Descending networks transform command signals into population motor control.

To convert intentions into actions, movement instructions must pass from the brain to downstream motor circuits through descending neurons (DNs). These include small sets of command-like neurons that are sufficient to drive behaviours1-the circuit mechanisms for which remain unclear. Here we show that command-like DNs in Drosophila directly recruit networks of additional DNs to orchestrate behaviours that require the active control of numerous body parts. Specifically, we found that command-like DNs previously thought to drive behaviours alone2-4 in fact co-activate larger populations of DNs. Connectome analyses and experimental manipulations revealed that this functional recruitment can be explained by direct excitatory connections between command-like DNs and networks of interconnected DNs in the brain. Descending population recruitment is necessary for behavioural control: DNs with many downstream descending partners require network co-activation to drive complete behaviours and drive only simple stereotyped movements in their absence. These DN networks reside within behaviour-specific clusters that inhibit one another. These results support a mechanism for command-like descending control in which behaviours are generated through the recruitment of increasingly large DN networks that compose behaviours by combining multiple motor subroutines.

Ultra-stable dry cryostat for variable temperature break junction.

We present the design of a variable temperature setup that uses a pulse tube cryocooler to perform break-junction experiments at variable temperatures ranging from 12 K to room temperature. The use of pulse tube coolers is advantageous because they are easy to use, can be highly automatized, and used to avoid wastage of cryogenic fluids. This is the reason why dry cryostats are conquering more and more fields in cryogenic physics. However, the main drawback is the level of vibration that can be up to several micrometers at the cold-head. The vibrations make the operation of scanning probe-based microscopes challenging. We implemented vibration-damping techniques that allow obtaining a vibration level of 12 pm between the tip and sample. With these adaptations, we show the possibility to perform break junction measurements in a cryogenic environment and keep in place atomic chains of a few nanometers between the two electrodes.