网范文：“Motor development and Maturation theory ”李约瑟(1959)提出,2017多年前的亚里士多德是第一个探讨胚胎发育的人。这篇医学范文讲述了这一问题。他检查不同的生物体的胚胎，英语论文范文，通过开放鸟类的蛋,和通过探讨哺乳动物甚至人类胚胎，在不同的发展阶段。在亚里士多德的哲学中,运动是一个核心的概念,这意味着不仅仅是一个机械过程。在胚胎发育的中枢神经系统,大型神经元,通常产生在小神经元，英语论文网站，在子宫内神经发育期间出现。
在子宫发育期间,大脑皮层的厚度不断增加，通过产后第一个月迅速生长,当孩子四岁左右达到成人厚度。人类的第一个突触出现在23周。下面的医学范文进行详述。
Motor development
Needham (1959) ed that over 2017 years ago Aristotle was one of the first people studying embryonic development systematically. He examined embryos of different organisms by opening up birds’ eggs, and by studying mammalian and possibly human embryos at different developmental stages. In Aristotle’s philosophy, movement is a central concept, which means much more than being a merely mechanical process, as emphasized by Tan (2017) in his psychomotor theory. During embryonic development of the CNS, the large neurons, such as the Purkinje cells in the cerebellar cortex, and the pyramidal cells in the cerebral cortex, are generally produced before the small neurons and the small neurons (interneurons) emerge last during neural development in utero. Also, motor nuclei commence their histogenesis and complete their cell populations before the sensory nuclei (Brown et al., 1997).
The thickness of the cerebral cortex steadily increases during in-utero development, and the Broca’s area rapidly grows through the first postpartum month, reaching the adult thickness when the child is around four years of age. The pyramidal cells in the cerebral cortex actually start to differentiate by 25 weeks of gestation, but without basal dendrites, with only tortuous apical dendrites. The first synapses in the human cerebral cortex appear at about 23 weeks of gestation (Molliver et al., 1973). The pyramidal cell morphology is completed at about three years of the postpartum development of the human cerebral cortex. The small pyramidal cells mature at the last trimester, during which spines also appear on the dendrites, with a tenfold increase in the dendritic trees during the same time span. Finally, as the neuronal projections approach their final structures, regressive events such as cell death, axonal pruning, and synaptic elimination, occur in the central nervous system (Cowan et al., 1984).
Actually, during the embryonic phase of in-utero development, many more neurons are produced than are present in the mature central nervous system. However, while making synaptic connections of their axons, many neurons (about half of them) are eliminated by selective cell death (Oppenheim & Chu-Wang, 1983), which rapidly occurs within a few days. Programmed cell death is genetically determined but may be influenced by functional changes such as motor activity or inactivity and by the size of the target group of neurons (Brown et al., 1997, p. 15). The purpose of the programmed cell death may be the elimination of axons incapable of reaching the target, to avoid or clean the connection errors. With regard to in-utero fetal movements, the first modern concept of the origin of the prenatal movements interestingly originated from Preyer as early as in 1885 (cited by Schröder & Young, 1995).
Preyer concluded that the spontaneous fetal movements may start long before the 12th week of gestation, not being associated with peripheral stimuli, suggesting an important function of this spontaneously generated fetal motility in the ontogeny of the organism. Children were classified according to the development of muscle response synergies, as follows (Woollacott & Sveistrup, 1992; Sveistrup & Woollacott, 1993): pre-sitting in 2-6 months, early pull-to-stand in 7-8 months, pull-to-stand in 9-10 months, independent stance in 10-12 months, independent walking in 12-14 months, and late independent walking in 14- 16 months. Postural control may be impaired in children with cerebral palsy, due to an injury to the cerebral cortex resulting from anoxic or hypoxic encephalopathy, intracerebral hemorrhage, and CNS neuropathy resulting from malformation (Olney & Wright, 1994).
