THE BRAIN FROM TOP TO BOTTOM

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Vision

Rods can be so sensitive that they can detect a single photon (the smallest unit of light). They also let us detect light sources that are more than a billion times weaker than the light we see outdoors on a bright, sunny day.

The morphology of rods and their high concentration of light-sensitive visual pigments (there are about 100 million in a single rod!) are what make them so extremely sensitive to light. But the type of connections that rods make with the other cells in the retina also contributes to this sensitivity. As a trade-off, however, the vision that rods provide is blurrier and does not include any colours.

PHOTORECEPTORS

The transduction (conversion) of light into nerve signals that the brain can understand takes placed in specialized cells in the retina called photoreceptors. Each photoreceptor has four parts: an outer segment, an inner segment , a cell body, and a synaptic ending.

The outer segment consists of a stack of discs embedded in the cell membrane. The photoreceptor's light-sensitive pigments are located on these discs.

It is the shape of the outer segment that distinguishes the two main types of photoreceptors: rods have a long, cylindrical, outer segment with many discs, while cones have a short, tapering outer segment with relatively few discs.

Because they have more discs, rods are over 1 000 times more light-sensitive than cones. That is why, at night and in other low-light conditions, your sense of vision comes from the rods alone. And conversely, in broad daylight, your cones are more active.

Your retina thus has dual capabilities: it can work in dim light, thanks to the rods, and in bright light, thanks to the cones. One of the other differences between the two types of photoreceptors is that only the cones are sensitive to colours.

When you move suddenly from the light into almost total darkness (for example, when you enter a darkened movie theatre), it takes a while before you can see anything. The reason is that for light to be converted into nerve impulses, it must break down molecules of a protein called rhodopsin. When you come in from a very brightly lit setting, you therefore no longer have enough of these molecules left to see effectively in the dimmer light. Before you can see again, you must wait for your eyes' supply of rhodopsin molecules to be replenished, and this is a relatively slow process. During this transitional period, your eyes are described as adapting to the darkness.

Similarly, when you put on a pair of sunglasses, at first everything seems to be the same colour as their lenses. But after a while, you no longer notice this colour. Your eyes have made another kind of adaptation, a chromatic adaptation.

HOW LIGHT GETS CONVERTED INTO NERVE IMPULSES

Light enters the visual system through the eye and strikes the retina at the back of it. The retina is composed of specialized cells, the rods and cones, which convert light energy into neural activity.

This conversion is made possible by light-sensitive pigments located on the discs in the outer segments of the rods and cones.When light strikes these pigments, they change form, causing a cascade of chemical reactions in these photoreceptors.

These reactions make the photoreceptors' membranes less permeable to certain ions, such as sodium. This change in permeability alters each photoreceptor's membrane potential and allows it to send a nerve signal to cells in the next layer of the retina.