C H A P T E R 28 Fundamentals of Motor Systems All mankind can do is to move things whether whispering a syllable or felling a forest Sherrington 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 maturation 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 immature Proper walking coordination followed by running appears later and the basic motor pattern actually continues to develop until puberty The fine details of the motor pattern are adapted to the surrounding world but also to modification by will The basic motor coordination underlying reaching and the fine control of hands and fingers undergo a similar characteristic maturation process over many years The newborn human infant is comparatively immature but other mammals such as horses and deer represent another extreme The gnu an African buffalolike 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 sufficiently mature and available at birth needing minimal calibration This is astounding A similar range of maturity 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 This 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 reflexes 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 reflexes that mediate coughing sneezing and touch avoidance These different patterns of motor behavior are thus available at birth and are due to innate motor programs Fig 28 1 During roughly the first 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 first 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 hundreds 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 sufficient Fundamental Neuroscience Third Edition 663 2008 2003 1999 Elsevier Inc 664 28 FUNDAMENTALS OF MOTOR SYSTEMS 9 Months 4 Months 0 Months 2 Months 15 Months 14 Months 10 Months FIGURE 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 finally walks independently The approximate time at which a child is able to perform these different motor tasks is indicated above each figure The variability in the maturation process is substantial Modified 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 sufficiently 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 SYSTEMS BASIC COMPONENTS OF THE MOTOR SYSTEM few weeks they leave the nest flying rather successfully for the first time in their life and thus without any previous experience In addition to the basic motor skills such as standing walking and chewing humans also develop skilled motor coordination allowing delicate hand and finger movements to be used in handwriting or playing an instrument or utilizing the air flow 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 3 BASIC COMPONENTS OF THE MOTOR SYSTEM Motoneurons and Motor Units The 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 fibers A motoneuron with its muscle fibers is referred to as a motor unit The muscle fibers of each motor unit have similar contractile properties and metabolic profile The muscle fibers 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 activated by interneurons of different motor programs or reflex centers and by descending tracts from the forebrain and the brain stem Thus motoneurons
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