Spinal motoneurons as active players in motor control

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Motoneurons are cells in the spinal cord that send nerve impulses to muscles to stimulate their contraction and are the only neurons that make direct contact with non neuronal cells outside the central nervous system. While the decision of action is initiated in the brain, motoneurons are the last neurons in the chain of connections controlling the most essential of behaviours: movement.Motoneurons are organized into separate columns innervating specific muscles, from those controlling the toes, to those controlling eyeball movements. Motoneurons' activation triggers muscle contractions and the many diseases affecting motoneurons cause progressive and fatal paralysis. Most research efforts in the past have been devoted to understanding the motoneurons cellular properties, in health and disease, since they have always been considered a mere output element necessary for translating motor command into action. My recent research however shows that motoneurons themselves form an interconnected network in which the output elements are connected with each other and form an excitatory loop, that has the potential role of an amplifier of the force output. The aim of the proposed research is to unravel the details of the pattern of connectivity between motoneurons. In particular, since motoneurons come in different sizes, with small motoneurons mostly employed for low intensity motor tasks, like walking, and large motoneurons called into action where greater force or speed are needed, as in jumping or lifting heavy weights, we will determine the architecture of the connections between different type of motoneurons, both between those innervating the same muscles and those innervating synergist or antagonist muscles. Furthermore, we know that motoneurons communicate with each other via the transmitter glutamate and we will be able to delete it selectively in motoneurons, in order to impair the excitatory loop. We will then perform physiological and behavioural experiments in order to determine how the motoneuron excitatory loop affect the performance of the animals in a variety of motor tasks and whether its deletion can alter the force output. Our finding that motoneurons can be considered active players in the generation of movement may have several implications in the study and management of many diseases affecting the motor system. It is in fact possible that some of the first symptoms associated with motoneuron degeneration might be due to loss of connectivity between motoneurons. This finding might eventually lead to targeted strategies for improving motor control in those affected by motor diseases.