The CS Pathway

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In honeybees, odors activate chemoreceptors on each antenna, which relay signals to the antennal lobes, where odor characteristics are neurally encoded (Lachnit et al. 2004; Flanagan and Mercer 1989) (fig. 3.1). The projection neurons of the antennal lobe form three main tracts, one of which innervates the calyces ofthe mushroom bodies. This projection from the antennal lobe to the mushroom bodies serves as the CS pathway for conditioning ofthe proboscis extension response (PER). Menzel and Miiller (1996) suggest that acetylcholine is the neurotransmitter in the CS pathway from the antennal lobes to the mushroom bodies because acetylcholine antagonists disrupt conditioning of the PER without disrupting olfactory perception (Cano Lozano et al. 1996; Gauthier et al. 1994). This result indicates that acetylcholine antagonists do not impair PER conditioning simply by eliminating the incoming olfactory CS from the antennal lobe, but instead disrupt the CS signal at a later stage of processing.

Figure 3.1. Schematic diagram of the CS and US pathways for olfactory conditioning in the honeybee. The olfactory CS detected by the antenna is relayed to the antennal lobe (AL) and then by acetylcholine-containing projections to the lateral protocerebral lobe (LPL) and the calyx (c) of the mushroom body (MB). The sucrose US detected at the proboscis is relayed to the subesophageal ganglion (s) and then by the octopamine-containing VUMmx1 nerve to the antennal lobe, the lateral protocerebral lobe, and the calyx of the mushroom body. The mushroom body, antennal lobe, and lateral protocerebral lobe are all bilateral structures that occur on both sides of the brain.

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Figure 3.1. Schematic diagram of the CS and US pathways for olfactory conditioning in the honeybee. The olfactory CS detected by the antenna is relayed to the antennal lobe (AL) and then by acetylcholine-containing projections to the lateral protocerebral lobe (LPL) and the calyx (c) of the mushroom body (MB). The sucrose US detected at the proboscis is relayed to the subesophageal ganglion (s) and then by the octopamine-containing VUMmx1 nerve to the antennal lobe, the lateral protocerebral lobe, and the calyx of the mushroom body. The mushroom body, antennal lobe, and lateral protocerebral lobe are all bilateral structures that occur on both sides of the brain.

Neural signals triggered by activation of chemoreceptors on the antennae thus deliver information about the odor of a nectar source to Kenyon cells of the mushroom bodies via projections from the antennal lobe (Mobbs 1982).

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