Brain and Optic Lobes

Nicholas J. Strausfeld

University of Arizona

Authors variously use the term brain either to include all neuropils located within the head capsule or, restrictively, to refer to only those neuropils (called preoral neuropils) that lie dorsal to the esophagus. These are considered to lie anterior to the mouth. Preoral neuropils are also known as the supra-esophageal ganglion, which comprises three fused ganglia: the protocerebrum, deutocerebrum, and tritocerebrum. The preoral brain of the larger Hymenoptera, such as the predatory wasp Pepsis thisbe, can contain well over a million neurons, with more than a third of a million neurons in each mushroom body. The extreme density of neurons packed into a small volume, and the likelihood that single nerve cells can be functionally divided into several discrete elements, suggests that the largest insect brains have impressive computational power.

The first definition of the brain includes neuropils of the subesophageal ganglion, which is composed of the fused ganglia from three postoral segmental neuromeres. These are located ventrally with respect to the digestive tract, as are ganglia of the thorax and abdomen. In most hemimetabolous insects, and in many paleopterans, the subesophageal ganglion is connected by paired circumesophageal commissures to the supraesophageal ganglion. In many crown taxa (those representing more recent evolved lineages) the subesophageal and supraesophageal ganglia are fused, as is the case in honey bees or the fruit fly Drosophila melanogaster, which is the taxon here used to summarize the major divisions of the brain (Figs. 1-6). A consequence of fusion is that tracts of axons that would otherwise form the circumesophageal commissures are embedded within a contiguous neuropil.

In insect embryos, the three preoral segmental neuromeres providing the proto-, deuto-, and tritocerebrum are contiguous

Drosophila Optic Lobe

FIGURES 1-6 Summary diagrams of the brain of the fruit fly D. melanogaster. The reader is referred to these searchable figures in the atlas of the Drosophila brain at http://www.flybrain.org/Flybrain/html/atlas/schematic/index.html. The higher centers of the mushroom bodies and central complex are shown in reds and oranges. Optic lobe regions are yellow. Antennal lobes are light green and their axon projections are dark green. The median bundle is shown in light blue. Other neuropil areas are gray/pink. (1) Posterior aspect, vertical section. According to the neuraxis (see Fig. 7), up is rostral in Figs. 1—3. (2) Middle aspect, vertical section, at the level of the central body and associated regions. (3) Frontal aspect, vertical section, at the level of the antennal lobes (green) and mushroom body lobes (red). Dark green profiles in Figs. 1—3 are the antennocerebral tracts. (4) Top-down view, looking onto the mushroom bodies and central complex. One mushroom body only is shown to the left, with the antennocerebral tracts from the antennal lobes to the lateral protocerebrum shown to the right. The front of the brain is down, the rear of the brain is up. (5) Top-down view of the deutocerebrum/tritocerebrum and the root of the ventral nerve cord. (6) Side-on view of the brain, emphasizing the ascending tracts (blue) from the subesophageal ganglion to the rostral protocerebrum via the medial bundle. Note the disposition of the mushroom body and central complex. Abbreviations used: a, a, ttc, (P, P',Pc, y) lobes, subdivisions of the mushroom body medial (p, p',pc, y) and vertical (a, a', ac) lobes; ant n, antennal nerve; ant lob, antennal lobe; a op tu, anterior optic tubercle (optic glomerulus); asc t vnc, ascending tracts of ventral nerve cord; ca, calyx of mushroom body; deu asc neu, deutocerebral neuropil receiving ascending terminals; d m pr, dorsal median protocerebrum; e b, ellipsoid body of the central complex; fb, fan-shaped body of the central complex; g d n, giant descending neuron (typifies descending pathways); inf l deu, inferior lateral deutocerebrum; i act, inner antennocerebral tract; int act, intermediate antennocerebral tract; l lob i pr, lateral lobe of the inferior protocerebrum; lat deu fasc, lateral deutocerebral fascicle; lab lob, labral lobe; lab com, labral commissure; lo, lobula; lo p, lobula plate; l ho, lateral horn; max su oes c, maxillary subesophageal connective; me, medulla; mech sens l deu, mechanosensory neuropil of the lateral deutocerebrum; mech sens, mechanosensory strand and neuropil supplied by the antennal nerve; m bdl, median bundle; no, noduli of the central complex; op lo eff, optic lobe efferents; ocl n, ocellar nerve; o act, outer antennocerebral tract; pr br, protocerebral bridge of the central complex; p l fasc, posterior lateral fascicle; p op fo, posterior optic focus (glomerulus); r act, root of antennocerebral tract; s a, superior arch of the central complex; s o g, subesophageal ganglion; s l pr, superior lateral protocerebrum; s m pr, superior median protocerebrum; s o g nerves, nerve bundles of subesophageal neuromeres; spur, spur of mushroom body; trito, tritocerebrum; tr str m bdl, tritocerebral strand of the median bundle; trito r m bdl, tritocerebral root of the median bundle; VS, HS, axons of giant vertical and horizontal cells (movement sensitive neurons); vs ax, visual interneuron axons; v bo, ventral body (also known as lateral accessory lobes); v sat MB, d sat MB, ventral and dorsal satellite neuropils of the mushroom bodies.

