Introduction to the Fly Visual System
The fly visual system, here in green, consists of three successive visual neuropils, the lamina, the medulla and the lobula-complex. The latter is divided into the lobula and lobula-plate. In the lobula-plate about 60 so-called tangential-cells (LPTCs) can be found.
Each of these cells can be identified individually due to its invariant anatomy and characteristic visual response properties. With their large dendrites they spatially pool the signals of thousands of local, columnar elements arranged in a retinotopic fashion. They, thus, have large receptive fields and respond to visual motion in a directionally selective way. Shown here is an intracellular recording from the axon of a VS1-cell. The cell responds best to vertical moving patterns. When the pattern moves down, the cell is excited: it responds with a graded depolarization of its membrane potential. This direction is called the preferred direction of this cell. Superimposed on this graded shift are spikes with a small and irregular amplitude. This is due to the fact that the VS1-cell, like all VS- and HS-cells, has voltage-dependent sodium and potassium channels that are mostly inactivated at the resting potential of these cells. When the pattern moves up, the cell is hyperpolarized. This direction is called the null direction of the cell.
Amongst the tangential cells, cells are found responding preferentially to a vertical motion like the VS-cells (Vertical System) as well as cells which are best activated by horizontal motion like the HS- and CH-cells (Horizontal System and Centrifugal Horizontal). There exist two CH-cells per brain hemisphere (a dorsal DCH and a ventral VCH), three HS-cells (the northern HSN, the equatorial HSE and the southern HSS-cell) and 11 VS-cells (VS1-VS11) together covering almost completely the visual space surrounding the animal. The different members of each family occupy different regions within the lobula plate and, due to the retinotopic organization, have different but often overlapping receptive fields. HS- and VS-cells are thought to be the major output elements of the lobula-plate. They convey information about large-field horizontal (the HS-cells) and vertical (the VS-cells) image motion onto descending neurons which finally control motor neurons for locomotion or head movements. Lesion experiments have shown that these neurons are directly involved in the course control system of flies.
Located directly underneath the rear surface of the brain, LPTCs are easily accessible after opening the head capsule. Shown here is a view on the fly´s head from behind. A small hole is cut into the cuticula and a tangential-cell is filled with the green fluorescent dye calcium green. The whole cell is in focus, since it is flat and branches right underneath the surface of the brain. This allows for optical imaging of these cells in vivo while the cells are stimulated by their natural synaptic input, i.e. visual motion.
