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Optic Lobe


Insect photoreceptor cells are arranged in a two-dimensional lattice in the compound eye, in a somewhat spherical sheet called the retina. . The optic lobe of the protocerebrum contains the neuroils concered with the preprocessing of visual information from the retinal photoreceptors. In Lepidoptera including the silkmoth, the optic lobe consists of lamina (La), medulla (Me), and Lobula complex. Information from photoreceptor cells is transmitted and processed in this sequence. The lobula complex consists of two parts, Lobula and Lobula plate, that perform information processing in parallel. The four neuropils mentioned above possess columnar structures, which are stratified in the vertical direction. In these columns, the constituent neurons are arranged two-dimensionally as the photoreceptor cells are, in a retinotopic (or visuotopic) fashion, similar to arrangements in the mammalian visual system. Visual information processed in the optic lobe is transmitted to higher protocerebral areas and integrated with other information, for example from the ocellar system and mechanoreceptive systems, in particular those sensitive to air currents (wind). Then the integrated information is transmitted further to motor circuits in the thoracic ganglia and employed for functions such as head movements and flight control.



Optic lobe function

Lamina (=lamina ganglionaris)
The lamina’s main function in visual information processing appears to be contrast enhancement. Lamina monopolar neurons, second-order visual neurons that receive direct input from photoreceptors (=retinula cells), which invoke excitatory responses when at the center of the receptive field of a monopolar cell but inhibitory responses when in the surround receptive field. This mechanism is called lateral inhibition and was first observed in the Limulus ventral eye, although it is a general feature probably found in all eyes used for image perception. Due to this mechanism, the lamina monopolar cells show only small responses to illumination of the whole eye but much larger responses when patterns are presented that predominantly excite a monopolar cells receptive field center.

Medulla
Some neurons in the medulla show excitatory responses when a presented object is moving in one direction but inhibitory responses when the object is moving in the opposite direction. Such neurons respond to the movement of an object within a very small area hence are called local motion detector neurons (I don’t know the name of such neurons). They are arranged orderly in the whole medulla so that an object moving in any direction can be detected by a subset of these neurons. These cells are supposed to be at the basis of movement detection in all areas of the visual field by insects. It has been reported that the medulla also processes color information in the bumble bee.

Lobula
In contrast to lamina and medulla, which process information from small areas of the visual field, the lobula integrates information from large areas of or the entire visual fields to abstract visual information from the whole retinal input. For instance, the lobula system can identify object shapes.



Neurons in the optic lobe

There are three classes of neurons that form the neural circuits in the optic lobe: photoreceptor neurons, intrinsic neurons and projection neurons. Intrinsic neurons arborize within the optic lobe and projection neurons connect optic lobe and central brain. Projection neurons are further classified into two types: centripetal neurons projecting from the optic lobe to the central brain and centrifugal neurons projecting from the central brain to an optic lobe (centrifugal neurons are less commonly reported). Neuron in the optic lobe has been investigated in great detail in Drosophila. Therefore, we describe neurons in the optic lobe of Drosophila as a representative example, despite the fact that there are significant differences in eye design between Diptera and other insects (and arthropods). Even in Drosophila the description and identification of neurons in the optic lobe is still incomplete. These two shortcomings should be considered when using the information in this section.

Lamina
Lamina intrinsic neurons (amacrine cells) are neurons whose entire processes are confined to the lamina. Photoreceptor neurons (R1-6) extend their axons to a lamina [It may be necessary to explain that R1-6 are part of the neural superposition system, maybe in a “retina” section?]. Some of photoreceptor neurons (R7,8) extend their axons to the medulla, bypassing the lamina. Lamina monopolar neurons extend their axons to the medulla through the external optic chiasm [explain that visual field is inverted by the chiasm?]. Lamina wide field neurons (La wf 1, 2) and T1-cells, both of which have their somata in a soma layer between the lamina and the medulla have axonal projections within the lamina. C2, 3 centrifugal neurons have axonal projections though the medulla and into the lamina. Lamina tangential neurons, and lamina projection neurons are centripetal neurons with axonal projections in the protocerebrum (Fischbach and Dittrich, 1989).

