Photoreceptors - Cones and Rods

What are photoreceptors

Photoreceptors are nerve cells found on the retina . These elements are sensitive to light waves and play an important transduction function, ie they are able to transform the light that reaches the bottom of the eye into information (first chemical, then electrical) to be transmitted to the brain by means of the optic nerve.

The photoreceptors of the retina are divided into rods and cones . Their structural differences are related to important functional characteristics. Rods, for example, transmit a less clear image, but have a greater number of photopigments than cones and are more sensitive in low light conditions. All rods also contain the same photopigment (rhodopsin), while the cones are not all the same. These last photoreceptors present, in fact, three different types of light-sensitive pigments (iodopsins), which guarantee the discrimination of the various colors (each cone of the retina contains only one of the three photopigments). Furthermore, cones are responsible for daytime vision and accurately capture details.

Features and functions

Cones and rods are highly specialized cells, which have the function of receiving light and adapting it to transmit it to the brain.

In the vision process, the photoreceptors share the tasks:

• The cones are dedicated to clear and central vision, they allow to see fine details and are mainly used in day vision (photopic) or in the presence of artificial light sources. There are three types of cones, each of which contains a pigment that makes them sensitive to different wavelengths in the visible spectrum; in particular, they have absorption peaks at 420, 530 and 560 nm, which correspond to blue, green and red respectively. For this reason, cones are capable of perceiving colors.
• Rods, on the other hand, have a great sensitivity to light and allow you to see even at night and in the presence of low light intensity (scotopic or crepuscular vision). These photoreceptors, however, are not able to construct images of good quality and are unable to distinguish colors. The rods intervene, in fact, in the achromatic vision, characterized only by white, black and shades of gray.

Cones and rods are therefore complementary and their work in sync guarantees perfect vision.

Distribution in the retina

The photoreceptors are not evenly distributed over the entire retina. The cones are about 6 million in the entire retina, so there are fewer than the rods; they have a very high density in the macular region (central area of ​​the retinal plane) and are the only photoreceptors present in the fovea.

Rods, on the other hand, occupy the entire retina (except for the foveal region) and are much more numerous than cones (on average 120 million in each retina). The percentage of rods increases, in particular, as the distance from the fovea increases, until it is maximum at the extreme periphery of the retina. This explains the reason why, in the presence of dim light, we can better observe objects if we do not look at them directly.

Color vision

The ability to perceive colors is based on the presence of three types of cones, which respond to particular wavelengths in the field of visible light. In these photoreceptors, in fact, there are three types of proteins (opsins), which are respectively sensitive to a stimulus of about 420 nm (sensitive to the blue spectrum), 530 nm (green) and 560 nm (red).

Based on the spectral composition of the radiation emitted by the observed object, the three types of cones are activated in various combinations and percentages.

The ability to distinguish the various colors results from this interaction and the final processing at the cerebral level. The contemporary and maximum stimulus of the cones provides the perception of white.

People without a specific type of cone obviously lose the ability to perceive certain colors, as happens in color blindness.

Note . Each type of cone picks up better at a specific wavelength, but each of them is also able to respond within a certain variation, within the same spectrum.

Furthermore, it should be noted that the absorption spectra of the three types of cones partially overlap, so many colors can be perceived.

How they are made

Structural characteristics of photoreceptors

The photoreceptors successively present an external segment and an internal segment in relation to the cells of the pigmented epithelium, an external fiber, the nucleus, an axon (or internal fiber) and a synaptic termination.

The outer segment of the cones has the shape of a truncated pyramid, while that of the rods is cylindrical and elongated; in both cases, this part is characterized by a stratified series of lamellae, which delimit membranous, flattened and discoidal sockets immersed in the cytoplasm of the cell. These "disks" contain the pigments that react to light and cause changes in the photoreceptor membrane potential (rhodopsin for rods and iodopsins for cones). The outer segment of cones and rods is in contact with the pigmented epithelium, the outermost layer of the retina, important because it provides a fundamental molecule for the phototransduction process: the retinal.

