Neurophysiology and vison By Richard Libertini

Contents Information processing and the eye The eye and lens The retina Photoreceptors and phototransduction Bipolar cells Receptive fields LG nucleus Visual cortex (Basic)

Information is encoded and decoded Encoding Decoding

Basic layout of information processing Light rays, photons Encoding Phototransduction- The conversion of light waves into electrical signals in the retina via rods and cones Processing/computation Information processing and computation starts in the retina (e.g on/off centres of retinal ganglion cells) LG performs further processing such as ‘summing’ the signals from the semifields as well as having more on/off centres. In the visual cortex a vast amount of processing occurs LG PVC

The Eye BASIC FUNCTIONS OF THE EYE 1) Captures light from the outside world 2) The cornea and lens focuses light on to the retina 3) The fovea is responsible for colour vison and high visual acuity 4) The optic nerve/tracts send the information to the visual cortex

Focusing of the image The cornea is responsible for the 2/3 of the focusing power (43 dioptres) The lens focuses and sharpens the image Ciliary muscle contract/relax adjusting the thickness and shape of the lens

The retina Light hits the retina at the back of the eye, It s 200 um thick. The photoreceptors are at the back of the retina and convert light into an electrochemical signal. Also the retina is capable of complex processing of visual information. The backward set up of the retina paradoxically allows for sharper images. The retinal pigment epithelium as the photoreceptors undergoes constant renewal or their membranes, also they capture lost photons. Photoreceptive cells -rods, cones and some retinal ganglion cells

Photoreceptors- rods and cones -only one type -Monochromatic dark adaptation vision -outnumber cones 16:1 Cones -Three subtypes - Responsible for colour vision

Fovea The fovea is the central area of the primate retina, 300-700 um in diameter. A 1:1 relationship with cones and ganglion cells. Inner layer neurons are laterally displaced to minimize light scattering Cones are at their greatest density within the fovea and fall dramatically with increasing distance from the centre. Ganglion cells have small receptive fields thus high resolution vision.

Photoreceptor in more detail Outer segment contains around 1000 tightly packed disks containing the photopigment rhodopsin Inner segment synthesizes photopigments and is densely packed with mitochondrion Photoreceptors have a resting membrane potential of around -40mv. They do not create action potential but a graded potential

Dark current and receptor potential Na+ ions flow into the photoreceptor via non selective channel This cause the receptor potential to be -40mv The dark current is mainly carried by the inward directed Na+ ions and the outwardly directed K+ ions This leads to an increase in Ca++ channel open state probability Light indirectly regulates the Na+ channel. So in darkness the channel is open. 90% of the dark current is due to this. This receptor potential in -40mv This causes neurotransmitter release

Let there be light!! Decrease inward flow of NA+ and continued efflux of K+ results in hyperpolarization Increased intracellular negative charge leads to a decrease in Ca++ channel open state probability K+ channel remains open leading to increasing intracellular – charge (hyperpolaraization) This leads to a decrease in neurotransmitter release Light cause Na+ channel to close via reduction in cGMP

Phototransduction 1 4) Phosphodiesterase hydrolyses cGMP to 5’ GMP 5) Decreased leves of cytoplasmic cGMP leads to an increase in closed states of cGMP gated ion channels 6) Reduced efflux of Na+ leads to hyperpolarization 1) Light cause a conformational change rhodopsin 2) Transducin is activated alpha subunit decouples 3) Alpha subunit stimulates phosphodiesterase

Phototransduction 2 Rhodopsin is a combination of retinol and opsin Light isomerizes 11- cis retinal to all – trans retinal This causes a conformational change of opsin that is now called metarhodopsin II

Phototransduction 3 Metarhodopsin II Activates transducin Metarhodopsin is then recycled in the pigmented epithelium

Phototransduction 4 When transducing is activated the alpha sub unit exchanges a bound GDP to a GTP This then diffuses from the membrane and activates phosphodiesterase Phosphodiesterase then hydrolyzes cGMP to 5’ GMP Photoreceptor hyperpolarizes and decreased glutamate is released

Bipolar cells responses to glutamate On bipolar cell Light decreases glutamate release from photoreceptor Glutamate usually inhibits On centre bipolar cell. The cell loses this inhibition Bipolar cell depolarizes releasing neurotransmitter

Bipolar cells continued Off centre bipolar cells Light decreases glutamate release from photoreceptor Off centre cell loses it’s excitation from glutamate Off centre bipolar cell hyperpolarizes and does not release neurotransmitter

Receptive fields Receptive fields are volumes of visual space. Light lands on the retina and can alter the firing of neurons. The organisation of retinal ganglion fields computes inputs form rods and cones and provides a way of detecting objects edges and contrast.

Receptive fields continued On centre and off centre ganglion cells and their responses to light by measuring action potentials This is achieved by the appropriate on/off bipolar cell firing and lateral inhibition

Lateral inhibition The light causes the centre photoreceptor to release less glutamate and this causes the on centre bipolar cell to fire The surrounding cone is not hyperpolarized so it releases glutamate that activates a horizontal cell The horizontal cell releases GABA onto the photoreceptor further hyperpolarizing it and inhibiting glutamate release it. This causes further activation of the on centre bipolar cell

Lateral inhibition As the horizontal cell releases GABA it further hyperpolarizes the centre photoreceptor This causes further depolarization of the on centre bipolar cell and further hyperpolarization of the off centre bipolar cell

Lateral geniculate nucleus

Visual pathways This is the two stream hypothesis, the “what” and the “where” stream

LG nucleus and visual cortex relations LGN sends projections to the PVS. Then further connections fro the PVC to V1 V2 occur. Content and colour are further processed here.

Visual cortex Any piece of primary visual cortex about 2 mm by 2 mm in surface area must have the machinery to deal completely with some particular area of visual field

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