The eye Domina Petric, MD

Eye structure The cornea is responsible for most of the light refraction that happens as light passes into the eye. About 80% of refractive power of the eye comes from the cornea. The remaining 20% comes as light passes through the middle structures of the eye: optical lense. Aqueous humor fills the anterior chamber of the eye, but it is produced in the posterior chamber of the eye. Aqueous humor helps support the curvature of the cornea.

http://www.glaucoma.org Anterior chamber of the eye Posterior chamber of the eye

In the unaccommodated vision the lens is relatively flat: far vision. Accommodation In the unaccommodated vision the lens is relatively flat: far vision. When ciliary muscle relax, the tension of zonule fibres is increased: flattening of the lense, unaccommodate vision or far vision. In accommodated configuration the lens is considerably fatter and more round: there is increase of lens refractive index for near vision. When ciliary muscle contracts, the tension of zonule fibres relax: rounding of the lense, accommodation or near vision.

Accommodation Elasticity of the lens is very important for the capacity of accommodation. Pupil increases diameter in far vision and constricts in accommodated vision.

Pupil constricts Near vision Lens becomes more round and accommodated for near vision. Ciliary muscle contracts Zonule fibres relax

Ciliary muscle relaxes Far vision Pupil dilates Ciliary muscle relaxes Lens becomes more flat and unaccommodated, far vision Zonule fibres tension rises

Retina Retina is part of the brain. Retina is derived from diencephalon´s optic vesicle. At the back of the eye there is tissue called pigmented epithelium. More posterior of the pigmented epighelium is neural retina. Neural retina comprises of five basic types of cells. Photoreceptors transduce the light energy into the electrical energy to make an action potential (phototransduction).

Recycling Outer tips of the photoreceptors make intimate contact with pigmented epithelium. The pigmented epithelium is involved in recycling the discs of membrane that contain the photopigments. Discs are bleached in interaction with photons and that is why the recycling is so important.

Retina Pigmenthed epithelium is the most anterior (outer) part of the retina. Next layer is the layer of photoreceptors: in the outer segment are the discs that contain the photo pigment. Next layer is outer nuclear layer which contains the cell bodies of the photoreceptors. Outer plexiform layer is the layer of synapses. Synaptic terminals of photoreceptors release neurotransmitters on the ending of the bipolar cell (in the outer plexiform layer).

Retina The cell body of the bipolar retinal cell is in the inner nuclear layer. The distant process of the bipolar cell makes the conntact with synaptic terminals of photoreceptors. The proximal (inner) process of the bipolar cell makes the conntact with amacrine cell and ganglion cell. The horizontal cell both recieves input from the photoreceptor and gives rise to input to the photoreceptor terminal. The major cell in the inner part of the retina is retinal ganglion cell.

Retina The ganglion cells recieve direct input from bipolar cells and indirect input from the amacrine cells. Amacrine cells give rise to horizontal connections within the inner plexiform layer. In the inner plexiform layer are connections between the bipolar cells and ganglion cells and also connections with amacrine cells. Cell bodies of the ganglion cells are in the ganglion cell layer.

Retina Photoreceptors and bipolar cells generate graded potentials. Ganglion cells generate action potentials. Muller cells are the glial cells of the retina: support the metabolism and the signaling activity of the neurons.

Photoreceptors and phototransduction Two types of photoreceptors are rods and cones. Rod cell is in the dark depolarised: dark current. Ion channels in the plasma membrane of the rod cell open in the dark: calcium and sodium ions influx into the cell. Other ion channels in the plasma membrane of the rod cell support the eflux of the potassium ions ouside the cell.

Photoreceptors and phototransduction Photopigment is a complex molecule which consists of OPSIN (large protein structure) that interacts with a smaller organic molecule 11-CIS RETINAL. Opsin is a metabotropic seven transmembrane structure. 11-cis retinal interacts with the photon directly. In cone cells there are three types of opsin that provide the fundation of color vision. Rod cells have only one type of opsin for scotopic vision.

