Researchers at the University of Cologne discovered that stick insects’ depressor muscle neurons are uniquely excited in a rhythmic pattern, challenging the belief that all motor neurons are uniformly activated by central pattern generators. This finding highlights the specialized neural control required for stabilizing walking movements.

A recent study conducted by researchers at the University of Cologne has shed light on how nerve cells (neurons) that regulate the movement of leg muscles in stick insects operate in a rhythmic manner. The team discovered that the neurons responsible for triggering the depressor muscle in the leg exhibit a rhythmic activation pattern, which is distinct from the activation patterns of neurons associated with other leg muscles.

So far, it has been assumed that all of these so-called motor neurons are activated in the same way by central neural networks. The study was published under the title ‘The synaptic drive of central pattern-generating networks to leg motor neurons of a walking insect is motor neuron pool specific’ in the journal Current Biology.

Understanding Central Pattern Generators

The UoC research team investigates the neural foundations of motion generation in animals, in particular those underlying locomotor activities such as walking. For this purpose, the team led by Professor Dr Ansgar Büschges analyses insects, among other arguments, as the requirements for the nervous system regarding the generation and control of walking movements are very similar across the animal kingdom.

In many animals, for example, there are networks in the central nervous system that form the basis for the generation of rhythmic activity patterns for many forms of movements, whether for rhythmic locomotor activity such as running, swimming, crawling, and flying or for vegetative functions such as breathing.

These highly specialized networks are referred to as central pattern generators (CPGs). They generate the rhythmic motor activity of the muscles for movement in interaction with information from sensory organs, neurons called proprioceptors; proprioceptors report movements and inform the central nervous system. In the case of walking, they are located on and in the insect’s legs.

The networks do this by activating the so-called motor neurons that innervate the muscles. So far, it was assumed that such CPGs have the same influence on all motor neurons they target. In their new study, Angelina Ruthe, Dr Charalampos Mantziaris, and Professor Büschges disproved this assumption about the locomotor activity of insects.

New Insights from Stick Insect Research

In their experiments, the scientists pharmacologically activated the CPGs in the central nervous system of the stick insect Carausius morosus and investigated their influence on the motor neurons that innervate its leg muscles. They found that all motor neuron groups of the leg muscles, except one, receive identical drive from the networks: rhythmic inhibitory signals from the CPGs. Only the motor neurons, which innervate the depressor muscle of the leg, are controlled by phasic excitatory drive.

Interestingly, the leg depressor muscle is precisely the muscle of the insect that is responsible for generating leg stance during any walking situation – regardless of whether the animal runs up or down horizontally, on the ceiling, or on a branch. “The rhythmic excitation and thus the specific activation of this motor neuron pool by the CPGs could serve to ensure the exact timing of the contraction of the depressor muscle and thus the start of the stance phase and its stabilization,” explained Professor Büschges.

Reference: “The synaptic drive of central pattern-generating networks to leg motor neurons of a walking insect is motor neuron pool specific” by Angelina Ruthe, Charalampos Mantziaris and Ansgar Büschges, 1 February 2024, Current Biology.
DOI: 10.1016/j.cub.2024.01.026

The study was funded by the German Research Foundation (DFG).