brain development

brain development - iii/M72 genome genes con— fie...

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Unformatted text preview: iii/M72 genome. genes con— fie' observable as height, eye uch as intel— ‘a'ins, puberty, 3lithe develop— primarily the Environmental behavior has lentical twins. ibility in their such as learn— {these suggest influenced by (chrdle et al. 'al growth and Etic contribu— Will give addi— 1e topic being 1 N a ti mi Chapter 2 Heredity and Neurological Changes at 39 agitai {hangars { injetthm 2.52 it Of all of the systems of the human body, the nervous system is one of the most important. Everything that takes place consciously or unconsciously, voluntarily or as a reflex, has its primary initiation within the nervous sys— tem. Growth, development, and motor behavior all depend on the efficient functioning of this system. The effectiveness of a motor response is signif icantly influenced by the quality and capability of the nervous system and the brain. The nervous system has three primary functions: (a) a sensory function; (h) an integrative function, which includes the memory and thought processes; and (c) a motor function. titre 132.3 a» A basic review of the anatomy of the nervous system is important to the further discussion of the neurological changes that occur across the life span. The nervous system has two major parts: the central nervous sys- tem (CNS), which consists of the spinal cord and brain, and the periph— eral nervous system (PNS), which is made up of all the nerve fibers that enter or leave the brain stem and spinal cord to supply the sensory recep- tors, muscles, and glands. Essentially, the PNS represents the lines of communication, whereas the CNS is the center of coordination and the mechanism that determines the most appropriate response to incoming impulses. {Iientrai Martians The structures of the CNS basically function to transmit information about the environment and the body to the brain, where it is recorded, stored in memory, and compared with other information (Figure 2.1 on page 40). The CNS also carries information from the brain to muscles and glands, thus producing motor responses and the body’s adaptations to environmen— tal demands. The following discussion provides a brief description of selected parts of the CNS deemed most relevant to the study of motor development. seiner. The spinal cord has an essential role in the input and response phases of information processing and motor behavior. Its primary function is to act as a transmission pathway; it carries to the brain all sensory infor— mation from the body and all motor commands sent down from the brain to muscles (and glands). The spinal cord also has an important function in reflex behavior. first; The brain is the principal integrative area of the nervous system. It is the location where memories are stored, thoughts are developed; emo— tions are generated, and complex control of motor behaviors is performed. i I g l I | . i l 40 u Part2 Biological Growth and Development Parts of the brain (side View) Cerebrum Cerebral cortex Cingulate cortex Corpus callosum Thalamus ——- Sensorimotor Basal ganglia cortex Hippocampus Prefontal cortex Cerebellum Pituitary Pons Brain stem Spinal cord The brain stem is that part of the brain primarily responsible for several involuntary (reflexes), metabolic functions, and the regulation of posture. The brain stem also sets the rhythm of breathing and controls the rate and force of breathing movements and heartbeat. Several important fiber tracts pass both upward and downward through the brain stem, transnntung sen— sory signals from the spinal cord mainly to the thalamus and transmitting motor signals from the cerebral cortex back to the spinal cord. The major structures of the brain stem are the pens, midbrain, medulla, diencephalon, and reticular formation. The medulla contains a number of sensory tracts for carrying information to the brain and motor tracts for carrying infor— mation to the muscles and glands. The medulla serves primarily to regulate vital internal processes, such as respiration, blood pressure, and heart rate. The ventral and dorsal parts of the pom contain several nerve tracts that allow for coordination and involuntary influences on automatic movement and posture. The midbraiu is involved with reflex movements caused by visual and auditory stimulation. The dieuceplaalou consists of two areas: the thalamus and hypothalamus. The thalamus is an important integration cen— ter through which most sensory information passes. The hypothalamus is the structure where neural and hormonal functions work to create a con— stant internal environment (body temperature). The retimlarformatiou plays an important role in attention and activation of the individual for cognitive and motor activity. The cerebral cortex is the outermost layer of the cerebrum and is come posed of an estimated 75 percent of the total neurons in the CNS. It is the functional head of the nervous system in its responsibility for higher—order critical thinking and information processing. Basically the cortex mediates (a) the reception and interpretation of sensory information, (b) the organi— zation of complex motor behaviors, and (c) the storage and use of learned experiences. The motor areas of the cortex (motor cortex) play an integral role in planning and executing coordinated movements. The motor cortex orimotor x Jeampus bellum :ord : for several of posture. he rate and fiber tracts nitting sen— Iansmitting The major encephalon, nsory tracts tying infor- ' to regulate i heart rate. : tracts that : movement 3 caused by '0 areas: the gration cen~ otbalamm is reate a con- mation plays or cognitive and is com— STS. It is the uglier-order :ex mediates the organi— e of learned ' an integral notor cortex , 5’ Chapter 2 Heredity and Neurological Changes in 41 occupies the posterior half of the frontal lobe and consists of the primary motor area, premotor area, and supplementary motor area. In general, this structure controls the specific fine motor muscles (e.g., muscles ‘of the hand, fingers, feet, and toes). The primary motor area is responsible for the actual execution of movements. In addition, this structure has a critical role in the control of speed and force of actions. The premotor area is linked to work- ing memory, making it possible to plan and guide movements. The pre— motor area also plays an important role in advance planning and