New research infers that motor neurons are much more important in the control of locomotion than previously believed.
The natural complexity of the nervous system is both awe-inspiring and bewildering.
Science slowly chips away at the mysteries of the complex interactions between brains, nerves, muscles and the world they inhabit. Progress is step by step, inch by inch, but, every so often, a surprising leap is made.
The present study, conducted by Prof. El Manira and his team at the Karolinska Institutet in Sweden, takes a fresh look at the role of motor neurons.
Locomotion is an essential part of an animal's behavioral repertoire; without it an animal might, quite literally, be a sitting duck.
Most humans take locomotion for granted. Because walking, skipping and running all come with a remarkable degree of natural fluidity, it is no surprise that we give them little consideration on a daily basis.
Our ease of locomotion belies its deep and impenetrable complexity. But, one only needs to look at the struggle a robot encounters when faced with a set of stairs to remind us how tricky planned movements can be.
The traditional role of the motor neuron
Despite the complexity involved in locomotion, most of the necessary patterns and rhythms are stored in spinal circuits. The higher brain only needs to get involved to start or stop the movement, or to make tweaks if, for instance, an obstacle appears in one's path.
A central pattern generator within the spine collects signals from multiple sources (sensory and higher brain centers) then forwards commands via motor neurons to the muscles.
The movements are fed back to the spinal cord, and the higher brain and adjustments are made as required. In this scenario, the motor neuron has historically been considered a lowly messenger. It ferries the information but makes no changes to the text along the way. This basic notion of the motor neuron seems set to change.
Zebrafish research and gap junctions
Prof. El Manira's study investigated motor neurons in zebrafish. These freshwater tropical fish are often used as lab animals; in fact, they were the first animal to be cloned.
Zebrafish are useful to science for a number of reasons: they are cheap and easy to maintain, their genome is fully mapped and their behaviors are well known and understood; also, zebrafish embryos are relatively large and transparent, and they are easy to genetically modify.
Using a variety of techniques and optogenetics (a method that allows genetically modified cells to be controlled by light), the researchers selectively silenced motor neuron activity in zebrafish. This allowed the team to observe the motor neuron's function during locomotion in detail.
A new role for motor neurons
This work on the modified zebrafish showed that, rather than motor neurons faithfully delivering an unedited message from the spinal cord to the muscle, they play a more detailed role. The team found, to their surprise, that motor neurons exerted a significant influence on locomotor activity via gap junctions.
Gap junctions are a separate institution to the more familiar synapse but, like synapses, they facilitate communication between cells; when found in nerves, they are also referred to as electrical synapses. A gap junction directly connects the cytoplasm of two cells enabling ions, molecules or electrical impulses to travel between.
Prof. El Manira explains the findings:
"We have now uncovered an unforeseen role of motor neurons in the elaboration of the final program for motor behavior.
Our unexpected findings demonstrate that motor neurons control locomotor circuit function retrogradely via gap junctions so that motor neurons will directly influence transmitter release and the recruitment of upstream excitatory interneurons."
Rather than information being blindly transmitted from the spinal circuitry to the muscles, the motor neurons added their own color to the proceedings. The team found that the gap junctions of motor neurons activated and recruited V2a interneurons that are vital in defining the rhythm of locomotion and its left-right swing.
It seems a little trite to declare that textbooks will need to be rewritten, but this discovery certainly is a major switch in our understanding of the role of motor neurons. As Prof. El Manira says:
"This study represents a paradigm shift that will lead to a major revision of the long-held view of the role of motor neurons. Motor neurons can no longer be considered as merely passive recipients of motor commands - they are an integral component of the circuits generating motor behavior."
Although zebrafish are known to be reliable models, further research in higher animals will need to be done before the precise functions of this neural feedback and integration are fully revealed.
Medical News Today recently covered research into the creation of serotonin-releasing neurons from human skin cells.