November 27, 2020

Visual system; an overview.

This week we will be learning about vision, one of the most important senses of the human body. This sense requires so much processing space that it has an entire lobe dedicated to it. Vision allows us to process the world around us and uses more than 30% of our brain everyday. To explore this sense in detail, we need to examine how it works, important structures, and the chemical signals between neurons.

Anatomy:

Before we learn about how the eye works, we need to understand where each part is located and what its function is.

Cornea: focuses the image (coarse focusing; ~80% of necessary focusing)

Lens: further focuses the image (fine tuning; ~20% of necessary focusing)

Iris: controls the size of the pupil

Pupil: controls the amount of light that enters the eye

Retina: absorbs the light and converts the information to neural signals to transmit to the optic nerve

Fovea: the central part of the retina where light focuses – only red and green cones

Macula: area around the fovea that is primarily used while reading or driving (degeneration of this area, macular degeneration, is the leading cause of blindness in the elderly population)

Lateral geniculate Nucleus: intermediate way station between the retina and the occipital lobe

Occipital lobe: where the signals from the light are processed and put together to form an image

Pathway:

Light passing through the cornea goes through a number of different steps before being converted into signals that can be transmitted to neurons. It starts in the cornea, which focuses the image. The light passes to the lens, which fine tunes the image. Finally, it hits the retina, where it is transformed into neural signals.

Signals:

Our retina has three layers, photoreceptors, interneurons and ganglion cells (in order from outer to inner).

There are over 125 million photoreceptors in the retina. They are divided into two groups: rods and cones. Rods make up over 95% of all photoreceptors and are extremely sensitive to light. They are used in dim light and can not “see” color. Cones account for most of our vision – acute detail and colors. There are three types of cones: red, blue, green. Green makes up most of our cones. The photoreceptors pass their signals they receive from the light to the interneurons. These neurons simply connect the photoreceptors to the ganglion cells. The ganglion cells’ axons make up the optic nerve, which transmits to the brain.

Once the neural signal has been transferred to the brain, the signal is transmitted to the lateral optic chiasma where the left and right signals cross over ( see diagram below). Then, it moves to the lateral geniculate nucleus and primary visual cortex which has layers that identify different characteristics in the picture. For example, cells near the middle respond to bars or edges at a particular angle in the “picture”. These signals are also separated into shape, color, movement, location, and spatial organization.

Disorders:

Strabismus: a disorder that occurs when the eyes are not properly aligned, more commonly known as cross eye; vision can be restored before the age of three using surgery, exercises, or an eye patch. If surgery is not performed early in life, one eye can be favored and lead to little or no vision in the other eye (pruning in example).

Loss of photoreceptors: a major cause of blindness, can be cured through gene therapy, or by bypassing the photoreceptors and sending the signals directly to the ganglion cells.

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