coordination of complex movement sequences. The supplementary motor area is involved in the preparation for movement, especially when the actions to be executed are internally generated as opposed to being elicited by sensory events. For example, the supplementary motor area is engaged when imagining (using motor imagery) before executing. The basal ganglia area of the brain is made up of a group of nuclei located in the inner layers of the cerebrum. The basal ganglia integrates the sensory motor centers and is involved with unconscious behavior, such as the maintenance of muscle tone required in upright posture. It also plays an important role in planning and coordinating movements. A major func— tion of the basal ganglia is to control very fundamental gross body move— ments, whereas the cerebral cortex plays prominently in the performance of more precise movements of the arms, hands, fingers, and feet. The cerebellum is an important part of the motor control system. Even though it is located far away from both the motor cortex and the basal gan— glia, it interconnects with both of these areas through special nerve path— ways. It also interconnects with motor areas in both the reticular formation and spinal cord. Its primary fimction is to determine the coordinated sequence of muscle contractions during complex movements. Its functions are also associated with vestibular awareness (balance), postural adjustments, and reflex activity. It should be stressed that the emerging motor behavior resulting from the processes described above is the outcome of an interaction among the many subsystems within the CNS. This interaction is further shaped by the goals of the task and environmental constraints (Rose & Christina, 2006). it? The peripheral nervous system (PNS) is a branching network of nerves. It is so extensive that hardly a single cubic millimeter of tissue anywhere in the body is without nerve fibers. The PNS is divided into two systems: the somatic and the autonomic. The somatic system controls all the skeletal mus— cles (contracted through voluntary initiation); the autonomic qstem is prima— rily responsible for regulating the automatic functioning of the smooth muscles of internal organs, such as the heart, liver, lungs, and endocrine glands. The activities of the autonomic system are seldom subject to vol— untary control. PNS nerve fibers are of two functional types: afiizreat fibers for trans— mitting sensory information into the spinal cord and brain and efiferemfibers for transmitting motor impulses back from the CNS to the peripheral areas, 42 Part 2 Biological Growth and Development Qhfieesiee 2.4 a» {Silageerive 2.5% Tn especially to the skeletal muscles. The peripheral nerves that arise directly from the brain itself and supply mainly the head region are called cranial nerves. The remainder of the peripheral nerves are spinal nerves. A neuron (nerve cell) is the basic structural unit of the nervous system. There are billions of neurons in the nervous system; the average neuron is a complex structure that has thousands of physical connections with other cells. Nerve impulses travel along neurons to relay information from one cell to another (and through the nervous system). The junction between two cells across which the information must pass is called a synapse. The three basic parts of the neuron are the cell body (soma), dendrites, and axon (Figure 2.2). The cell body (ram) is the metabolic center of the cell. It contains the nucleus, which is responsible for regulating the various processes of the cell. Neuron cell bodies are located mostly within the central nervous sys— tem. Within the CNS, clusters of cell bodies are called nuclei. A dendrite is a nerve fiber that extends from the cell body. Each neuron can have from one to thousands of dendrites. Dendrites are the receiving part of the neu— ron, serving the important function of collecting information and orienting it toward the cell body. Although many dendrites branch from the cell body of a neuron, there is only one axon. The axon is the nerve cell struc— ture that carries information away from the cell body to other cells. As pre— viously noted, nerve impulses travel along neurons to transmit information. An impulse is an electrochemical process that travels in a chainlike sequence from the dendrite or cell body of one neuron to its axon, to the dendrite or cell body of another neuron, and so on, through the length of the nerve tract. The functional connection between the axon and another neuron is the synapse. Neurons are generally classified according to their function. Afferent neurons, also referred to as sensory neurons, carry nerve impulses from the sensory receptors into the spinal cord or brain of the central nervous sys~ tern. Efiferenr neurons transmit impulses from the central nervous system to the muscles and glands. Efferent neurons passing impulses to muscles are commonly called maronenmnr. Over 95 percent of all neurons are classified as inrennenrans. This type of neuron originates and terminates solely within the CNS. Along with the neuron, the other basic types of nervous system tissues are supporting and insulating cells. These cells have the important function of holding neurons in place and preventing signals from spreading between the neurons. In the CNS these cells are collectively called nenroglia’ and referred to specifically as glz'nl cells. in the PNS they are referred to as Schwann cells. Sebwann cell: wrap myelin sheaths around the large nerve fibers, thus insulating the pathway for an electrochemical nerve impulse. ‘ {Miriam} Each motoneuron axon branches into several synaptic terminals, and each of these terminals provides the nerve supply to a muscle fiber. A neuron and all the muscle fibers innervated by ‘ise directly died mania] be nervous he average :onnections nformation 1e junction :d a synapse. dendrites, ontains the sses of the ervous sys— t dendrite is have from of the neu— d orienting m the cell : cell struc— :lls. As pre— iformation. :e sequence 1e dendrite If the nerve r neuron is )n. Afferent as from the ervous sys— 5 system to nuscles are re classified )lely within :tem tissues nt function [1g between uroglz'a and erred to as large nerve impulse. inches into s the nerve rervated by lagers The basic parts of a neuron Chapter 2 Heredity and Neurological Changes 3% 43 Dendritic ZOnB Cell body Nucleolus Neurofibrils Collateral Axonai zone Direction of d t' CO“ “C Ion Schwann cell