Cones: Structure, Function & Diseases

Cones are the photoreceptors on the retina of the eye responsible for color and sharp vision. They are highly concentrated in the yellow spot, the area of color vision and also the area of sharpest vision. Humans have three different types of cones, each of which has its maximum sensitivity in the blue, green, and red frequency ranges 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 also located in the fovea centralis. The yellow spot is centrally located in the visual axis of the eye for “straight looking” and is equipped with about 140,000 color photoreceptors per qmm. These are so-called L-, M- and S-cones, which have their highest light sensitivity in the yellow-green, green and blue-violet range. Although the L cones have their maximum sensitivity of 563 nanometers in the yellow-green range, they also take over the red range, so that they are usually called red receptors. In the innermost part of the fovea centralis, the foveola, which is only about 0.33 mm in diameter, only M and L cones are present. In total, there are about 6 million color receptors (cones) on the retina. In addition to the cones, the retina is mainly equipped outside the yellow spot with about 120 million additional photoreceptors, the so-called rods. They are similar in structure to the cones, but are much more sensitive to light and can only distinguish between light and dark tones. They are also very sensitive to moving objects in the peripheral visual field, i.e., outside the fovea centralis.

Anatomy and structure

The three different types of cones and the rods, which are present in only one type in the retina, convert received light packets into electrical nerve signals in their function as photoreceptors. Despite slightly different tasks, all photoreceptors work according to the same biochemical-physical principle of action. The cones consist of an outer and an inner segment, the nucleus and the synapse for communication with bipolar cells. The outer and inner segments of the cells are connected 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 segments and to transport matter. The outer segment of the cones has a large number of membrane invaginations, the so-called discs. 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. At the endoplasmic reticulum protein synthesis takes place and in the nucleus a multitude of mitochondria takes care of energy metabolism. Each cone has contact with its “own” bipolar cell via its synapse, so that the visual center in the brain can display a separate pixel for each cone, enabling 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. 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 composed of cone opsin, the protein portion of a different visual pigment depending on the type of cone, and retinal, a vitamin A derivative. An incident photon of the “right” wavelength leads to a conversion of the retinal into another form, causing the two molecular components to separate again and the opsin to be activated, initiating a cascade of reactions and biochemical conversions. 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 responds, the cone continuously produces the neurotransmitter glutamate. If the signal transduction cascade is initiated by appropriate light input, the release of glutamate is inhibited, causing the ion channels at the synapse-connected bipolar cell to close. This results in new action potentials in the downstream retinal ganglion cells, which are transmitted as electrical impulses to the visual centers of the CNS for further processing.Thus, the actual signal is not produced by activation of a neurotransmitter, but due to its inhibition. Another peculiarity is that unlike most nerve impulses, where the “all-or-nothing principle” prevails, in transduction the bipolar cell can produce gradual signals, depending on the strength of the inhibition of the glutamate. Thus, the strength of the signal emitted by the bipolar cell corresponds to the strength of light incidence at the corresponding cone.

Diseases

The most common symptoms of dysfunction associated with cones in the retina of the eye are color vision deficits, color blindness, and impairment in contrast vision and even visual field loss. In color vision deficiencies, the corresponding type of cones is limited in function, while in color blindness, the cones are absent or have a total functional failure. The visual defects may be congenital or acquired. The most common genetic color vision deficiency is green deficiency (deuteranopia). It occurs predominantly in males due to a genetic defect on the X chromosome. About 8% of the male population is affected. Impaired perception of colors in the blue to yellow range is the most common visual defect in color vision loss acquired by lesions on the optic nerve due to an accident, stroke or brain tumor. In some cases, congenital cone-rod dystrophy (ZSD) is present with slowly progressive symptoms to visual field loss. The disease begins in the yellow spot and initially causes degeneration of the cones, and only later are the rods affected as the dystrophy spreads to other parts of the retina.