CHAPTER28Fundamentals of Motor Systems“All mankind can do is to move things . . . whether whisper-ing a syllable or felling a forest.”—SherringtonThis brief quote is a reminder of the basic fact that all interactions with the surrounding world are through the actions of the motor system. When a human baby is born it is a sweet but very immature survival machine, with a limited behavioral repertoire. It is able to breathe and has searching and sucking refl exes so that it can be fed from the mother’s breast. It can swallow, vomit and process food, and cry to call for attention if something is wrong. A baby also has a variety of protective refl exes that mediate coughing, sneezing, and touch avoidance. These different pat-terns of motor behavior are thus available at birth and are due to innate motor programs (Fig. 28.1).During roughly the fi rst 15 years of life the motor system continues to develop through maturation of neuronal circuitry and by learning through different motor activities. Playing represents an important element both in children and in young mammals such as kittens and pups. During the fi rst year of life the human infant matures progressively. It can balance its head at 2–3 months, is able to sit at around 6–7 months, and stand with support at approximately 9–12 months. The coordination of different types of posture, such as standing, is a complex motor task to master, with hun-dreds of different muscles taking part in a coordinated fashion. Sensory information contributes importantly, in particular from the vestibular apparatus, eyes, muscle, and skin receptors located at the soles of the feet. This development represents to a large degree a maturation process following a given sequence, but with individual variability among different children. When the postural system has evolved to a suffi cient degree, a child is able to start walking, which requires that the body posture be maintained while the points of support are changing by the alternating movements of the two legs. In common language the child is said to “learn” to walk, but in reality a progressive matura-tion of the nervous system is taking place. Identical twins start to walk essentially at the same time, even if one has been subjected to training and the other has not. At this point the motor pattern is still very imma-ture. Proper walking coordination followed by running appears later, and the basic motor pattern actually con-tinues to develop until puberty. The fi ne details of the motor pattern are adapted to the surrounding world, but also to modifi cation by will. The basic motor coor-dination underlying reaching and the fi ne control of hands and fi ngers undergo a similar characteristic maturation process over many years.The newborn human infant is comparatively imma-ture, but other mammals, such as horses and deer, represent another extreme. The gnu, an African buffalo-like antelope, needs to run away in order to survive attacks of predators such as lions. The young calf of the gnu can stand and run directly after birth and has been reported to be able to gallop ahead 10 minutes after delivery, tracking the running mother (Fig. 28.2). Clearly the neural networks underlying locomotion, equilibrium control, and steering must be suffi ciently mature and available at birth, needing minimal cali-bration. This is astounding. A similar range of matu-rity is present in birds. To get out of the egg, a chick makes coordinated hatching movements to open up the eggshell to subsequently lift off the top of the egg and to stand up to walk away on two legs following the mother hen and start picking at food grains. Most birds are more immature when hatching, but after a Fundamental Neuroscience, Third Edition 663 © 2008, 2003, 1999 Elsevier Inc.664 28. FUNDAMENTALS OF MOTOR SYSTEMS V. MOTOR SYSTEMS0 Months2 Months4 Months9 Months10 Months14 Months15 MonthsFIGURE 28.1 Motor development of the infant and young child. The pattern of maturation of the motor system follows a characteristic evolution. Two months after birth a child can lift its head, at 4 months it sits with support, and subsequently it is able to stand with support; later it crawls, stands without support, and fi nally walks independently. The approximate time at which a child is able to perform these dif-ferent motor tasks is indicated above each fi gure. The variability in the maturation process is substantial. Modifi ed from M. M. Shirley.FIGURE 28.2 Some animals are comparatively mature when they are born. Ten minutes after the calf of the gnu, a buffalo-like antelope, is born it is able to track its mother in a gallop. This means that the postural and locomotor systems are suffi ciently mature to allow the young calf to generate these complex patterns of motor coordination at birth. There is thus little time to calibrate the motor system after birth and obviously no time for learning. Courtesy of Erik Tallmark.V. MOTOR SYSTEMSfew weeks they leave the nest fl ying rather success-fully, for the fi rst time in their life, and thus without any previous experience.In addition to the basic motor skills such as stand-ing, walking, and chewing, humans also develop skilled motor coordination, allowing delicate hand and fi nger movements to be used in handwriting or playing an instrument or utilizing the air fl ow and shape of the oral cavity to produce sound as in speech or singing. The neural substrates allowing learning and execution of these complex motor sequences are expressed genetically and characteristic of our species. What is learned, however, such as which language one speaks or the type of letters one writes, is obviously a function of the cultural environment.BASIC COMPONENTS OF THE MOTOR SYSTEMMotoneurons and Motor UnitsThe motoneurons that control different muscles are located in different motor nuclei along the spinal cord and in the brain stem. Each motoneuron sends its axon to one muscle and innervates a limited number of muscle fi bers. A motoneuron with its muscle fi bers is referred to as a motor unit. The muscle fi bers of each motor unit have similar contractile properties and metabolic profi le. The muscle fi bers in different muscles are composed of three main types specialized for different demands, such as a continuous effort as in long-distance running (slow motor units) or fast explosive movements, such as lifting a heavy object (fast motor units (two subtypes)).Motoneurons are
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