Spinal interneurons providing input to the final common path during locomotion

Abstract

As the nexus between the nervous system and the skeletomuscular system, motoneurons effect all behavior. As such, motoneuron activity must be well regulated so as to generate appropriately timed and graded muscular contractions. Accordingly, motoneurons receive a large number of both excitatory and inhibitory synaptic inputs from various peripheral and central sources. Many of these synaptic contacts arise from spinal interneurons, some of which belong to spinal networks responsible for the generation of locomotor activity. Although the complete definition of these networks remains elusive, it is known that the neural machinery necessary to generate the basic rhythm and pattern of locomotion is contained within the spinal cord. One approach to gaining insights into spinal locomotor networks is to describe those spinal interneurons that directly control the activity of motoneurons, so-called last-order interneurons. In this chapter, we briefly survey the different populations of last-order interneurons that have been identified using anatomical, physiological, and genetic methodologies. We discuss the possible roles of these identified last-order interneurons in generating locomotor activity, and in the process, identify particular criteria that may be useful in identifying putative last-order interneurons belonging to spinal locomotor networks.

Figure 1

Diagram depicting last-order spinal interneurons and some of their known excitatory inputs from descending and muscle afferent pathways. It is likely that each population receives inputs from many more types of afferents than those depicted. Filled circles represent classes of neurons identified using molecular biological approaches. Unfilled circles represent classes of neurons identified through anatomical and electrophysiological techniques. These are not mutually exclusive: for example, Renshaw cells and Ia inhibitory interneurons belong to the V1 class of spinal interneurons (indicated by yellow text) and some excitatory interneurons expressing EphA4 belong to the V2a class of interneurons. Note that not all neurons of each class are depicted; for example, some EphA4+ neurons may be last-order inhibitory interneurons and some V0 interneurons are excitatory. Red designates excitatory neurons and boutons, blue designates inhibitory neurons and boutons and green designates neuromodulation. Solid lines represent monosynaptic inputs, and dashed lines represent supraspinal descending inputs that are either mono- or oligo-synaptic.