The nervous system is no slacker: each constituent synapse only takes a few milliseconds to transfer its information from one cell to the next. This allows an animal to rapidly sift through the sensory world in order to determine the best behavioural response. However, sometimes even this process takes too much time, as can be the case when an animal has to respond to a fast-approaching predator. A recent study by researchers at the Janelia Farm Research Campus, USA, has elucidated a neural mechanism that allows an animal to speed up its behavioural response for these occasions.
The team noticed that the fruit fly Drosophila melanogaster responds to looming threats with one of two escape behaviours: a slow, stable response, in which the fly raises its wings before it jumps off and flies away, and a faster, far less stable response, in which the fly does not elevate its wings before it jumps off, shaving 8 ms off the response time. How does a fly choose between these two responses? A well-known cell that is involved in the initiation of escape responses in many species of animal is the giant fibre (GF) interneuron, though its precise function is unclear. The authors therefore decided to genetically activate or inhibit this neuron and see what the behavioural response would be.
They noticed that activation of this neuron led to the initiation of the fast, unstable escape behaviour, while inactivation rendered the flies incapable of performing this fast response. This suggests that the GF is both necessary and sufficient for this behaviour, and raises the possibility that the choice between the fast and slow modes is simply a matter of turning this cell on or off. To test this possibility, the researchers decided to record the activity of this neuron in flies performing these behaviours.
To their surprise, the GF was active not just when the fly performed the fast escape response; it also fired during some of the slower responses, raising doubt about how it is involved in the choice between fast and slow escape responses. In order to figure this out, the authors performed a detailed analysis of the correlation between the activity of the GF neuron, the properties of the visual stimulus and the exact behavioural action selected at any given time.
The researchers found that the timing of the activity of the GF neuron relative to the stimulus was crucial: the approach of a predator initiates an escape response in the fly that, by default, is the slower one; only if the predator approaches with sufficient speed is the GF activated, which then overrides the slower response to produce a faster escape. However, only if the GF is activated during a particular time window during the behavioural sequence is it capable of overriding the slower response, which explains why the GF can be active even during some of the slower responses.
Flies in which the GF was genetically inhibited were not able to escape from damselflies, one of their natural predators, when they approached the flies at their fastest attack speeds. This shows that the seemingly insignificant 8 ms difference that the faster response makes can actually save the life of a fruit fly.