Nodes of Flanvier (gaps) End tufls Myelin zone (boutons or buttons) it are referred to as a motor unit because all the muscle fibers contract as a unit when stimulated by the motoneuron (Figure 2.3 on page 44). Each of the muscle fibers making up the small delicate muscles of the eye may be supplied by a rnotoneuron, but larger postural muscles may only have one motoneuron to supply as many as 150 muscle fibers. The nerve tracts of the spinal cord together with the spinal nerves provide a 2—way line of communication between the brain and parts of the body outside of the nervous system. The tracts that conduct sensory impulses “to the brain” are called ascending tmctr. Tracts that conduct motor pulses “from the brain” to motor neurons that control muscles are the descending tracts. Both tracts are comprised of axons. Typically, all the axons within a given tract originate fiom neurons located in the same part of the nervous system and end together in some other part. For example, 44 fl Part 2 Biological Growth and Development figure A motor unit fingertiva as Ea»- Motoneuron a spinothalamic (ascending) tract begins in the spinal cord and carries sensory impulses associated with pain and pressure (touch) to the thalamus of the brain. Another neuron then relays the information to the cortex. The corticospinal (descending) tract originates in the cortex and carries motor impulses downward via the spinal cord and spinal nerves. These impulses control skeletal muscle movements. Ascending tracts are also referred to as afferent or sensory pathways, while descending tracts are associated with efferent or motor pathways. HEN;§>§.E€:'?§{}al. Neurons send information through their axons in the form of brief impulses, or waves, of electricity in the form of single electrical clicks called action potentials. Nerve conduction velocity is the speed at which information travels; this rate is greatly affected by the pres— ence or absence of a material called myelia around the axon. Myelin is a fatty material that forms a sheath around many axons both within and out- side of the CNS. The myelin sheath is formed from a type of insulating cell called a Srbwann cell. These cells wrap around the axon and form a jelly—roll-type structure that serves as an effective insulator of electrical currents. The myelin sheath is interrupted every millimeter or so by gaps called males of Raneier (see Figure 2.2 on page 43). The neuron membrane is active only at the nodes, so the impulse is conducted when the action potential jumps from one of these nodes to another. This type of impulse conduction is known as salmtmy conduction and is significantly faster than conduction in nonmyelinated axons. Saltatory conduction also requires less metabolic energy, enabling myelinated axons to fire nerve impulses at higher frequencies for longer periods of time. Research has also indicated that there is higher conduction velocity, lower threshold, shorter latency, and higher amplitude coincident with the time of appearance and the degree of myelination. Another factor in conduction velocity is the size of the axon. Basically, the larger the cross— sectional diameter, the faster the speed of conduction. Apparently axon conduction velocity is related to the urgency of the information it is called upon to transmit. Axons that have greater speed potential are concerned Chapter 2 Heredity and Neurological Changes in 45 with the control of movement, especially in the mediation of rapid reflexes; % . axons that transmit visceral information are small, generally unmyelinated, and slow. iifliilfz’ i}EifEiflPii/iiii‘llméii fifiibfififli ,m % vegetative 2,? it A mature brain contains more than 100 billion neurons intricately con~ and carries he thalamus the cortex. and carries rves. These cts are also g tracts are air axons in ‘m of single Deity is the by the pres— Myelin is a iin and out- )f insulating and form a of electrical T so by gaps 1 membrane a the action 3 of impulse r faster than ry, enabling s for longer r conduction e coincident . iother factor ‘er the cross— irently axon n it is called e concerned nected with one another in ways that make possible the amazing func— tions that underlie our behavior. Perhaps most remarkable is the precision of neural circuitry (wiring) that develops between connections that occur; each neuron links up with thousands of others to form trillions of. con— nections. The total length of wiring between neurons is estimated at 62,000 miles (Coveney 8t Highfield, 1995)! It was once believed that the wiring diagram for each person was predetermined by one’s genetic blue- prints. However, the contemporary perspective is that although the main circuits may be prewired for responses such as breathing, control of heart— beat, and reflexes, stimulation from the environment can shape the tril~ lions of finer connections that complete the architecture of the brain. This critical effect, which has profound implications for our nature—nurture theme and motor development in general, will be discussed in the latter part of this section. Normal CNS development appears to follow a dynamic sequence of integrated biological events; these include the processes of cell proliferation, migration, intogmtz'm and dzfiéremiotion, myelimtim, and cell death. Basically the developmental process begins with immature neurons (cell prolifera— tion). These cells become specified regarding their function and location within the system. When their location has been determined, the different cell types migrate to various sites and integrate with other cells. At the site of integration, neurons begin to elaborate axons and dendrites in prepara— tion for establishing the functional connection (synapse) between cells (dif— ferentiation). In their final stage of morphological development, most nervous system pathways become coated with myelin to allow them to transmit impulses more effectively. Many neurons are eliminated during early development of the nervous system (cell death), which is believed to be a normal part of the development process. It should be reemphasized that while the developmental. sequence is rather exact, it is also a complex and intricate process. The sequence described only represents highlights in the process. The following discus— sion elaborates in greater detail significant events that occur Within the sequence of early neurological development. {Zeii itersiiteration and Neurons first appear-in the brain during the