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Mammalian vision

The eyeball is a magnificent contraption of brain tissue that protrudes during development to form said ball. It works to capture and process photons in light, and transmit the compressed information via the optic nerve to the visual cortex in the brain. This journey starts in the retina, which is the innermost layer at the back of the eye containing the light receptors and cells involved in this process.





Having two eyes provides binocular vision which enhances depth perception, gives a wider visual field and better distance perception.

The eye has many different components that work in unison to enable vision. On its surface and lining the eyelids, the conjunctiva secretes mucus and tears, lubricating the eye and providing immune surveillance via the many microvessels. The sclera is the white of the eye and contains collagen and elastic fibre that protects the eye.

The ciliary body allows the eye to focus, while the suspensory ligament ensures the eye doesn't fall downwards. The ability to focus is given by the added curvature given to the lens. The cornea covers the central part of the eye and refracts light, giving the eye's optical power. Alongside the cornea, the lens, of course, refracts light.




The pupil allows the light to strike to retina, with the amount controlled by the contracting iris.

The aqueous humour is replaced frequently and consists of a plasma-like solution without proteins. On the other hand, the vitreous humour is a gel and does not get refreshed. As such, debris can accumulate in it, causing floaters. With age, it can also liquefy and collapse.

The choroid supplies the retina with oxygen and nutrients via the blood supply. Additionally, it contains pigmented cells (with melanin) to prevent internal reflection. The retina contains photoreceptor cells like cone cells which perceive visual information. This is transmitted to the brain via the optic nerve. The optic nerve creates a region without photoreceptor cells, called the blind spot because one cannot perceive images through it. The fovea is an area dense in cone cells, and serves as the key processor of central vision. This vision provides high focus on detail e.g. in reading.

The retina lines the inside of the eyeball. The macula is the high-density photoreceptor area of the retina found towards the back of the eye. The structure of the retina, width-wise, contains several types of cell. These are interconnected one to the next towards the optic nerve side, as well as horizontally connected amongst them.



Notice the "vertical" connections from the rods and cones to bipolar cells (lime) to ganglion cells (purple/red), as well as the "horizontal" connections from amacrine cells to multiple ganglion cells, and from horizontal cells to multiple rods and cones. The rainbow beam represents the entry and direction of light. 

The signals transmitted from the rods and cones travels down this pathway of multiple cells, as it is being processed before reaching the brain's visual cortex via the optic nerve. Some of these cells, such as the ganglion and amacrine cells, are able to generate action potentials, while the other only transmit them.

They are the two main specialised eye photon receptors and differ in their distribution and sensitivity.

Rods are extremely sensitive in low light and can perceive as few as 6 photons, while cones sense more abundant light and therefore can identify different colours.

How?

The signal starts in the rods and cones. They contain photoreceptors such as rhodopsin which chemically shift under different light environments. Rods are responsible for low-light vision and hence contain rhodopsin, which is very sensitive to light (cones contain photopsin). In daylight, it is completely photobleached and it can take half an hour for a human eye to chemically process it back to original functionality under low light (adaptation).




The photons in light initiate a response from rhodopsin that cascades down an enzyme pathway (G-coupled protein) to result in membrane depolarisation and the generation of an electrical signal.



Have a look above: action potentials are generated by polarised membranes as a result of light entering the eye and causing a chemical reaction in the respective cells.

Note the cone cell is connected to a single neurone while a few rod cells share the same neurone. Why could this be? When very little light enters the eye, it isn't sufficient to generate an action potential per rod cell. So the cumulative light from a few rods creates a response great enough to trigger an action potential in one neurone.

The downside of this arrangement for rod cells is that visual acuity, that is the ability to tell two points apart, decreases.

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