Visual System

The optic disc is nasal temporal( nasal, temporal ) to the fovea, so signals from the fovea are processed in the ipsilateral contralateral( ipsilateral, contralateral ) hemisphere of V1.

From V1 a ventral pathway goes to the temporal parietal occipital frontal( temporal, parietal, occipital, frontal ) lobe with information about what where how big( what, where, how big ) the stimulus is and its color motion shape speed( color, motion, shape, speed ).

A dorsal pathway goes to the parietal occipital frontal temporal( parietal, occipital, frontal, temporal ) lobe with information about where what( where, what ) the stimulus is and its motion color shape( motion, color, shape ).

Visuotopic maps are more precisly organized at early higher( early, higher ) levels of the pathway.

Retinal ganglion encode information about contrast brightness color movement( contrast, brightness, color, movement ) in the visual field.

In most vertebrates the cornea lens( cornea, lens ) is fixed and dynamic focusing is achieved by a flexible lens cornea( lens, cornea ) and the ciliary flexing extending( ciliary, flexing, extending ) muscle.

The vertical pathway down the retina is excitatory and inhibitory excitatory inhibitory( excitatory and inhibitory, excitatory, inhibitory ) and goes from the photoreceptors retinal ganglion cells( photoreceptors, retinal ganglion cells ) via bipolar cells horizontal cells amacrine cells( bipolar cells, horizontal cells, amacrine cells ) to the retinal ganglion cells photoreceptors( retinal ganglion cells, photoreceptors ).

Horizontal pathways can be excitatory or inhibitory are excitatory are inhibitory( can be excitatory or inhibitory, are excitatory, are inhibitory ) and include horizontal cells retinal ganglion cells bipolar cells( horizontal cells, retinal ganglion cells, bipolar cells ) and amacrine cells photoreceptor cells( amacrine cells, photoreceptor cells ).

Retinal Pigmental Epithelium Epiderm Epimeninges( Epithelium, Epiderm, Epimeninges ) (RPE) is located between the light-sensitive outer segments of the photoreceptors bipolar cells retinal ganglion cells( photoreceptors, bipolar cells, retinal ganglion cells ) and the blood vessels of the choroid cornea( choroid, cornea ).

Bruch's membrane is the innermost outermost( innermost, outermost ) layer of the choroid and acts as a diffusion barrier light absorbing barrier( diffusion barrier, light absorbing barrier ) between Retinal Pigmental Epithelium (RPE) the photoreceptors( Retinal Pigmental Epithelium (RPE), the photoreceptors ) and the blood vessels bipolar cells( blood vessels, bipolar cells ).

There are approximately 100 million 100 billion 10 million( 100 million, 100 billion, 10 million ) rods and 6 million 60 million 20 million( 6 million, 60 million, 20 million ) cones.
There is only 1 type of rods cones( rods, cones ) but 3 types of cones rods( cones, rods ).

The fovea contains no rods cones( rods, cones ) but is densely packed with small cones rods( cones, rods ).

A few millimeters outside the fovea rods greatly outnumber cones cones greatly outnumber rods( rods greatly outnumber cones, cones greatly outnumber rods ).

Receptive field of a visual neuron:

The area in visual space on the retina( in visual space, on the retina ) 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 inner segment outer segment cell body synaptic terminal( inner segment, outer segment, cell body, synaptic terminal ).

Absorption of a photon hyperpolarizes depolarizes( hyperpolarizes, depolarizes ) photoreceptors and generates OFF ON( OFF, ON )-responses.

Phototransduction

Rhodopsin is densely packed in the disk membranes inner segment( disk membranes, inner segment ) of rods. It consists of the protein opsin and the light-absorbing part retinal retinol metarhodopsin( retinal, retinol, metarhodopsin ).
In the dark retinal is in the 11-cis all-trans( 11-cis, all-trans ) configuration but absorption of a photon causes a flip to the all-trans 11-cis( all-trans, 11-cis ) configuration.
This causes a change in the opsin to an activated inactivated( activated, inactivated ) state called metarhodopsin II metarhodopsin rhodopsin II( metarhodopsin II, metarhodopsin, rhodopsin II ).

Metarhodopsin II can then activate hundreds of molecules exactly one molecule( hundreds of molecules, exactly one molecule ) called transducin. Transducin in turn activates deactivates( activates, deactivates ) phosphodiesterase which hydrolizes cGMP and leads to a decrease an increase( a decrease, an increase ) of the cGMP level.
PDE hydrolizes more than 1000 cGMP approximately 1 cGMP( more than 1000 cGMP, approximately 1 cGMP ) per second.

This causes the Na+ and Ca2+ channels to close open( close, open ) and a sharp decrease increase( decrease, increase ) in glutamate release.