second prenatal month, and virtually all of the cell proliferation process (growth in number) is completed by birth. At this time, a baby’s brain contains about 100 billion neurons and 46 a Part 2 Biological Growth and Development figure- The environment will play a major roie in developmental outcome. a trillion glial cells, which form a honeycomb that protects and nourishes the neurons. Not only do neurons multiply very rapidly during early devel— opment, but they also grow in size as well. The period of dramatic increase in neuron size seems to occur from the sixth prenatal month through the first year of life. Beginning around the third trimester of gestation and continuing into at least the fourth year is a period of rapid brain growth and development called the brain growth sport. The general period also includes development in terms of cell proliferation, myelination, dendritic and synaptic growth, and refinement of certain enzyme systems. A considerable body of evidence suggests that the brain has a critical period for laying down its foundation (wiring of circuits) for optimal development. After this critical period, there are limits to the brain’s ability to reorganize (plasticity). It is also important to note that the timing of the growth spurt cannot be altered; the time line for development is predetermined genetically. However, as will be dis— cussed later, the extent of development is activity—dependent, that is, it is sig— nificantly affected by experience and stimulation. This fact points to the importance of prenatal care and a stimulating postnatal environment, espe— cially during the early years. d nourishes early devel— itic increase hrough the inning into evelopment evelopment utic growth, of evidence foundation eriod, there 3 important 1e time line will be dis— is, it is sig— )ints to the ment, espe— Chapter 2 Heredity and Neurological Changes a 47 The genetic structure of the cell nucleus plays a primary role in con— trolling cell proliferation and differentiation. About half of the body’s total genes are involved in forming and maintaining the CNS. Although their role during growth is vital to the wiring of basic functional circuits, there are simply not enough of them to specify the trillions of finer connections made after birth. Along with the process of cell proliferation, several developmental changes occur in the axon and dendrite structures. Once the neurons have migrated to their final location in the CNS, they begin to elaborate their axon and dendritic structures in readiness to accept impulses through synap~ tic interconnections. Again, neurons may be categorized as sensory (affer— ent, sending signals to the CNS), motor (efferent, sending signals from the CNS to the muscles), or central (originating and terminating in the brain or spinal cord; also known as interneurons). This specification is likely bio— chemically set. The axons of sensory neurons must often travel relatively long distances to reach their synaptic conjunction. Terminal targets of motor neurons do not appear to be as specific as those for sensory neurons, but complex specificity is still evident in the growth process. The growth of the axon of a motor neuron is guided some— what by the chronological order in which it matures and differentiates as well as by its biochemical properties. When a motor neuron innervates a muscle, the axon makes contact and induces biochemical specificity into the muscle, thus allowing the two structures to match up biochemically. Once specificity of the neuromuscular junction is established, sensory and central neurons form their synaptic connections. One of the major events in cortical neuronal differentiation is the elab— oration of dendritic structures. Dendrites are important because they are the main receptors for the neuron. At the synaptic junctions the dendrites of each neuron can receive signals from literally thousands of other neu— rons. It has been estimated that dendrites of cortical neurons provide more than 95 percent of the targets for transmitting information through the system. Not until approximately the eighth fetal month do the first signs of thick dendrites that have conspicuous spines start to appear on cells in the visual cortex. Even though the motor cortex is noted as being more advanced in dendritic development than the visual cortex at that time, the appearance of dendritic spinedevelopment is not as evident. At around 30 weeks of gestation, the dendritic spines show obvious immaturity; they are few in number and irregular in shape. But by 8 months (postnatal) the number of well—formed, lollipop—shaped, stubby spines has multiplied. Fig— ure 2.5 on page 48 represents the various stages of dendritic development in large pyramidal neurons (Lund, 1978). integratinn and As cells begin to multiply, migrate to their final location, and elaborate, they also undergo the concurrent processes of integration and differentiation (i.e., wiring of the brain). Integration refers to the intricate interweaving of 48 as 'Part 2 Biological Growth and Development Representation of dendritic development neural mechanisms toward their target destination. Differentiation, on the other hand, is the process by means of which structure, function, or forms of behavior become more specialized. This term also refers to the progression of motor control from gross, poorly controlled movements to precise, com— plex motor behavior. Differentiation cannot occur until synaptogenesis has taken place (i.e., the synapses between neurons have formed). Estimates for the average number of neuronal connections range from 1,500 to 15,000, depending on the type and function of the neuron, pro— ducing trillions of connections. The formation of synapses occurs at differ— ent times and in different parts of the brain, with the sequence tied with the emergence of various functions and skills (Chugani, I998). Neuronal connections begin to form during embryonic development, when each dif— ferentiating neuron sends out an axon (the transmission lines of the nerv~ ous system) tipped at its leading point by a gmwa cane. The cone migrates through the dense environment to its synaptic targets. Current theory sug- gests that the growth cone navigates itself (at times, relatively long dis— tances) to the target using guidance mechanisms (cues) involving various chemical attractor and repulsion processes (Tessier—Lavigne & Goodman, 1996). By the time of birth, the infant’s brain has already