Clarity in the
cellular thicket. Four classes of nerve cell (Tm9, 4, 1 and 2)
are instrumental
in calculating directionally selective signals in T5 neurons (yellow).
© MPI of
Neurobiology
(February 6, 2016) Neurobiologists
characterize nerve cells that detect motion by light changes
The ability to see the direction in which something is
moving is vital for survival. Only in this way is it possible to avoid
predators, capture prey or, as humans in a modern world, cross a road safely.
However, the direction of motion is not explicitly represented at the level of
the photoreceptors but rather must be calculated by subsequent layers of nerve
cells. Scientists from the Max Planck Institute of Neurobiology in Martinsried
have now discovered that, in fruit flies, four classes of nerve cell are
involved in calculating directionally selective signals. This is strikingly
different from mathematical models of motion detection discussed in the
literature so far.
When crossing a road, it’s advantageous to know the
direction in which nearby cars are moving. However, the individual light
sensitive cells in the eye only signal local changes in brightness, whether an
image point becomes brighter or darker. The direction of motion is detected in
a downstream neuronal network.
Alexander Borst and his team at the Max Planck Institute of
Neurobiology have unravelled cell by cell how the brain calculates motion from
light changes. Their model is the fruit fly, a master in motion vision,
possessing a relatively small brain. Although there are more than 50,000 nerve
cells in the area of the fruit fly brain responsible for motion vision, the
researchers believe that the network is “simple” enough to allow them to
understand the circuitry at the cellular level. In previous studies, they have
shown that in flies, similar to vertebrates, motion is detected in two parallel
pathways, one for moving bright edges (ON-pathway) and one for moving dark
edges (OFF-pathway).