Optic Neuropil Research Articles

Goldfish retinotectal transmission in vitro: component current sink-source pairs isolated by varying calcium and magnesium levels

Field potentials and radial current source-densities (CSDs) evoked by optic tract stimulation were observed in goldfish tectum in vitro. The effects of different concentrations of calcium and magnesium were studied to improve understanding of component events in retinotectal transmission and interpretations of effects of pharmacological agents. Responses were of 3 forms: ‘non-synaptic’, ‘subthreshold’, and ‘complex’. Subthreshold responses occurred when amplitudes were less than 40% of their maximal levels. They consisted of a sink-source pair with the sink in the superficial optic neuropil and the source 100–150 μm deeper. They were monophasic, rising in 1–2 ms and decaying as simple exponentials with time constants between 3.3 and 4.5 ms. Several other conditions which reduce amplitudes (besides low [Ca 2+] and/or high [Mg 2+]) produce responses of the subthreshold form. Complex responses, observed when amplitudes were 40–100% of maximal, were characterized by more rapid rise and decay and included, in the most extreme cases, a late, long, low-amplitude sink-source pair of opposite polarity. We propose that the time course of decay of subthreshold responses is determined by the passive cable properties of bi- or multistratified neurons with one dendritic arbor in the optic neuropil and a second arbor 100–250 μm deeper. Complex responses probably include recurrent inhibition to depolarized dendrites of the optic neuropil, latent by 4 ms to the monosynaptic excitation. Pharmacological assessments of retinotectal transmission may be made more precise by using low [Ca 2+]/high [Mg +] media to isolate monosynaptic activity.

The locus of optic nerve head representation in the retinotopic projection over nucleus geniculatus lateralis ventralis and nucleus griseum tectalis in the chick also lacks a retinal projection.

Recently we have demonstrated the presence of a gap in the avian retinotectal projection, that corresponds with the locus of retinotopic representation of the elongated optic nerve head. The present report describes an equivalent gap in the optic neuropiles of the avian ventral geniculate nucleus and griseum tectalis formation, detected after filling optic terminals anterogradely from the contralateral eye with peroxidase. A projection-less strip appears at the expected retinotopic position in both grisea intersecting radially all the strata of the corresponding neuropiles. It is speculated that a common synaptogenetic mechanism must account for these gaps.

Morphological and physiological characterization of individual olfactory interneurons connecting the brain and eyestalk ganglia of the crayfish.

1. In order to understand the functional organization of the crustacean olfactory system, we are using intracellular recording and staining techniques to correlate the structure and function of single, odorant-sensitive interneurons in the brain of the crayfish Procambarus clarkii. We describe here the anatomy and physiology of interneurons that connect the brain with the medullae terminales or other eyestalk ganglia. 2. All of the interneurons in our study are at least third-order olfactory neurons (second-order olfactory interneurons) because they respond to chemostimulation of the olfactory organ (the antennules) but do not branch in the olfactory lobe (the neuropil to which primary olfactory receptor cells of the antennules project). 3. Much of the central nervous system, including the three main divisions of the brain (protocerebrum, deuterocerebrum, tritocerebrum) and the medullae terminales, are involved in integrating olfactory or multimodal (including olfactory) information, since these areas contain neurites of olfactory interneurons. Previous studies have indicated that regions involved in such processing include the olfactory lobes and accessory lobes of the deuterocerebrum, and regions I, II, IV, and VII (in some species) of the medullae terminales. Our results show that also prominent among regions involved in olfactory or multimodal (including olfactory) integration are the anterior and posterior optic neuropils of the protocerebrum, the lateral and medial antennular neuropils of the deuterocerebrum, the tegumentary neuropils and the antennal neuropils of the tritocerebrum, and neuropils III, VI, XII of the medullae terminales. 4. These olfactory interneurons were sensitive to chemostimulation (unimodal), chemo- and mechanostimulation (bimodal), or chemo-, mechano-, and photostimulation (trimodal). Responses could be excitatory or inhibitory, even for a given neuron. Morphologically complex interneurons (those having bilateral branching) were more likely to have complex response characteristics (trimodal sensitivity) than were morphologically simpler interneurons (those having unilateral branching). Olfactory interneurons with a soma in the medulla terminalis showed the most complex response profiles: they were trimodal, and were exicted by odorants but were inhibited by touch and/or light. This finding suggests that these are complex, high order interneurons. 5. Our studies reveal that olfactory and other sensory information is transmitted between the brain and the medullae terminales (and possibly other eyestalk ganglia) by a coactivated, parallel array of structurally and functionally diverse neurons.

