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The compound eye is one of the classic subjects in sensory physiology and neuroscience. The loved one (or assumed) simplicity that the eye and associated nervous mechanism has encouraged investigators because the start of the 20th century. Electrophysiological investigations really began with intracellular recording methods from 1960 onwards (Burkhardt and also Autrum, 1960). Modern-day additions to compound eye researches come native the use of mutant models (Drosophila) and also computer modeling, i m sorry have more increased the generality that the research on insect vision.

2 Photoreceptor function3 Synaptic transmission to 2nd order cells3.1 Synaptic transmission to LMCs in flies4 visual signal handling in higher brain centers

straightforward structure and function


Figure 1: Schematic structure of the insect compound eye. The size and also detailed structure of the different neuronal ganglia and centers might vary from types to species. The structure presented is closest come dipteran flies, although the number of retinotopic facets (facets and also corresponding components in deeper structures) is generally much larger.
Compound eyes are organs the vision in arthropods (insects and crustaceans). A compound eye is identified by a change number (a few to thousands) of little eyes, ommatidia, which role as elevation photoreception units with an optical system (cornea, lens and some accessory structures) and normally eight photoreceptor cells. The link eyes perform not kind an image like the huge lens eyes of vertebrates and also octopi, however a "neural picture" is formed by the photoreceptors in ommatidia, which space oriented to receive light from various directions, characterized by the optics the the ommatidia, the curvature the the eye and the spacing arrangement and density of the ommatidia (Fig. 1). The optical device shows plenty of variations, depending how isolated the ommatidia room from each other and how light is concentrated onto the photoreceptors. Key variants are the apposition eye, whereby the ommatidia space optically isolated (e.g. In locusts and also beetles; generally in day-active insects), the superposition eye, wherein theommatidia space not optically diverted (e.g. In butterflies; commonly in crepuscular or night-active insects), and also the neural superposition eye, v the ommatidia optically isolated but neuronal plan causes partial summation of pixels (found in diurnal flies)(reviews: Land, 1981; Stavenga 2006).

Figure 2: basic compound eye designs. (A) A focal distance apposition link eye. Irradiate to photoreceptors come through small corneal lens in each small eylet. (B) A refracting superposition compound eye. A variety of optical aspects focus light to photoreceptors in the retina (cz, the clear zone of the eye). Follow to Warrant 2004

Light stimulation create depolarizing graded potentials in insect photoreceptors (as opposed to hyperpolarizing in vertebrate rods and cones). Action potentials perform not exist, generally, although lock may have actually a function in photoreceptors of some varieties (e.g. In the cockroach, Heimonen et al. 2006). The signals space processed in the very first synaptic layer, the lamina, and in the more neural centers (e.g. The medulla) in a retinotopic fashion. This method that the "pixels" created by the anatomical organization of the retina space being preserved. The signals and their info content adjust continuously, however. In deeper visual centers the retinotopic company is disrupted come the benefit of greater level analysis, like motion detection, pattern recognition and visual orientation (Strausfeld 1976).

Photoreceptor function


The molecular communication of insect phototransduction is finest known in Drosophila melanogaster (Hardie and also Raghu 2001). Absorption of irradiate quanta through rhodopsin molecules leader to activation that a G-protein-coupled phosphoinositide pathway. This takes ar in the microvillar component of the photoreceptor in a very little compartment, whereby all participating molecule are an extremely close to every other. The molecule mechanism involves activation that two varieties of cationic ion channels in the microvillus, creating a light-induced present (LIC) that is measurable with voltage-clamp methods, prefer the patch-clamp. Opened of the channels (products the trp and trpl genes) create a Ca2+ and also Na+ conductance, depolarizing the photoreceptor. Insect photoreceptors, favor their vertebrate counterparts, the rods and cones, room able to respond with so-called quantum bump to single photons, but with rapid kinetics. Fusing the voltage responses to solitary quanta creates the (graded) receptor potential, i beg your pardon is passively conducted along the axon in most cases.

Spatio-temporal filtering through photoreceptors


Figure 3: Transduction current and also filtering by the non-transductive membrane. A) diagram of the key ionic currents affiliated (ion pumps and also exchangers have been omitted). Light stimulation induces a cationic existing from the microvillus come the rest of the cell, part of the return currents going with Kv-channels, b) illustration of the idea the the membrane filter, developed by a combination of passive membrane (with its normal RC-structure) and also the Kv-channels.

The small eye optics of photoreceptors create a situation, whereby a point-like thing moving across the receptive ar of a photoreceptor produce a near-Gaussian intensity circulation in duty of the angle of incidence (Stavenga, 2006). The totality of the compound eye functions in this fashion, an interpretation that the the initial component of the eye performs a spatial low-pass filtering operation to the intuitive image, in addition to the sampling dictated by the density of optical elements. In the very same time, the photoreceptor signals room necessarily minimal by both slowness the transduction chin but also by the membrane time constant, to produce a temporal low-pass filtering (Fig. 3.; van Hateren 1992). The photoreceptor (non-transductive) membrane is specifically slow, due to the fact that the microvillar membrane boosts the membrane area nearly 5-6 fold, not developing a conductive pathway come the very same extent. Both filtering operations might be regulated. The receptive field may be small or widened somewhat by subtle transforms in the optics. On the various other hand, the photoreceptor membrane has voltage-dependent (Kv-type) K+ channels that lower membrane resistance through depolarization (Weckström and also Laughlin, 1995). This way that when stimulated, the photoreceptors become faster with sharper receptive fields.

