Figure 8.
Schematic of information flow to and from the Drosophila mushroom body. Olfactory and gustatory stimuli are processed in the antennal lobe and the gustatory center of the subesophageal ganglion (sog), respectively. The feedback loop (behavior) indicates that motor actions feed back as new sensory stimuli. Projections from the antennal lobe convey information to the lateral horn (via iACT and mACT) and to the mushroom body (predominantly via iACT, with a few fibers via mACT). Projections via the oACT and broad root (BR) send information to the inferior lateral/medial protocerebra. Projections from the gustatory center join the iACT (Ito, unpubl.). Kenyon cells of the mushroom body supply thick axons to γ and α′ lobes, and slender axons to β and α. The two types of Kenyon cells are biochemically heterogeneous (represented by different shades). Extrinsic neurons form presynaptic (small circles) or postsynaptic (small rectangles) connections with the Kenyon cell fibers. The direction of information is shown with a small arrowhead. Note that each extrinsic neuron does not necessarily interact with all the Kenyon cell fibers, and the strength of each connection is unlikely to be the same. Efferent and afferent neurons (1–4) connect the medial (β and γ) lobes with surrounding mushroom body-linked neuropils; e.g., the anterior portion of the i m pr. Input to the medial lobes is also likely to be provided from the ventral body (5), although the direction of signals is yet unknown. Interior neurons (6) connect, or are common to, the vertical (α and α′) and medial lobes. Relatively few extrinsic neurons link vertical lobes to the surrounding mushroom body-linked neuropils (anterior portion of the s m pr and s l pr). Some of them (7) connect both vertical and medial lobes with the surrounding neuropil. A few neurons (8, 9) connect the lateral horn and the head of the α lobe; these neurons also have projections in the s m pr near the lobe head. Mechanosensory and visual information is sent to the lateral horn and other protocerebral regions, from where signals might be relayed to the mushroom body-linked neuropils and then to the mushroom body lobes. Actual projections between these areas remain unknown. These mechanosensory and visual neuropils also have direct connections to the posterior slope, which contains premotor neuropil supplying descending neurons to the subesophageal ganglion and thoracic–abdominal ganglia (ventral nerve cord). Although suggested (dotted line), direct projections from the ant lob via the broad root (BR: Power 1946) to the posterior slope have not been observed. The posterior slope receives fibers from protocerebral neuropils (thick arrow), but little is known about their origin. Connections within and between protocerebral neuropils are left essentially uninvestigated (within area designated as containing mushroom body-linked neuropils, lateral horn, and other protocerebral regions). Information from the mushroom bodies is sent to these neuropils, rather than directly to the posterior slope.