Visual SystemBy: Jordan Koehling

What is the Visual System● Photoreceptors are the type of sensory neuron involved in sight.

● It is part of the CNS and allows us to see our surroundings using visual light

● Psychologically known as Visual Perception or our ability to interpret the light

our photoreceptors ‘see’. Not being able to interpret light is also known as

blindness.

The Eye

Anatomy of Sight● The eye is the primary mechanical part of the visual system as well as where

the sensory neurons are located.

● The Visual System also includes the optic nerve, optic Chiasma, optic tract, Lateral Geniculate Nucleus(LGN), and the Occipital Lobe/Visual Cortex

● The Retina is made up of Photoreceptors known as Rods and Cones that phototransduce light into electrical signals.

● Once turned into electrical signals, the Optic Nerve, a bundle of aprx. 1 million axons, is what transfers the signals.

Anatomy cntd.● The Optic Chiasma is where the optic nerve axons partially crossover.

○ Only the nasal side of the retina crosses over.

● Optic Tract is the optic nerve after it goes through the Chiasma and leads primarily to the LGN.

○ The Optic Tract also has several smaller branches that branch off too:■ Superior Colliculi(Roof of Midbrain)-Rapid eye movements and target tracking■ Suprachiasmatic Nucleus (SCN)-Circadian Rhythm■ Pretectum(Between Midbrain and Thalamus)- Controls pupillary reflex

● The LGN is the primary relay center for the signals from the retina. It receives the majority of Optic Tract.

Occipital Lobe/Visual Cortex● The Visual Cortex is the part of the brain where electrical signals from the eye

are interpreted.● There are six distinct parts of the Visual Cortex-V1, V2, V3, V3a, V4, and V5.● The main areas are V1, V2, and V5. ● V1 is the Primary Visual Cortex and interprets the signals in the areas of

color, form, space, and orientation. ● V2 is a Secondary Visual Cortex and analyzes an image’s color and form

more in depth than V1● V5 is where object recognition happens

Photoreceptors● There are 2 main types of Photoreceptors-Rods and Cones

● There is a third type of Photoreceptor known as Photosensitive Retinal Ganglion which are not well known about they do however play a role in unconscious eye movements and pupil reflexes.

Rods● Low Light Vision

● Very Light Sensitive

● 120 million rods in the retina

● Only found in the peripheries, not in the fovea(Center of the Retina)

Cones● Used in high light conditions

● Only sensitive to direct light

● Sees three different pigments(How we see colors)

● 6 million cones in each retina, found in the Fovea

How We See1. Visual light enters the eye through the pupil and is focused through the lens.

2. After the light is focused it travels to the back of the eye to the Retina, which is made up of Photoreceptors. The Photoreceptors turn the light into electrical signals using phototransduction.

3. Once the light is transduced into electrical signals, it leaves the eye using the Optic Nerve and travels to the Optic Chiasm where the Optic Nerves cross over each other. After the Optic Chiasm the Optic Nerves become the Optic Tract.

How We See contd.. 4. As the signal travels along the Optic Tract it then reaches the Lateral Geniculate Nucleus or LGN. The LGN is the primary terminal for the Axons of the Optic Tract.

5. The LGN Axons, also known as Optic Radiations travel deep through the brain, carrying the electrical signals to the Primary Visual Cortex or V1 in the Occipital lobe of the brain.

6. In the Occipital Lobe the signals are analyzed by the various areas of the Visual Cortex to form an understandable and recognizable image.

Homeostatic Imbalances1st Disorder:Nyctalopia

-Night Blindness

2nd Disorder: Glaucoma

-Vision loss/Blindness

Nyctalopia ● Rod Cells lose their ability to respond to light

● Can be either genetic or through a deficiency of Retinol/Vitamin A

● Also causes slower eye reaction to light and reduced peripheral vision

Glaucoma ● Several types with the

most common being Open-Angle Glaucoma

● Open-Angle is painless and causes only vision loss

Sourceshttps://www.livescience.com/33664-amazing-optical-illusions-work.html

https://emedicine.medscape.com/article/1948576-overview#a2

https://en.wikipedia.org/wiki/Visual_system

http://www.bioon.com/bioline/neurosci/course/basvis.html

https://en.wikipedia.org/wiki/Nyctalopia

https://en.wikipedia.org/wiki/Glaucoma