The Sight – Reception

Eyes’ main job is to detect patterns of light. Then, they work with the brain to turn those patterns into images. Let’s take a closer look :

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DID YOU KNOW THAT ?

Humans sees less well than the chameleon, which can look forward and back at the same time.

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Structure :

Light rays bounce off an object that we are looking at. The light reflects off the object to your eye. Then, the light enters through the outer part of your eye, called the cornea. The cornea is clear, like a window : it helps your eye focus the light to make things look sharp and clear. Next, the light rays pass through an opening called the pupil (dark part).

• The eye is surrounded by 3 distinct layers :

1. the fibrous layer : the outermost fibrous layer is made of connective tissue. Most of it is the sclera, while the most anterior part is the transparent cornea.
2. The vascular layer: it contains the posterior choroid, a membrane that supplies all of the layers with blood. It contains also cells charged with a pigment that absorbs light: melanin. On the front of the eye, the choroid becomes the iris.
3. the inner layer

• The iris (color part) controls how wide the pupil is and how  much light can pass into the eye.

• behind the iris is the lens of the eye. It focuses on the retina the light that enters the eye. The lens is a transparent protein disc that divides the eye into two parts. In front of the lens is the aqueous humor, a transparent and colourless liquid. The aqueous humor is produced by ciliary body. Behind the lens is the vitreous humor, a gelatinous substance. The lens flattens and bends, allowing us to see things far away or up close.

• The light is focussed on the back of the eye, where the retina is.

• Located inside the choroid, the neurons and photoreceptors of the retina constitute the deepest layer of the eye, or eyeball.

• The optic nerve exits the eye through the optic disc.  The axons of all the ganglion cells weave together to create the optic nerve. The optic nerve is thick ropey.

• The optic disc is part of the retina as it lacks photoreceptors, it forms a « blind spot« , that is to say an area that does not perceive light.

• The macula is the portion of the eye at the center of the retina that processes sharp, clear straight-ahead vision.

• There is a depression at the center of the macula that provides the greatest visual acuity because it contains only cones: this is the fovea. The fovea allows light to be less distorted by the tissues it must pass through before reaching the photoreceptors.

RECEPTION:

The retina contains 5 types of neurons:

• ganglion cells
• bipolar cells
• amacrine cells
• horizontal cells
• photoreceptors

These 5 types of neurons have subtypes (sous-types) of neurons, which increase the variety of neurons.

Photoreceptors:

There are two types of photoreceptors: rods (more numerous, more light-sensitive, but can’t pick up real color) and cones (detect fine details and colors, red, green and blue sensitive types based on how they respond to different types of light).

common structure of cone and rod photoreceptors:

There are four parts in a photoreceptor:

• an inner segment, with organelles
• an outer/cuter segment

=> segments are linked by microtubules

• a cell body which contains the nucleus of the cell
• synaptic termination.

The outer segment consists of a stack (empilement) of discs, which are themselves embedded (enchâssés) in the plasma membrane of the cell. Light-sensitive photopigments are present in these discs. They are the ones that absorb light and generate changes in the photoreceptor membrane potential.

The rods have a long cylindrical outer segment with many discs. (that’s why they are more light-sensitive)

The cones have a shorter, tapered outer segment with few discs.

The rods all contain the same photopigment, but there are three types of photopigments for cones, so three types of cones. Thus, they are sensitive to different wavelengths (green, blue and red). That’s why they ensure color vision.

A hundred of different rods may connect to a single ganglion cell. However, they all send their information to the ganglion at once. Therefore, the brain cant’t tell which individual rods have been activated. This is the reason they are not very good at providing detailed images. (they give information about object’s general shape, or if it is light or dark)

Each cone, by contrast, gets its own personal ganglion cells to hook up with, which allows for very detailed color vision, at least if the conditions are bright enough.

Bipolar and ganglionary neurons:

The retina itself has two layers, the outer pigmented layer that helps absorb light so it doesn’t scatter around the eyeball, and the inner neural layer. This layer, as the name indicates, contains neurons, not only the photoreceptors but also bipolar neurons and ganglion neurons. These two kinds of nerve cells combine to produce a sort of pathway for light.

Bipolar neurons have synapses at both ends, forming a kind of bridge. At one end, synapses linked with a photoreceptor, and at the other, synapses linked with a ganglionic neuron which goes on to form the optic nerve.

Horizontal and amacrine cells:

Horizontal cells and amacrine cells are specialized in lateral interactions between photoreceptors and bipolar cells.

Arrival of light in the eye:

The light hits the posterior retina and spreads from the photoreceptors to the bipolar cells just beneath them, to the innermost ganglion cells, where they then generate action potentials.

Eyes photoreceptors convert light energy into nerves impulses that the brain can understand. The retina is loaded with million of photoreceptors which do the crucial work of converting light energy into the electrical signals that your brain will receive.

Transduction of visual information by the nervous system:

The energy of each photon (light components) is captured by a change in the configuration of a chemical bond in the retina: the cis retinal is converted into a trans retinal. A retinal is a molecule that forms visual pigments and absorbs light.

This molecule is linked to a membrane protein called opsin. This change in retinal configuration causes a change in the opsin configuration from an angular to a straight shape.

We saw that photoreceptors are made of discs. Seven alpha helixes of each opsin molecule pass through the membrane of a disc. A retinal and an opsin form a visual pigment in the rods called rhodopsin.

This energy conversion activates rhodopsin. Then, rhodopsin activates a G protein called transducin. The transducin then activates an enzyme: phosphodiesterase. Once activated, this enzyme activates the GMPc (GMP cyclical) of the Na+ ion channels of the plasma membrane of the rod, hydrolyzing it to GMP.

Clarity (Clarté): GMPc is degraded. This causes the Na+ ion channels to close. The membrane is therefore less permeable to Na+, and the rod becomes hyperpolar.

Darkness: GMPc binds to Na+ ion channels and keeps them open. Thus, the rod depolarizes.

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DID YOU KNOW THAT ?

It is said that cats, owls and other nocturnal animals see in the dark: that is wrong! They have a special ability to accommodate that allows them to see in very low light, but no animal can see in complete darkness!

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