Postembryonic development of serotonin-immunoreactive neurons in the central nervous system of the blowfly, Calliphora erythrocephala

The postembryonic development of serotonin-immunoreactive (5-HTi) neurons was studied in the optic lobe of the blowfly. In the adult fly there are 24 5-HTi neurons invading each optic lobe. The perikarya of two of these neurons are situated in the dorso-caudal part of the protocerebrum (LBO-5HT neurons; large bilateral optic lobe 5-HTi neurons). The cell bodies of the remaining 22 neurons are located anteriorly at the medial base of the medulla (2 innervating the lobula, LO-5HT neurons; and 20 neurons innervating the medulla, ME-5HT neurons). The two central neurons (LBO-5HT neurons) are derived from metamorphosing larval neurons, while the ME- and LO-5HT neurons are imaginai optic lobe neurons differentiating during pupal development. The 5-HTi neurons of the optic lobe seem to have different ancestors. The LBO-5HT neurons are probably derived from segmental protocerebral neuroblasts, whereas the ME-and LO-5HT neurons are most likely derived from the inner optic anlage. The first 5-HTi fibers to reach the imaginal optic lobes are seen in the late third instar larva and are derived from the LBO-5HT neurons. The first ME- and LO-5HT neurons become immunoreactive at 24 h (10%) pupal development. At about 96 h (40%) of pupal development all the 5-HTi neurons of the optic lobes have differentiated and attained their basic adult morphology. The further development mainly entails increase in volume of arborizations and number of finer processes. The differentiation and outgrowth of 5-HTi processes follows that of, e.g., columnar neurons in the optic lobe neuropils. Hence, 5-HTi processes invade neuropil relatively late in the differentiation of the optic lobe.

Pharmacology of retinotectal transmission in the goldfish: effects of nicotinic ligands, strychnine, and kynurenic acid

The goal of this study was to evaluate different neurotransmitters and their receptors that might be involved in retinotectal transmission in the goldfish. Sections of tectum were isolated and maintained in vitro while pharmacological agents were administered via the tissue bath. Field potentials were elicited by electrical stimulation of the optic nerve and recorded at 50 micron depth intervals, and profiles of current source densities (CSDs) were computed from the second spatial derivatives of these potentials. The preparations were treated with low [Ca2+]/high [Mg2+] media, various cholinergic agonists and antagonists, eserine, strychnine, or kynurenic acid, via the tissue bath. Prior to treatment, depth profiles of these in vitro field potentials and CSDs closely resembled those previously reported in vivo, including 2 prominent sink-source pairs with their sinks in the superficial optic neuropil, followed by a smaller and more prolonged sink-source pair of opposite polarity. These were rapidly and reversibly eliminated by low [Ca2+]/high [Mg2+] bathing media, and substantially reduced by 0.5 or 1.0 mM kynurenic acid. By contrast, d-tubocurarine (d-TC; up to 0.16 mM) reduced peak response amplitudes by less than 40%, eliminated the third sink-source pair, and more than doubled the duration of decay of sink-source pairs 1 and 2 in a concentration-dependent manner. Strychnine had a similar action to d-TC but was slightly more potent. The time course and amplitudes of responses were not much affected by the following nicotinic agonists or antagonists (concentrations in microM): mecamylamine, 50; dihydro-beta-erythroidine, 50; nicotine, 200; tetramethylammonium, 500; ACh (protected by eserine, 20), 200; alpha-bungarotoxin, 2 microM for 2.5 hr, and 0.4 microM for up to 10.5 hr; and lophotoxin, 32 microM for up to 94 min. Eserine (20 microM) and carbachol (200 microM) increased peak response amplitudes by up to 80% within 5-10 min, and amplitudes remained elevated during 20-33 min of continued treatment. The onset of the effects of d-TC, strychnine, and kynurenic acid began in 5-10 min and was completed in 30 min or less, indicating that test substances could adequately penetrate into the interior of the isolated sections of tectum. The failure of these cholinergic ligands to prevent postsynaptic responses indicates that excitatory retinotectal transmission does not depend on an intact nicotinic (or other cholinergic) system, as previously proposed. The action by kynurenic acid suggests the involvement of an excitatory amino acid neurotransmitter in retinotectal transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

Postembryonic differentiation of serotonin-immunoreactive neurons in fleshfly optic lobes developing in situ or cultured in vivo without eye discs.

