Handbook of Psychophysiology: The Anatomy and Physiology of the Motor System in Humans

The behavioral repertoire of humans is broad, extending from simple behaviors such as sensory perception to more complex cognitive behaviors like language or creativity. Interestingly, no matter how simple or complex are these behaviors, they share without exception the common feature that their expression is a motor act. For the motor system to implement this large variety of cognitive behaviors, there must be an evolutionary bias toward a highly sophisticated and expanded organization of the system. Indeed, according to some (Freund, 1983), association cortical motor regions are among the cortical areas that have changed the most from non-human to human primates. They further manifest a parallel increase in complexity of their connectivity patterns (Zilles et al., 1995). This means that while subcortical motor regions (from spinal cord to cerebellum and basal ganglia) might not be that dissimilar among mammals, cortical motor regions have changed significantly, reflecting a link between motor output and cognitive processing. This chapter on motor physiology intends to provide an overview on how the nervous system has implemented the ability to produce complex motor behaviors in the human. Although the following sections will place special emphasis on “skilled” movements, some comments on rhythmic movements will also be included. Coordination of movements between the upper limbs is a function highly distributed across the animal kingdom, from tetrapod taxa to primates (Iwaniuk & Whishaw, 1999). Although this coordination can be considered to be homologous across species, and although the movements per se are not very different between animal species and humans, there are two aspects of this homology to highlight: First is that there are at least two types of limb synchronization, one involving simpler, generally rhythmic, movements and another incorporating more complex movements. The simplest movements are generally associated with posture and locomotion and are, at least in part, controlled subcortically (Poppele & Bosco, 2003), generated in specialized circuits originally described as “motor pattern generators” (Grillner & Wallen, 1985; Pearson, 1995). By contrast, more complex movements, sometimes called “skilled” movements, have evolved to the highest levels in humans. In terms of evolutionary biology, motor skill refers to the “ability to solve a motor problem correctly, quickly, rationally and resourcefully” (Bernstein, 1996, cited in Wiesendanger, 1999). According to Wiesendanger (1999), the high degree of development of hand dexterity in hominids is reflected by the fact that they not only have the ability to use tools but are also able to fabricate them. In this context, the concept of hand dexterity implies goaldirected action that has been previously learned and practiced. Throughout evolution, the increase in hand skill, tool making and vertical locomotion (that freed the hands), has been accompanied by a parallel increase in the size of cortical motor association areas. Anatomically, this increase is specifically related to an increased volume of neuropil, reflecting an increase in the complexity of connectivity (Zilles et al., 1995). Albeit somewhat artificial, an anatomical division of regions involved in the production of movement can be instructive in understanding the physiology of the human motor system (Figure 22.1). The brain stem and spinal cord can be viewed as the regions of the central nervous system that contain the interneuronal-motoneuronal networks responsible for the production of reflexive and rhythmic motor behaviors. These networks can be regulated by descending modulation from the cerebral cortex (to produce voluntary movements) or from sensory afferents from the peripheral nerves to produce reflexes. Sherrington (1906) called this the final common pathway to denote the fact that this is the final output from the whole system to the muscles. The Basal Ganglia and the Cerebellum are important regions of the Central Nervous System that play a crucial role in the coordination of