Synaptic infection to 2nd order cells

Synaptic infection to LMCs in flies

Visual information in the form of neural voltage signal are further processed in the first neuropil, the lamina, whereby the second order neurons, the big monopolar cell (or LMCs) form the post-synaptic elements. These room often, many conspicuously in Dipteran flies, group in retinotopic neural cartridges, lined by glial cells. The 2nd order cells receive signals native the photoreceptors in the type of histamine transmitter that opens fast Cl- networks in the LMCs (Hardie, 1989), thus developing hyperpolarizing responses come depolarizing photoreceptor input.
Figure 4: Responses of paris photoreceptors to light pulses once dark-adapted (A) and also to comparison pulses as soon as light-adapted (C and also D); responses of the first-order interneurons, the LMCs display inverted responses to the same (B, E and also F). Modified native Juusola et al. 1995.
. The Cl- equilibrium potential is very an unfavorable in LMCs, definition that depolarizations in photoreceptors are turned right into hyperpolarizations in LMCs, i.e. The signals change sign.
Figure 5: Adaptive adjust in the synaptic frequency response function (gain part), i.e. The signal deliver from photoreceptors to LMCs. Arrows show the direction of readjust by increasing ambient illumination. Modified indigenous Juusola et al. 1996.
Temporal differentiation

In enhancement to sign readjust the signal in LMCs room further readjusted by a process akin come differentiation (or temporal antagonism) (Laughlin, 1987). In order to the median intensity of illumination changes the personality of synaptic transmission: in dim light the synapse has temporal qualities like those that the photoreceptors, in bright light the synapse changes into a high-pass filter (Fig. 3.). This adaptation of temporal characteristics enhances temporal contrasts and apparently is required for more processing.

Lateral inhibition

Also spatial handling takes ar right in the periphery of the compound eye, in the lamina. Over there a process called lateral inhibition create spatial anagonism (similarly as in the vertebrate retina; Laughlin, 1987). The cellular communication of lateral inhibition may be a combination of direct synaptic feedbacks native the lamina to the photoreceptors, but likewise in the poorly investigated extracellular potentials linked with the regulation the the glial permeability or currents. The lateral inhibition is able come attenuate signals at any type of pixel (one ommatidium) that are probable, i.e. Feasible to suspect on the communication of the adjoining pixels. The spatial details processing may thus fullfil the requirements of predictive coding (Srinivasan et al.,1982)

Feedbacks and network processing

The photoreceptors at least in fly link eye obtain feedback from the second-order cells with a network in the lamina (Zheng et al, 2006). Viewed more broadly, this is component of what can be dubbed network adaptation, through which the neural aspects downstream from photoreceptors change their duty according to the intake properties. This enhances temporal power of the visual system. However, that is no known, just how wide-spread these mechanisms room in other pets apart native flies.

intuitive signal handling in higher brain centers

Signal processing in medulla

Very tiny experimental data is available on signal processing in the medulla, and mostly our knowledge comes from inferences based upon anatomical investigation of the synaptic connections and local microcircuits. The retinotopic organization of the signals is likely to it is in retained, however the signals of numerous outputs of the lamina neurons room segregated to different pathways, probably performing functions such as colour discrimination, elementary activity detection (see below) and intensity coding.

activity detection

Neurons responding predominantly to activity in the visual field ( Insect movement vision neurons) are found in the lobula or lobular key ganglion. They are supposed to get retinotopic entry from hypothetical neural aspects called EMDs (elementary activity detectors)residing most more than likely in the medulla and also calculating activity from the pixel-based details with a mechanism dubbed Reichardt correlation (Hassenstein and also Reichardt, 1956). The cellular substrates of EMDs have not yet been found, but circumstancial evidence for the mechanisms and the existence of EMD-like elements is fairly strong. The movement detection neurons come in assorted flavors, but roughly they have the right to be divided into detectors the horizontal or vertical motion (Hausen, 1981). The outputs that the cell are used in indict of movements, either terrestrial or flying (like the so-called optomotor responses). Some cells may also be associated in detection of in-depth objects.

Looming neurons

Some insects, notably locusts, have been displayed to have a unique system, through which they have the right to avoid collisions, and generally, detect objects approaching lock in your visual field (e.g. Rind and also Simmons, 1992). In locusts a lobula neuron dubbed LGMD (lobula large movement detector) has been described, which, v some relays, has actually output to movement managing neural circuits. That does not react to the activity of the totality visual field, however vigorously come enlarging (looming) objects. This response is quickly habituated away.


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