Recurrent excitation between motoneurones propagates across segments and is purely glutamatergic

Spinal motoneurones (Mns) constitute the final output for the execution of motor tasks.In addition to innervating muscles, Mns project excitatory collateral connections to Renshaw cells (RCs) and other Mns, but the latter have received little attention.We show that Mns receive strong synaptic input from other Mns throughout development and into maturity, with fast-type Mns systematically receiving greater recurrent excitation than slow-type Mns.Optical recordings show that activation of Mns in one spinal segment can propagate to adjacent segments even in the presence of intact recurrent inhibition.While it is known that transmission at the neuromuscular junction is purely cholinergic and RCs are excited through both acetylcholine and glutamate receptors, here we show that neurotransmission between Mns is purely glutamatergic, indicating that synaptic transmission systems are differentiated at different postsynaptic targets of Mns. Author summaryMotoneurones (Mns) are the last elements of the networks that command, coordinate, and actuate the most essential of behaviours: movement.Their activation triggers muscle contractions, and the many diseases affecting Mns cause progressive and fatal paralysis.We show here that Mns themselves form an interconnected network and that activity in one segment of the spinal cord can propagate reciprocally to neighbouring segments, thus constituting a positive feedback loop that can amplify the strength of motor output.Remarkably, while Mns excite muscles through release of the neurotransmitter acetylcholine, our data show that the synapses between Mns operate through glutamate, providing a rare example of differentiation of transmitter systems according to the postsynaptic targets.