Photoreceptors and phototransduction When light strikes the rod cell, there is reduction in the concentration of cGMP. When there is low concentration of cGMP, channels for sodium and calcium are closed. Potassium channels remain open and allow the eflux of the potassium ions out of the cell: hyperpolarisation occurs (-65 mV). Light hyperpolarises photoreceptors.

Photoreceptors and phototransduction When light strikes 11-cis retinal, there is rotational twist between carbon atoms 11 and 12: all-trans configuration. All-trans configuration change activates transducin (G proteins) and then the phosphodiesterase enzyme is turned on. Phosphodiesterase enzyme cleaves the cGMP: hyperpolarisation of the photoreceptor.

Cones Rods and cones Rods extremely sensitive to light very low spatial resolution scotopic vision (low light conditions) relatively insensitive to light very high spatial resolution color vision (very bright conditions)

Rods and cones In the rod cells there is graded response, graded by the intensitiy of the light stimulus. There is a very sustained response in rod cells that adapts relatively slowly. Cone cells respond in a graded fashion, but the adaptation is very quick.

About 15-30 rods connect with a single bipolar cell. Rods and cones About 15-30 rods connect with a single bipolar cell. One cone cell connects with a single bipolar cell.

Illumination can be: scotopic mesotopic photopic

Rods are very numerous in the pheripheral parts of the retina. Rods and cones Rods are very numerous in the pheripheral parts of the retina. In the central part of the retina there is a complete exclusion of rods, there are only cone cells: fovea centralis. In the middle of the fovea centralis there is a pit called foveola and it is avascular zone: maximal efficiency of phototransduction. Foveola is the center of fovea centralis and fovea centralis is the center of macula lutea.

Macula lutea Macula lutea Fovea centralis Foveola

Fundoscopy Optic disc is medial. Macula lutea and fovea centralis are lateral. Lateral Medial www.studyblue.com

Ganglion cells Receptive field in vision is a patch of photoreceptors that can modulate the activity of ganglion cells. Illumination and changes in illumination can lead to the modulation of retinal ganglion cells firing. Visual receptive field has central region sensitive to changes in luminance and surrounding region with antagonistic relationship with the center.

There are on-center ganglion cells and off-center ganglion cells. On-center ganglion cell responds best when light strikes at the center of its receptive field. Off-center ganglion cell responds best when light strikes at the periphery of its receptive field (the surround).

Bipolar cells Cone cell synapses with one on-center bipolar cell and one off-center bipolar cell. On-center bipolar cell synapses with on-center ganglion cell and off-center bipolar cell with off-center ganglion cell. On the on-center bipolar cell receptor is metabotropic glutamate receptor. On the off-center bipolar cell receptor is AMPA ionotropic glutamate receptor. In dark conditions glutamate keeps the on-center bipolar cell hyperpolarised and off-center bipolar cell depolarised. In bright conditions on-center bipolar cell is depolarised and off-center bipolar cell is hyperpolarised.

Light in the center Photoreceptor is hyperpolarised and releases less glutamate. On-center bipolar cell is depolarised because metabotropic glutamate receptors cause hyperpolarisation when glutamate binds: no glutamate means depolarisation. On-center ganglion will then be activated. Less glutamate causes the off-center bipolar cell hyperpolarisation because the AMPA receptors need glutamate to depolarise the cell. Off-center ganglion will be deactivated.

Photoreceptor is depolarised and releases the glutamate. Dark in the center Photoreceptor is depolarised and releases the glutamate. Glutamate binds to on-center bipolar cell metabotropic glutamate receptors what leads to hyperpolarisation and finally supression of on-center ganglion cell. Glutamate binds to off-center bipolar cell AMPA receptors what leads to depolarisation and finally off-center ganglion cell activation.

Horizontal cells act in antagonism with photoreceptors. Horizontal cells are the key to mediating the center surround interaction. Horizontal cells act in antagonism with photoreceptors.

Conclusion Retina is conveying to the brain the contrast defined by edges of light and shadow.

Literature https://www.coursera.org/learn/medical-neuroscience: Leonard E. White, PhD, Duke University http://www.glaucoma.org www.studyblue.com