Phototransduction and Vision

Distribution of Rods

• As you get closer to the Macula, rods become denser.

Distribution of Cones

• Cones are densest in the macula.
• Cones are most dense in the middle, called the fovea.

Visual Words

• Central vision relates to the middle of the world.
• Hemifield is half of visual world when looking at the nose.
• The left side correlates with the left hemifield.
• A point of light in the left temporal hemifield is focused onto the left nasal retina.
• A point of light in the left nasal hemifield is focused on the right temporal retina.
• The right side correlates with the right hemifield.
• A point of light in the right temporal hemifield is focused on the right nasal retina.
• A point of light in the right nasal hemifield is focused on the left temporal retina.
• The nasal part of the retina can be seen with both eyes and is binocular.
• The temporal part of the retina is closer to the ears and is monocular.
• The left visual world is processed by the right brain.
• The right visual world is processed by the left brain.

Retina to Brain (Thalamus) Pathway

• The optic nerve is exiting the retina.
• The optic tract goes into the thalamus.
• The optic chiasm allows information to cross over.

Decussation

• The left temporal hemifield focuses on the left nasal retina.
• Decussation must occur for the left temporal hemifield to go through the optic chiasm and down into the right LGN.
• Ipisilateral refers to the same side.
• The right temporal hemifield focuses on the right nasal retina.
• Decussation must occur for the right temporal hemifield to go through the optic chiasm and down into the left LGN (Thalamus nucleus).
• The optic chiasm is only activated through decussation, meaning crossing.
• The right nasal hemifield is focused on the left temporal retina.
• Contralateral refers to the opposite side.
• The left nasal hemifield is focused on the right temporal retina.

LGN

• Two regions exist in the LGN.
• Two layers exist in the 1st LGN region and 4 layers in the 2nd region, totaling 6.
• Nasal retina goes to layers 1, 4, and 6.
• Temporal retina goes to layers 2, 3, and 5.
• Ganglion cells are in the LGN.
• M type ganglion cells activate layers 1 and 2 and relate to movement.
• P type ganglion cells activate layers 3, 4, 5, and 6 and relate to color.

Cortex

• The cortex contains 6 layers.
• Layer 4 receives information from the thalamus (visual layer).
• Layer 4 consists of 3 layers called A, B, and C.
• Layer C contains alpha and beta.
• Alpha is activated by information processed by M type ganglion cells (movement).
• Beta is activated by information processed by P type ganglion cells (color).

Visual System

• Most parts of the eye are optical and focus light onto the retina.
• The retina converts it into brain signals.
• Retinal cells release neurotransmitters in response to light, generating action potentials.
• Photopic vision occurs in well-lit conditions, such as during sunlight.
• Scotopic vision occurs in low light conditions, detecting small amounts of light from external sources.

Optical properties

• The cornea is the main optical element and has a fixed focal length that matches the eye's length.
• The lens takes in light and focuses it with an adjustable focal length.
• Focal length is the distance from the lens to where the light is focused.
• Refracting light involves bending light.
• The cornea and lens focus light rays onto the retina.
• Diverging light refers to light rays from objects closer than 9 meters (30ft) and creates a pupillary reflex.
• The lens and iris help see near clearly.
• Accommodation involves adjusting the lens curvature to focus on close objects.
• The iris is a muscle with a hole (pupil) that controls the amount of light entering the retina, and it helps see near and far at the same time.
• Two parts of the iris function to control the size of the pupil.
• The parasympathetic part includes the circular muscle that contracts the pupil and makes it smaller.
• The sympathetic part includes the radial muscle that dilates the pupil and makes it larger.
• The parasympathetic and sympathetic parts work together to control the pupil.

Gross Anatomy of Eye

• The sclera is the tough, white, outer layer of the eye.
• The optic nerve carries visual information from the retina to the brain, surrounded by the sclera.
• The ophthalmic artery and vein are blood vessels running through the center of the optic nerve and supply blood to the eye.
• The choroid is a vascular layer underneath the sclera.
• The retina is the innermost layer containing photoreceptor cells (rods + cones) that convert light into neural signals.
• The ciliary body is a circular muscle that controls lens shape.
• Contraction of the ciliary body reduces tension on the zonular fibers allowing the lens to curve for near vision.
• Relaxation of the ciliary body pulls on zonular fibers flattening the lens for distant vision.
• Zonular fibers connect the lens to the ciliary body and exert tension on the lens.
• The anterior chamber is a fluid-filled space between the cornea and lens, containing aqueous humor.
• The posterior chamber (vitreous chamber) is a large space behind the lens, containing vitreous humor (gellike).
• The posterior chamber is primarily made up of sclera, choroid, and the retina

