Photoreceptor cell

As a PR or photoreceptor is referred to a cellular level, the specialized light- sensitive cells of the eye, such as the rods and cones of the retina.

Secondly, certain light-sensitive dyes ( photopigments ) are so named at the molecular level, which are composed each of a different protein content (eg opsin ) and various chromophores (eg, retinal), for example, the visual pigments rhodopsin and iodopsin in a photoreceptor cell ( Cellula optica bacilliformis than rod cell or coniformis as Zapf cell) of animals. Also in plants or fungi are those photoreceptors to find out how the phototropins, phytochromes and cryptochromes, and also in unicellular algae and bacteria.

• 3.1 Structure of the photoreceptors in the compound eye

Basics

The absorption of light of sufficient energy in these cells leads to the formation of an electrical voltage signal.

Here is a basic distinction between the photoreceptors of invertebrates and vertebrates:

• The photoreceptors in the compound eye of invertebrates react to light with a voltage decrease ( depolarization ).
• Conversely, in the photoreceptors of the retina of vertebrates light absorption to a voltage increase ( hyperpolarization ). However, the action potential does not occur in the receptor itself, but only in downstream neurons, therefore the photoreceptors of vertebrates are secondary receptors.

Photoreceptors in vertebrates

Construction

In the retina of the human eye, there are three types of photoreceptors: rods, cones, and photosensitive ganglion cells. Of these, only the rods and the cones have a function in pattern recognition. The photosensitive ganglion cells serve to regulate the day - night cycle ( circadian rhythm ).

The rods are more numerous and more sensitive to light than cones. The human retina contains about 120-130 million rods. They allow the 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 ) are located exclusively in humans journal, approximately 70,000.

In construction rods and cones are similarly organized. In the outer segments ( " Outer segment", OS ) phototransduction takes place by means of the coupled with retinal seven-transmembrane protein rhodopsin ( " visual purple "). This is in many membranous " disks" (rods ) or Membraneinfaltungen ( cones) embedded. The outer segments of the rods are long, narrow and adjacent to the retinal pigment epithelium ( RPE), which phagocytosed tied-off, old membrane stack. The outer segments of the cones are just like the entire cone photoreceptor cell wider than the rods and taper to.

An outer segment is a modified cilium in a decentralized location, Verbindungscilium ( "Connecting cilium ", CC), connected to the inner segment. Nine microtubule doublets in nonagonaler arrangement, the internal structure of such immovable cilium. At this, the metabolically active inner segment ( "Inner segment", IS) follows, which is sub-divided into the mitochondria -rich ellipsoid and the myoid with the endoplasmic reticulum ( ER). Here, among other things, the protein biosynthesis. The next layer is the outer nuclear layer ( " Outer nuclear layer", ONL ), which include the nucleus to the cell body. From this an axon goes out, which with a synapse in the outer plexiform layer ( " Outer plexiform layer", OPL ) ends. The synapses of the photoreceptors are specialized, so-called " ribbon synapses ," in reference to a band-like or plate-like structure directly at the active zone of the presynaptic terminal. On the ribbon structure many synaptic vesicles are coupled and it can have a far higher number of vesicles are released per unit time compared to " normal" synapses. Cones have much more ribbons as chopsticks.

Function

The different photoreceptor types 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 in this case determines the spectrum of perceived colors. Man possesses, for example, as trichromatic three types of cones.

In the dark, a continuous release of the neurotransmitter occurs at the synapses of photoreceptor cells glutamate. This usually affects inhibitory synapses to the post of horizontal and bipolar cells. When light hits a photoreceptor cell, ion channels are closed in the cell membrane, triggered by the signal transduction cascade. The photoreceptor cell is hyperpolarized and pours the neurotransmitter to a lesser extent from. As a result, the ion channels of the postsynaptic nerve cells ( bipolar and horizontal cells ) is opened and transferred to the pulse of this that this excitation to other nerve cells ( amacrine cells, ganglion cells ) and finally pass on to the brain.

Photoreceptors in invertebrates

Structure of the photoreceptors in the compound eye

The compound eyes (also called compound eyes ) of insects, crustaceans, and even in some annelids consist of many individual eyes, the ommatidia. This form seen from the outside with their lenses a honeycomb -like hexagonal pattern.

Each ommatidium consists of a lens, the underlying crystal cones 6 to 8 as well as the pigment cells photoreceptors, which shields each ommatidium from the others. Each photoreceptor cell has a brush-like brush border on one side of the photoreceptor cell and continues in an axon. The rhodopsin is embedded in the membrane of this brush border, which is called rhabdomeric. These rhabdomeres "grab" in the middle of a Ommatidiums interdigitated and so form the rhabdome. In many insects (for example, bees) are rhabdomeres in a compact structure grown (closed rhabdome ), the microvilli adjacent cells often are perpendicular to each other oriented. The symmetrical structure of the rhabdomeres is the basis for the polarization vision. Flies ( Musca, Calliphora ) have open rhabdomeres.

Light is focused by the lens towards the rhabdome. The visual signal transduction is then here in slightly modified form, instead, in which at the end of the opening of ion channels a depolarization ( in contrast to vertebrates, as is done hyperpolarization ) of the cell causes.

Photoreceptors in plants

Plants have a number of photo receptors that mediate information about the light quality and quantity of the environment. Phytochromes measure the ratio of light and dark red light and control of growth and development processes of plants, such as seed germination, the greening of parts of plants or the flower induction. In addition, a perception of blue light by cryptochromes, which are also involved in photomorphogenesis, and phototropins.