Fruit Flies Unravel Brain Mysteries

Through a fly's eye

The fruit fly's compound eye (pictured below) contains 760 unit eyes or ommatidia. Researchers consider this to be an ideal model system for studying how the visual system extracts information from the environment. Drosophila have only tiny brains that consist of a relatively small number of neurones; however, they are able to perform tasks involving highly complex visual orientation far better than other animals - including humans.

Scientists can study the fly’s neural computations and its visual responses relatively easily in the living system (in vivo). They can monitor the signals between neurones though intracellular recordings, which involves measuring voltage and/or current across the membrane of a cell.

One such procedure is called the sharp electrode technique, which measures the potential inside the cell membrane with a sharp electrode. First the fly is mounted with its head protruding through the open tip of conical holder (pictured, below left). The fly is fixed with beeswax to prevent it from moving, whilst the wax keeps the fly intact and gives it room enough to breathe. Secondly, a hole, the size of a few ommatidia, is cut in the fly’s eye, and a microelectrode is inserted through the hole into the cell.

A reference electrode is inserted into the fly’s head, and this closes the circuit (pictured, below right). Light is then flashed at the eye of the fly, and the responses can be measured.

With this technique, combined with genetic manipulation, scientists can investigate different cells in the eyes and the brain of the flies. They have undertaken experiments where they vary the duration of the light flashes and the wavelength of the light, in order to study the way that the first visual synapse works. This nerve-cell junction is the first step in the pathway that communicates - frm the eye to the brain - how the creature sees motion.

The main goal of these studies has been to better understand in vivo how networks of nerve cells in the eye act together to "compute" how objects the fruit fly is seeing are actually moving.

Researchers are also using Drosophila to better understand eye diseases in humans, such as hereditary retinal degeneration. By looking at the molecular and genetic basis of the process of degeneration in flies, scientists hope to provide information that doctors can use to find ways of slowing the progression of the disease in humans, including drug treatments or gene therapy.

How do fruit flies make their choices in life? Read on to find out more....