The Visual System and Eye Anatomy

• The visual system is important for primates and well-studied due to its simple organization and precise retinotopic mapping.
• This allows for straightforward prediction of damage consequences.

Eye Considerations:

• The retina is an outgrowth of the CNS, surrounded by meningeal outgrowths.
• Alternatively, the eye is like a camera with tissues acting as parts.

Tissue Layers of the Eye:

• The eye is a three-layered structure with a lens inside.
• The outermost layer is thick and collagenous (dura mater continuation), providing mechanical strength.
• The anterior sixth of the outermost layer is the transparent cornea, continuing as the white sclera.
• The middle layer (uveal tract) is pigmented and vascular (arachnoid and pia mater continuation).
• The middle layer consisting of the choroid (sclera lining), ciliary body, and iris are components of the pigmented vascular layer.
• The lens is suspended from the ciliary body by zonules.
• The iris separates the space between the cornea and lens into anterior and posterior chambers.
• The innermost layer is a double layer from a diencephalon outgrowth.
• The outer layer is retinal pigment epithelium, from optic disc to pupil edge.
• The inner layer forms the photoreceptive neural retina in the posterior eye part.
• Anteriorly, both layers continue over the ciliary body and iris as an epithelial layer accompanying the pigment epithelium.

Eye: Functional Parts: Camera Body

• The sclera and cornea offer rigid mechanical support, with shape maintained by fluid pressure.
• The Ciliary body is a loose connective tissue structure covered by a double-layered epithelial continuation of the optic cup, functioning like choroid plexus by leaking water, solutes and transporting across the epithelial covering into the posterior chamber as aqueous humor.
• From here the aqueous humor moves to the anterior chamber, and it is forced into a circular venous sinus at the corneoscleral junction.
• The eyeball behind the lens contains vitreous humor (gel with hyaluronic acid and collagen fibrils), providing mechanical rigidity, and it has a smooth surface for eyeball rotation and insertion points for extraocular muscles.

Functional Parts of the Eye: Focusing

• Corneal collagen fibers must be highly organized into sheets to ensure transparency.
• Epithelial layers maintain corneal dehydration, crucial for transparency, with no blood vessels and trigeminal nerve endings accounting for irritability and the blink reflex afferent limb.
• The cornea has a refractive index similar to water, so the air-cornea interface refracts in terrestrial animal eyes, while the lens, composed mostly of water in aqueous humor, provides additional refractive power.
• Various species accommodate by adjusting focus using muscles that change corneal shape or move the lens or retina.
• Terrestrial vertebrates use the ciliary muscle to change lens shape.
• At rest, the eye focuses at infinity and the lens is flattened.
• The contraction of the ciliary muscle relaxes tension and allows the lens to thicken, focusing on closer objects, this process is Innervated by the parasympathetic nervous system.
• The system is an autofocus system where choice dictates focus.

Functional Parts of the Eye: Regulation of Light

• The Iris (similar to diaphragm) regulates light amount
• The anatomical composition consists of: Loose connective tissue continuous to the choroid + double layered epithelium continuous with the retina and pigment epithelium.
• The anterior layer contains pigment in brown-eyed individuals.
• The double layered pigment is always pigmented for the iris's function
• Two muscles from the pigment epithelial layers control the pupil size
• The pupillary sphincter constricts it and is stronger.
• The pupillary dilator widen it and is radially arranged
• Sphincter innervation is parasympathetic: oculomotor n.(CN III) → ciliary ganglion → short ciliary nerves → sphincter.
• Dilator innervation is sympathetic: Spinal cord → superior cervical ganglion → long ciliary nerves → dilator
• Parasympathetic damage causes ipsilateral dilated pupil without constriction to light while sympathetic damage causes ipsilateral constricted pupil that constricts further in response to light.

Functional Parts of the Eye: Antireflection Lining

• The Choroid (lines inner sclera) is pigmented, spongy, and vascular with pigment (melanin), which helps prevent light bouncing.
• The inner margin contains the choriocapillaris capillary bed, supplying the retinal pigment epithelium and part of the photoreceptors.
• The choroid provides a route through which nerves and blood vessels can spread.

Functional Parts of the Eye: Film

• The analogy breaks down by this part
• Retinal images are self-developing, erasable, and reusable, but it is not silicon- or polyester-based and designed to extract important features versus duplicating the outside world.

RETINA structure

• The retina takes information landing on photoreceptors and extracts important features.
• The information is funnelled into an action potential pattern for an approximate 1 million optic nerve axons.
• Interneurons exist is a layer positioned between the photoreceptor layer and ganglion cells (the axons form optic nerve)
• Types of cells include bipolar cells and horizontal cells, where the former transmit signals, and the latter affects transmission
• Retinal photoreceptors develop close to the extension of the neural tube cavity into the optic cup.
• Photoreceptors face away from incoming light

