Photoreceptors: Structure, Function & Diseases

Photoreceptors are light-specialized sensory cells on the human retina. They absorb various electromagnetic light waves and convert these stimuli into bioelectrical excitation. In hereditary diseases such as retinitis pigmentosa or cone-rod dystrophy, the photoreceptors perish bit by bit until blindness occurs.

What are photoreceptors?

Photoreceptors are light-sensitive sensory cells that are specialized for the visual process. An electrical potential is generated from light in the sensory cells of the eye. The human eye contains three different types of photoreceptors. In addition to rods, they include cones and photosensitive ganglion cells. Biology distinguishes between the photocells of vertebrates and invertebrates. Depolarization takes place in the photocells of invertebrates. This means that the cells react to light by lowering their voltage. In contrast, hyperpolarization takes place in those of vertebrates. Their photoreceptors therefore increase their voltage when exposed to light. Unlike those of invertebrates, the photoreceptors of vertebrates are secondary receptors. The conversion of the stimulus into an action potential therefore takes place outside the receptor. In addition to humans and animals, plants also contain photoreceptors to oppose the incidence of light.

Anatomy and structure

There are about 120 million of the rods on the retina of the eye. The cones add up to about 6 million out of about one million ganglion cells in the eye, about one percent is photosensitive. The most light-sensitive photoreceptors are the rods. The blind spot of the eye contains no receptors except the cones. Therefore, a person should actually see a hole where the blind spot is located. This is not the case only because the brain fills the gap with perceptual memories. The rods of the retina contain so-called discs. The cones, on the other hand, contain membrane folds. In these areas they are equipped with the so-called visual purple. Overall, rods and cones have a similar structure. They each have an outer segment in which their most important tasks are performed. The outer segments of the cones are conical and wider than the long and narrow outer segments of the rods. A cilium, or plasma membrane protrusion, connects each of the outer and inner segments of the receptors. The inner segments each consist of the ellipsoid and a myoid with endoplasmic reticulum. The outer granular layer of the photoreceptors connects the cell body with the nucleus. An axon with a synaptic end in ribbon or plate form attaches to each cell body. These synapses are also called ribbons.

Function and tasks

The electromagnetic waves of light are converted into bioelectric excitation by the photoreceptors of the human eye. Thus, the function of all three types of photoreceptors is to absorb and convert light. This process is also known as phototransduction. To do this, the receptors absorb the photons of light and initiate a complex, biochemical reaction to change the membrane potential. The change in potential corresponds to hyperpolarization in vertebrates. The three different receptor types have different absorption limits and thus differ in their sensitivity to certain wavelengths. The main reason for this is the different visual pigment in each cell type. Thus, the three types differ somewhat in their function. The ganglion cells, for example, regulate the day-night rhythm. The rods and cones, on the other hand, play a role in image recognition. The rods are mainly responsible for light-dark vision. The cones, on the other hand, play a role only in daylight and enable color recognition. Phototransduction takes place in each of the outer segments of the photoreceptors. In darkness, most photoreceptors are in the unstimulated state and have a low resting membrane potential due to their open sodium channels. At rest, they permanently release the neurotransmitter glutamate. However, as soon as light enters the eye, the open sodium channels close. As a result, the potential of the cells increases and hyperpolarization takes place. During this hyperpolarization, receptor activity is inhibited and fewer transmitters are released. This retrograde release of glutamate opens the ion channels of the downstream bipolar and horizontal cells.The impulse from the photoreceptors is transmitted via the open channels to the nerve cells, which then activate ganglion and amacrine cells themselves. Thus, the signal from the receptors is transmitted to the brain, where it is evaluated with the aid of visual memories.

Diseases

With regard to the photoreceptors of the human eye, many kinds of complaints and diseases can occur. Many of these manifest as progressive loss of eyesight. For example, cone-rod dystrophy is a form of inherited retinal dystrophy that causes photoreceptors to perish. In this hereditary disease, the patient continuously loses cones and rods due to retinal pigment deposition. This process manifests itself in the early stages as decreased visual acuity, increasing sensitivity to light, and incipient color blindness. Sensitivity in the central visual field decreases. In the later course, the disease also attacks the peripheral visual field. Symptoms such as night blindness may develop. After some time, the patient will probably go completely blind. Retinal pigmentosa, also known as rod-cone dystrophy, must be distinguished from this disease. In this form of retinal disease, the symptoms are the same as in cone-rod dystrophy, but the symptoms are reversed. This means that retinitis pigmentosa first manifests itself in night blindness, while night blindness for cone-rod disease is symptomatic only in the later course. The course of retinal pigmentosa is usually less severe than that of cone-rod dystrophy. In addition to these degenerative diseases, the sensory cells of the visual system can also be affected by inflammation or be damaged by accidents.