The stretch reflexes are always exaggerated in children with cerebral palsy. The functional significance of these hyperactive stretch reflexes is the subject of debate. Accordingly, Nashner et al. (1983) did not find a correlation between stretch reflex abnormalities and functional ability, and questioned the role of the stretch reflexes in postural control. This is expected from the system perspective: many subsystems are dynamically interacting during postural development within the context of the task and environment. Humans with or without quadrupedalism may share similar CPGs with other quadrupeds, because they are all using the common neuronal control mechanisms for locomotion (Shapiro & Jungers, 1994). Accordingly, Dominici et al. (2017) selected the basic discharge patterns of lumbosacral motoneurons during stepping of neonates, toddlers, preschoolers, and adults.
These authors found two basic patterns of stepping neonates are retained through development… Markedly similar patterns were observed also in the rat, cat, macaque, and guinea fowl, consistent with the hypothesis that, despite substantial phylogenetic distances and morphological differences, locomotion in several animal species is built starting from common primitives, perhaps related to a common ancestral neural network…(p.997). For animals with diagonal-sequence quadrupedal locomotion, these ancestors may be extended to the first emergence of tetrapods during the Devonian period, about 400 million years ago. In other words, the basic neural networks that emerged about 400 million years ago in the first tetrapods during transition from water to land seem to be conserved in animals with diagonal sequence quadrupedal locomotion from primitive quadrupeds up to the primates including human beings. Stuart (2017) emphasized the need to “… give equal attention to issues raised concerning the motor control of a variety of animal phyla, classes and species (Stuart, 1985, p.96; Stein, 1999). It has proven to be heuristic value to indicate when common principles of motor control are present across phyla (Pearson, 1993), when evolutionary deviations have occurred (Fetcho, 1992).
Maturation theory
During the first half of last century motor development was approached by longitudinal studies to show the sequence of motor behaviors in infants and young children. This maturation theory was mainly elaborated by Gessel (1928), Shirley (1931), and McGraw (1932, 1943), who searched for rules governing the order of changes during motor maturation. Konner (1991) stated motor development sequences are largely genetically programmed. Forssberg (1985) explained the disappearance of the neonatal stepping and its reemergence by the maturation of the supraspinal centers, suggesting that the neural code has been retained in humans since the earlier evolutionary periods.
The development of early motor behaviors was also attributed to the maturation of the cortico-spinal pathways (von Hofsten, 1984; Jeannerod, 1988). Although the maturation theory added some valuable information to the basics of motor development, it was far from explaining the dynamics of locomotor development, especially with regard to the behavior in real time and the process of change. In this context, Ulrich (1997) stated it is not at all clear how genetic codes can be translated into even simple patterned neural organization… behavior is much more than a simple neural pattern (p.321). To be able to solve the problems related to development, one must consider the properties of complex systems with many dynamically interacting individual parts. Dynamic systems theory is involved in such an approach, which seeks to understand the behavior of a system, not by taking it as separate parts, but by taking these parts to see under what circumstances they dynamically cooperate to induce a whole pattern (Thelen, 1996). The most important basic characteristic of dynamic systems theory is that behavioral patterns such as locomotion can emerge spontaneously from the dynamic interaction of multiple subsystems or components; detailed plans or neural codes are not represented a priori in the brain, nor are the movement patterns, such as walking and running, from the maturation of fixed CPGs.
From the dynamic systems perspective, the developmental emergence of human locomotion is also self-organizing, as in other complex systems. Multiple subsystems, intrinsic and extrinsic, contribute to behavioral outcomes. for example, neural organization, muscle strength, joint structures and ranges of motion, motivational and arousal levels, the support surface, and the task… the coordination pattern emerges spontaneously, and is self-organized and opportunistic (Ulrich, 1997, p.324). The classical example of self-organization is Thelen’s (1986) discovery showing a-month-old child exerting stepping on a motor-driven treadmill when supported upright. The child’s stepping on the treadmill occurred under the influence of multiple subsystems. That is, the sensory receptors first detect the dynamics of the context, and then send the information to the motor neurons through interneurons to activate the muscle synergies, comprising the intrinsic subsystems that contribute to the locomotor behavior. The treadmill belt, the supplemental postural control and weight support provided by the experimenter are extrinsic subsystems.
