Of rods and cones

Copyright: Glowimages/ Glowimages/ Getty Images
Von Stäbchen und Zapfen
Author: Sascha Karberg

Intricate molecular mechanisms in the rod and cone cells in our retinas transform the energy of light into nerve impulses. But these two types of cells do much more...

Scientific support: Prof. Dr. Jan Kremers

Published: 19.12.2017

Das Wichtigste in Kürze
  • Weil bei schwachem Licht nur die Stäbchen in der Netzhaut aktiv sind, sehen wir nachts alles grau.
  • Bei starkem Lichteinfall sorgen drei Zapfentypen für Farbsicht.
  • Die Umwandlung von Lichtenergie in elektrische Signale erfolgt beim Stäbchen dank des Rhodopsins, einem Fotopigment. Den entsprechenden Vorgang nennt man Fototransduktion.
  • Die Zapfen-Typen haben unterschiedliche Opsine, die jeweils auf eine andere Licht-Wellenlänge ansprechen.
  • Die unterschiedliche Verschaltung von Stäbchen und Zapfen mit Bipolarzellen verstärkt den jeweiligen Seheffekt.

Stäbchen

Stäbchen/-/rod cells

Die Stäbchen sind Lichtsinneszellen mit hoher Lichtempfindlichkeit. Sie reagieren schon auf schwaches Licht und sind so für das skotopische Sehen, das Schwarz-​Weiß-​Sehen und das Sehen in der Dämmerung zuständig. Die Stäbchen liegen gehäuft in den äußeren Bereichen der Netzhaut und vermitteln daher keine große Sehschärfe.

Netzhaut

Netzhaut/Retina/retina

Die Netzhaut oder Retina ist die innere mit Pigmentepithel besetzte Augenhaut. Die Retina zeichnet sich durch eine inverse (umgekehrte) Anordnung aus: Licht muss erst mehrere Schichten durchdringen, bevor es auf die Fotorezeptoren (Zapfen und Stäbchen) trifft. Die Signale der Fotorezeptoren werden über den Sehnerv in verarbeitende Areale des Gehirns weitergeleitet. Grund für die inverse Anordnung ist die entwicklungsgeschichtliche Entstehung der Netzhaut, es handelt sich um eine Ausstülpung des Gehirns.
Die Netzhaut ist ca 0,2 bis 0,5 mm dick.

Fotopigment

Fotopigment/-/photopigment

Fotopigmente sind lichtempfindliche Moleküle in den Rezeptoren der Netzhaut. Durch Einfall von Photonen (den Teilchen des Lichts) zerfällt das Fotopigment und löst so eine Kaskade diverser Prozesse aus. Auf diese Weise wird Licht in einen Nervenimpuls umgewandelt. Die drei Zapfentypen des Auges verfügen genauso wie die Stäbchen über je eine eigene Art.

Zapfen

Zapfen/-/retinal cones

Die Zapfen sind eine Art von Fotorezeptoren der Netzhaut. Die drei unterschiedlichen S-​, M– und L-​Zapfen sind jeweils durch kurz-​, mittel und langwellige Frequenzen des sichtbaren Lichts erregbar und ermöglichen so Farbsehen.

Bipolarzellen

Bipolarzelle/-/bipolar cell

Die Bipolarzelle ist ein bipolares Neuron, also ein Neuron mit einem Axon und einem Dendriten das in der mittleren Schicht der Netzhaut liegt. Es übermittelt die sensorische Information von den Photorezeptoren zu den Ganglienzellen.

The dazzling displays from a light organ at a concert, the flashes of lasers in a disco and the colorful advertisements in the city show how adept people have become at decorating the darkness with light and color. Yet anyone who has been surprised by the arrival of dusk during a walk has noticed the way the saturated greens of fields and trees fade into dull grey. This isn't due to the trees – whose inherent visual characteristics remain the same night or day. What has changed is the way our visual perception works, which renders everything – not just cats – in shades of grey.

Photoreceptors as translators

Even in dim light we recognize our environment without stumbling blindly about. This is made possible by specialized light-receptive cells in the retina called rods, which arose later in evolution than the second type of photoreceptor cell (cones). The first mammals used the cover of night to escape the larger reptiles that ruled the earth because the temperatures were uncomfortable for cold-blooded animals. Early mammals adapted by developing long, cylindrical rod cells out of shorter cells shaped like bowling pins.

Both types of cells transform the energy of light into electrical signals, thus translating visual stimuli into the language of nerve cells. Rods required much less energy to trigger this process of phototransduction, which is the basis of vision. They owe this capacity to a highly sensitive molecule called rhodopsin, which derives from the Greek word "rhodon" (rose), due to the pinkish violet color of photoreceptors – leading to a second designation of the molecule as "visual purple." Rhodopsin is a photopigment, a substance whose properties change under the influence of light, and it's the form active in rods. It is found in the disk-shaped membranes that make up the outer segment of these photoreceptor cells.

Rhodopsin

Rhodopsin/-/rhodopsin

Ein bestimmtes Opsin, das in den Stäbchen der Netzhaut vorkommt.

Light releases nerve cells from their leash

With the help of an effector enzyme, activated opsin passes a signal to a channel protein in the membrane of the rod cell. As a result the channel, called transducin, reduces the amount of positively charged sodium ions that enter the cell. This changes the distribution of electrical charges – the membrane potential – of the cell membrane. When it gets dark, rod cells contain fewer positive ions than the extracellular environment and thus have a negative membrane potential. Normally this leads to an inflow of the ions, but any light closes the channels to prevent this. This increases the negative membrane potential and hypoerpolarizes the cell; as a result, the rod cell lowers its secretion of a signaling molecule called glutamate.

