Mushroom Body Evolution

Mushroom Body Evolution - Figure 15

Click on inline image to retrieve larger version (approx. 208k)

Figure 15: Relationships amongst mushroom body neuropils and other brain areas. Central neuropils (mauve) such as the mushroom body, protocerebrum, central body complex have diverse and complex connections with other parts of the nervous system.

Round-cornered boxes: sensory organs grouped according to modality.
Blue-colored boxes: neuropil regions relatively well investigated.
White boxes: regions essentially uninvestigated.
Thick lines: major tracts connecting neuropil regions.
Arrows: supposed direction of information (omitted where evidence for polarity is lacking).

Information about ambient light intensity is received by the ocelli, preprocessed in the ocellar neuropil and sent to the caudal part of the deutocerebrum (posterior slope) via the ocellar nerve (ocellar n). Visual information is received by the compound eyes and processed in nested retinotopic optic lobe neuropils (lamina, medulla, and lobula/lobula plate) before entering the central brain. The ventro-lateral protocerebrum (v l pr) receives visual signals almost exclusively, and thus can be considered to contain the primary optic foci. Other areas (e.g., lateral horn, l ho; superior lateral protocerebrum, s l pr; inferior lateral protocerebrum, i l pr) are also supplied by the optic lobes, but they also receive inputs from other sensory neuropils. A direct connection from the medulla and lobula to the calyces is observed in certain Hymenoptera (dashed arrow). Certain efferents from the lobula/lobula plate project directly to premotor neuropil of the posterior slope.

Mechanosensory receptors on the antennae (including Johnston's organ supplying acoustic information in flies and mosquitoes) enter the brain (via the antennal nerve, ant n) terminating in the antennal mechanosensory neuropil (dorsal lobe) of the deutocerebrum. Mechanosensory axons from the head and proboscis project to specific neuropils in suboesophageal ganglia (sog) via labial (lb n), pharyngeal (phy n), and accessory pharyngeal (ac phy n) nerves. Mechanosensory axons from the body and extremities (wings, legs, and genitalia) project to defined regions in the respective thoracic or abdominal neuromere (vnc: ventral nerve cord). Connections with somatic motor circuits provide appropriate local computations for reflexive motor actions (see Burrows, 1992). Tactile and acoustic relays (in crickets, grasshoppers, certain Diptera) reach the brain via the cervical connective (cv con) and median bundle (m bdl) to reach superior medial protocerebrum (s m pr).

The third antennal segment and maxillary palps detect airborne (olfactory) chemical stimuli. The former sends axons through the antennal nerve (ant n) to the antennal lobe. Axons from the latter enter the suboesophageal ganglion via the labial nerve (lb n) and project to the antennal lobe, via the antenno-suboesophageal tract (AST). Olfactory receptor terminals have odortypic segregation to specific glomeruli (Rodriguez and Buchner, 1984; Rodriguez and Pinto, 1989; Stocker, 1994).

Contact (gustatory or taste) sensory neurons in the labial palps project via the labial nerve (lb n) to a gustatory center in the sog, which, unlike the antennal lobe, does not show prominent glomerular structures. Gustatory neurons from the ventral cibarial sense organ (VCSO) and the labral sense organ (LSO) project to the gustatory center via the accessory pharyngeal nerve (ac phy n), and those from the dorsal cibarial sense organ (DCSO) via the pharyngeal nerve (phy n). In Blattoidea and Orthoptera, a second glomerular neuropil, the lobus glomerulatus, receives inputs from the mouthparts (Ernst et al., 1977) and provides axons to the calyces via a parallel strand of the i ACT (Weiss, 1981). Gustatory neurons in the fore, middle and hind legs, wings, and female genitalia project to the thoracic and abdominal ganglia (vnc) via respective segmental nerves. It is likely that the somatic gustatory signal is conveyed to the brain via the cervical connective (cv con).

Projection neurons from the antennal lobe, lobus glomerulatus, and sog gustatory centers contribute to inner antennocerebral tracts (iACT). These project to the lateral horn (l ho) sending collaterals to the mushroom body calyx (ca). Projection neurons in the middle antennocerebral tract (mACT) project directly to the l ho with a small subset of mACT entering the pedunculus and terminating in the calyx. The outer antennocerebral tract (oACT) contains fewer fibers connecting the ant lob and inferior lateral protocerebrum (i l pr). The "broad root" (Power, 1946) is supposed to contain a few fibers that project posteriorly from the ant lob.

Vertical lobes of the mushroom bodies are connected with the anterior region of the superior medial and lateral protocerebra (s m pr and s l pr). Medial lobes are connected to the anterior part of the inferior medial protocerebrum (i m pr). The mushroom bodies receive multimodal sensory information from protocerebral regions to their lobes and send output back to the same neuropil regions.

The central complex is connected to many protocerebral regions but has no direct connection with the mushroom bodies nor receives direct information from any primary sensory neuropils. The ventral body is connected to various regions of the protocerebrum and has major connections with the central complex.

Mushroom bodies and the central complex possess easily identifiable structures. The surrounding neuropils (lateral horn, ventro-lateral protocerebrum, and superior and inferior medial/lateral protocerebra) show a much more ambiguous organization. Although often (erroneously) referred to as "diffuse neuropils," these areas do have characteristic but complex fibroarchitectures whose neural networks within and between them are scarcely known.

Behavior is accomplished by the organized contraction of muscles resulting in the further stimulation of sensory organs (indicated by feedback loop "behavior"). Muscles are innervated by motor neurons originating in the sog and in thoracic and abdominal ganglia. In the brain (three pre-oral supraoesophageal ganglia), only the deutocerebrum contains motor neurons, which control antennal movements. The brain's deutocerebrum, and parts of the protocerebrum, possess premotor neuropils that contain dendrites of descending neurons that supply somatic motor circuits. Details of connections between most brain regions and descending neurons are not yet known.

AAxxxxx