Delineating the Diversity of Spinal Interneurons in Locomotor Circuits

Abstract

Locomotion is common to all animals and is essential for survival. Neural circuits located in the spinal cord have been shown to be necessary and sufficient for the generation and control of the basic locomotor rhythm by activating muscles on either side of the body in a specific sequence. Activity in these neural circuits determines the speed, gait pattern, and direction of movement, so the specific locomotor pattern generated relies on the diversity of the neurons within spinal locomotor circuits. Here, we review findings demonstrating that developmental genetics can be used to identify populations of neurons that comprise these circuits and focus on recent work indicating that many of these populations can be further subdivided into distinct subtypes, with each likely to play complementary functions during locomotion. Finally, we discuss data describing the manner in which these populations interact with each other to produce efficient, task-dependent locomotion.

Keywords: CPG; development; interneuron; locomotion; spinal cord.

Figure 1.

Genetic identity, locomotor function, and synaptic connectivity of molecularly defined subpopulations in the ventral spinal cord. A, Table indicates the name of the cardinal populations, the subsets derived from each (including the defining transcription factors), and their function during locomotion. B, Schematic summarizing initial experimental findings and predictions regarding synaptic connectivity of each population listed in A. The non-V2a Shox2+ cells (orange) have also been included in B. Synaptic connectivity that has been demonstrated experimentally is indicated by a solid line; connectivity that has been predicted is indicated by a dashed line. Glutamatergic synapses are indicated by “I,” inhibitory synapses by “●”, and cholinergic synapses by “<.” Motoneurons on the left (L MNs) and right (R MNs) are represented by the brown boxes on either side of the midline (gray horizontal line). Note that the connectivity presented here is not exhaustive.