Rod or rod cell , anatomically neuron bacilliferum , is the name of a type of photoreceptor in the retina of the vertebrate eye with rod-shaped appendage , the rod , anatomically Bacillum retinae ( Latin bacillum , rods). Rod cells are neurons that serve as specialized sensory cells for scotopic vision at low brightness, night vision or twilight vision . With these very sensitive light sensory cells, even weak light stimuli from the outside world can be converted into a signal that the brain can use. The evaluation of the signals from receptor cells with rods alone enables light-dark vision ; Since the receptors of this one type all react to light of the same specific wavelength range, one speaks here of monochromatic vision. Many animals also have the analog, less sensitive cones , of which there are different types that are necessary for color perception , known as photopic vision .
Layout and function
The structure of rods and cones is similarly organized and consists 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 by the visual pigment molecules. These are composed of a chromophoric group ( retinal ) and a glycoprotein ( opsin ). These molecules are embedded in many (> 1000) membranous discs ("disks"). The outer segments of the rods are long, narrow and border the retinal pigment epithelium ( RPE ), which phagocytes constricted, old stacks of membranes . 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 ), which 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 layer is the outer nuclear layer ("Outer nuclear layer", ONL ), which contain the cell body with the cell nucleus (nucleus, N ). An axon extends from this and ends with a synapse ( S ) in the outer plexiform layer (“Outer plexiform layer”, OPL ). The synapses of the photoreceptors are so-called "ribbon synapses", band- or plate-like structures 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. In the dark, the neurotransmitter glutamate is continuously released . This usually has an excitatory 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 hyperpolarizes and stops releasing the neurotransmitter . As a result, the ion channels of the downstream cells are opened and the impulse is transmitted to them.
Human rods contain a form of the visual pigment rhodopsin , which is most sensitive to light with a wavelength of around 500 nm (blue-green). These sensory cells are mainly important for seeing at dusk and at night, as they work even with low light intensity. No colors can be differentiated by the rods because, in contrast to the cones, all rods have the same sensitivity spectrum. In the outer area of the center of the retina (5–6 mm), the number of rods predominates, which means that people can see better in the periphery than in the center at dusk . In total there are around 120 million rods and around 6 million cones in the human eye.
The greater light sensitivity of the rods compared to the cones has two main reasons:
- On the one hand, the light-sensitive pigment discs in the upper part of the rods are more light-sensitive. Even a single absorbed photon leads to a membrane voltage change of around 1 mV after a series of intracellular processes. Cones, on the other hand, require a much larger number of photons (at least around 200) in order to pass an excitation on to the downstream cells.
- The second reason is the neuronal interconnection of the receptors with downstream cells. Roughly many sticks conduct its signal to a single ganglion cell (over bipolar cells, etc.) on, while a pin in many cases even to each one derives ganglion. This means that the information from the rods converges much more strongly than that from the cones. This is also the reason for the poor spatial resolution of rod vision (for example at night). If a ganglion cell (via which the information is ultimately passed on to the brain) receives a rod signal, this can come from many different rods that form synapses with it, and the point on the retina where the image is displayed is therefore relative vague. If, on the other hand, a ganglion cell receives cone information, the point of light can be localized very well on the retina, since only very few cones are connected to it.
Transmission of excitation in rods and cones
The vast majority of nerve cells (neurons) pass their information on to other neurons via so-called action potentials . Put simply, the stimulation of a neuron causes a change in voltage (the actually negatively charged cell is positively charged for a short time), which leads to the neuron releasing messenger substances ( neurotransmitters ) via a synaptic connection . These neurotransmitters bind to receptors of the downstream neuron and also lead to voltage changes there, etc. In this usual type of excitation transmission, the information is not encoded by the strength of an action potential (the change in voltage), but solely by the frequency of the action potentials in a certain period of time.
The transmission of excitation in rods and cones works in a different way, however: They code the light information not via the frequency of action potentials, but via the extent of their transmembrane voltage change. Most of the other neurons are in their rest position (when no stimulus is received) negatively charged with about -65 mV. If a stimulus acts on it, the charge rises for a short time to around +10 to +30 mV, and an action potential is triggered by this depolarization. In their rest position (when no light is incident), rods and cones are less negatively charged at around –40 mV - that is, slightly depolarized. As soon as light acts on them, they are charged even more negatively (up to a maximum of about -65 mV) - that is, hyperpolarized - instead of becoming more positive like the other neurons. Roughly speaking, each neuron releases more messenger substances, the more positively it is charged. While normal neurons suddenly release a lot more messenger substances when a stimulus (which causes depolarization), this reaction is exactly the opposite with photoreceptors: If a light stimulus occurs, they become even more negative (hyperpolarized) and release fewer messenger substances than in a resting position. The light stimulus is not signaled to downstream cells by more, but by less messenger substances. The intensity of the light stimulus is communicated to the downstream cells (bipolar cells) through the extent of the reduction in messenger substances - the fewer messenger substances, the stronger the light stimulus.
- ↑ Terminologia Histologica (TH, current nomenclature ), see entry H3.11.08.3.01030 p.109 .