Anatomy of the mushroom bodies in the honey bee brain: The neuronal connections of the alpha‐lobe

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Abstract
Neural connections between the mushroom body (MB) and other protocerebral areas of the honeybee's brain were studied with the help of cobalt chloride and Golgi staining methods. Focal injections of cobalt ions into the α‐lobe neuropil of the MB reveal seven clusters of somata located in the protocerebrum and deutocerebrum of each brain hemisphere. These neurons connect the mushroom body neuropil with protocerebral areas and number approximately 400. They contact the layered organization of the α‐lobe at different locations. Some project not only into the α‐lobe, but also into the β‐lobe and pedunculus neuropils. Fifteen cell types which form intraprotocerebral circuits are morphologically described. They can be divided into three categories: (1) unilateral neurons, with projection fields restricted to the ipsilateral protocerebrum; these neurons connect the α‐lobe with areas in the protocerebral lobe and ramify with densely layered arborisations arranged perpendicularly to the longitudinal axis of the α‐lobe; (2) recurrent neurons, which interconnect subcompartments of the MB, forming loops at different leveles of the neuropil; their arborisations are mainly restricted to the α‐lobe, β‐lobe, pedunculus, and calyces of the ipsilateral MB; they also ramify sparsely around the neuropil of the α‐lobe; and (3) bilateral neurons, which either interconnect both α‐lobes or connect the ipsilateral α‐lobe and protocerebral lobe with the dorsolateral protocerebral lobe of the conntralateral hemisphere. The connections of different compartments of the MB with other parts of the protocerebrum as revealed in this study are discussed in the context of hypotheses about the functional role of MBs in the honeybee brain.