Rod cell

Chopsticks (English rod cells, or rods ) are the photoreceptors in the retina of the eye that serve the vision in low light conditions, the scotopic vision, night vision or night vision. With these specialized, highly sensitive cells, a light signal from the outside world is converted into a usable signal for the brain. Chopsticks allow monochromatic vision, since the receptor cells respond only to the light of a specific wavelength range ( black and white vision). Many animals also have the analogue construction, less sensitive cones, which are important for color perception, the so-called Photopic vision, necessary.

Design and function

In construction rods and cones are similarly organized and consist of a cell body, a synapse and a cell specialization: the inner and outer segment. In the outer segment ( " Outer segment", OS ) the visual signal transduction takes place through the Sehfarbstoffmoleküle. These consist of a chromophoric group together ( retinal) and a glycoprotein ( opsin ). These molecules are in many (> 1000 ), membranous discs ( " discs " ) stored. The outer segments of the rods are long, narrow and adjacent to the retinal pigment epithelium ( RPE), which phagocytosed tied-off, old membrane stack. 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 following layer, the outer nuclear layer ( " outer nuclear layer " ONL ), which contain the cell bodies with nuclei (nucleus, N). From this an axon goes out, which ( OPL "The Outer plexiform layer") ends with a synapse (S ) in the outer plexiform layer. The synapses of the photoreceptors are so-called " Ribbon synapses ", band -like or plate -like structures directly at the active zone of the presynaptic terminal.

On the ribbon structure many synaptic vesicles are coupled, and it may be paid per unit of time compared to normal synapses a far higher number of vesicles. In the dark, a continuous release of the neurotransmitter glutamate occurs. This usually affects excitatory 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 neurotransmitters not elaborate. Subsequently, the ion channels of the cells are opened and downstream of the pulse as transmitted to it.

Photosensitivity

The rods contain a human form of the visual pigment rhodopsin, which is most sensitive to light having a wavelength of about 500 nm ( blue-green). These sensory cells are mainly for vision at dusk and at night important because they are active at low light intensity. By the stick no colors can be distinguished, because in contrast to the pin rods all have the same sensitivity spectrum. Outside of the central retina (5-6 mm) outweighs the number of rods, whereby man at dusk in the peripheral looks better than in the center. Altogether there are in the human eye about 120 million rods.

The greater light sensitivity of the rods against the pin has two main causes:

• First, the light-sensitive pigment in the upper part of the rod washers are photosensitive. Already absorbed single photon leads to a number of intracellular processes into a membrane voltage change of approximately 1 mV. Pin, however, require a substantially larger number of photons ( of at least about 200 ), to route a reliable optical signal to the downstream cells.
• The second reason lies in the neuronal circuitry of the receptors with downstream cells. Roughly speaking many sticks conduct their signal to a single ganglion cell ( about bipolar, etc.) continue while a stud in many cases derived to only one ganglion cell. That is the information of the rods converges much stronger than that of the cones. Therein lies the reason for the poor spatial resolution of the rod vision ( for example, at night). Where a ganglion cell ( about which the information is eventually redirected towards the brain ) a rod signal, this can come from many different rods that form with their synapses, and the point on the retina where the image is mapped, is thus relatively vague. Where a ganglion cell, however PTO information, so the light spot can be very well localized on the retina, as very few pins are connected to it.

Excitation forwarding with rods and cones

The vast majority of nerve cells ( neurons ) derives its information via so-called action potentials to other neurons on. Put simply, is the stimulation of a neuron in it causes a change in voltage (which is actually negatively charged cell is positively charged for a short time ), which causes the neuron secretes a synaptic connection messengers ( neurotransmitters). These neurotransmitters bind to receptors of the downstream neuron and lead there to voltage change, etc. In this ordinary type of excitation forwarding the information is not by the strength of the action potential ( the induced voltage change ) codes, but only by the frequency of action potentials. The downstream neuron thus is not interested in how much the action potential of another neuron is or how many messengers are distributed but only for how often an action potential occurs in a given period of time. This corresponds to the frequency of action potentials, so it is frequency modulated. The distributed from the previous neuron neurotransmitter concentration may be regarded as approximately proportional to the following potentials to the following receptors. These find their coding in the action potential frequency at follow- neuron, provided that the receptor potentials exceed a certain threshold. Due to the proportionality of neurotransmitters and receptor potential compensates for this frequency of the previous neuron.

However, the excitation forwarding in rods and cones work in another way: Do not code the light information about the frequency of action potentials, but on the strength of their intracellular voltage change. Most of the other neurons are in their rest position (if no appeal is received ) to about -65 mV negatively charged. Acts on it a stimulus one, quick charge for a short time to about 10 to 30 mV upward, and an action potential is triggered by depolarization. Rods and cones are in their rest position ( when no light arrives ) but with about -40 mV less negative charge - so slightly depolarized. When light is applied to them, they will be even more negative charged ( up to about -65 mV. ) - Ie hyperpolarized - instead of being like the other positive neurons. Roughly speaking showered each neuron from the more messengers, the more positive it is loaded. While normal neurons also ( causes depolarization ) at a stimulus at once pour out a lot more messengers, this reaction proceeds at photoreceptors exactly reversed from: Where a light stimulus, they are even more negative ( hyperpolarized ) and pour fewer messengers than in the rest position from. Downstream cells of the light stimulus is thus not signaled by more, but by less undistributed messengers. In contrast to the other neurons of not the frequency, but the magnitude of the voltage change at the photo receptor plays a critical role in the stimulation intensity coding. The intensity of the light stimulus is communicated to the downstream cells by the amount of neurotransmitter reduction - the less messengers, the stronger was the light stimulus.