with the three postoral neuromeres that will give rise to the subesophageal ganglion. These neuromeres are, in turn, contiguous with fused neuromeres of the thorax and abdomen. In many species of hemimetabolous insects, such as locusts and cockroaches, the sub- and supraesophageal ganglia separate postembryonically and are connected by paired tracts. In cockroaches, each segmental ganglion is separate from the next, except for the last three abdominal ganglia, which are specialized to serve receptors of the cerci and contain the dendrites of giant ascending neurons and local networks of interneurons that mediate escape reactions. However, in many holometabolous insects there are various degrees of ganglion fusion, one of the most extreme being in certain Heteroptera such as the water strider Gerris sp. In Gerris, the supraesophageal, subesophageal, and thoracic—abdominal ganglia comprise a contiguous mass perforated by the gut. This arrangement is reminiscent of the nervous systems of another group of arthropods, the chelicerates. In adult cyclorrhaphan flies the three thoracic ganglia and all abdominal ganglia are fused into a single mass connected to the sub- and supra-esophageal ganglion by long neck connectives (this has also been achieved in the nervous systems of crabs).

The subesophageal ganglion, which comprises the mandibular, maxillary, and labial neuromeres, has a ground pattern organization comparable to that of the thoracic and abdominal ganglia. The roots of motor neurons (the exit point of motor neuron axons) are generally dorsal with respect to incoming sensory axons. This arrangement is the opposite of that in the vertebrate spinal cord.

The names of the subesophageal ganglia reflect the appendages that their motor neurons control and from which they receive sensory supply. However, this relationship is not a strict one. For example, in flies mechanosensory neuropil extending into the subesophageal ganglion also receive afferents from mechanosensilla on the head, including between facets of the compound eyes, around the margin of the eyes, the frons, between and flanking the ocelli, and at various positions on the rear of the head capsule. As on the thorax and abdomen, or on the limbs, wings, and halteres (modified wings in Diptera that are organs of balance), sensilla on the head provide receptor neuron axons to defined locations in their target ganglia. Principles underlying the development and organization of the central representation of sensilla are best known from Walthall and Murphey's 1988 studies of cricket cerci or studies on the central projections of receptors to discrete regions of the thoracic ganglia of dipterans, also by Murphey and colleagues in 1989. In flies, groups of receptors encoding different modalities at a segment supply axons to modality-specific regions within the ganglion. In such regions, the peripheral locations of receptors within a sensory field can be represented as a map of axon terminals onto the dendritic trees of postsynaptic neurons. Burrows and Newland have shown that such maps play important roles in the activation of the postsynaptic elements that participate in circuits controlling limb actions and position.

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