V3 neurons are excitatory commissural interneurons that extend caudally projecting primary axons. The V1 neurons are inhibitory interneurons with axons that project ipsilaterraly and rostrally. V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ispilaterally and caudally across multiple spinal cord regions. The class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons. The integration of the sensory feedback signals and central motor commands at several levels of the central nervous system plays a critical role in controlling movement. Research on cat's spinal cord has shown that at the spinal cord level sensory afferents and descending motor pathways converge onto common spinal interneurons. Human studies since the 1970s have documented how this integration of motor commands and sensory feedback signals is used to control muscle activity during movement. During locomotion, the sum of convergent inputs from the central pattern generator (CPG), sensory feedback, descending commands and other intrinsic properties turned on by different neuromodulators give rise to the activity of the interneurons. Further, this interneuronal activity was either recorded directly or inferred from the modulation of response in their postsynaptic targets, most often motoneurons. The most efficient way to gate sensory signals in reflex pathways is to control the firing level of interneurons. For example, during locomotion, the interneuronal activity is modulated via excitation or inhibition depending on the reflex pathways. Thus, different patterns of interneuronal activity will determine which pathways are open, blocked, or modulated. The sensory information that is transmitted to the spinal cord is modulated by a complex network of excitatory and inhibitory interneurons. Different neurotransmitters are released from different interneurons, but the two most common neurotransmitters are GABA, the primary inhibitory neurotransmitter and glutamate, the primary excitatory neurotransmitter. Acetylcholine is a neurotransmitter that often activates interneurons by binding to a receptor on the membrane. Renshaw cells are among the very first identified interneurons. This type of interneuron projects onto α-motoneurons, where it establishes inhibition by expressing its inhibitory neurotransmitter glycine. However, some reports have indicated that Renshaw cells synthesize calcium-binding proteins calbindin-D28k and parvalbumin.