formed trillions of neuronal connections (synapses). Areas already Wired prior to birth include functions for breathing, circulation, heartbeat, reflexes, and basic (spontaneous) movement. In the first months of postnatal life, the number of connections will increase 20—fold. Synaptogenesis begins in the motor cortex at about 2 months of agewa time when basic reflexes (e. g, startle and ‘ ‘m '“as C»). m tion, on the , or forms of progression recise, com- :genesis has : range from reuron, pro— urs at differ- .ce tied with 3} Neuronal Len each dif— of the nerv— one migrates t theory sug— aly long dis— lving various 3: Goodman, med trillions tior to birth es, and basic , the number in the motor g, startle and figure Average glucose utilization levels in 4 areas of the cortex SOURCE: Adapted from Chugani (1998) Chapter 2 Heredity and Neurological Changes % 49 rooting) begin to phase away and purposeful movement emerges (e.g., reach— ing). Tied closely to this event is synaptic formation in the visual cortex at about 3 months, allowing the infant’s eyes to focus on an object. Shortly after, the brain, in a phase of biological exuberance, begins to produce tril— lions more connections than it can possibly use. Around the age of 10 years, the brain selectively eliminates (also referred to as pruning) connections that are infrequently or never used. Some interesting data reported by Chugani (1998) provide what has been referred to as a glimpse of the period of exuberant connectivity and a testament to the general critical period for wiring of the brain (Figure 2.6). These data show the human brain’s consumption of glucose (metabolic rate of glucose utilization) from birth to adulthood, As shown, the rates at birth through about 10 years are more than twice that of adults. The suggestion is that these high levels of consumption are devoted to extraordinary activ— ity in neurogenesis, more specifically, an increase in the niunber of den— drites per neuron and synaptogenesis. Supportive data comparing the brains of children who died early show similar age—related developmental change (Huttenlocher 8r Dabholkar, 1997). For example, in one layer of the visual cortex, the number of synapses rise from around 2,5 00 per neuron at birth to as many as 18,000 about 6 months later. While different areas of the brain develop at different rates, in general, there is a rise in the rates of glucose utilization from birth until about age 4, at which time the child’s brain uses more than twice as much glucose as that of an adult. From 4 to about 9 to 10 years these levels are maintained, after which there is a grad— ual decline. Once again, comparative data with animals and cadavers show that after this period of exuberance begins a pruning phase, in which sur— viving neurons show a decline in synaptic density Hypothetically, neurons LCMFigIc (umol/minHOOg) 0 2 4 6 8 10 12 14 16 Adult Age (years) 50 a Part 2 Biological Growth and Development stimulated adequately during the critical period show less elimination and support the idea of activity—dependent stabilization. Correlations between regional glucose consumption and behavior provide much of the basis for what has become known as the optimal windows of opportunity for wiring and efficient learning. As alluded to in Chapter 1 and in this section, scientists now believe that to achieve the precision of the mature brain, neural function and stimulation during infaHCy and early childhood are necessary. That is, optimal development is activity—dependent (e.g., Nelson, 2000). Dr. Bruce Perry, a noted researcher in this field, sums up the find— ings on this topic by stating, “Experience is the chief architect of the brain” (Time, February, 1997; refer to the set of articles in the suggested readings regarding early brain development for further reference). The general notion is that while genetics plays a major role in deter— mining the main neural circuits of the brain, positive stimulation signifi— cantly influences the trillions of finer connections made after birth. Experience appears to exert its effects by strengthening synapses. That is, stimulation from the environment produces a signal that forms various connections. Connections that are not made, or are weak, are pruned away. If the neurons are used, they become integrated into the circuitry of the brain. Thus, due to differences in experience, not even identical twins are wired the same. How do scientists know this? They invasively examine (invade tissue of ) the brains of animals after experimental manipulation and noninvasiver study infant brain structure and activity using sophisticated neuroimaging techniques. Most research on this topic has been conducted with regard to development of the visual system in mammalian species (cat, rat, and fer— ret). Although the measurement techniques are more sophisticated, several modern studies have followed the general experimental manipulation pop— ularized by Hubel and Wiesel in the 19705. That is, either one or both eyes of the animal are deprived of stimulation prenatally or postnatally for a period as long as several weeks. Scientists then make comparisons of anatomical and physiological changes to those of control animals, or they compare visual cortex areas within the animal. Common observations include (for example) number of synapses, neuron and axon size, and bio— chemical characteristics. The general conclusion from these experiments suggests that optimal functional and structural organization of the brain (wiring of the visual system) requires a level of stimulating experience. In addition, several interesting experiments have compared rats raised in vari— ous environmentally enriched settings (e.g., toys, treadmills, and obstacle courses) to rats kept in deprived environments (e.g., isolated and confined movement). These studies also confirm that stimulation (of this general type) is a significant factor in brain development (Cotman & Engesser— Cesar, 2002; Helidelise et a1., 2004). Studies with humans provide a provocative parallel. For example, researchers have measured the size of cortical and subcortical areas of a iination and His between he basis for y for wiring this section, 3.er brain, ildhood are :.g., Nelson, up the find— if the brain” ted readings )le in deter— tion signifi— after birth. ses. That is, rrns various ironed away. :uitry of the :al twins are nvade tissue .oninvasively euroimaging th regard to rat, and fer— ated, several ulation