Retina: Distinctive areas

• A second consequence of the inside-out orientation of the retina is that ganglion cell axons must travel along the vitreal surface to leave the eye.
• Axons converge and leave the eye at the optic disc, where the sclera continues over the optic nerve as a sheath continuous with the dura mater.
• Layers of the retina are absent at the optic disc, which causes a blind spot of the size of a golf ball, half a meter away, in each visual field.
• The optic disc is located a few millimeters medial to the retinal intersection of the optic axis
• The cornea and lens reverse everything (side to side and top to bottom) meaning the blindspot is lateral to the visual center
• The disc is devoid of neurons.
• Mammals have rods, and cones for photoreception.
• Rods are long and have cylindrical outer segments containing photopigment while cones have short, tapered outer segments and are less sensitive to light.
• There are 100 million rods and 5 million cones per human retina
• At the center of the retina there is an optic axis intersecting the Fovea, a yellow pigmented zone known as the macula lutea specialize in vision of spatial acuity

Retina :Rod and cone function

• Both outer cone/rod segments have stacked-up membranous discs.
• Rod outer segments have more surface and length, and those discs become pinched off, while cone outer segments have smaller and fewer discs, and they stick with the surface membrane.
• Vertebrate photoreception is based on closely related opsin proteins, similar to G protein-coupled norepinephrine receptor, that binds 11-cis retinal (the 11-cis isomer of vitamin A aldehyde) that gives it light-absorbing properties.
• The Differences in amino acid sequence dictate what is the best wavelength for light absorption.
• A photon absorption causes 11-cis retinal to photoisomerize to it's all-trans form which the changes conformation of opsin allowing It to activate nearby G proteins.
• This will activate and enzyme (phosphodiesterase) that hydrolyzes GMP
• After hydrolyzing cGMP , cGMP close some channel gated by cGMP.
• Photoreceptors have cGMP gated cation channels that are open and that release glutamate, but Light cause the channels to close

Retina : Receptive Fields of Retinal Neurons

• A receptive field of a neuron in the visual system is a real-world area (or retina part where its image falls) altering a neurons activity upon illumination change .
• Rod's and cone's receptive field is a spot that covers the receptor, causing it to hyperpolarize.
• Ganglion cells are broken into types with complementary center-surround characteristics
• ON-center cells: fire faster when light falls on the field's center, slower when on the periphery, and do little with diffuse illumination.
• OFF-center cells: slower when light falls on the field's center, faster when on the periphery, and do little with diffuse illumination.

Central Visual Pathways:

• Axons project different pathways, including the superior colliculus, midbrain, and thalamus.
• Visual info from the optic nerve consists of all shape, color, ,location and movement information of observed objects
• Optic nerve fibers consists of ganglion cells classes emphasizing different properties of visual stimulus, and these different properties begin to be sorted in the six-layered lateral geniculate nucleus of the thalamus.
• Geniculate layers projects to primary visual cortex above and below the calcarine sulcus (striate cortex).
• Primary visual cortex picks apart attributes and parcels them semiselectively to distinct areas of visual association cortex in the occipital and temporal lobes.

Central Visual Pathways: Optic Nerve, Chiasm, and Tract:

• Somatosensory information projects to the contralateral postcentral gyrus.
• Visual information from one side of the world winds up in the contralateral occipital lobe.
• Each eye looks at most of the right and left half of the complete visual field, so half the output of each retina crosses in the optic chiasm, and half stays uncrossed.
• The crossed and uncrossed fibers emerge from the chiasm as an optic tract
• The optic nerve can be damaged causing at most complete blindness of the ipsilateral eye
• Damage to the optic tract (or any part to the visual system behind the chiasm) can cause the loss of the contralateral half of the visual field bilaterally
• Fibers from the nasal half of each retina cross at the optic chiasm, so pituitary tumor means deficits in mirror parts of the temporal fields

Central Visual Pathways: Getting to Primary Visual Cortex:

• Fibers from the optic tract end in retinotopic pattern and make geniculation
• Similar receptive fields concerned with color vs contrast, and that focus on small areas
• The Lateral Geniculate Neurons project to the striate cortex where things change

Central Visual Pathways: Primary Visual Cortex

• Geniculate axons through the retrolenticular and sublenticular parts of the internal capsule, which make the optic radiation (sweeps occipital).
• Inferior visual fields are in upper half of the optic radiation, and superior fields are in lower half, with the fovea having axons in the middle zone.
• Terminates systemically in the striate cortex
• Cortical cells sensitive on the contrast, responds to diffuse illumination depending on where they are
• The stimulus will cause for the eye to look at stripes or bar orientation
• Some are concerned with color, motion, or disparities between the 2 retinas
• The retinotopic map is divided in many functional units

Central Visual Pathways: Beyond Primary Visual Cortex:

• After attributes are partially separated a series of portions can get this information
• The analysis is then carried out on color and form
• Motion and form takes place
• All is made if its on damage to the gyrus without the visual fields

VISUAL REFLEXES:

• The eye has an mechanisms mediated by the CNS which involves both pupil and ciliary control
• The eyes reflex to light and the ears reflex to sound

Visual Reflexes: Pupillary Light Reflex

• Light shone in one eye causes pupillary light reflex, constricting both the ipsilateral and the contralateral pupil.
• It's independent with respect to the retinal cone and rods for instead subset that of photosensitive ganglion cells for that provide for the Afflerent Limb with AX that can distribute bilaterally while at the Optic Chiasm and terminal in the Pretectal Area
• The pupillary muscles will constrict equally under any given condition of Ilunnation.

Visual Reflexes: Accommodation Reflex

• This is done so that images land on the foveae
• There occurs responses the pupil constricts