Connectivity of dI3 Interneurons In the Development of Mice Spinal Cord

Abstract

Maturation of motor control, including movements that can be autonomously generated by spinal circuits, relies on the development of key inputs to spinal circuitry. In particular, the development of supraspinal, sensory and motor fibers come together to form organized spinal circuits capable of producing skilled movements that are volitionally controlled. Primitive reflexes such as the palmar grasp reflex (PGR) are known to disappear during development; presumably giving way to more volitional control of hand grasping. However, the underlying changes to the spinal circuitry responsible for this transition remain to be determined. dI3 INs, a class of dorsal spinal interneurons, have positioned themselves as key mediators of reflexive grasping in early development and grasping in adult mice. The first aim of the study focused on determining the developmental time point at which the PGR disappeared. Our studies demonstrated that the PGR was lost by the third week of development. The second aim of this study focused on identifying changes in sensory innervation, presynaptic inhibition and supraspinal excitation to dI3 INs that might account for the loss of this reflex. Our studies demonstrated that while sensory innervation remained constant during development, presynaptic inhibitory terminals onto sensory afferents were found to increase during development. In addition, we report that dI3 INs receive decreasing corticospinal (CST) input during development. While these developmental changes do not fully account for the disappearance of the PGR, they provide valuable insights into how a reflex centered on a particular population develops.