The differentiation of serotonin-immunoreactive (5-HTi) neurons in the optic lobes of fleshflies was studied during in situ development and in in vivo cultures. All 5-HTi neurons with cell bodies in the imaginal optic lobes differentiate during postembryonic (pupal) development. These are local anaxonal neurons. In addition there are two large 5-HTi bilateral neurons that connect all optic lobe neuropil regions on both sides of the brain and have their cell bodies in the midbrain proper. Deafferentation of optic lobes cultured in vivo leads to drastic reduction in optic lobe volume and increased cell death. All the 5-HTi neurons differentiate after deafferentation but their morphology changes. The neuropil receiving the photoreceptor inputs, the lamina, degenerates but a disorganized "pseudolamina" is formed by the processes of the two large 5-HTi neurons. The layering of the optic lobe neuropils cannot be distinguished and 5-HTi processes form novel projectional patterns. Hence, the 5-HTi neurons do not require afferent inputs from the retina for their differentiation and survival, but the effect on other optic lobe interneurons is reflected in the morphological plasticity of the 5-HTi neurons.

Color and spatial antagonism in the visual system of the larval swallowtail butterfly

Many neurons exhibiting an antagonistic interaction between signals from one particular stemma (center) and neighboring stemmata (surround) were found in the second optic neuropil (medulla) of the larval swallowtail butterfly. Furthermore, most of the neurons showed a color opponent interaction in the center: combinations of opponent colors were blue vs green, UV vs green, and blue vs UV + green. Thus, photoreceptor signals are integrated into chromatically and spatially antagonistic mechanisms, even in the invertebrate.

Evolutionary aspects on the construction of the first optic neuropil (lamina) in Crustacea

A Golgi study of the neuronal morphology of the first optic neuropil (lamina) in three anostracan species revealed a close similarity in the lamina construction and neuron types. The lamina architecture of decapod and mysid crustacean species, as revealed by the Golgi method, conformed to previous studies and differed from the anostracans. The comparison was made at the level of resolution comprising retinal projection on the lamina, structural entities such as columns and layers and neuron position, branching pattern and terminal fields. It was shown that different types of compound eyes and variation in the habitat of the animals were of less importance for the lamina design than common descent as expressed in the present taxonomic groups.

Distribution of GABA-like immunoreactivity in the brain of the honeybee.

The distribution of GABA-like immunoreactivity in the brain of the honeybee was investigated with antisera generated against GABA protein conjugates. The binding of the antisera in paraffin serial sections was studied with the peroxidase-antiperoxidase method. GABA-like immunoreactive fibers appeared in all main neuropile areas. The staining of the optic lobes showed pronounced stratification. The receptor cells of compound eyes, ocelli, and antennae were not labelled. Several prominent fiber tracts showed GABA-like immunoreactivity, whereas other tracts were devoid of staining. There are no major immunoreactive commissures linking the two brain hemispheres with the exception of small commissures that bridge short distances between the beta-lobes and the antennal lobes. Several fibers in the cervical connective were also labelled; some of those may descend from the suboesophageal ganglion to the thoracic ganglia. The dense reactivity seen in the optic and antennal neuropiles implies that GABA is more important in mediating local rather than more distant neural interactions.

Internuclear connections between the pretectum and the accessory optic system in Salamandra salamandra

Application of horseradish peroxidase into the posterior thalamic and basal optic neuropils of Salamandra salamandra (L.) revealed strong reciprocal connections between the pretectum and the accessory optic system. Pretectal neurons located within the periventricular gray matter project to the basal optic neuropil distributing their terminals over the whole extent of this neuropil. A well developed nucleus of the basal optic neuropil, with its neurons within and medial to this neuropil, projects to the posterior thalamic neuropil. Its terminals appear to be located selectively within the core of the posterior thalamic neuropil which receives no ipsilateral retinal afferents.

