BISC 421 1st Edition Lecture 18 Outline of Current LectureI.EyeballsCurrent LectureEyeballsSurface Features of theEye•Surface feature- what you see when you look in the mirror.•Can look through the pupil and see to the back of the eye.Eyeball•Know the parts of the eye•Rigid thick lens-pupil•Iris attached to ciliary muscle•Vitreous means glasslike- helps maintain shape of the eye•Sclera is the white of the eye- protects the eye•Choroid nourishes the neural portion of the eyeball (retina)Focal planes with respect to the retina(A) Emmetropia (norma l ) (B) M yopia (nearsighted) (C) Hyperopi a (fa rsighted )•Want to focus light on the back of the retina•3 different types of eye problems•most common is hyperopiaa) Ciliary muscles relax a') Ciliary muscles contract b) Zonule fibers stretch b') Zonule fibers are slack c) Far objects are focused on the retina c') Near objects are focused on the retina•Can't do much with the cornea because it is fixed•The flexible lens helps fine tune the focusThe Flexible Lens and AccommodationUnaccommodatedAccommodated•lens wants to be round- the ciliary muscles contract and the zonule fibers grow relaxed good for near objects•when ciliary muscles relax the zonule fibers get stretched- good for seeing things from afarThe RetinaCiliary body•Neural component•The photoreceptors are actually embedded in the choroid so light must pass through ha lot of tissue•The retina is inside out because the photoreceptors have a high metabolic demand•Back= distal•Vertical circuit and horizontal circuit•Outer segments make up the photoreceptor layerThe Retina is Thick Centrally and Thin Peripherallycentral retina•Visual acuity is determined by how tightly spaced these photoreceptors are•More neural processing power in the fovea- thicker layers in the center•Don't need to know each of these layersThe FoveaFovea, coronal sectionHuman foveaPhotoreceptor mosaic, tangential view•Specialized part of the fovea called foveal pit- only cones and they are tiny and tightly packed•Visual acuity is best hereThe Ratio of Cones to Rods Decreases with Eccentricity• Distribution of rods and cones• More rods as you move to the peripheralDemonstration of Changes in Acuity From the Fovea to the PeripheryThe lef hand picture shows the San Diego skyline. The right hand picture is progressively blurred from the center to the periphery. When fixated at their respective centers, both pictures look equally sharp because the progressive blurring in the right hand picture just matches the progressive loss of acuity with eccentricity caused by the increasingly coarse grain of the peripheral retina.• What our acuity is in the center versus the outsideRetina in Health andDiseaseHealthyRetinaThe macula includes the foveal pit, foveal slope, para- and perifovea.•Healthy retina versus diseased retina•Macular degeneration- start to have degradation of the retina around the fovea- lose tissue in photoreceptors•Diabetic retinopathy- capillaries start to die- clouding of foveal regionTransmission Electron Micrograph of Rods and Cones•Transmit light energy into electrical signalDisks in Cones vs. RodsConeFor cones, the outer surface of disks are embedded in the cell membrane.For rods, the disks are stacked inside of the cell membrane.•Cones: disks embedded in cell membrane can transduce light more quickly•Rods: disks stacked inside of cell membrane•Current that is hyperpolarizing the neuron for rod•Signal is much faster for conesNo matter where a photon lands, the whole outer segment hyperpolarizes cell attached patch, Bwhole-cell patchRecordings from a rod outer segment duringstimulation of punctate sites with a laser•Attached 2 electrodes to the outer segment of a rod•The small change in current is propagated throughout the whole cell•Very large signal no matter where the light hits the photoreceptor- anywhere where light hits is read through the entire outer segment and transmitted throughout the entire photoreceptorThe Origin of Receptor Currents•How does light change the voltage?•Rods relatively depolarized when sitting in the dark- specialized Na+ channels kept open by cAMP•With light there is a cascade where the cAMP falls away and then the membrane is hyperpolarizedThe Phototransduction CascadePhototransduction Cascade: The photoreceptor is sitting in the dark with lots of cGMP in the cytoplasm available to keep specialized cGMP activated Na+ channels in the outer membrane open.Then there is A:A small moiety on the opsin molecules ( e.g rhodopsin, in rods) embedded in the disks of the outer segments switches from the 11-cis to all-trans configuration ( e.g. metarhodopdin). In this all-trans state, the opsin activates a GTP-binding protein called transducin (cyan) such that its alpha subunit binds GTP (yellow) and floats away to activate a phosphodiesterase (putty). The phosphodiesterase hydrolyzes cGMP (purplish) to GMP (green). As the concentration of cGMP drops, many molecules fall away from the ion channel (ghastly pink). Thus Na• channels close and the membrane voltage becomes more hyperpolarized.What is particularly amazing is what that one photon of light absorbed by one molecule of opsin leads to the activation of 800 molecules of transducin1:800The 800 transducin molecules activate one phosphodiesterase apiece. 1:1Each molecule of phosphodiesterase hydrolizes -6 molecules of cGMP.1:6By some mysterious stoichiometry, one photon leads to the closure of 200 sodium channels and a drop of 1mV in the membrane potential.Adaptation:The great sensitivity of the phototransduction cascade could lead to early saturation. Thus, mechanisms of adaptation that extend the dynamic range of response are important.AdaptationAdaptation is largely dependent on calcium. The retinal sodium channel is also permeable to calcium. Calcium levels decrease when the channels close; this decrease in calcium concentration leads to increased guanylate cyclase activity* that produces more cGMP etc. There are other restorative mechanisms, as mentioned in the book. A decrease in calcium increases activity of rhodopsin