The metarhodopsin II splits within minutes to opsin and free all-trans retinal 11-cis retinal( all-trans retinal, 11-cis retinal ). 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 the retinal pigment epithelium the outer plexiform layer the inner plexiform layer Bruch's membrane the outer nuclear layer the inner nuclear layer( the retinal pigment epithelium, the outer plexiform layer, the inner plexiform layer, Bruch's membrane, the outer nuclear layer, the inner nuclear layer ).

Termination of the phototransduction cascade

(1) Metarhodopsin is first phosphorylated and its interaction with transducin phosphodiesterase cGMP( transducin, phosphodiesterase, cGMP ) is blocked by the protein arrestin cGMP( arrestin, cGMP ).

(2) Active transducin has GTPase GDPase cGMPase( GTPase, GDPase, cGMPase ) activity which leads to inactivation reactivation( inactivation, reactivation ) of phosphodiesterase cGMP( phosphodiesterase, cGMP ).

(3) A negative feedback by decrease increase( decrease, increase ) of Ca2+ Na+( Ca2+, Na+ ) influx leads to synthesis of new cGMP GTP GDP GMP( cGMP, GTP, GDP, GMP ).

Adaptation to light is modulated by changes in the influx of Ca2+ Na+( Ca2+, Na+ ).

ON- and OFF-type responses occur for the first time in bipolar cells photoreceptors retinal ganglion cells( bipolar cells, photoreceptors, retinal ganglion cells ). The lateral inhibition is mediated by inhibitory excitatory( inhibitory, excitatory ) horizontal cells amacrine cells( horizontal cells, amacrine cells ) which use electrical chemical( electrical, chemical ) synapses.

Rod ON bipolar cells connect to AII amacrine cells horizontal cells retinal ganglion cells( AII amacrine cells, horizontal cells, retinal ganglion cells ) which make excitatory inhibitory( excitatory, inhibitory ) gap junctions glutamatergic synapses( gap junctions, glutamatergic synapses ) to ON-cone bipolar cells and glycinergic glutamatergic GABAergic( glycinergic, glutamatergic, GABAergic ) inhibitory excitatory( inhibitory, excitatory ) synapses to OFF-cone bipolar cells.

Termination of the phototransduction cascade:

Rhodopsin-kinase phosphorylates hydrolates( phosphorylates, hydrolates ) metarhodopsin, which then binds arrestin that blocks the interaction with transducin phosphodiesterase cGMP( transducin, phosphodiesterase, cGMP ) (GDP-GTP GMP-GDP( GDP-GTP, GMP-GDP ) exchange).

Active transducin has an intrinsic GTP GDP GMP( GTP, GDP, GMP )-ase activity that splits its own GTP to GDP and thereby inactivates phosphodiesterase cGMP( phosphodiesterase, cGMP ).

The decrease increase( decrease, increase ) in Ca2+ Na+ K+( Ca2+, Na+, K+ ) leads to the activation of Guanylate-cyclase by GCAP GCAT GBAP GBBT( GCAP, GCAT, GBAP, GBBT ). This leads to the synthetization of cGMP from GTP GDP 5'GMP( GTP, GDP, 5'GMP ).

Horizontal Amacrine Photoreceptor Retinal ganglion( Horizontal, Amacrine, Photoreceptor, Retinal ganglion ) cells are responsible for the OFF-surround response of bipolar cells.

Three proposed mechanisms for negative feedback from horizontal cells to photoreceptor cells:

(a) GABA glycine acetylcholine( GABA, glycine, acetylcholine ) release
(b) Ephaptic modulation by hemi-gap junctions in horizontal cell dendrites axons( dendrites, axons )
(c) H+ Phosphor units NO( H+, Phosphor units, NO ) modulates presynaptic photoreceptor calcium sodium potassium( calcium, sodium, potassium ) currents

OFF retinal ganglion cells have their synapses above below( above, below ) ON retinal ganglion cells in the inner plexiform layer outer plexiform layer inner nuclear layer outer nuclear layer ganglion cell layer( inner plexiform layer, outer plexiform layer, inner nuclear layer, outer nuclear layer, ganglion cell layer ).

Metarhodopsin II consists of all-trans 11-cis( all-trans, 11-cis ) retinal retinol( retinal, retinol ) bound to opsin. It helps to exchange GDP to GTP GMP to GDP GTP to cGMP( GDP to GTP, GMP to GDP, GTP to cGMP ) on hunderds of transducin molecules to activate them (first amplification step).

Phosphodiesterase has an inhibitor activator( inhibitor, activator ) which is removed produced( removed, produced ) by active transducin.

The alpha beta gamma beta-gamma( alpha, beta, gamma, beta-gamma )-subunit of active PDE hydrolyzes more than thousand cGMP to 5'GMP cGMP to GDP GDP to GTP GTP to cGMP( cGMP to 5'GMP, cGMP to GDP, GDP to GTP, GTP to cGMP ) per second (second amplification step).