Back33The Organization of MovementClaude GhezJohn KrakauerIN THE PRECEDING PART of this book we considered how the brain constructs internal representations of the world by integrating information from the different sensory systems. These sensory representations are the framework in which the motor systems plan, coordinate, and execute the motor programs responsible for purposeful movement. In this part of the book we shall learn how the motor systems of the brain and spinal cord allow us to maintain balance and posture, to move our body, limbs, and eyes, and to communicate through speech and gesture. In contrast to sensory systems, which transform physical energy into neural signals, motor systems produce movement by translating neural signals into contractile force in muscles.Just as our perceptual skills reflect the capabilities of the sensory systems to detect, analyze, and estimate the significance of physical stimuli, our motor agility and dexterity reflect the capabilities of the motor systems to plan, coordinate, and execute movements. The accomplished pirouette of a ballet dancer, the powered backhand of a tennis player, the fingering technique of a pianist, and the coordinated eye movements of a reader all require a remarkable degree of motor skill that no robot approaches. Yet, once trained, the motor systems execute the motor programs for each of these skills with ease, for the most part automatically.The ability of humans to carry out skilled movements while still performing cognitive tasks—such as thinking while using tools or speaking while walking—requires flexibility and skills no other animal has. A striking aspect of motor function is the effortlessness with which we carry out the most complicated motor tasks without a thought given to the actual joint motions P.654or muscle contractions required. Although we are consciously aware of the intent to perform a task, such as driving a car, and planning certain sequences of actions, and at times we are aware of deciding to move at a particular moment, the details of our movements generally seem to occur automatically. The tennis player need not consciously decide which muscles to contract to return a serve with a backhand or which head motions and body parts must be moved to intercept the ball. In fact, thinking about each body movement before it takes place would disrupt the player's performance. Thus, conscious processes are not necessary for the moment-to-moment control of movement.The graceful and effortless quality of normal movement carried out automatically depends on a continuous flow of visual, somatosensory and postural information to the motor systems. The “effortless” quality of normal motor control is frequently lost if the motor systems are deprived of a continuous flow of sensory information, from vision, somatic sensation, and vestibular inputs. Vision is particularly important to guiding movement and provides critical cognitive information about the location and shape of objects. The blind must explore space using tactile and kinesthetic cues, a more lengthy process, and they need to rely more on memorized representations of the locations of objects than do sighted persons. Similarly, movements become inaccurate and posture unstable when somatic sensation is lost from the limbs and posture changes. Loss of vestibular input also impairs ability to maintain balance and orientation.Successively higher levels of the motor hierarchy specify increasingly more complex aspects of a motor task. This hierarchy of motor representations depends on a parallel hierarchy of sensory input; more complex sensory information is extracted, at each level, from the spinal cord to the motor cortex. The crucial insight that the components of the motor systems are organized hierarchically was first obtained in the eighteenth century in studies that showed the spinal cord severed from the brain stem and forebrain is capable of organized behaviors. These relatively automatic behaviors include rhythmic behaviors, such as breathing or running as well as reflexes, such as the knee jerk or coughing. These patterned responses to sensory stimuli differ according to the level at which the neuraxis is transected. These differences therefore provide useful clinical indicators of the level of a lesion and of the integrity of afferent and efferent pathways.Because these movements are so stereotyped, in contrast to the endless variety of voluntary movements, reflex and voluntary movements were originally thought to be controlled by qualitatively different neuronal mechanisms. At the beginning of the twentieth century, however, Charles Sherrington in England proposed that voluntary movements represent chains of reflex responses linked together by the brain. Although this is not correct, the spinal cord does contain local circuits that coordinate reflexes, and these same circuits participate in more complex voluntary movements governed by higher brain centers.In this chapter we first review the principles that govern various classes of movement and action. We shall learn how motor psychophysical studies of movement describe the relationships between intended actions and performance, just as sensory psychophysical studies relate physical stimuli to sensory experience in a quantitative way (Chapter 2). The lawful relationships emerging from these studies provide critical insights into how motor systems operate. Finally, we review the overall anatomical organization of the motor systems, from local spinal reflex circuits to the systems of the brain stem and the cerebral cortex that coordinate simple muscle contractions into elaborate purposeful actions.The Motor Systems Generate Reflexive, Rhythmic, and Voluntary MovementsJust as there are distinct modalities of sensation, there are three distinct categories of movement: reflexive, rhythmic, and voluntary.Reflexive and Rhythmic Movements Are Produced by Stereotyped Patterns of Muscle ContractionReflexes are involuntary coordinated patterns of muscle contraction and relaxation elicited by peripheral stimuli. They are typically isolated in animals in which motor pathways from higher brain centers to the spinal cord have been cut (such animals are called decerebrate or spinal animals depending on the level of the cut). The spatial and temporal patterns of muscle contraction vary in different reflexes, depending on the type of sensory receptors that are stimulated. Receptors in muscles
View Full Document
Unlocking...