Table 1.

Interneuronal populations engaged in the mouse locomotor central pattern generation

Neuronal Population	Neurotransmitter	Target Neurons	Proposed Role in Locomotor CPG
Ipsilateral excitatory interneurons
Chx10-expressing V2a INs	Glutamate	Dbx1-Evx1-expressing V0v INs	High-speed left-right coordination*
Shox2-expressing, nonV2a INs	Glutamate	MNs	Rhythm generators
		Shox2 INs	Low-speed left-right coordination
		Flexor-related MNs
		Commissural INs
Pitx2-expressing V0c INs	Acetylcholine	Extensor-related MNs	Task-dependent modulation of motor outputs*
Hb9-expressing INs	Glutamate	Hb9 INs	Rhythm generators
		Flexor-related MNs
		unidentified INs
EphA4 INs (dI4, dI5)	Glutamate	MNs	Non-weight-bearing left-right coordination (dI4 and dI5)
Ipsilateral inhibitory interneurons
En-1-expressing V1 INs	GABA/glycine		Locomotion speed*
Numerous (>19) subpopulations		MNs to specific muscles
20% comprise:
RCs		Reciprocal MNs
		Ia INs
Ia INs (V1)		Flexor-related MNs	Flexor-extensor alternation (V1+V2b)*
GATA3/4-expressing V2b INs	GABA/glycine	Extensor-related MNs	Flexor-extensor coordination (V1+V2b)*
GAD65-expressing dorsal INs	GABA/glycine	MNs	Termination of rhythmic motor output
Commissural excitatory interneurons
Dbx1-Evx1-expressing V0v INs	Glutamate	Contralateral inhibitory INs	High-speed left-right coordination*
		Contralateral RCs
		Contralateral Ia INs
Sim1-expressing V3 INs	Glutamate	<15% Ipsilateral MNs
Sim1-expressing V3 INs	Glutamate	Contralateral MNs	Left-right coordination*
		Contralateral RCs
		Contralateral Ia INs
Comprised of:
Ventral V3 (V3v)		Contralateral MNs	Left-right coordination*
Dorsal V3 (V3d)		Contralateral INs	Weight bearing
			Left-right coordination*
Commissural inhibitory interneurons
Dbx1-Pax7-expressing V0d INs	GABA/glycine	Contralateral MNs	Low-speed left-right coordination
WT1-expressing dI6 INs	GABA/glycine		Left-right coordination?
Subpopulation dI6	?	Ipsilateral INs?	Rhythm generation?
Subpopulation dI6	?	Ipsilateral MNs?	Pattern formation?
GAD67-expressing ventral INs	GABA/glycine	Contralateral MNs	Left-right coordination
		Contralateral RCs
		Neurons in numerous laminae
Bilateral inhibitory interneurons
Dmrt3-expressing INs	GABA/glycine	Ipsilateral + contralateral MNs	High-speed movements
			Flexor-extensor, left-right coordination*

CPG, central pattern generation; INs, interneurons; MNs, motoneurons; RCs, Renshaw cells.

^*Locomotor activity examined in juvenile/adult mice.