Retinofugal projections in salamanders of the family Plethodontidae

A comparison of the retinofugal projections in 14 species of plethodontid salamanders by means of the horseradish peroxidase (HRP) technique revealed almost identical contralateral projections. In all species studied three optic tracts were found. Behind the chiasma opticum the basal optic tract runs to the peduncle region, there forming the basal optic neuropil. The marginal optic tract courses from the chiasma over the thalamus to the tectum opticum where it covers the entire surface. In the anterior thalamus the marginal optic tract innervates the neuropil Bellonci-pars lateralis and the corpus geniculatum thalamicum, and more caudally the neuropil posterior thalami. The medial optic tract supplies the neuropil Bellonci-pars lateralis and pars medialis in the anterior thalamus from where it runs medial to the marginal optic tract as a separate tract to the uncinate field in the posterior thalamus. The ipsilateral projections show differences among the species studied, although the global organization remains constant. The differences mainly concern the marginal optic tract which varies from being weakly labeled and restricted to the rostral part of the tectum opticum, to being heavily labeled and innervating the entire tectum to its caudal edge. Species with the heaviest ipsilateral projections all belong to the plethodontid tribe Bolitoglossini, all of which show direct development, a highly projectile tongue, rather frontally oriented eyes and excellent depth perception. In these species the thalamic ipsilateral projection areas are equal in size and shape to the contralateral one. The ipsilateral projections to the tectum show two distinct layers, a superficial and a deep one, which intermingle with the contralateral projections. The two other ipsilateral tracts do not differ significantly among the plethodontid species: the medial optic tract is always heavily and the basal optic tract always weakly labeled.

Immunocytochemical demonstration of?-amino butyric acid and glutamic acid decarboxylase in R7 photoreceptors and C2 centrifugal fibres in the blowfly visual system

Neurons within the compound eye of the flyCalliphora erythrocephala, suspected of containing gamma-aminobutyric acid were revealed immunocytochemically, using antibodies directed against gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). The GABA content within putative GABAergic neurons was increased by high affinity uptake of GABA and selective blocking of GABA metabolism with Gabaculine. Only neuronal populations which were labelled with the GABA as well as the GAD antibodies were presumed to be GABAergic. The first optic neuropil (lamina) exhibited two distinct GA-BAergic fibre populations amongst a larger population comprised of fourteen cell classes. One fibre population was formed by the axons of the photopic photoreceptors R7 which pass through the lamina and terminate in the second optic neuropil (the medulla). The identity of R7 was established from longitudinal and transverse sections of the retina where R7 can be unequivocally distinguished from the six scotopic photoreceptors R1-6 and the other photopic receptor, R8.

Factors guiding regenerating retinotectal fibres in the frog Xenopus laevis

I have examined the pathways of retinotectal fibres regenerating back to the contralateral tectum, and also to innervated and 'virgin' ipsilateral tecta in postmetamorphic Xenopus. The fibres were visualized by HRP labelling of either the whole optic nerve, or a selected quadrant of the retina. Most fibres grow into either the ipsilateral or contralateral optic tract, although a small proportion go down the outside of the contralateral optic nerve. In the tracts, many fibres grow superficially, close beneath the pia, but a variable proportion runs more deeply. Axonal growth is not, therefore, restricted absolutely to the subpial region in the postmetamorphic Xenopus brain. Fibres growing onto the contralateral, or a 'virgin' tectum mostly grow straight onto the rostral margin of the tectal lobe, without growing around its margin in the form of a medial or lateral brachium. Most of these fibres grow through the deeper part of the tectal layer which normally contains optic neuropil, but a proportion of them grow immediately deep to the pia. Fibres regenerating to an innervated ipsilateral tectum mostly enter either the medial or lateral brachium of the optic tract, and only leave this close to their site of termination. In the brachia the fibres run superficially under the pia, but when they leave the brachia they mostly run through the deeper retinorecipient layers. These observations provide further evidence that ingrowing optic fibres have their pathways influenced by the axons which have preceded them.

Visual projections in larval Ichthyophis kohtaoensis (Amphibia: gymnophiona)

The visual projection patterns of retinal efferents were studied in larval Ichthyophis kohtaoensis by means of anterogradely transported HRP. Our results show in all larvae a projection contralateral to a thalamic terminal field, a pretectal terminal field, and a basal optic neuropil, but only a sparse innervation of the contralateral tectum. In addition, all larvae possess an uncrossed projection to a thalamic and a pretectal terminal field. The fibers are bilaterally almost confined to the medial optic tract with only a few fibers running in the marginal and basal optic tract. The ipsilateral and contralateral tracts and terminal fields seem to enlarge during larval life. Comparison with other amphibian orders reveals that larval Ichthyophis are unique in that they develop the medial optic tract and the related thalamic and pretectal terminal fields very early in larval life. In addition they possess only a very sparse tectal projection, though it is the largest projection in larval urodeles and anurans. This suggests a selective phylogenetic loss of those ganglion cells or collaterals which project mainly to the tectum in other amphibian orders and a change in the ontogenetic program leading to an earlier development of the medial optic tract in Ichthyophis as compared to urodeles and anurans.

