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.

Origin of excitation underlying locomotion in the spinal circuit of zebrafish

“…The ability of spinal circuits to generate the locomotor rhythm emanates from the existence of an intrinsic source of excitability that sets the activity tone of the constituent neurons of these circuits. The molecular identity of the excitatory interneurons underlying the locomotor rhythm has been unclear (6,18,22). Our analysis now suggests that V2a interneurons represent a possible source of excitation that contributes to generating swimming activity.…”

“…V2a interneurons may have acquired a different role in the mouse compared with zebrafish and seem to be mostly involved in ensuring left-right alternation at high locomotor frequency (33,34,51). In addition, the generation of the rhythm underlying locomotion in mice could involve overlapping classes of interneurons and elimination of one of these classes (i.e., V2a interneurons) is not sufficient to prevent the normal expression of the rhythmic activity in the spinal circuits (6,18,22).…”

“…The spinal cord acts as an interface to process descending commands from the brain and sensory inputs from the periphery (5,8,(10)(11)(12)(13)(14)(15)(16). Many insights into the organization and function of circuits underlying motor behavior have been gained from studies of spinal networks controlling locomotor movements (6,13,(17)(18)(19)(20)(21)(22)(23)(24). The basic locomotor activity requires the interplay of ipsilateral excitatory drive and crossed inhibition, which ensures the alternating pattern between the two sides of the spinal cord.…”

Gender-specific perturbations in modulatory inputs to motoneurons in a mouse model of amyotrophic lateral sclerosis

“…Analyses of synaptic inputs have revealed imbalances in excitatory and inhibitory inputs to motoneurons, although inconsistencies exist in the reported nature of this imbalance (Schutz, 2005;Avossa et al, 2006;Hossaini et al, 2011;Sunico et al, 2011). Given such discrepancies and the abundance of synaptic inputs to motoneurons (Brownstone and Bui, 2010), it is important that the fates of specific classes of synaptic inputs to motoneurons are now addressed. (Conradi and Skoglund, 1969;Li et al, 1995;Hellstrom et al, 2003;Muennich and Fyffe, 2004;Zagoraiou et al, 2009).…”

Body stability and muscle and motor cortex activity during walking with wide stance

“…The mechanisms of such contribution are not well understood; they might involve synaptic gaiting in 1) descending and ascending pathways between the motor cortex and other brain areas and spinal cord, 2) spinal interneuron circuits comprising the locomotor rhythm generator and/or pattern formation networks, and 3) interneurons mediating proprioceptive input, etc. (Brownstone and Bui 2010;Drew et al 2008;Markin et al 2012;Stecina et al 2013).…”