pop— one or both ustnatally for 1parisons of 1als, or they )bservations ze, and bio— experiments of the brain :perience. In llSBCl in vari~ and obstacle lIlCl confined this general 3t Engesser— or example, 511 areas of a Chapter 2 Heredity and Neurological Changes 51 child’s brain, as related to stimulation or environmental deprivation. These studies, in general, found that the human brain grows in size, develops complexity, makes synaptic connections, and modifies as a function of the quality and quantity of sensory experience. It is speculated that during infancy and early childhood, exercise and play provide vital sensory and physiological stimulation resulting in increased nerve connections. This general effect (of stimulation) has also been used to explain results of stud- ies conducted in more naturalistic settings. In addition, Craig Ramey found that intensive early education using blocks, beads, and a variety of games has long—term positive effects on IQ and academic achievement. This con— clusion is the result of three separate studies conducted on children aged 4 months to 8 years who live in disadvantaged environments (Ramey & Ramey, 1994). Perhaps most noted is the finding that the earlier the chil— dren enrolled in the program, the more enduring the long—term result; those enrolled after age 5 derived little comparative benefit, suggesting a critical period effect. There is no question that this line of research has added a great deal to our understanding of brain development, and the future looks quite promising. However, at this point, it is still quite unclear as to the specific types of experience and how much is optimal to stimulate the formation of particular neural connections. . , . ~: , Recall from our discussion of neuron proliferation that virtually all neurons are present at the time of birth. A long—standing belief was that neurons were produced only during the embryonic and fetal periods and for only a short time after birth, if then. However, due in large part to new techniques for observing brain processes, that belief has changed dramatically. Researchers now know the brain does indeed generate new neurons on a rather regular basis throughout much of the lifespan. The number of neurons is relatively small, and the location appears to be limited to the hippocampus, which is involved in memory and learning processes. Most interesting, however, is compelling evidence that environmental manipulations affect adult neuro— genesis and improve learning (Van Praag et 31., 2005). This has been shown in animal studies as evidenced by exercising mice (rimning on a treadmill) that produce 2.5 times the growth of new neurons compared to animals housed in more sedentary conditions. This line of research supports the theory discussed earlier that environmental manipulations may affect neu— rogenesis in a positive manner. a a ,. seiyeiumtinn The developmental process of myelination has been one of the most exten— sively examined indicators of neurological growth. .Much of the early growth in brain size and weight can be attributed to myelination. Evidence has already been presented to show that neurons have slower transmission rates before myelination, are more prone to fatigue, and are more limited in 52 % Part 2 Biological Growth and Development liliivement ."m.ttag'ttgtiaag"“attain; Giants mumps—saver and Trainer (2005) found that having 7~1nonth—old babies move with their mother to various beats influenced rhythm per— ception. It was suggested that movement played an important role in helping Wire babies’ brains to hear rhythm. The researchers tested the infants by having them listen to music made by a snare drum and sticks that had an ambiguous rhythm—no accented beats. Half of the moth— bounced to a song or rocked to a lullaby suggest that there is a strong vestibular— auditory interaction that is critical for the deveiopment of human musical behavior. It has long been known that infants are attracted to music and responsive to its emotional content. This work provides evidence that the experi- ence of body movement plays an important role in musical rhythm perception. ers bounced their infants on every second heat, in a marchlike rhythm, and the other half bounced their infants on every third beat, in a waltzljke rhythm. Then the researchers played the music again, this time with the beats accented in either the march or waltz pattern. The infants preferred to listen to the pattern that matched how they had been bounced. The researchers concluded that the perception and development of musical rhythm comprise a muiusensory experience that likely reflects a strong interaction between auditory and vestibular (movement) information in the human nervous system. Underscoring the importance of movement in developing rhythm, the researchers contend that how we move may influence what we hear. Early development of the vestibular system and infant delight at vestibular stimulation when SOU RCE Phiilips—Silver, J., 8: Trainer. L. J. (2005). Feeling the beat in music: Movement influences rhythm perception in infants. Sciatica, 308, 1430. their rate of repetitive firing. The relative rates of myelination in different areas of the brain give a rough estimate of when these areas reach adult levels of functioning. The degree of myelination is also closely related to maturation (and readiness) in acquisition of motor skills during early child— hood. The ages at which rnyelination begins and ends seem to vary from one brain structure to another, as does the time required for the myeiina— tion process itself. The formation of myeiin begins in the spinal cord about halfway through fetal development, then continues through adolescence and adults hood and, in some areas of the brain, perhaps into old age, Figure 2.7 pro— vides an approaimate time line of the myelination process in selected structures and pathways within the CNS. Both sensory and motor roots begin myelination 4 to 5 months before birth. The motor roots are the first parts to develop myelin; the sensory structures exhibit a rapid increase about 1 month later. Motor mechanisms of the spinal cord appear to be fuliy t. It has )ntent. :peri— tant role 1g the beat 3 eption in , in different reach adult 1y related to 5 early child— to vary from the myelina— Jout halfway :e- and adult— gure 2.7 pro— 5 in selected motor roots :5 are the first ncrease about 1r to be fully figure: Structura/ Pathway Motor Rants Sensory Roots Optic Radiation