Mention of C.elegans always makes me smile. It is a small simple worm. It has exactly 302 neurons (each is named) and its connectome is completely known. And yet the relationship between the actions of those neurons and the animal’s behaviour is not yet understood. In a recent paper reviewed by NeuroScienceNews (here) researchers have found a multitasking neuron (AIY by name).
Multitasking neurons have been suspected in other animal and humans, but ways for them to do this have not be understood. The researchers found that AIY sends an analog excitatory signal to one circuit having to do with speed of movement and a digital inhibitory signal to another circuit having to do with switching direction. The neurotransmitter is the same for both signals but the receptor that receives the signal is a different type in the two circuits.
Here is their diagram and abstract for Z. Li, J. Liu, M. Zheng, S. Xu; Encoding of Both Analog- and Digital-like Behavioral Outputs by One C. elegans Interneuron; Cell 159-4 Nov 2014:
Model organisms usually possess a small nervous system but nevertheless execute a large array of complex behaviors, suggesting that some neurons are likely multifunctional and may encode multiple behavioral outputs. Here, we show that the C. elegans interneuron AIY regulates two distinct behavioral outputs: locomotion speed and direction-switch by recruiting two different circuits. The “speed” circuit is excitatory with a wide dynamic range, which is well suited to encode speed, an analog-like output. The “direction-switch” circuit is inhibitory with a narrow dynamic range, which is ideal for encoding direction-switch, a digital-like output. Both circuits employ the neurotransmitter ACh but utilize distinct postsynaptic ACh receptors, whose distinct biophysical properties contribute to the distinct dynamic ranges of the two circuits. This mechanism enables graded C. elegans synapses to encode both analog- and digital-like outputs. Our studies illustrate how an interneuron in a simple organism encodes multiple behavioral outputs at the circuit, synaptic, and molecular levels.