The Eye and Retina

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The retina is a light-sensitive layer of tissue that lines the rear surface of the eye. Light from one's visual field passes through the eye and projects onto the retina to create an image. Subsequently, retinal neurons detect this image, which initiates a cascade of biochemical and electrical processing that is sent through the optic nerve and eventually to the visual cortex of the brain. These biochemical and electrical signals provide the basis for vision.

Hey EyeWirers!

As most of us know, we are tracing a sample from the retina of a mouse. Why a mouse and not a human? Why the retina and not a piece of the brain?

We decided to start a blog series with some information on the anatomy and physiology of the human eye and vision. Hopefully you guys will enjoy it. We will start this journey with a simple anatomy of the parts of the eye we. From this point, we will travel inwards the same way light does. Along the way, we will talk about some structures along the way and any other curious information.”

Introduction to the Eye 001/101 - outline

Introduction to what is ahead

Basic anatomy of the human eye

Look into my eyes and tell me what you see

Eyelids and eyelashes Tear duct Sclera White part of the eye Iris Color of the eye Pupil Dark area at the center of the iris

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More anatomy - Going deeper into the human eye

Situated in the orbit of the skull

Cornea Transparent surface of the eye that covers the pupil and iris First refractive surface that light goes through on its way to the retina 3 layers (epithelium, stroma, endothelium) 5, possibly 6 layers, (Epithelium, Bowmans layer, Stroma, Dua’s layer (Discovered last year, still awaiting official confirmation), Descemet’s membrane, endothelium) medical: corneal abrasions, ametropia (any refractive area can be cause), etc Iris Muscular diaphragm that controls the size of the pupil and amount of light that can enter the eye Color of the iris is due to quantity and type of melanin Pupil Aperture that allows the light to enter the eye Lens Second refractive surface that light goes through on its way to the retina tidbit: no blood supply to the lens medical: cataracts, presbyopia Ciliary body Ciliary muscle (with zonule fibers) Changes the shape of the lens and allows for accommodation Ciliary epithelium Makes the aqueous fluid that fills the anterior and posterior chambers Three fluid chambers Anterior Between cornea and iris Posterior Between iris and lens Vitreous Between lens and retina Humours (hehehe) Aqueous Clear, watery fluid that supplies nutrients to the structures it surrounds. Movement of aqueous humor is from posterior chamber to anterior chamber through the pupil. Drainage out of the anterior chamber via canal of Schlemm and trabecular meshwork into venous system. The equilibrium maintains appropriate intra-ocular pressure. medical: glaucoma (open and close angle) Vitreous thick, gel-like fluid that maintains the shape of the eye 80% of the volume of the eye medical: floaters, retinal detachment Three layers of the eye sphere (outside to inside) Sclera Outermost layer of the globe White fibrous layer that becomes transparent at anterior part of the eye and forms the cornea Choroid Middle layer between sclera and retina It is the vascular layer and source of oxygen and nutrients to the outer layers of the retina (photoreceptor layer) Bruch’s membrane Retina Inner layer of the globe Light-sensitive neurons transmit visual signals Macula and Fovea Site of high acuity of vision Optic nerve (add this here?) Retinal arteries and veins (add this here?)

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Extraocular muscles of the eye

Muscles insert into the sclera and move the eyeball and keep image focused on the fovea 6 main muscles: superior rectus inferior rectus medial rectus lateral rectus superior oblique inferior oblique medical: lazy eye, etc.

