Photoreceptor or photoreceptor refers to a light-sensitive receptor cell (also called visual cell ) at the cellular level . These include the rod cells and cone cells of the retina and photosensitive ganglion cells as specialized sensory cells of the eye .
A photoreceptor cell contains special structures at the molecular level that are changed by light and thus make the cell sensitive to light. These molecular structures are called visual pigment , photopigment or also photoreceptor . They are composed of a protein component (e.g. an opsin ) and a chromophore (e.g. retinal ), for example the visual pigments rhodopsin or iodopsin in a light- sensing cell ( Cellula optica bacilliformis as a rod cell or coniformis as a cone cell) of animals . Light receptors such as phototropins , phytochromes and cryptochromes can also be found in plants or fungi , and also in unicellular algae and bacteria.
- The photoreceptors in the compound eye of invertebrates react to light with a decrease in voltage ( depolarization ).
- Conversely, in the photoreceptors of the retina of vertebrates, light absorption leads to an increase in voltage ( hyperpolarization ). The action potential does not arise in the receptor itself, but only in downstream nerve cells, which is why the photoreceptors of vertebrates are secondary receptors .
Photoreceptors in vertebrates
There are three types of photoreceptors in the retina of the human eye: rods , cones and photosensitive ganglion cells . Of these, only the rods and the cones have a function in image recognition. The photosensitive ganglion cells regulate the day-night rhythm ( circadian rhythm ).
The rods are more numerous and more sensitive to light than the cones. The human retina contains around 120 to 130 million rods. They enable light-dark vision ( scotopic vision ). The approximately 6 million cones are used for daylight vision ( photopic vision ) and color recognition. In the so-called yellow spot ( macula lutea ) there are only cones in humans, about 70,000.
The structure of the rods and cones is similar. In the outer segments ("Outer segment", OS) the phototransduction takes place by means of the retinal-coupled seven transmembrane protein rhodopsin ("visual purple"). This is stored in many membranous “disks” (a rod) or membrane folds (a cone). The outer segments of the rods are long, narrow and border the retinal pigment epithelium (RPE), which phagocytes constricted, old stacks of membranes . The outer segments of the cones, like the entire cone photoreceptor cell, are wider than the rods and are tapered.
An outer segment is connected to the inner segment via a modified cilium in a decentralized position, the connecting cilium ("Connecting cilium", CC). Nine microtubule doublets in a nonagonal arrangement form the inner structure of this immobile cilium. This is followed by the metabolically active inner segment ("Inner segment", IS), divided into the ellipsoid, which is rich in mitochondria, and the myoid with the endoplasmic reticulum (ER). Among other things, protein biosynthesis takes place here.
The following retinal layer, called the outer nuclear layer (ONL), contains the cell nucleus with the cell body. A short neurite emerges from this, which ends in the outer plexiform layer ("Outer plexiform layer", OPL) with a complex synapse . The synapses of the photoreceptors are specialized, so-called “ribbon synapses”, in relation to a band- or plate-like structure directly on the active zone of the presynapse. Many synaptic vesicles are coupled to the ribbon structure and a much higher number of vesicles can be released per unit of time compared to “normal” synapses. Cones have significantly more ribbons than chopsticks.
The different types of photoreceptors each have a different visual pigment ( rhodopsin ). These differ in their absorption maxima and thus in their sensitivity to certain wavelengths of light. This is the basis of color vision . The number of types of cones determines the spectrum of perceived colors. The human possesses z. B. three types of cones as trichromat .
In the dark there is a constant release of the neurotransmitter glutamate at the synapses of the photoreceptor cells . This usually has an inhibiting effect on the postsynapses of horizontal and bipolar cells . When light hits the photoreceptor cell, ion channels in the cell membrane are closed, triggered by the signal transduction cascade . The photoreceptor cell becomes hyperpolarized and releases the neurotransmitter to a reduced extent. As a result, the ion channels of the downstream nerve cells ( bipolar and horizontal cells ) are opened and the impulse is transmitted to them, which pass this excitation on to other nerve cells ( amacrine cells , ganglion cells ) and finally to the brain .
Photoreceptors in invertebrates
Structure of the photoreceptors in the compound eye
The compound eyes (also called complex eyes ) of insects , crustaceans and even some annelids consist of many individual eyes , the ommatidia . Seen from the outside, these form a honeycomb-like hexagon pattern with their lenses.
Each ommatidium consists of a lens, the underlying crystal cone, 6 to 8 photoreceptors and the pigment cells that shield each ommatidium from the others. Each photoreceptor cell has a brush-like microvilli border on one side of the photoreceptor cell and continues in an axon. The rhodopsin is stored in the membrane of this microvilli border, which is known as the rhabdomer . These rhabdomers “interlock” like fingers in the middle of an ommatidium and thus form the rhabdom. In many insects (for example bees) the rhabdomers have grown together to form a compact structure (closed rhabdom), whereby the microvilli of adjacent cells are often oriented at right angles to one another. The symmetrical structure of the rhabdomere is the basis for polarization vision. Flies (Musca, Calliphora) have open rhabdomers.
Light is focused on the rhabdom through the lens. The visual signal transduction takes place here in a slightly different form, in that at the end the opening of the ion channels causes a depolarization (in contrast to vertebrates, there is hyperpolarization) of the cell.
Photoreceptors in plants
Plants have some photoreceptors that convey information about the quality and quantity of light in the environment. Phytochromes measure the ratio of light and dark red light and control the growth and development processes of plants, such as seed germination , the greening of plant parts or the induction of flowers . In addition, blue light is perceived by cryptochromes , which are also involved in photomorphogenesis , as well as phototropins .