03. Vision

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"We do not see with our eyes, we see with the brain."

All sensory systems that are available, the vision [ 52 , 57 , 133 ] is probably the meaning that marks us most. It is thanks to the vision we can observe, analyze and properly interact with the world around us. Without vision life would be black!

1. Reception: section_oeil

1.1. Anatomy:

The receptor organ of vision is the eye (eye) [ 43 , 45 ]. This spherical body is composed of three envelopes [ 42 ] which are outside to inside: The sclera, choroid and retina.

Sclera (white of eye) [ 146 ] is a white, resistant casing that keeps the internal pressure and protects the eye against mechanical stresses. It extends forward by a non-vascularized and richly innervated thin transparent envelope,: The cornea [ 64 ], it protrudes in front of the eyeball.

Choroid [ 5 , 64 , 146 ]: A black jacket and richly vascularized nourishes photoreceptor cells of the eye and keeping the inside of the eye in the darkroom. The choroid extends forward by the ciliary body [ 94 , 122 , 147 ] and iris [ 133 ] gives us the eye color and defines an opening: the pupil. All these vascular structures form what is called the uvea [ 72 ].

The retina [ 57 ]: A thin membrane around 0.5 mm [ 154 ] highly vascularized. This is nervous tissue that extends through the optic nerve to the optic papilla. The retina is the sensory part which contains photoreceptor cells [ 39 , 94 ].

Behind the iris, there is the lens [ 99 , 148 ], a kind of transparent biconvex lens and which contributes to the convergence of light beams on the retina.

The eye is filled inside substances that maintain its globular shape: In front of the lens there is the aqueous humor [ 104 , 148 ], a clear low viscosity liquid that nourishes the cornea. Behind the lens is the vitreous [ 116 , 148 , 149 ], a transparent gelatinous substance holding the retina in place against the eye wall and absorbs a large quantity of ultraviolet rays.

tuniques_oeil 1.2. Optics:

The cornea is the first surface that light must pass to reach the retina, it is curved and thus participate fully in the convergence of light rays.

The diaphragm is the pupil of the eye [ 52 ], it sets the amount of incoming light with its diameter which varies according to the light intensity due to antagonist muscle of the iris system [ 150 ] The radial dilate the fibers pupil and the circular fibers shrink.

The lens is the lens of the eye [ 43 ], biconvex, it allows, thanks to its flexibility to change its curvature and thus make a correct focus distance objects, we call this phenomenon: The accommodation [ 3 ].

The eye plays the role of a camera [ 43 ] with diaphragm (pupil) [ 52 ] automatic exposure control, a lens (cornea and lens) [ 43 ] autofocus and a photosensitive surface ( the retina). The resulting image will focus on the retina (for an emmetropic eye), it will be reduced and reversed.

1.3. The retina [ 3 ]: les_deux_retines

1.3.1. Cells in the retina:

The retina (nervous tissue from the diencephalon during development [ 41 , 72 ]) consists essentially of three layers of nerve cells [ 1 , 70 , 151 ]. The center to the periphery of the eye, we first find the ganglion cells [ 4 , 99 , 152 ], those whose axons form the optic nerve, there are about 1 million in each eye [ 75 , 96 , 133 , 149 , 152 ].

And bipolar cells [ 153 ], intermediate between photoreceptors and ganglion cells, they form the intermediate layer. Paradoxically photoreceptor neurons [ 99 ] form the outermost layer of the light, they are in direct contact with the epithelium of the choroid.

There are also two types of neurons in the retina: the horizontal cells and amacrine cells [ 99 ] which are interposed between the three layers of the retina and can improve the contrast and resolution of the transmitted image the brain.

1.3.2. Photoreceptors:

There are two types of photoreceptor cells: rods and cones. Each human eye contains about 125 million photoreceptors [ 99 ] 5 million [ 75 ] are only cones. Cones: uncone

Even though they are very few compared to rods, cones that are shaping our visual acuity [ 48 ]. Indeed, the central region of the retina (the macula) [ 154 ] contains a central region in the center (the fovea [ 4 , 39 , 148 ]), which is completely devoid of rods [ 38 , 39 ], there cones that at this level.