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Anatomy of the human retina

Funduscopic examination of the retina

Optic cup and disc retinal ganglion axons converge here central area are retinal artery and veins “Blind spot” Macula and Fovea high quantity of ganglion cells and cones for visual acuity and color perception Interactive site: Image credits:

10 histological layers of the retina Retinal pigment epithelium Single layer of hexagonal cells Located between the choroid and the photoreceptor layer Forms a blood-retina barrier with tight junctions with the choroid It is not firmly attached to the the neural aspect of the retina (photoreceptor layer) medical: a potential site of retinal detachment Photoreceptor layer Composed of rods and cones Outer limiting “membrane” Site of connection between photoreceptors and Müller cells Outer nuclear layer Nuclei of photoreceptor cells Outer plexiform layer Photoreceptor fibers Bipolar cell dendrites “Two important synaptic interactions that occur at the outer plexiform layer are: the splitting of the visual signal into two separate channels of information flow, one for detecting objects lighter than background and one for detecting objects darker that background the instillation of pathways to create simultaneous contrast of visual objects In the first synaptic interactions, the channels of information flow are known as the basis of successive contrast, or ON and OFF pathways, respectively, whereas the second interaction puts light and dark boundaries in simultaneous contrast and forms a receptive field structure, with a center contrasted to an inhibitory surround.” Inner nuclear layer Bipolar cell nuclei Horizontal cells Amacrine cells Interplexiform cells Muller cells Inner plexiform layer Presynaptic dendrites of bipolar cells (axons) Postsynaptic dendrites of ganglion cells Amacrine cell dendrites Ganglion cell layer Nerve fiber layer Axons of Ganglion cells Inner limiting “membrane” Ends of Muller cells

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Cells of the retina Retinal Pigment Epithelium Contain pigment granules and absorbs scattered light Regenerates 11-cis-retinal the chromophore used in photoreceptors Responds to oxidative stress Clearing up shed discs of rods and cones Blood-retinal barrier functions Medical: albinos lack pigment in this layer macular degeneration retinitis pigmentosa Photoreceptors Segments (outer, inner, fiber) Rods: role in peripheral vision, night vision contain rhodopsins more rods than cones in the retina no rods in fovea = night blind more sensitive to dim light increases in quantity peripherally Cones: role in visual acuity and color vision concentrated at center (fovea and macula) and less at periphery contains different types of opsins 3 types - each absorb one of 3 colors of light S-cone: short wavelength - blue M-cone: medium wavelength - green L cone: long wavelength -red

Medical: if issues with one or more cones types - colorblindness most common - can’t differentiate red and green usually X-linked recessive - affects men more Ishihara test Vitamin A deficiency Retinitis pigmentosa Dark adaptation (discuss in the future) Phototransduction (discuss somewhere else in the future) Bipolar cells Several types rod-specific bipolar cells (1) cone-specific bipolar cells (10) Transmit signals from photoreceptor cells to ganglion cells processes/neurites are called dendrites ON and OFF layers need to explain this* “We know that a photoreceptor neurotransmitter (which is glutamate, see Dowling (24) and Massey (25) for reviews) is released in the dark in the vertebrate retina (26). Thus, the photoreceptor, whether it be rod or cone, is in a depolarized state in the dark. On light stimulation, the photoreceptor responds with a hyperpolarization; transmitter release ceases, but the postsynaptic bipolar cells respond with either hyperpolarization or depolarization of their membranes. The hyperpolarizing type of bipolar cell is called an OFF-center cell, whereas the depolarizing bipolar cell is called an ON-center cell (27, 28).” Ganglion cells Bipolar cells contact both dendrites and soma of ganglion cells Axons → forms optic nerve (CN II) Association neurons (interneurons) Modify synaptic transmission in retina Horizontal cells located between OPL and INL 3 types on human retina (HI, HII, HIII) dendrites contact synaptic terminals of photoreceptor cells and with the dendrites of bipolar cells, which they inhibit

Amacrine cells located between INL and IPL all dendrites emerge from same side of the cell to branch out and terminate in synaptic complexes between bipolar, ganglion, etc cells thought to lack axons. some morphologies might have axons but do not leave retina Interplexiform cells post-synaptic to amacrine cells and pre-synaptic to horizontal and bipolar cells feedback loop Neuroglial cells cells of Müller - principal glial cells of the retina extend through the whole thickness of the retina Provide architectual support supporting role and other functions (communications?) Astroglia Microglia

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