Neuronal group selection theory
In the field of motor control, better understanding of neurophysiology caused a gradual shift from the concept that motor behavior is largely controlled by reflex mechanisms towards the notion that motility is the net result of complex spinal or brainstem activity. It was assumed that motor control of rhythmical movements like locomotion, respiration, sucking, and mastication may be based on CPGs. The activity of the networks, which are usually located in the spinal cord or brainstem, is controlled from supraspinal areas via descending motor pathways. The supraspinal activity itself is organized in large-scale networks in which cortical areas are functionally connected through direct recursive interaction or through intermediary control or subcortical (striatal, cerebellar) structures.
Consequently, theoretical frameworks for the processes involved in the development of motor control include two major but current and conflicting theories: neuronal maturationist theories and the dynamic systems theory. There is also a third theory, the neuronal group selection theory, which combines the “nature” part of the neural-maturationist theories with the “nurture” part of the dynamic systems theory (Hadders-Algra, 2017). It was believed in the mid-1990s that the maturation of the CNS progressively occurred through the genetically predetermined neural patterns, in the cephalocaudal and central-todistal direction; the locomotor development was regarded as a result of the progressively maturated and hence increased cortical control on the spinal reflexes.
That is, it was believed that standing and walking result from the cerebral maturation, which is genetically predetermined and not learned by experience. McGraw (1943) considered the locomotor development from a convergent action of “nature” and “nurture”. More controversially, Thelen and Ulrich (1991) did not accept the neural-maturationist theories, asking how can the timetable of motor solutions be encoded in the brain or in the genes? The contemporary ideas about neuronal maturation actually originated from Bernstein (1935), who tried to understand how the CNS solves the problem of locomotor coordination, and argued that the production of locomotion involves hundreds of muscles and joints, which require specific computational techniques of the nervous system. Kelso et al. (1981) utilized the dynamic systems theory to explain the developmental emergence of locomotion in human beings.
These authors suggested that a behavior, such as a locomotor pattern (quadrupedal or bipedal), may result from the combined dynamic effects of, for instance, muscle strength, body weight, postural support, motivation, and brain development, in addition to the environmental initial conditions and task requirements (see also Thelen, 1996; Ulrich, 1997). Transitions in locomotor behavior, best exemplified in the transition from quadrupedalism to bipedalism in some UTS patients (see above), may occur due to the innate dynamics of the complex systems. Thelen (1996) suggested that the locomotor development as a dynamic system may be considered as a self-organizing process, a series of states of stability, instability, and phase shifts in the attractor landscape, reflecting the probability that a pattern will emerge under particular constraints.
In summary, the neural maturationist theories consider the maturational state of the nervous system as the main constraint for developmental progress, whereas in the dynamic systems theory the neural substrate plays a subordinate role (Hadders-Algra, 2017). The variability in developmental processes, such as motor performance, developmental sequence, or the duration of developmental stages, was emphasized to explain the development by the Neuronal Group Selection Theory (NGST) (Edelman, 1993; Sporns & Edelman, 1993). Neuronal groups are collections of many hundreds or thousands of neurons interconnected by excitatory and/or inhibitory synapses as well as the excitatory and/or inhibitory recurrent feedback circuits. According to the NGST, the functional and structural properties of neuronal groups are determined by evolution. However, the repertoires are variable because of the dynamic epigenetic mechanisms regulating cell division, adhesion, migration, apoptosis, and extension and retraction of neuronal arborizations. Behavior and experience, such as during locomotor development, produce afferent information for the cerebro-spinal locomotor system.
This afferent information is used for the neuronal selection induced by changes in the excitatory and inhibitory levels of the synapses and in the intergroup connections within a particular neuronal group. The experiential afferent information induces modifications in the strength of the synaptic connections within and between the neuronal groups resulting in the variable secondary repertoire. The changed connectivity within the secondary repertoire allows for a situation-specific selection of neuronal groups. Thus, the secondary neuronal repertoires and their associated selection mechanisms form the basis of mature variable behavior, which can be adapted to environmental constraints (Hadders-Algra, 2017). The NGST emphasizes the role of the complex information processing originating from an intertwining of information from genes and the environment, which is inconsistent with the “nature-nurture” debate. During motor development in early fetal life, the spontaneous movements (primary variability), i.e., the self-generated motor activity with the consequent self-generated afferent information, may explore all the locomotor possibilities within the neurobiological and anthropometric constraints within the CNS, preserved during evolution.()
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