Samesrherlc Radiation Motor Tracts (Descending) Great Cerebral Commiuuros Assodafiun Areas Reticuiur Formation Birth am —"III ' V2 "" Chapter 2 Heredity and Neurological Changes & 53 #llllli *Widlh and length of bars indicate increasing density of myelinai'lon; blacked-in areas at end of bars indicate approximate age range of completion of myeilnel'ion process. An estimated time line of the myeiination process myelinated and functional by the end of the first month after birth. Once again, the sensory mechanisms lag behind somewhat and do not show sig— nificant myelin growth in the cord until approximately 6 months after birth. The primary sensory tracts to the cortex appear to mature at slightly different times during the life span. The myelination of the visual pathways begins around the time of birth. However, once the developmental process of the visual pathways begins, it proceeds very rapidly and is completed sometime during the first 5 months of postnatal life. The higher somes— thetic pathways related to touch show myelin growth around the eighth prenatal month, and by birth these pathways are myelinated through to the cortex. Evidence of this level of development is exhibited by newborns who are normally quite sensitive to touch stimuli. Significant growth continues in the somesthetic pathways until approadmately 2 years of age. The descending motor tracts, the major efferent pathway from the motor cortex, begin myelination a month before birth. Although the pathway does not achieve full maturity for about 2 years, it is probably functional by 4 to 5 months, when intentional (voluntary) motor behavior can be observed in the infant (Bushnell, 1982; Kinney et al., 1988). The part of the brain that integrates information (cerebral commissures) exhibits a rapid increase in myelin about the third month after birth, and this process continues until approximately age 10. The reticular formation associated 54 a; Part 2 Biologicai Growth and Deveiopment with the attention processes begins a period of rapid myelination at birth and continues to mature in an individual unn'l sometime after the second decade. This observation has led to the belief that a person’s capability to selectively attend to a task is still being modified until early adulthood. Last to undergo myelination are the areas of the brain associated with memory. These structures, called association areas, begin to show significant myelin growth that starts around the third month after birth and continues into and beyond the third decade of life. The development of myelin and the characteristics of early motor behav— ior present an interesting developmental parallel. Prior to the initiation of voluntary motor control, movement is exhibited in the form of myagenic, newngem’c, and reflex behaviors. Myogemr behavior is movement that is the reSult of direct muscle stimulation rather than stimulation that arrives at the muscle through some intervening neural structure. Myogenic movement can sometimes be elicited in the fetus prior to the appearance of myelin. After the appearance of myelin in the spinal cord at about the third or fourth fetal month, movement can be elicited through the motor neurons connection with the muscle. This type of behavior, which is affected by neu— ral structures, is called mamgenir behavior. When myelination is more com— plete in the spinal cord, reflex am appear, meaning that the fetus can receive sensory input and reflexively translate the information into a behavior or motor response. Touch stimulation can elicit such movement behaviors as the primitive grasp reflex of the hands and the Babinski reflex (see Chap— ter 8) of the feet. By the eighth fetal month the spinal cord is close to fully myelinated, and the direction of growth proceeds upward toward the higher brain regions such as the medulla, Inidbrain, and thalamus. At this point several important developmental motor responses can be observed: respira— tory movements, primitive sucking, the Moro reflex, tonic neck reflexes, and other righting reflexes. At birth and during the neonate period, and as the motor pathways from the cortex are myelinated, voluntary motor behavior becomes possi— ble. Since the somatosensory pathways are the most advanced of the sen— sory tracts, the infant responds readily to tactile stimulation on almost any part of its body. Although myelination of the optic tracts trails somewhat behind at this point, the neonate is capable of visual fixation and simple tracking movements as well as some form and depth perception awareness. Myelination of the auditory pathways develops at a slower rate than the other sensory tracts; therefore, in comparison, the infant’s responses are less mature (e.g., exhibiting primarily gross reactions to a sharp, loud sound) during the first month of life. Although we may never be able to get an honest count of neuronal loss, researchers do believe that there is considerable cell death, or natural elim— ination of neurons, during early development and as we age. During early development some structures may lose 40 to 75 percent of neurons initially :ion at birth ' the second capability to lthood. Last ith memory. icant myelin ntinues into notor behav— initiation of of myagenic, .t that is the irrives at the c movement of myelin. the third or ntor neuron’s :cted by neu— s more com~ s can receive behavior or behaviors as K (see Chap— close to fully rd the higher at this point 'ved: respira— eck reflexes, :or pathways comes possi— i of the sen— n almost any ils somewhat 1 and simple in awareness. 'ate than the esponses are , loud sound) ieuronal loss, natural elim— During early irons initially Chapter 2 Heredity and Neurological Changes fi 55 1. With proliferation, the brain produces many more neurons than it needs, then eliminates the excess neurons. 2. These neurons migrate to their final location. 3. Once cells have reached their target destination (integration), they spin out axons that connect to other neurons via dendrites (synoprogenesis). Spontaneous bursts of electrical activity strengthen some of these connections, while others die. 4. During this process, myeiinotion (insulation of the neural pathway) and differen- tiation (greater specialization of function) occur. 5. After birth. the brain experiences a second growth spurt (in neuronal size) re- flected by an explosion in the number of dendrites, that is, new connection sites. Additional sensory stimulation fine—tunes the brain’s circuitry by making and strengthening new connections. 