Medulla
Medulla intrinsic neurons, distal medulla (Dm) and proximal medulla (Pm) neurons are local neurons of the medulla. Two types of photoreceptors (R8, 9) [not 7,8???] and lamina monopolar neurons (L1-5) extend their axons into the medulla. These neurons project to different strata of the medulla. Transmedullary neurons (Tm1-26) project to the lobula through the internal (inner? Second?) optic chiasm. Transmedullary Y-cells (Tm1-12) also extend their axons to lobula and lobula plate through the internal [see before] optic chiasm. As input neurons to the medulla, T-cells (T2a, b, T3, T4) extend their axons into the medulla. The types T2 and T3 have collaterals in the lobula, also. Somata of the T-cells are located in the zone between the medulla and the lobula complex. Y-cells (Y) also extend their axons to both medulla and Lobula. Somata of Y-cells are located in the rind of (or surrounding?) the Lobula plate. Medulla tangential neurons (Mt1-11) are centripteral neurons and extend their axons to the protocerebrum or into the contralateral optic lobe. It has been reported that some neurons of the medulla have axonal projections in the mushroom body.

Lobula plate
Transmedullary Y neurons (TmY1-12) project to the lobula plate. Lobula plate amacrine neurons (Lpi) are intrinsic to the lobula complex. Y-neurons have axonal projections in both medulla and lobula. T5-cells connect lobula and lobula plate. Somata of T-cells are located in between the Lobula and Lobula plate. The information may thus flow from Lobula to Lobula plate, implying that parallel processing is not fully independent between these neuropils. Lobula-complex columnar neurons (Lccn1, 2) have axonal arborisations in the lobula. HS (horizontal system) and VS (vertical system) neurons are electrophysiologically well investigated especially in larger flies (Musca, Calliphora) and are a part of the optomotor pathway.

Lobula
Lobula intrinsic (Li) neurons have all their processes within the lobula. Transmedullay neurons (Tm1-26) and transmedullary Y neurons (TmY1-12) have axonal projections inthe Lobula. Y-neurons (Y) and T-cells (T5a-d) extend their axons from Lobula plate to Lobula (is that axons in both or axons to the lobula?). Lobula columnar neurons (Lcn6-8) are centrifugal and extend their axons to the protocerebrum (and thus cannot be centrifugal!!!). Lobula-complex columnar neurons (Lccn1, 2) innervate in both Lobula and Lobula plate and extend their axons to the protocerebrum. Some neurons of the Lobula extend their axons to the mushroom body (Heisenberg, 1998, 2003).



Neurotransmitters

Acetylcholine, GABA, Glutamate, Taurine, Dopamine, Noradrenaline, serotonin, octopamine and histamine have been detected in the insect visual system and all appear to act as transmitters or neuromodulators (see review Nässel, 1991). Photoreceptor neurons (R1-6, 8) are probably histaminergic in larger files (Hardie, 1988) . Histamine acts on a chloride channel in lamina monopolar neurons (Geng et al., 2002). Histamine is also detected in cockroach photoreceptors (Pirvola et al., 1988). R7 photoreceptor neurons in Drosophila are probably GABAergic (Datum et al., 1986). In the lamina of blowfly columnar C2 centrifugal neurons (originating in the medulla) are GABA- immunoreactive (Datum et al., 1986; Meyer et al., 1986). In the medulla C2-neurons contain GABA (Nässel, 1991). In the lobula at least four types of GABA- immunoreactive neurons are present (Nässel, 1991). Four neurons inervating the lobula plate are GABA-immunoreactive (Nässel, 1991). Similarly, the optic lobe of the honey bee and the sphinx (maybe it is tobacco hornworm?) moth contain large numbers of GABA-immunoreactive neurons (Meyer et al., 1986; Schäfer and Bicker, 1986: Homberg et al., 1987). In the lamina of Drosophila possibly L1 and L2 monopolar neurons and the centrifugal C2 neurons show choline acetyletransferase (ChAT), which is a suitable marker for cholinergic neurons). In the medulla at least four layers are ChAT immunoreactive (Nässel, 1991). In the lobula four distinct layers are ChAT immunoreactive whereas in the lobula plate ChAT immunoreactivity is diffuse (Nässel, 1991). In the honey bee optic lobe acetylcholinesterase (AchE), the enzyme catalyzing acetylcholine hydrolysis, activity is seen intensely in the medulla and lobula (Nässel, 1991). Acetylcholine-like immunoreacitivity is seen on lamina monopolar neurons (Nässel, 1991). Glutamate-immunoreactive neurons are found in the optic lobe of the honey bee (Bicker et al., 1988). Lamina monopolar neurons are glutamate-immunoreactive. No other glutamate-immunoreactive neurons are found in the medulla. Some wide field lobula neurons are found to be glutamate-immunoreactive. Lamina monopolar neurons in Drosophila contain both acetylcholine vesicles and glutamate vesicles. It is still a matter of debate which is used as the neurotransmitter (Kolodziejczyk et al., 2008). In the blowfly about 20 amacrine neurons in the lobula and a few amacrine neurons in the medulla are serotonin-immunoreactive (Nässel and Klemm, 1983;; Nässel and Elekes, 1984; Nässel, 1988). In the optic lobe of Drosophila the antennal lobe octopaminergic neurons project to the medulla, lobula and lobula plate (Sinakevitch and Strausfeld, 2006). In sphinx moth Manduca sexta it has been reported that centrifugal neurons projecting to the lamina, medulla, and lobula contain octopamine, FMRFamide and SCPB (Homberg and Hildebrand, 1989). There is also evidence that dopamine and NO may act as neurotransmitters or neuromodulators in the insect visual systems (Nässel, 1991; Settembrini et al., 2007).



