Photoreceptors are sensitive to light, making this possible See. They turn the stimulus into a electrical signal so that an image can be perceived in the brain.
Occurrence of photoreceptors
In vertebrates such as humans, the photoreceptors are located in the Eye. It is therefore also called light-sensing organ designated.
To be precise, photoreceptors are found on the retina (retina), i.e. on the back wall of the eye, so to speak.
Figure 1: Structure of the eye
Before light hits the retina, it has to pass through various other layers of the eye. This includes the cornea (cornea) and lenswhich break the light so that it can reach the retina in a bundle.
However, the photoreceptors are not evenly distributed on the retina. At the central point of the so-called yellow spot (macula lutea), the central pit (central fovea), they occur in high density. That’s why this spot is also called point of sharpest vision.
In addition, overlying layers of nerve cells are pushed aside above the central fossa: the light hits the photoreceptors much more directly here.
On the other hand, where the nerve tracts of the photoreceptors bundle and leave the eye as the optic nerve, there are no photoreceptors. This place is called papilla or blind spot.
Figure 2: Fundus. Source: medicalgraphics.de
In fact, we cannot perceive rays of light that hit the eye at the blind spot. However, this is not a problem in everyday life, since the loss is small and can be compensated for by the other eye.
Structure of photoreceptors
There are two types of photoreceptors: rod and cones. Both are fundamentally similar. However, their differences qualify them for being seen in different brightness ranges.
rod take light of a wavelength of 500nm best up. Due to their high sensitivity to light, they enable vision at dusk and in the dark (mesopic and scotopic region). On the other hand, the resolution is low: We are less able to perceive sharp edges and outlines, as well as colors.
To compensate for this deficit, a second type of photoreceptor exists:
the cones serve the color perception. For this they need enough light (photopic area), because their sensitivity to light is low. There are cones for blue, green, and red light. Each of these cone types has a different one absorption maximumi.e. a wavelength at which the cones are particularly excited.
All the cones together cover the entire range of light that is visible to us humans (approx 400-700nm).
Figure 3: Absorption range of the different photoreceptors
The table below gives you an overview of the differences in the photoreceptors and why they work so well at the different brightness levels:
- rather small
- pointed, cone-shaped outer segment
- Membrane indentations to increase the surface area
- rather big
- cylindrical outer segment with discs stacked on top of each other
distribution on the retina
- in the central fovea there are only cones, mainly red and green cones
- Blue cones further out
- no chopsticks in the central fovea
- greater density in the periphery
Photosensitivitysmallvery highresolutionvery highrather low
The structure of the two types of photoreceptors is quite similar. Both have a so-called outer segment, also known as the photosensitive appendage. In the chopsticks there are so-called disks. It refers to membrane invaginationswhich serve to increase the surface area.
The discs contain the visual pigments, i.e. the dyes that convert the photons of light into an excitation. In the outer segments of the cones there are so-called slatswhich have a similar function as the chopstick discs.
At the other end of the cell, with rods and cones, is the inner segment. This actually consists of the cell body with the cell nucleus and other cell organelles and ends in a short axon that is connected to downstream neurons via several synaptic endings.
Figure 4: Structure and classification of photoreceptors
The visual pigment mentioned above is rhodopsin:
rhodopsin is composed of the vitamin A derivative retinal and the protein opsin together. The structure of the carrier protein opsin varies slightly between rods and cones, but also between the three types of cones. These slight differences provide the specific absorption maxima of the photoreceptors.
The opsin of the cones becomes photopsin called.
photoreceptors function
Now you know how a photoreceptor is constructed and what types of these components exist. But what exactly happens when light hits a photoreceptor?
Conversion into electrical impulses at photoreceptors
The first question is how the photoreceptors convert the light into an electrical signal. This process is also known as signal transduction and it runs equally in the rods and cones:
- When light hits the rhodopsin, the spatial structure of the retinal changes. From the 11-cis retinal becomes all-trans retinal.
- The activated rhodopsin interacts with a special protein: Transducin. This is a G protein that is now also activated.
- Active transducin activates the phosphodiesterase (PDE). The activated phosphodiesterase cleaves the second messenger cGMP (cyclic GMP) too GMP. cGMP normally ensures that Sodium and calcium ion channels of the cNMP family are kept open in the membrane.
