University of Michigan scientists have come up with a possible explanation for the impressive ability of neurons to perform a wide range of functions.

They explored this using the C. elegans* roundworm. They found that a single neuron in C. elegans regulates both the speed and direction in which the worm moves, shedding light on how the human brain works, say investigators in the lab of Shawn Xu, a faculty member in the University of Michigan Life Sciences Institute.

The trick: the neuron is apparently able to route information through multiple downstream neural circuits, with each circuit controlling a specific behavioral output.

“Understanding how the nervous system and genes lead to behavior is a fundamental question in neuroscience, and we wanted to figure out how C. elegans are able to perform a wide range of complex behaviors with their small nervous systems,” Xu said.”Scientists think that even though humans have billions of neurons, some perform multiple functions.”

Both analog and digital

Xu’s team used a multifaceted approach, integrating calcium imaging, optogenetics,
molecular genetics, laser ablation, and electrophysiology at single-neuron resolution. They found that C. elegans synapses encode both analog- and digital-like behavioral outputs.

The model neuron studied, called AIY, regulates at least two distinct motor (movement) outputs: locomotion speed and direction-switch. AIY interacts with two circuits, one that is inhibitory and controls changes in the direction of the worm’s movement, and a second that is excitatory and controls speed.

“It’s important to note that these two circuits have connections with other neurons and may cross-talk with each other,” Xu said. “Neuronal control of behavior is very complex.”

Xu is also a professor of molecular and integrative physiology at the U-M Medical School.at the University of Michigan Life Sciences Institute

The findings were published online in the journal Cell. The research is also featured on the cover. The research was supported by the National Institutes of Health.

* C. elegans has a simple nervous system, containing only 302 neurons, making it ideal as a model for neurological functions.

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Abstract of Cell paper

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 theC. 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.