Anatomy & Organs

Cones – structure, function & diseases

Cones

The photoreceptors on the retina responsible for color and sharp vision are called cones . They are strongly concentrated in the yellow spot , the area of ​​colored and at the same time sharpest vision . Humans have three different types of cones, each of which has its maximum sensitivity in the blue, green and red frequency range of light.

What are the cones?

The zone of sharpest vision is concentrated in the human retina in the yellow spot (fovea centralis) with a diameter of about 1.5 mm. At the same time, color vision is located in the fovea centralis. The yellow spot is located in the center of the eye’s visual axis for “looking straight” and is equipped with approx. 140,000 color photoreceptors per square mm. These are the so-called L, M and S cones, which are most sensitive to light in the yellow-green, green and blue-violet range.

Although the L cones have their sensitivity maximum at 563 nanometers in the yellow-green range, they also take on the red range, so that they are usually referred to as red receptors. In the innermost area of ​​the fovea centralis, the foveola, which is only about 0.33 mm in diameter, only M and L cones are represented. There are a total of around 6 million color receptors (cones) on the retina.

In addition to the cones, the retina is equipped with about 120 million other photoreceptors, the so-called rods , mainly outside the yellow spot . They have a similar structure to the cones, but are much more sensitive to light and can only distinguish between light and dark tones. They also react very sensitively to moving objects in the peripheral field of vision, i.e. outside the fovea centralis.

Anatomy & Structure

The three different types of cones and the rods, of which there is only one type in the retina, convert light packets received into electrical nerve signals in their function as photoreceptors. Despite slightly different tasks, all photoreceptors work according to the same biochemical-physical active principle.

The cones consist of an outer and an inner segment, the cell nucleus and the synapse for communication with bipolar cells. The outer and inner segments of the cells are connected to one another by a fixed cilium , the connecting cilium. The cilium consists of microtubules in a nonagonal arrangement (nine-sided polygon). The microtubules serve to mechanically stabilize the connection between the outer and inner segment and to transport substances. The outer segment of the cones has a large number of membrane indentations, the so-called disks.

They form flat, densely packed vesicles, which – depending on their type – contain certain visual pigments. The inner segment with the cell nucleus forms the metabolically active part of the photoreceptor. Protein synthesis takes place in the endoplasmic reticulum and in the cell nucleus a large number of mitochondria are responsible for energy metabolism. Each cone is in contact with its “own” bipolar cell via its synapse , so that the visual center in the brain can display a separate image point for each cone, which enables high-resolution, sharp vision.

Tasks

The most important task of the cones is the transduction of light impulses, the conversion of received light stimuli into an electrical nerve impulse. The transduction takes place largely in the outer segment of the cone in the form of a complex “visual signal transduction cascade”.

The starting point is iodopsin, which is made up of cone opsin, the protein component of a visual pigment that varies depending on the type of cone, and retinal, a vitamin A derivative. An incident photon of the ‘right’ wavelength causes the retinal to transform into another form, causing the two molecular components to separate again, activating the opsin and setting in motion a cascade of reactions and biochemical transformations. Two features are important here. As long as a cone does not receive light pulses of the length wave to which its type of iodopsin reacts, the cone continuously produces the neurotransmitter glutamate .

If the signal transduction cascade is set in motion by the appropriate incidence of light, the release of glutamate is inhibited, with the result that the ion channels on the synapsed bipolar cell close. This creates new action potentials in the downstream retinal ganglion cells, which are conducted as electrical impulses to the visual centers of the CNS for further processing . The actual signal is therefore not created by the activation of a neurotransmitter, but rather by its inhibition.

Another peculiarity is that unlike most nerve impulses , where the “all or nothing” principle prevails, during transduction the bipolar cell can produce gradual signals depending on the strength of the inhibition of glutamate. The strength of the signal emitted by the bipolar cell thus corresponds to the strength of the incident light at the corresponding cone.

Diseases

The most common symptoms of a dysfunction associated with cones in the retina of the eye are color blindness, color blindness and a reduction in contrast vision up to visual field defects . In the case of color blindness, the corresponding type of cones is restricted in its function, while in the case of color blindness the cones are either absent or have a total functional failure.

The visual disturbances can be congenital or acquired. The most common genetic color blindness is green weakness (deuteranopia). It occurs mainly in men because it is a genetic defect on the X chromosome. About 8% of the male population is affected. Impaired perceptions of the colors in the blue to yellow range are the most common visual disturbances in color blindness acquired through lesions on the optic nerve as a result of an accident, stroke or brain tumour .

In some cases, congenital cone-rod dystrophy (CSD) is present with slowly progressing symptoms up to loss of visual field. The disease begins in the yellow spot and leads first to degeneration of the cones and only later to the rods when the dystrophy spreads to other parts of the retina.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.