Structural organization of neurosecretory cells terminating in the sinus gland of the terrestrial isopod, Oniscus asellus, revealed by paraldehyde fuchsin and cobalt backfilling

The morphology of the brain – sinus gland neurosecretory system in the terrestrial isopod, Oniscus asellus, is described with paraldehyde fuchsin and with cobalt backfilling of the sinus gland. Paraldehyde fuchsin stained the A, B, and β cells located medially in the protocerebrum and the γ cells located in the optic lobe. Cobalt applied to the sinus gland delineates an axon tract that extends from the sinus gland medially along the posterior surface of the protocerebrum and descends into the protocerebrum at the level of the central protocerebral neuropile. Cobalt backfilled to the B, β, and γ cells but not to the A cells. One cell group located distally to the most distal optic lobe neuropile filled with cobalt, but was not stained with paraldehyde fuchsin. It is argued that the B and β cells together comprise the equivalent of the decapod "X-organ." Varicosities, which may represent additional storage and (or) release sites for neurosecretion, appear in the axon tract over the region of the optic lobe. Extensive dendritic arborizations of the B and β cells occur along the anterior-medial side of the central protocerebral neuropile. Additional arborizations of these cells occur in the contralateral protocerebral lobe, suggesting a pathway for neural coordination of left and right sinus glands. Further observations on changes in the staining properties of the β and γ cells during the moult cycle suggest the involvement of β cells with moulting and the involvement of γ cells with egg development.

Presynaptic inhibition in the crayfish brain

1. Ascending interneurons were functionally identified and injected with Lucifer Yellow and horseradish peroxidase. The distribution of their axon terminals in the supraesophageal ganglion was examined with light and electron microscopy 2. The terminals of all cells examined invade the ipsilateral parolfactory neuropil. Most of the cells also project processes to the ipsilateral antennal and/or optic neuropil and a few cells terminate in the optic ganglia. Thus ascending interneurons may distribute their actions to all three neuromeres of the brain and to distal elements of the visual pathway (Figs. 1–3). 3. The axons and secondary processes of ascending interneurons are distinguished by the presence of microtubules. Both processes exhibit protuberances (Fig. 5) which in the electron microscope have all the features of subsynaptic zones (Fig. 6). 4. HRP labeled processes in the synaptic neuropil region contain small round vesicles and large dense core vesicles (Fig. 7). The two vesicle types are segregated. Only the small round vesicles are found at synaptic release sites. 5. Presynaptic sites containing small round vesicles (Fig. 8) are identifiable along the secondary and tertiary process of the HRP labeled ascending axon terminals. We infer that these synapses are the basis of the excitatory functional connection between ascending and descending interneurons. 6. Vesicle-containing neural processes in the antennal and antennule neuropils make axo-axonic contacts with the HRP labeled ascending cell terminals (Figs. 6 and 9). A preponderance of these synapses contain elongated vesicles which imply an inhibitory function. We infer that these axoaxonic synapses are the structural correlates of the functional inhibitory inputs to ascending interneuron terminals. 7. The axo-axonal synapses are principally located on the terminal arborization between the axon proper and the release sites of the ascending interneuron. This location is consistent with inhibitory action on events intermediate between the axonal action potential and the transmitter release mechanism.

Retinal degeneration in rdgA mutants of Drosophila melanogaster meigen (diptera : drosophilidae)

To better understand the roles of particular defective genes affecting the Drosophila (Diptera : Drospphilidae) visual system and the ensuing cell biology they order, we utilized 2 mutant alleles of the rdgA gene, namely rdgA PC47 and rdgA BS12 . Each allele causes the expression of a different pattern of retinal degeneration. We examined ultrastructurally the time course and cell specificity of degeneration. The rdgA PC47 mutant has a relatively normal compound eye and lamina ganglionaris (1st optic neuropile) upon pupal emergence. Photoreceptors, especially R1–6, degenerate in one week and their terminals in the lamina are largely phagocytosed by glia. R7/8 receptors and 2nd-order interneurons are somewhat spared by comparison. By contrast, the cytology of the photoreceptors of rdgA BS12 is abnormal upon eclosion. However, even though abnormal, R1–6 and R7/8 do project axons to higher centers; there are even some normal properties in the optic cartridges (synaptic glomeruli of R1–6 terminals) in the lamina. A week later, there is little further demise of R1–6 terminals in rdgA BS12 , revealing a surprising survival of structure in the lamina.