- The cGMP concentration decreases, so that the close channels. It comes to hyperpolarization.
- Due to the hyperpolarization or the decrease in the calcium concentration, the glutamate release at the synapse to the downstream bipolar cell.
- Eventually, one will be trained action potentialwhich is then sent to the brain.
Figure 5: Signal transduction in a photoreceptor.
To ensure that the resulting sensor potential is large enough to later be able to generate an action potential, two important amplification steps are built into the transduction:
- 1. Reinforcement step: An active rhodopsin can activate several hundred transducin molecules.
- 2. Reinforcement step: A phosphodiesterase can also inactivate several hundred cGMP molecules.
As a result, the signal, which originally consisted of a photon, is amplified – one also speaks of one signal transduction cascade.
Photoreceptors are special sensory cells: nerve cells usually produce one when they are activated depolarizationn. In the case of photoreceptors, an exception occurs hyperpolarization.
This hyperpolarization can only occur in downstream neurons, the bipolar cells, be converted into a depolarization. the final ones action potentials are only one neuron layer later in the ganglion cells generated.
Regeneration of the photoreceptor
After the signaling cascade has ended, the cell must return to its original state return to remain light sensitive. For this, rhodopsin must be inactivated and regenerated, and the ion channels opened again. How does this happen? Active rhodopsin is theoretically able to become inactive again on its own. This works faster, however, by using the enzyme rhodopsin kinase phosphorylated and then by the protein arrestin is completely deactivated. This is how opsin and all-trans retinal can be detached from each other. Now here it is pigment epithelium in the game:
That pigment epithelium is a layer in the retina that lies behind the photoreceptors. It’s for that, among other things Retinal recycling responsible. All-trans-retinal is taken up by the pigment epithelium and retinal isomerase converted back to the cis form.
Also opsin can be reused. Together with new or recycled 11-cis-retinal, functional rhodopsin is created again. Now the only thing missing is the opening of the ion channels. Due to the hyperpolarization, the calcium concentration in the cytosol, so that calcium from Calcium-binding proteins solves. This will make them in their active state shifted.
This is also one of these proteins Guanylate cyclase activating protein. As the name suggests, it activates guanylate cyclase. This increases the cGMP synthesis: The channels are reopened.
Light and dark adaptation
The different photoreceptors serve to be able to see something with as many levels of brightness as possible. However, switching from light to dark does not work from one second to the next.
You are probably familiar with the following situation: you turn off the light in the evening and suddenly it is pitch black. At first you can’t see your hand in front of your face, but a little later you can suddenly even see the outlines of furniture in the room again.
Your eye and therefore also your photoreceptors have adapted. It works the same the other way around. When you come out of a dark room, e.g. B. a cinema hall. From the bright light outside are you blinded before you can see normally again.
Thus, when the luminance changes, different ones run in the eye adaptation processes away:
dark adaptation
The eye adapts to the darkness with different mechanisms:
- dilation of the pupil: More light can fall on the retina
- accumulation of rhodopsin (continues to regenerate and is not immediately «bleached» by light): Increased sensitivity
- Temporal summation of signals by looking for a long time
- If the luminance is too low, switch to pure rod vision
In the so-called dark adaptation curvewhich assigns a corresponding adaptation time to a luminance, this change from cones to rods is called Kohlrausch kink.
Because the point of sharpest vision, the central foveacontains only cones, sharp vision in the dark is severely restricted.
- Magnification of the receptive fields: Connection of more photoreceptors to a downstream nerve cell causes reduced resolution
- Purkinje shift: The blue-green color area is perceived better
The Purkinje shift occurs because the wavelength of the blue-green color range is closer to the optimal wavelength for rods (approx. 500 nm).
The dark adaptation is basically rather slow. Until we have adapted optimally, we can 30-60 minutes pass away
light adaptation
The size of the pupil can also be set first with light adaptation: so that sharp vision is possible and not too much light hits the retina, the pupil reduced.
The transition from rods to cones is very smooth due to their high sensitivity to light quickly. The bleaching of the rhodopsin in the rods mentioned above occurs. The cones take over and color vision is possible again without any problems.
Other photoreceptors
Photoreceptors can take on other functions and are not only found in vertebrates.
Photoreceptors in ganglion cells
Also located in the eye, but…