Fly, photoreceptor axon terminals

Structure and Function of Gap Junctions in the Photoreceptor Axon Terminals of the Fly... 

Figure 5. Light micrograph of a few facets of a fly s compound eye. Dark spots that represent the rhabdomeres of the photoreceptors have been superimposed onto each corneal lens to demonstrate the principle of neuro-superposition. The central corneal facet has been removed from the photograph to depict the underlying lamina cartridge. Anatomically, six peripheral photoreceptor axon terminals (R1-R6) synapse with two second-order cells (LI and L2) in the underlying neuropil that is called the lamina. Each of these six photoreceptors is illuminated by a different lens, but optically they share the same visual axis that is, they look at the same point in space. This lamina subunit is known as neuroommatidium. Axons of the central receptor cells (R7 and R8) from the overlying ommatidium pass close to this cartridge, but simply bypass the lamina and do not contribute synapses at this neural level.

Figure 7. Fluorescence micrograph of a 1-pm-thick section of the lamina of a male fly that shows stained photoreceptor axon terminals cut parallel to their long axis. Gap junctions between the six peripheral cells occur in the outer 10 pirn of the axon terminal, just below the somas of the monopolar neurons. Because recorded cells are located in reference to the corneal facet matrix, the location and orientation of that cell in the underlying neuropil is predetermined, so it can be sectioned in any given plane and recovered for histological and histochemical examination.

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