Physiological and anatomical properties of optical input-fibres to the mushroom body in the bee brain

More than 150 neurones in the nushroom body area of the bee brain were recorded and stained intracellularly with either Lucifer Yellow or Cobalt-Hexamminochloride (III). Among them 12 neurones have been characterized physiologically and anatomically which connect the medulla and the lobula with the mushroom bodies. All neurones responded to stationary or moving light stimuli exclusively. Movement-sensitive neurones were all direction-selective. Excitatory and inhibitory responses occurred in response to moving stripe patterns in the preferred and null directions respectively. Anatomically, the neurones could be clearly distinguished as belonging to three types depending on their input features in the optic lobes: (a) Neurones with small dendritic fields (up to 100 μm) in the lobula; (b) Neurones with large dendritic fields (up to 400 μm) in the lobula; (c) Neurones with small dendritic fields (up to 100 μm) in the medulla. The axons of all three cell types run from the optic lobes on each side to the outer ring tract around the pedunculus-calyx-transition and arborize in the collar region of the ipsilateral calyces. Additional branches invading the basal ring of the calyces had been observed; endings in the lip region were not found. The endings in the calyces often exhibited bleb-like specializations indicating their presynaptic nature. Retinotopic organization of the optic inputs into the calyces could not be proven. The results are compared with the characteristics of multimodal mushroom body output fibres and are discussed in context with the complex information processing and storage functions ascribed to the mushroom bodies.