Rods are the retinal photoreceptors responsible for light-sensitive monochromatic night vision and peripheral vision. The main concentration of rods is outside the yellow spot (fovea centralis) located centrally on the retina, which is mainly populated with three different types of cones for color and sharp vision during the day and in bright twilight.
What are rods?
The approximately 110 million rods on the retina are photoreceptors that are much more sensitive to light pulses than the approximately 6 million cones. The rods are therefore predestined for night vision (scotopic vision) and vision in dark twilight. Because there is only one type of rod, which is particularly sensitive to light in the blue-green spectral range, vision becomes monochromatic below a certain brightness. Different colors are no longer perceived. The high sensitivity to light is partly at the expense of contrast. Because up to 20 rods report light impulses to the same ganglion via bipolar cells, the visual center in the brain can no longer locate the light impulse as precisely as with the cones, which are often interconnected with “their” ganglia in a 1:1 ratio. Although the principle of conversion of light impulses into electrical nerve signals is in principle almost the same for rods and cones, the messages from rods are significantly faster than those from cones because there are fewer intermediate connections. As a result, rods are extremely sensitive not only to light but also to moving objects in the peripheral visual field.
Anatomy and structure
The structure of rods is similar to those of cones, but rods are more slender and use rhodopsin as their visual pigment, whose highest sensitivity in the blue-green range is 498 nanometers. Rods consist of a cell body, synapse, inner segment, connecting cilium, and outer segment. The inner segment provides cell metabolism and, by means of thousands of mitochondria in the nucleus, energy metabolism, while the outer segment is where the conversion of light pulses into electrical nerve signals, visual signal transduction, takes place. The outer segment contains more than 1,000 so-called discs in which the visual pigment rhodopsin is stored. The discs have developed from former membrane invaginations that have detached from the outer membrane in the course of evolution. In contrast, the membrane invaginations in the outer segments of the cones are still recognizable as such because they have remained part of the membrane. The marginal connecting cilium, which consists of nonagonal microtubules (9-sided polygon), serves to mechanically stabilize the connection between the inner and outer segments and to transport matter between the two segments.
Function and Tasks
The main function of the rods is to convert (weak) light impulses into electrical nerve impulses. The process involves a complex signal transduction cascade and occurs mainly in the outer segment. The first stage consists of the reaction of the visual pigment rhodopsin, which is composed of opsin and the carotenoid 11-cis-retinal. After light exposure, the 11-cis-retinal isomerizes to the all-trans isomer and detaches again from rhodopsin. Unlike the activation of other neurons, which are usually stimulated to release a neurotransmitter by a brief depolarization from -65 mV to +10 to +30 mV, this works the other way around in photoreceptors; the synapses, which are negatively charged at about -40 mV, are briefly hyperpolarized to -65 mV, causing them to briefly reduce or stop releasing glutamate, their specific neurotransmitter. Thus, the generation of the corresponding nerve impulse occurs not by the release of a neurotransmitter, but by the reduction of its release. If no light hits the receptors (resting position), glutamate is constantly released at the synapses of the photoreceptors. This has the advantage that the downstream ganglia can vary the nerve stimulus gradually according to the strength of the light incidence, i.e. generate a kind of analog signal that allows the visual centers not only to spatially assign the light spots, but also to determine their brightness. The property of the rods to react extremely sensitively to objects in the peripheral visual field that move relative to their surroundings originally served our protection. Enemies or predators approaching from the side were noticed early.Today, this ability of rods plays a role in visual aviation by noticing laterally approaching objects early and initiating evasive maneuvers.
Diseases
Rod dysfunction is most noticeable in impaired night vision. Widespread reversible night blindness presents with an undersupply of vitamin A because insufficient visual pigment rhodopsin can then be deposited at the discs in the outer segment of the rods. Symptoms of a dysfunction of the rods can also be recognized by an increased sensitivity to glare, e.g. due to oncoming traffic. Apart from vitamin A deficiency and nerve lesions due to traumatic brain injury (SHT), brain tumor or other injuries, rod dysfunction is mostly due to genetic defects. These are usually genetic defects that lead to retinal dystrophies of various types and cause gradual destruction of the photoreceptors in the retina. Retinitis pigmentosa is a retinal dystrophy that progresses from the outside in. This means that the rods are the first to be affected and the typical night blindness and sensitivity to glare develop although daytime vision is (still) unimpaired in terms of sharpness and color vision. Other retinal dystrophies, such as cone-rod dystrophy (ZSD), progress from the inside out, so that the cones are affected first and the rods later.