Reference


Bicker G, ShäferS, Ottersen, OP, Storm-Mathisen J. (1988) Glutamate-like immunoreactivity in identified neuronal populations of insect nervous systems. J Neruosci. 8: 2108-2122.

Datum K.-H, Weiler R, Zettler R. (1986) Immunocytochemical demonstration of g-amino buturic acid and glutamic acid decarboxylase R7 photoreceptors and C2 centrifugal fibers in the blowfly visual system. J Comp Physiol. 159: 241-249.

Fischbach KF, Dittrich APM. (1989) The optic lobe of Drosophila melanogaster. I. A Golgi analysis of wild-type structure. Cell Tissue Res. 258, 441-475.

Geng C, Leung H.-T, Skingsley DR, Iovchev MI, Yin Z, Semenov EP, Burg MG, Hardie RC, Pak WL. (2002) The target of Drosophila photoreceptor synaptic transmission is a histamine-gated chloride channel encoded by ort (hclA)* J Biol Chem. 277: 42113-42120.

Heisenberg M. (1998) What do the mushroom bodies do for the insect brain? an introduction. Learn Mem.5:1-10.

Heisenberg M. (2003) Mushroom body memoir: from maps to models. Nat Rev Neurosci. 4(4):266-75.

Homberg U, Kingan TG, Hildebrand JG. (1987) Immunocytochemistry of GABA in the brain and suboesophageal ganglion of Manduca sexta. Cell Tissue Res. 248: 1-24.

Homberg U, Hildebrand JG. (1989) Serotonin immunoreactivity in the optic lobes of the sphinx moth Manduca sexta and colocalization with FMRFamide and SCPB immunoreactivity. J Comp Neurol. 8;288(2):243-53.

Kolodziejczyk A, Sun X, Meinertzhagen IA, Nässel DR. (2008) Glutamate, GABA and acetylcholine signaling components in the lamina of the Drosophila visual system. PLoS One. 7;3(5):e2110.

Meyer EP, Matute C, Streit, P, Nässel DR. (1986) Insect optic lobe neurons identifiable with monoclonal antibodies to GABA. Histochemie 84:207-216.

Nässel DR. (1988) Serotonin and serotonin-immunoreactive neurons in the insect nervous system. Prog. Neurobiol. 30: 1-85.

Nässel DR. (1991) Neurotransmitters and neuromodulators in the insect visual system. Prog Neurobiol. 37(3):179-254.

Nässel DR,Elekes K. (1984) Ultrastructural demonstration of serotonin- immunoreactivity in the nervous system of an insect (Calliphora erythrocephala). Neurosci Lett. 48: 203-210.

Nässel DR, Klemm N. (1983) Serotonin-like immunoreacitivity in the optic lobes of three insects. Cell Tissue Res. 232: 129-140.

Mizunami M. (2006) Konchu -kyouinobishounou  Chuko shinsho.

Pirvola U, Tuomisto L, Yamatonani A, Panula P. (1988) Distribution of histamine in the cockroach brain and visual system: an immunochytochemical and biochemical study. J Comp Neurol. 276: 514-526.

Settembrini BP, Coronel MF, Nowicki S, Nighorn AJ, Villar MJ. (2007) Distribution and characterization of nitric oxide synthase in the nervous system of Triatoma infestans (Insecta: Heteroptera). Cell Tissue Res. 328(2):421-30.

Sinakevitch I, Strausfeld NJ. (2006) Comparison of octopamine-like immunoreactivity in the brains of the fruit fly and blow fly. J Comp Neurol. 494(3):460-75.

Shafer S, Bicker G.(1986) Distribution of GABA-like immunoreactivity in the brain of the honeybee. J Comp Neurol. 246: 287-300.

Takemura SY, Lu Z, Meinertzhagen IA. (2008) Synaptic circuits of the Drosophila optic lobe: the input terminals to the medulla. J Comp Neurol. 509(5):493-513.


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