The internal segment is characterized by the presence of intracellular organelles, such as mitochondria and granular endoplasmic reticulum membranes, which are indispensable for cellular metabolism. Indeed, it is their task to produce new pigment molecules as they are broken down. This portion continues shrinking into an outer fiber, followed by the part of the cellular body containing the nucleus. The latter is connected by the axon (or internal fiber) to the synaptic termination, which has a bulb shape (spherical) in the rods, flooded and branched (pedicel) in the cones.

The synaptic termination allows the transmission of signals from the photoreceptor to the bipolar cells by synapses, ie by biochemical transmission between nerve cells. This part is, in fact, analogous to the synaptic button of the axonal terminals of neurons, where vesicles containing the neurotransmitter are present.

Relationships with other cells of the retina

The retina is a membrane placed on the inner surface of the eye, formed by three layers of nervous tissue, composed of various types of cells:

• An inner layer consisting of ganglion cells;

• An intermediate layer containing bipolar cells;

• A more external layer, in contact with the pigmented epithelium, in which the photoreceptors are found.

Cones and rods are arranged perpendicularly to the retinal surface; if exposed to light or darkness they undergo conformational changes, which modulate the release of neurotransmitters. These perform an excitatory or inhibitory action on the bipolar cells of the retina.

The bipolar cells are connected on one side to the photoreceptors and on the other hand to the ganglion cells of the innermost layer, whose axons give rise to the optic nerve. Bipolar cells are capable of transmitting graduated potentials.

The ganglion cell axons form a beam that converges on the optic disk and exits from the eyeball, proceeding towards the diencephalon as an optic nerve (the pair of cranial nerves); in response to retinal receptor transduction, ganglion cells generate action potentials aimed at the central nervous system.

In the retina there are also amacrine and horizontal cells that modulate communication in the retinal nervous tissue (for example, by lateral inhibition).

At the rear of the retina, however, there is the choroid.

Note . Rods and cones are not exposed to the vitreous humor, but are positioned in the outer layer of the retina, so they are excited by light after it has passed through the inner and middle retinal layer.

phototransduction

Phototransduction represents the process by which light energy is converted into electrical signals, then transmitted to the brain through the optic nerve. This phenomenon sees photoreceptors as protagonists, whose functioning is based on photochemical reactions.

The first event of phototransduction is represented by the absorption of the light signal by photopigments. Each of these molecules is characterized by a peak of light absorption, corresponding to a particular wavelength (in the case of cones, for example, it makes it more sensitive to a given color). Each photosensitive pigment contains a component called retinal (common to all photopigments) and a protein called opsin.

Therefore, due to light radiations, photopigments change their molecular structure triggering biochemical reactions from which nerve stimulation originates. This is then transmitted to the adjacent retinal cells (bipolar and ganglionary).

The cascade of events in the rods

The rod photopigment (rhodopsin) is located in the membrane of the outer segment discs. Here we also find a G protein (called transducin) and an enzyme, phosphodiesterase, which catalyzes the degradation of the second cyclic GMP messenger (cGMP).

In the dark :

• The levels of cGMP are elevated within the cytosol of the outer segment of the rod, thus opening the sodium channels located in the photoreceptor membrane.
• Sodium ions enter the cell and determine a depolarization that travels from the outer segment to the photoreceptor terminal.
• In response to depolarization, calcium channels are opened.
• Calcium entry triggers an exocytosis process leading to neurotransmitter release.
• The neurotransmitter acts on bipolar cells, generating graduated potentials.

In light :

• Rhodopsin absorbs light.
• The retinal changes its conformation and dissociates itself from the opsin (the pigment present in the rods becomes "discolored"), which activates the transducin which, in turn, activates the phosphodiesterase.
• Phosphodiesterase catalyzes the cleavage of cyclic GMP.
• The levels of cGMP in the cytosol of the outer segment decrease, so the sodium channels close.
• The lower sodium intake hyperpolarises the cell (due to the release of potassium).
• The hyperpolarization causes the closure of the calcium channels in the inner segment, therefore less neurotransmitter is released from the photoreceptor terminal.

The process of phototransduction that occurs in the three types of cones is similar to that of rods, even if three different photopigments are involved.