Organization of visual afferents to the mushroom bodies

In some insects the mushroom bodies, a central brain structure of arthropods, receive afferents from the optic lobes. Such afferents have been described in crickets (Honegger and Schürmann, 1975), cockroaches (Strausfeld and Li, 1999), but are especially noticable in Hymenoptera. Electrophysiological recordings of such neurons have been described only once for a single neuron as responsive to stationary, but not moving light stimuli (Gronenberg, 1986). Another approach towards the possible function of those neurons is to attempt to map their organization in the mushroom bodies, which should show at least if the retinotopic organization is preserved in this central neuropil. In the honey bee Apis mellifera neurons from the medulla and lobula, but not from the lamina terminate in the collar region of the mushroom bodies. Rather than a strict retinotopic organization these afferents segregate into five layers that receive alternating input from the dorsal or ventral medulla. A sixth, innermost layer of the collar receives input from lobula neurons. Corresponding to the dorsal/ventral separation of the afferents in the mushroom bodies, the medulla-mushroom body neurons show a difference in their arborization pattern between the dorsal and ventral part of the medulla. Dorsal-ventral differences within the medulla are found also in other neurons as in fibers which are immunoreactive for serotonin. This can be assumed reflects functional differences between the dorsal and ventral eye, which are revealed in behavioral discrimination experiments (Lehrer, 1999).

Afferents from the reight and left eye are not organized into different layers.

Afferents from the dorsal (green) and ventral (red) medulla are organized in five alternating layers in the collar of the mushroom bodies.

Afferents from the lobula (red) terminate in a inner collar (co) layer. Green: afferents from the medulla.

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