Monocular and binocular optic inputs to salamander pretectal neurons: intracellular recording and HRP labelling study.

Intracellular recordings and horseradish peroxidase injections were performed in the pretectum and adjacent tegmentum of Salamandra salamandra, while both optic nerves were electrically stimulated. In approximately half of the recorded units no spikes could be evoked but rather graded postsynaptic potentials. The latter type morphologically showed features of interneurons. From a total of 48 recorded units, nearly 60% were excited only by the contralateral optic nerve, whereas approximately 40% were binocular. For the most part (10/19) the binocular cells were excited by the contralateral and inhibited by the ipsilateral optic nerve. Fewer neurons (7/19) received excitatory inputs from both optic nerves. The latency distribution of the monocular cells shows a maximum of 20-30 ms. The same maximum exists for the contralateral inputs to the binocular cells, whereas the ipsilateral inputs to these units were nearly as frequent with latencies of 20-30 and 40-50 ms. Since neurons with the short ipsilateral latencies always had parts of their dendrites within the ipsilateral ocular projection field, a feature which was lacking in the cells with long ipsilateral latencies, it is possible that the longer latencies are due to indirect ipsilateral inputs. Efferents of labelled dorsal pretectal cells reach the contralateral pretectum via the posterior commissure, the basal optic neuropil of the accessory optic system and the tegmental white substance. More ventrally located cells often reach the pretectal and the basal optic neuropil with their dendrites. Axons of this type descend to the medulla oblongata via the medial longitudinal fasciculus.

Cellular and ultrastructural features of the adult and the embryonic eye in the marine gastropod, Ilyanassa obsoleta.

Light and electron microscopic techniques show that the eye of the marine prosobranch gastropod, Ilyanassa obsoleta, is composed of an optic cavity, lens, cornea, retina, and neuropile, and is surrounded by a connective tissue capsule. The adult retina is a columnar epithelium containing three morphologically distinct cell types: photoreceptor, pigmented, and ciliated cells. The retina is continuous anteriorly with a cuboidal corneal epithelium. The neuropile, located immediately behind the retina, is composed of photoreceptor cell axons, accessory neurons, and their neurites. The embryonic eye is formed from surface ectoderm, which sinks inward as a pigmented cellular mass. At this time, the eye primordium already contains presumptive photoreceptor cells, pigmented retinal cells, and corneal cells. Several days later, just before hatching, the embryonic eye remains in intimate contact with the cerebral ganglion. It has no ciliated retinal cells, neuropile, optic nerve, or connective tissue capsule and its photoreceptor cells lack the electron-lucent vesicles and multivesicular bodies of adult photoreceptor cells. As the eye and the cerebral ganglion grow apart, the optic nerve, neuropile, and connective tissue capsule develop.

Termination profiles of photoreceptor cells in the larval eye of the swallowtail butterfly.

Profiles of the axonal endings of seven retinular cells (R1-7) from each of six stemmata (I-VI) of the butterfly Papilio xuthus were examined. Their axons enter the brain anteroventrally and project dorsally along the brain's lateral surface before entering the optic neuropil. Larval optic neuropil has two distinct areas, a (distal) lamina and a (proximal) medulla, connected by a chiasma. Intracellular administration of cobalt shows that distal retinular cells (R1-3 of stemmata I-IV, R1-3 and R6 of stemmata V and VI) terminate as short axons with plug or toothbrush-like endings in the lamina. Proximal retinular cells (R4-7 of stemmata I-IV, R4, R5 and R7 of stemmata V and VI) terminate as long axons in the chiasma or in the medulla. The long axons have fine branches in the lamina, chiasma and medulla. The two proximal cells (R5 and R7) are distinguished by their deeper endings and longer branches. The terminals of photoreceptors correspond to their photoreceptive properties and provide further evidence for the functional specialization of the photoreceptors.