Meeting Abstract
To survive, animals must respond to sensory cues in a context-specific manner. Even innate sensorimotor responses are flexible, such that an identical cue can elicit different actions in different situations. How the brain achieves this context-dependent flexibility, or ‘makes decisions,’ is unclear. In the fruit fly, Drosophila melanogaster, frontal looming stimuli elicit an escape takeoff if the fly is standing and a landing response if the fly is flying. While the neuronal basis of the escape response is relatively well-known, and relies, in part, on giant-axon descending neurons, little was known about the control of landing in any species. We created a collection of 130 transgenic lines that target individual, identified descending neurons. We conducted an optogenetic activation screen of this collection and identified two descending neuron bilateral pairs whose activation drives extension of all six legs with kinematics that resemble the landing response of the fly. Genetically silencing either of these descending neuron types significantly reduced landing responses elicited by frontal visual looming stimuli. Using whole-cell patch-clamp recordings, we show that landing descending neurons integrate visual and mechanosensory cues and control leg extensions in a graded fashion while the fly is flying. Critically, landing descending neuron visual responses are eliminated or severely attenuated when the fly is not flying. This gating occurs by separate mechanisms (neuromodulation or efference copy) in the two different landing neuron types. Our findings show that state-dependent gating of descending pathways is one mechanism that controls the brain’s access to different motor networks, thus enabling flexible, context-dependent action selection.