Photo Transduction

• In the absence of light, a rod increases cGMP, causing Na+ channels to open, leading to Na+ influx, depolarization, opening of voltage-gated Ca2+ channels, Ca2+ influx, and neurotransmitter release.
• In the presence of light, a rod decreases cGMP, preventing Na+ channels from opening and resulting in no neurotransmitter release.
• Photo transduction is converting light energy into a brain signal to bipolar and Ganglion Cells.
• Photoreceptors are the only cells to respond to light energy and transduce it.
• In no light conditions, rods produce a messenger called Cyclic Guanosine Monophosphate (cGMP).
• cGMP levels rise and open sodium channels, allowing sodium to enter rods.
• Sodium influx depolarizes the rod, which opens voltage-gated calcium channels, causing calcium influx in rods and release of neurotransmitters.
• Absence of light is converted into "no signal", rods are specialized to absorb light in the dark.
• In light conditions the rod builds up, blocking of cGMP and causing levels to go down that does not open channels
• This leads to no sodium influx, no deplore, no CA2+ channel, opening and no calcium influx
• Rod is a type of photo receptor.

Retina System Duplex

• The scotopic system allows to see in the dark.
• Rods are the photoreceptor in scotopic system.
• Rhodopsin is a pigment in the rod that allows it to absorb light.
• Rhodopsin is 1000x more sensitive to light than opsin.
• Rhodopsin is purple color, shines light ,white color and takes 4-5 minutes to reset to purple
• Conversion circuitry occurs, There may not be many photons of light, so only same rods are activated
• Mesoptic vision: ability to occur between scotopic/photopic conditions

Mesoptic vs Photopic condition

• Mesopic: Rods inactive, cones active.
• Mesopic: Rods never turn off.
• Mesopic: Cones need a lot more light to be active to recognize something, color but not vibrant

Photopic System

• Is the system allowing to see in light.
• cones are the photoreceptors used in the photopic system.
• Opsin is the photopigment used.
• 3 types of opsin absorbs different ranges: Blue, Green, Red
• There are enough photons of light to activate all cones.
• Circuitry here is 1:1 meaning: 1 cone >> 1 bipolar cell >> 1 ganglion cell.

Macula and Fovea

• Macula: regions where photoreceptors are dense.
• Fovea: central vision where cells converge and are exclusively are cones.
• Everything else is peripheral retina by vision and primarily rods.

Cellular Organization of Retina.

• Macula: the central area of the retina responsible for sharp, detailed vision and densely packed with cones.
• Cones: photoreceptor cells that detect color.
• Fovea: located within the macula and the central point that has the highest concentration of cones resulting in clearest sight.
• Optic Disc (Blind Spot): the area where the optic nerve exits the eye.
• Optic Disc: Lacks photoreceptors resulting in a blind spot in each eye.
• The brain compensates for the blind spot. Axons from retinal ganglion cells form the optic nerve that supply visual information to the brain, in relation to optic
• The blood supple to retina enter/exits via optic disc.

Retina Layers

• Surface to deep consists of: sclera >> choroid >> retina.

3 Primary Layers

• Ganglion Cell Layer is made of ganglion cells, that are a collection of cell bodies.
• Ganglion Cell Layer is the only cells that have action potentials and leave through optic disc enter optic nerve which (most superficial).
• Inner Nuclear Layer is primarily, made of bipolar cells.
• Outer Nuclear Layer is primarily deepest made and consists of photoreceptors.
• Photoreceptors have a photo pigment that absorbs light and outer segment part of the outer nuclear layer not separate.
• Light passes through other layers before being absorbed.
• Ratio to brighten light is 1:1:1
• Outer plexiform layer consists, with 1 photoreceptor, communicating with a 1 bipolar cell, that communicates with 1 ganglion.

Intermediate Layers

• Outer Plexiform Layer contains Horizontal cells.
• Horizontal cells take signal from 1 photo receptor to distribute it to multiple Bipolar cells that (between outer --> inner nuclear layer)
• Inner Plexiform Layer : Amacrine cells take signal from 1 bipolar cell and activate multiple ganglion cells (between ganglion to inner nuclear)
• In low light signalling, it to speak that it has to signal more cells, so output is greater, allowing us to see in low resolution lighting

Pigmented Epithelium

• Any light not absorbed by is absorbed in cells below the retina.