Spinal and Neuromechanical Integration: Overview

To interact with the environment or other organisms, the nervous system must move. Whether it is a fundamental protective reflex, a stabilizing postural adjustment, a rapid prey strike, or an expressive dance gesture, the motor repertoire of an organism defines the nature of its environmental interactions. Structures in the periphery of the motor system, the musculoskeletal system and spinal cord, most directly mediate these environmental interactions. The actions of central brain regions such as cortex, cerebellum, or basal ganglia all ultimately have to pass through these peripheral structures. Understanding the properties of these peripheral systems is therefore critical for our understanding of the neural control of movement. How should we consider these peripheral components of the motor system? In one common perspective, these systems are problems that the CNS must overcome. In this perspective, the complex properties of muscles, limb mechanics, motor neurons, and spinal circuits require that central motor systems develop strategies that invert, bypass, or suppress these complexities. Complexity could therefore lead to greater complexity: evolutionary changes to the periphery could require the co-evolution of more complex mechanisms to maintain performance. In an alternate perspective, peripheral systems might simplify for motor control. The complexities of spinal systems or nonlinear properties of the musculoskeletal systemmight reflect adaptations that allow simplified control by descending systems. For example, passive mechanics can be used to assist movements(Collins et al. 2005), muscle properties can contribute to stability (Nichols and Houk 1973), basic reflexes allow for rapid control (Loeb et al. 1999), and defining adaptive muscle coordination patterns can potentially simplify movement (Tresch and Jarc 2009). In this perspective, energetic costs associated with inefficient, complex control might lead to evolutionary adaptations that simplify control and neural processing. These two perspectives are not mutually exclusive, and both emphasize the importance of understanding the contribution of peripheral systems to motor control and brain function. Entries in other sections of this Encyclopedia and the Encyclopedia of Neuroscience describe the basic physiological properties of muscles (Muscle http://www.springerreference.com/docs/html/ chapterdbid/117204.html), motor units (Motor units http://www.springerreference.com/docs/html/ chapterdbid/117164.html), reflexes (Integration of Spinal Reflexes http://www.springerreference. com/docs/html/chapterdbid/116032.html), and pattern generating spinal networks (▶Vertebrate Pattern Generation: Overview). The entries in this section expand on those descriptions, characterizing how these systems function and how they interact with descending systems. Spinal interneuronal systems have been studied extensively for more than a century, and although much remains unknown about their role in behavior, these studies have provided basic information that can be used to guide computational studies of motor control (▶ Spinal Cord, Integrated (Non CPG) Models of). Proprioceptors provide the nervous system with information about body state – the position of the limb, the forces produced by muscles, and the interactions with the environment. Understanding