Rod cells are connected to bipolar cells, which are particularly responsive to levels of glutamate. Normally the continual release of this neurotransmitter inhibits their activity – like a sort of leash that keeps them under control. But when levels of glutamate sink, biopolar cells trigger a sequence of events that stimulate the optic nerve. It's as if the glutamate signal has been kept on a leash, and a light stimulus sets it free. Now another special feature of bipolar cells comes into play. While other types of nerves are activated through an all-or-nothing mechanism, bipolar cells experience a gradual change in their membrane potential, and thus transmit information about the strength of a signal to the optic nerve. This, in turn, can be traced back to incremental drops in the amount of glutamate they sense. All of these events combine in such a way that as more light is sensed by the rod, the signal transmitted by a bipolar cell becomes stronger.

Rhodopsin is made of two subcomponents: retinal, a derivative of Vitamin A, and opsin. Retinal is so sensitive that it may respond a single photon by altering its structure from a form known as "11-cis" to "all-trans." This leads the two modules of the protein to move apart, activating the opsin. The process has also been called "bleaching" because it alters the color of the photopigment from violet to yellow. The result is something like the way a cascade of dominos falls when the first stone is nudged, ultimately leading to the generation of a nerve impulse.

With the help of an effector enzyme, activated opsin passes a signal to a channel protein in the membrane of the rod cell. As a result the channel, called transducin, reduces the amount of positively charged sodium ions that enter the cell. This changes the distribution of electrical charges – the membrane potential – of the cell membrane. When it gets dark, rod cells contain fewer positive ions than the extracellular environment and thus have a negative membrane potential. Normally this leads to an inflow of the ions, but any light closes the channels to prevent this. This increases the negative membrane potential and hypoerpolarizes the cell; as a result, the rod cell lowers its secretion of a signaling molecule called glutamate.

Rod cells are connected to bipolar cells, which are particularly responsive to levels of glutamate. Normally the continual release of this neurotransmitter inhibits their activity – like a sort of leash that keeps them under control. But when levels of glutamate sink, biopolar cells trigger a sequence of events that stimulate the optic nerve. It's as if the glutamate signal has been kept on a leash, and a light stimulus sets it free. Now another special feature of bipolar cells comes into play. While other types of nerves are activated through an all-or-nothing mechanism, bipolar cells experience a gradual change in their membrane potential, and thus transmit information about the strength of a signal to the optic nerve. This, in turn, can be traced back to incremental drops in the amount of glutamate they sense. All of these events combine in such a way that as more light is sensed by the rod, the signal transmitted by a bipolar cell becomes stronger.

Rhodopsin

Rhodopsin/-/rhodopsin

Ein bestimmtes Opsin, das in den Stäbchen der Netzhaut vorkommt.

Retinal

Retinal/-/retinal

Eine Chemikalie, die aus Vitamin A synthetisiert wird. Gemeinsam mit Opsin bildet es Rhodopsin.

Specialists with strengths and weaknesses

Nature seems to have come up with a complicted solution in transforming light into nervous energy – the biochemical cascade is highly complicated, with a lot of steps. This turns out to be an elegant way to amplify signals, because a single photopigment molecule can activate many transducin proteins. The effector enzymes that they release go on to block hundreds of sodium channels and ultimately the influx of millions of sodium ions. At the end of the day – often quite literally – it is this amplification system that allows us to perceive a single photon, the smallest unit of light.

The phototransduction process also occurs in cone cells. While all rod cells are limited to perceiving light with a maximal wavelength of 500 nanometers, the human retina has three types of cones, each of which contains a unique type of opsin. This makes them respond to different wavelengths of light or, more technically speaking, each type of cone is sensitive to a different part of the light spectrum. The first is optimally triggered by blue light, the second by green, and the third by red. It is this specialization that gives us the ability to perceive colors. But the photopigments of cone cells require significantly more energy than rods to respond. This means that they only function during the day, which is important because once the light intensity reaches a certain level, they shut down. At that point they are operating at maximal level and are so "saturated" that additional light doesn't prompt a hyperpolarization reaction. One reason for this is the fact that the cells contain a huge amount of photopigment. A single rod can contain up to 10 million copies of the rhodopsin molecule – many times the number of opsins found in cones. This makes the rods up to a thousand times more sensitive to light than their color-receptive counterparts.

Amplification through connectivity

The structure of rods alone suggests that they might have something to do with vision under conditions of poor lighting. But our ability to make out most of our surroundings even in the moonlight requires a further mechanism that also first evolved with the appearance of mammals. A single bipolar cell is connected to many rods. And just as a dozen solar cells can collect more energy than a single panel, this feature of cells increases the probability that enough light will be collected by the eye to trigger a nerve impulse.

Cones, on the other hand, are often only connected to a single bipolar cell. This is optimal given their role in the sharpness of vision: it permits an image to be broken down into a large number of spots. The price of this precision is sensitivity, because it means that a cone cell must generate all the energy it needs to activate a bipolar cell. The process is much easier for a connected network of rod cells, which capture light as a team. They do so at a cost in resolution, because a light signal that falls on the retina can't be matched to a single rod – it can only be assigned to a group. This explains not only why a cat seen at night seems to be grey, but also why it is blurry compared to a sighting during daytime.

Veröffentlichung: am 03.11.2010
Aktualisierung: am 19.12.2017

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