Glutamate (cortex, spinal cord) and acetylcholine (spinal cord) modulate excitatory input within neurons, whereas GABA and glycine facilitate inhibitory neurotransmission (Ramírez-Jarquín et al., 2014). At the neuromuscular junction (NMJ), only acetylcholine acts at the synapse but interestingly, synaptic transmission between MNs in the spinal cord involves both acetylcholine and glutamate (Bhumbra and Beato, 2018). Renshaw cells are excited through both acetylcholine and glutamate receptors and spinal MNs co-release glutamate to excite Renshaw cells and other MNs, but not to excite muscles (Nishimaru et al., 2005; Bories et al., 2007; Bhumbra and Beato, 2018). Hence, different synaptic transmission systems are present at different postsynaptic targets of MNs (Bhumbra and Beato, 2018).