Figure 2.

Schematic diagrams of the mushroom bodies and their connections. A, A schematic diagram of the Drosophila brain (shown in gray) with the mushroom body and antennal lobe of one hemisphere shown in green and blue, respectively. B, An expanded view of the boxed region of A. Kenyon cell (KC) bodies are represented as small green circles. KCs extend dendrites to the calyx, and extend axons down the peduncle to α/β, α′/β′ and γ lobes. Olfactory sensory information is transmitted to the calyx and the lateral horn (LH) from the antennal lobe (AL) via projection neurons, and reward and punishment information has been proposed to be transmitted to the mushroom body lobes via PAM and PPL1 dopaminergic neurons. Mushroom body output neurons (MBONs) transmit signals from the mushroom bodies to various other brain regions. C, Summary of the Heisenberg model. Odor information from the AL stimulates a subset of MB Kenyon cells represented as large green circles. Odor‐activated synaptic connections are shown as small blue circles. Kenyon cells connect to MBONs that influence conditioned approach or avoidance responses. However, odor‐dependent activation of KCs is not sufficient to activate approach or avoidance programs in naïve animals. When odor pathways are activated at the same time as aversive electrical shock pathways (shown as activating synapses represented by red circles), or appetitive reward pathways (not shown), the combined CS and US activation strengthens synaptic connections between odor‐encoding KCs and appropriate avoidance or approach‐inducing MBONs (filled green circles). Due to this synaptic strengthening, when flies are re‐exposed to the odor, they activate appropriate motor response pathways. (Modified from Heisenberg et al., 2003)12