Cones give us color vision [ 1 , 39 ] with their three pigments (opsins [ 38 ]) sensitive to blue, green and red. Each preferably contains a cone of these pigments. Cones allow us to see the details of images [ 1 , 39 ], because each cone is linked to a single bipolar cell which itself is linked to a single cell lymph node [ 41 ]. Sticks: unbatonnet

The rods [ 153 ] are very sensitive to light [ 1 ], and a stick can react to absorb only a photon [ 5 , 41 ]! They contain an essential pigment (rhodopsin [ 57 ]), each stick contains about 1000 records with 40 million pigment rhodopsin [ 155 ].

Rhodopsin contains a molecule (the retinene: derivative of vitamin A [ 4 , 42 ]) that changes shape whenever it absorbs light, it triggers a chain reaction that will hyperpolarize the membrane and stimulate a bipolar cell.

Rods are distributed mainly at the periphery of the retina, they allow us to detect the movement of objects [ 149 ]. Their high sensitivity allows us to see in the dark (scotopic vision) [ 41 ], unlike cones that provide us photopic vision.

1.3.3. The optic disc:

A fundus [ 67 ], we can see the area where gather all before the nerve fibers form the optic nerve, called this area: the optic nerve [ 148 ]. At this level there are no photoreceptors, so it's a blind spot [ 41 ]. How is it then that we can see no visual field?

The answer is that the brain is responsible for filling the information vacuum that level by the information it collects areas of the environment [ 39 ].

1.3.4. Retina and visual hemi-fields:

There are two parts of the retina [ 72 ]: nasal temporal retina and retina, the visual field is thus also divided into two half-fields each corresponding to the region of the retina which received [ 41 ]. The nasal retina of the right eye and the left eye temporal receive the light of the visual hemi-field right and vice versa.

2. Transmission:

All nerve fibers from the ganglion cells form the optic nerve [ 41 , 116 ], it is the only nerve in the body that belongs to the CNS and not the SNP [ 41 ].

Indeed, from an anatomical point of view this is the only nerve to be surrounded by three tunics of the meninges [ 64 ]. Embryological point of view, it develops from the diencephalon [ 50 ]. Cell view point, it does not include Schwann cells, oligodendrocytes but, this is why it is often reached at a multiple sclerosis.

The optic nerve behind the eye is born and ends at the optic chiasm just above the pituitary gland.

The optic chiasm [ 119 ] is an area of intersection where the fibers of each nasal retina will cross the midline to join the fibers from the temporal retina of the contralateral eye and thus form a pair of two optic tracts. Each optic tract contains information from the contralateral visual hemi-field [ 50 ].

Both optic tracts will bypass the brainstem (cerebral peduncles) and will lead to the lateral geniculate nuclei of the thalamus. From there, go from several bundles of fibers (optical radiation) [ 41 , 149 ] which will be projected on the primary visual cortex in the occipital lobe, but also on other structures such as the superior colliculi to the brainstem where there is some development of reflex phenomena.

voies_de_transmission_visuelle 3. Perception:

The primary visual cortex (V1, Brodmann's area 17) [ 49 , 73 ], located at the occipital cortex is the first cortical relay of nerve fibers in the visual system. The primary visual cortex receives visual information and is responsible for the primary treatment.

Each visual cortex analyzes the visual hemifield contralateral. There is retinotopy [ 5 ] with a very large area of the visual cortex corresponding to the fovea (central macula area).

Since the primary visual cortex, other nerve fibers will rejoin other regions of the cerebral cortex called secondary visual cortex [ 57 ]: V2, V3, V4, V5, MT ... [ 57 ] To study the visual properties such as color, shape, texture, movement and terrain.

Other fibers will project on remote regions of the cortex, these regions associative areas [called 57 ]. There are two main types of these projections [ 57 ]:

  • The backend (the where, occipito-parietal pathway), this system analyzes the motion, depth and topography, so it allows the spatial location of objects.
  • The ventral system (what, occipito-temporal pathway), it analyzes the shape and color and therefore allows us recognition and object perception.