Grading Movement Strength by Changes in Firing Intensity versus Recruitment of Spinal Interneurons

Abstract

Animals can produce movements of widely varying speed and strength by changing the recruitment of motoneurons according to the well-known size principle. Much less is known about patterns of recruitment in the spinal interneurons that control motoneurons because of the difficulties of monitoring activity simultaneously in multiple interneurons of an identified class. Here we use electrophysiology in combination with in vivo calcium imaging of groups of identified excitatory spinal interneurons in larval zebrafish to explore how they are recruited during different forms of the escape response that fish use to avoid predators. Our evidence indicates that escape movements are graded largely by differences in the level of activity within an active pool of interneurons rather than by the recruitment of an inactive subset.

Control of visually guided behavior by distinct populations of spinal projection neurons

“…Having identified which neurons control particular motor patterns, we can now ask how their activity is decoded in the spinal cord to produce the associated behavior. Substantial progress has been made recently in understanding the organization and function of specific cell types and circuits in the zebrafish spinal cord [38][39][40] , encouraging the hope that a connection between descending motor commands and the resulting motor patterns is within reach. More obscure, however, is the upstream circuitry that leads to the selective activation of these descending control neurons.…”

Relationship between the Phases of Sensory and Motor Activity during a Looming-Evoked Multistage Escape Behavior

“…Some animals have evolved fast motor circuits devoted to the generation of such behaviors, such as the giant fiber system in flies, the Mauthner cell in fish, or the lateral giant neurons of crayfish (Wyman et al, 1984;Edwards et al, 1999;Korn and Faber, 2005). Although much is known in these and in other systems about how escape behaviors are generated in response to abrupt stimuli such as mechanical disturbances, air puffs, or light flashes (Levi and Camhi, 2000;Fayyazuddin et al, 2006;Bhatt et al, 2007), we still know very little about how escape behaviors are generated in response to objects approaching on a collision course, as may be expected from potential predators (Yamamoto et al, 2003;Preuss et al, 2006;Santer et al, 2006;Oliva et al, 2007;Hammond and O'Shea, 2007).…”

A mechanism for graded motor control encoded in the channel properties of the muscle ACh receptor

“…These segments contain multiple classes of muscle fibers, including slow, fast, and intermediate types, with distinct mechanical and metabolic properties that constitute a "gearing" system adjustable to a wide range of speeds. The activity of a given segment is mostly independent of the others, and when, which, and how many fibers will be activated are determined by a neural network in the spinal cord (1)(2)(3). This neural mechanism by which contraction magnitude is varied is based on the logic for recruiting motor neurons into the active population, called the size principle (1)(2)(3)(4).…”

“…The activity of a given segment is mostly independent of the others, and when, which, and how many fibers will be activated are determined by a neural network in the spinal cord (1)(2)(3). This neural mechanism by which contraction magnitude is varied is based on the logic for recruiting motor neurons into the active population, called the size principle (1)(2)(3)(4). This control system for compound muscle fibers is likely a key innovation in vertebrates, but how far this basis can be traced back in invertebrate systems is poorly understood.…”