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The Senses

Help Link : Rods Link : Cones Link : Rods and Cones

The light that enters the eye must pass through all the layers of the retina before reaching the outer segments of the photoreceptors. The inner segments of the cones, located just in front of the photosensitive pigments on the light rays' path, seem to act like optic fibres, guiding the light rays to the outer segments. This would explain why the cones are far more sensitive to light rays from the centre of the eye. These rays are better aligned with the cones' inner segments and hence pass through them more easily than rays that strike them obliquely. The cones thus take advantage of the better optics at the centre of the eye.

In contrast, for the rods, because of their essential role in low light conditions, every photon is important, and they cannot afford to lose any. That is why the rods do not have a selective-direction system like that found in the inner segments of the cones.

Link : Anatomy and Physiology of the Retina

New discs are constantly being produced at the base of the outer segments of the rods and cones, so these segments are constantly growing longer. That is why the pigment epithelium that is in contact with the old discs at the tip of the outer segment phagocytoses ("eats") and destroys them.

Link: Outer segment generation. Link: Phagocytosis of outer segments by pigment epithelium.


The transduction (conversion) of light signals into nerve impulses is accomplished by some 125 million photoreceptors located in the deepest part of the retina. This task is divided between two types of photoreceptors that are very different from each other. The 120 million receptors called rods let you see shades of grey in low light conditions (whence the saying "All cats are grey in the dark"). The other 5 million receptors, called cones, are smaller and wider, and sensitive to colour in bright light conditions.

The outer segments of the rods are cylindrical, while those of the cones are cone-shaped. But shape is not the only feature that distinguishes the two. They also differ in the number and arrangement of the discs formed by the folding of their membranes. In the rods, there is a stack of about 900 of these discs, which become completely detached from the membrane and float freely inside it. In the cones, there are far fewer discs, and instead of becoming detached from the outer segment membrane, they remain attached to it.

These photoreceptors are actually nothing more than highly specialized cilial cells whose outer and inner segments are joined by a connecting cilium. The inner segment of each photoreceptor contains the cell's nucleus and organelles such as mitochondria and Golgi bodies that are essential for the functioning of any cell. In the inner segments, as in the outer ones, there are some notable anatomical differences between rods and cones (see sidebar).

The distribution of rods and cones varies from one point to another on the retina's surface. There are very few cones around the periphery, where rods predominate. In contrast, in the central region of the retina, called the fovea, there are no rods at all. That is why you turn your eyes to make an object that you want to look at fall within this area of greater acuity within your field of vision.

Lastly, the most important functional distinction between rods and cones, the one that makes cones sensitive to colours whereas rods are not, comes from their differing photopigments. While all rods have the same kind of photopigment, called rhodopsin, the outer segments of cones contain one of three different opsins that have absorption peaks in the short, medium, and long wavelengths of light, respectively. These three pigments, with their differing spectral sensitivities, are the basis for human colour vision.


Link : Phototransduction, the movie Link : Mechanism of transduction Link : G Protein Coupled Receptor

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The eye's sensitivity to light is not constant. Instead, it adjusts to light levels in various ways. Dark adaptation occurs, for example, when you enter a darkened movie theatre when the film has already started. At first you feel as if you can't see anything. But very quickly, your irises open to let more light reach your retinas. A slower adaptation also takes place, involving the photosensitive pigment in the rods, rhodopsin. Over the first 20 to 25 minutes that you spend in a dark environment, because your rods' stores of rhodopsin are no longer being bleached by the light, they regenerate more readily, causing the rods' sensitivity to light to increase about a million-fold!

When you come back out of the theatre into the daylight after your eyes have adapted to the darkness, you are temporarily blinded, until they have completed the reverse process, light adaptation. In the first step of this process, your irises close rapidly to reduce the amount of light entering the eyes. Next, the other biological changes that occurred in dark adaptation are reversed as well, so that after just a few minutes, your vision has adjusted to the bright light of a sunny day. As this implies, the cones adapt to light more quickly than the rods adapt to darkness.

Lien : Dark adaptation

The photosensitive pigments that are broken down by light take about 1/12 of a second to re-form. This time lag is responsible for the phenomenon of retinal persistence, which for a long time was incorrectly thought to be the basis for the illusion of movement in motion pictures.




The function of the photoreceptors is to transduce (convert) light energy into membrane potential. In many ways, the mechanics of this process are comparable to those found in synapses that use metabotropic receptors to achieve transduction chemically. When a neurotransmitter binds to a metabotropic receptor, it activates G proteins that in turn stimulate various enzymes . These enzymes alter the intracellular concentration of a second messenger, which results in a change in the conductance of certain ion channels and hence a change in membrane potential.

The transduction of light by the photoreceptors in the retina involves the same basic steps. But before we describe them, it must be noted that while the resting potential of most neurons is usually around
–65  mV, the membrane potential of the outer segment of rods is about – 30 mV in the dark. The reason for this depolarization is that in the absence of light, there is a continuous flow of sodium ions into each rod's outer segment through sodium-specific channels in its membrane. These channels are kept open by the presence of the second messenger cyclic guanosine monophosphate (cGMP), which in dark conditions is produced continuously by the enzyme guanylate cyclase. This phenomenon is known as the dark current.

When photons of light strike the light-sensitive pigment in the rods or cones, it changes form and thus activates a G protein called transducin. This transducin in turn activates the enzyme phosphodiesterase, which metabolizes cGMP and thus reduces the level of cGMP in the photoreceptor. This drop in cGMP in turn reduces the outer segment's sodium conductance and, consequently, the dark current that is responsible for its unusually high membrane potential.

The result is thus the contrary of what might be expected: the presence of light hyperpolarizes the photoreceptor cell and consequently causes it to release fewer neurotransmitter molecules into its synapse with the bipolar cells.

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