Spinal motoneurons as active players in motor control

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.

Motoneurons integrate inputs from spinal interneurons, supraspinal tracts and sensory afferents, to generate the motor command that translates into muscle contraction. We have recently shown that motoneurons are not only output units, but make powerful, purely glutamatergic synapses onto each other, forming a recurrent excitatory feedback loop.
The extent and strength of connectivity between motoneurons led us to the hypothesis that recurrent excitation between motor units is an essential amplification mechanism that is employed to generate strong muscle contraction. This hypothesis is supported by the widespread synaptic connectivity among motoneurons in young and mature mice and by the preliminary observation that animals with genetically ablated recurrent excitation showed an impaired performance in motor tasks requiring fast and strong muscle contraction in vivo.
We propose to establish the connectivity pattern between motoneurons of different type (fast and slow) within and across nuclei innervating different muscles, using electrophysiological experiments and optical mapping of synaptic connections. We will analyse the firing activity of motoneurons during stereotyped motor tasks in vitro, using calcium imaging of the entire spinal cord and high density multi electrode array recordings. We will also perform EMG recordings in vivo and behavioural tasks that involve forceful and ballistic movement, like jumping or grasping. By comparing the motor performance between wild type animals and those in which recurrent excitation between motoneurons is genetically ablated, we will determine how recurrent excitation shapes the motor output.

During the course of the tenure and beyond we expect to generate an impact on clinical scientists, industrial stakeholders and the general public.

Clinical beneficiaries
The proposed research is designed to address the specific involvement of the recently described synaptic connections between motoneurons in the control of motor behaviour. Many motor related pathologies directly affect motoneurons and it is possible that one of the early failure points during the evolution of such diseases might occur at the synaptic contacts between motoneurons. Our data will help elucidate the structure of connectivity between motoneurons and have a potential impact on clinical practice. This impact will be achieved by extensive dissemination of results outside the circle of basic neuroscientists and between the more clinically oriented researchers, within and outside the host institution.
One of the hypothesis tested in the research plan is that the excitatory loop between motoneurons could serve as an amplifier of muscle contraction when maximum force is required. This idea could have an impact on studies of sport physiology. In fact, it is possible that the known phenomenon of short term post-exercise potentiation might be at least partly explained by a synaptic potentiation at the level of the spinal cord. Once data on the animal model are collected, we will involve experts in human motor unit recordings (Prof. Iannetti at UCL has extensive experience of work on human subjects) and try to extend our measurements to humans.

Industrial beneficiaries
Part of the project rely on the refinement and improvement of an existing setup for optical stimulation that makes use of a holographic pattern to sculpt the light and maximize selective excitation at the neuronal surface. During the tenure of the project, we will improve and modify the hardware and software of the setup in close interaction with Bruker Ltd. and we expect to contribute to the design of a commercial version. Within this collaboration we also aim at obtaining a case studentship (LIDo) in partnership with Bruker to facilitate the testing and optimization.
We will also design new probes for extracellular recordings, specifically suited to address our research question. This will be done in collaboration with Neuronexus, a company with a long history of probe design. Newly conceived probes normally become part of the catalogue and become available to the wider community after testing.

General public
I will target high school children, by giving talks in secondary school, based on the results of the proposed research. I will continue my commitment to the In2Science project, that organizes summer placement for high school pupils from disadvantaged background. This scheme has an enormous social value because it facilitates access to higher education to pupils that would be otherwise cut out of the competition.
I will also apply for a stand at the Royal Society Science exhibition, that is a great showcase of advancement in science and is attended by a large number of people every year.