon[blank_end] synapses. Their function is not entirely clear but they serve as a hotspot of synaptic vesicle exocytosis as well as a replenishment station. They disassemble when there is a high concentration of [blank_start]Ca2+[blank_end].

Which one is true?

Horizontal cells form synapses to [blank_start]photoreceptors[blank_end] in the [blank_start]outer plexiform layer[blank_end], while amacrine cells form synapses to [blank_start]retinal ganglion cells[blank_end] in the [blank_start]inner plexiform layer[blank_end].

The synapses of cones connect [blank_start]to ON and OFF[blank_end] bipolar and horizontal cells and form [blank_start]multiple small ribbons[blank_end]. The synapses of rods connect [blank_start]only to ON[blank_end] bipolar and horizontal cells and form [blank_start]a single large ribbon[blank_end].

Which of these cells fire action potentials?

ON- and OFF-type responses occur for the first time in [blank_start]bipolar cells[blank_end]. The lateral inhibition is mediated by [blank_start]inhibitory[blank_end] [blank_start]horizontal cells[blank_end] which use [blank_start]electrical[blank_end] synapses.

Rod ON and cone ON bipolar cells express [blank_start]mGluR6[blank_end] which leads to [blank_start]inhibitory[blank_end] glutamate receptors. Cone OFF bipolar cells express [blank_start]AMPA-/kainate[blank_end] which leads to [blank_start]excitatory[blank_end] glutamate receptors.

Horizontal cells provide negative feedback to cone terminal which leads to a short peak in the cone response and a subsequent smaller steady level. This is related to the transient response of retinal ganglion ells.

There are ON and OFF [blank_start]cone[blank_end] bipolar cells but only [blank_start]ON[blank_end] [blank_start]rod[blank_end] bipolar cells.

Rod ON bipolar cells connect to [blank_start]AII amacrine cells[blank_end] which make [blank_start]excitatory[blank_end] [blank_start]gap junctions[blank_end] to ON-cone bipolar cells and [blank_start]glycinergic[blank_end] [blank_start]inhibitory[blank_end] synapses to OFF-cone bipolar cells.

Responses of retinal ganglion cells ...

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Alphabetic: Low-level processing: [blank_start]Color[blank_end] [blank_start]Contrast[blank_end] [blank_start]Movement direction[blank_end] [blank_start]Orientation[blank_end] Intermediate-level processing: [blank_start]Contour integration[blank_end] [blank_start]Object motion and shape[blank_end] [blank_start]Surface depth[blank_end] [blank_start]Surface properties[blank_end] High-level processing: [blank_start]Object identification[blank_end]

Development of the eye: The [blank_start]optic cup[blank_end] and the [blank_start]ecto[blank_end]derm form the eye. The inverted optic cup forms the [blank_start]retina[blank_end], the outer layer the [blank_start]pigment epithelium[blank_end]. The retina is a protrusion of the [blank_start]diencephalon[blank_end].

For near vision the lens is [blank_start]round[blank_end] and the ciliary muscle [blank_start]contracted[blank_end], for far vision the lens is [blank_start]flat[blank_end] and the ciliary muscle [blank_start]relaxed[blank_end].

Shortsightedness: [blank_start]Myopia[blank_end] Farsightedness: [blank_start]Hyperopia[blank_end]

Diffusion barriers of the mammalian retina

Cortical magnification: The central [blank_start]10[blank_end] degrees of the retina project to [blank_start]50[blank_end] % of the primary visual cortex.

Termination of the phototransduction cascade: Rhodopsin-kinase [blank_start]phosphorylates[blank_end] metarhodopsin, which then binds arrestin that blocks the interaction with [blank_start]transducin[blank_end] ([blank_start]GDP-GTP[blank_end] exchange). Active transducin has an intrinsic [blank_start]GTP[blank_end]-ase activity that splits its own GTP to GDP and thereby inactivates [blank_start]phosphodiesterase[blank_end]. The [blank_start]decrease[blank_end] in [blank_start]Ca2+[blank_end] leads to the activation of Guanylate-cyclase by [blank_start]GCAP[blank_end]. This leads to the synthetization of cGMP from [blank_start]GTP[blank_end].

[blank_start]Horizontal[blank_end] cells are responsible for the OFF-surround response of bipolar cells.

Cone-OFF bipolar cells and horizontal cells are excited by glutamate via [blank_start]ionotropic[blank_end] glutamate receptors.

Three proposed mechanisms for negative feedback from horizontal cells to photoreceptor cells: (a) [blank_start]GABA[blank_end] release (b) Ephaptic modulation by hemi-gap junctions in horizontal cell [blank_start]dendrites[blank_end] (c) [blank_start]H+[blank_end] modulates presynaptic photoreceptor [blank_start]calcium[blank_end] currents

OFF retinal ganglion cells have their synapses [blank_start]above[blank_end] ON retinal ganglion cells in the [blank_start]inner plexiform layer[blank_end].

P retinal ganglion cells have [blank_start]small[blank_end] receptive fields with [blank_start]high spectral sensitivity[blank_end] and project to the [blank_start]ventral stream[blank_end]. M retinal ganglion cells have [blank_start]large[blank_end] receptive fields with [blank_start]broad spectral sensitivity[blank_end] and project to the [blank_start]dorsal stream[blank_end].

Metarhodopsin II consists of [blank_start]all-trans[blank_end] [blank_start]retinal[blank_end] bound to opsin. It helps to exchange [blank_start]GDP to GTP[blank_end] on hunderds of transducin molecules to activate them (first amplification step). Phosphodiesterase has an [blank_start]inhibitor[blank_end] which is [blank_start]removed[blank_end] by active transducin. The [blank_start]alpha[blank_end]-subunit of active PDE hydrolyzes more than thousand [blank_start]cGMP to 5'GMP[blank_end] per second (second amplification step).