6. Connections not made or stimulated adequately are pruned away. 7. Ceiideoth is a natural, lifelong process. generated. Cell death of neurons in this instance is believed to be a normal part of the developmental process of establishing synaptic. connections (i.e., differentiation), first in a phase of synapse overproduction, followed by selective elimination (i.e., pruning) or preservation to yield a more spe- cific neural connection. It has been hypothesized that proliferating cells compete with each other for a limited number of synaptic sites or for some function that is vital to their existence. Table 2.2 provides a summary of the sequence of early neurological development. {fjriticei {Wintitwaa {songwriters} One of the strongest implications of the research described is the observa— tion that during the developmental period of exuberant connectivity, there is a relatively high degree of brain plasticity. That is, there are critical periods in neuronal development in which experience may be most effec~ tive in forging connections (wiring the brain). From another perspective, these critical periods have more recently been referred to as windows of opportaaigr, the theory that nature opens certain Windows for the experi~ ence effect starting before birth and then closes each opportunity, one by one. In theory, there are a series of windows for developing motor control, vision, language, feelings, and so on. With increasing age, the brain’s plas— ticity declines. The child who misses an opportunity may not develop the brain’s circuitry to its full potential for a specific function. For an applied example, recall the Ramey research noted in the previous section; the-crit— ical period for early intervention was before age 5 (not in the 5— to 8-year— olds). Figure 2.8 on page 5 6 shows estimates of the critical periods (windows of opportunity) for different functions in which neuronal development is most receptive to experience. Keep in mind that these projections are esti« mates of general functions. Part 2 Biological Growth and Development (Age in years) 3 4 5 Motor development: Gross motor Fine motor Vision Math logic Vocabulary Music Second language _J figure 2.8 Windows of opportunity SOURCE: Data compiled from Newsweek (1997) and Time (1997) articles noted in Suggested Readings at the end of the chapter Most relevant to our discussion is the general window of opportunity estimated for motor development. For basic motor skills, it appears to be from the prenatal period to around the age of 5 years. Keep in mind once again that this is a period in which experience is vital to forging the foun— dation of brain circuits dedicated to motor control. Other factors such as myelination are also important. The main circuits for reflex (involuntary) behavior are wired prenatally. The window for the primary circuits that control posture and general movement, housed in the cerebellum, is open for about the first two years. It is during this time that children begin to gain considerable experience in the world as they move about in the envi— ronment. Once again, this suggests that physical activity may be a strong determinant in early development of the brain, not just motor control. In regard to finer muscle control and timing (which follow gross—motor devel— opment), it seems reasonable that the general window would be open from shortly after birth to about age 9. Also evident in Figure 2.8 is general clo— sure of the window of opportunity around the age of 10, especially for sec— ond language development. This is the period when the balance between synaptic connectivity and pruning abruptly shifts. caresses as a— reassess as as Throughout brain growth from early periods, the appearance of function approximates maturation in structure. Keep in mind that structure is influenced by experience, therefore making age predictions gross estimates. However, general trends have been docu- mented. The adult brain weighs about 31/2 pounds, and although it makes up only about 2.5 percent of the total body weight, it requires 15 percent 10 " opportunity rppears to be n mind once ,ng the foun— ctors such as (involuntary) circuits that llum, is open lren begin to t in the envi— y be a strong )r control. In -motor devel— be open from s general clo— ;cially for sec— ance between vrh from early 1 in structure. refore making re been docu— augh it makes res 15 percent Chapter 2 Heredity and Neurological Changes 57 of the body’s blood supply and about 25 percent of all the oxygen con" surned. From 2 to 8 weeks following conception, the nervous system begins to develop as a long, hollow tube on the back of the embryo. As the sys— tem develops, brain size increases into a mass of neurons, losing its primi— tive tubular appearance. At birth, the infant’s brain weighs about one fourth of its adult weight. After birth, nerve cell size increases, other supporting cells called neuroglia are formed, and the myelin develops, causing the brain to double in volume. By age 3 the brain has reached nearly 90 per— cent of its adult size, and by age 6 it has basically achieved its full size. In contrast, total body weight at birth is just 5 percent of young adult size, and by age 10 body weight is only 50 percent of the weight that will even— tually be attained. The part of the brain most fully developed at birth is the midbrain. As previously mentioned, the midbrain is the part of the CNS that controls much of the early reflex behavior. The midbrain, pons, and medulla occupy approximately 8 percent of the total brain volume at 3 fetal months of age, but by birth this proportion has fallen to around 1.5 percent. During the first decade of life, these percentages increase slightly due to fiber tract growth. After the midbrain (in developmental progression) comes the cere— brum and, considerably later, the cerebellum. Growth and differentiation of the cortical regions of the brain are land— mark features in the functional maturity of the CNS. As previously noted, the cortex is composed of an estimated 75 percent of the total neurons found in the brain; this is an estimated 75 billion, interconnected by thousands of miles of axons and dendrites. Yet in actual size, the cortex accounts for only a very small portion, forming the outermost layer of the cerebrummabout one—fourth inch in thickness. While thickness of the cortex normally reaches adult levels by approximately 21/2 years of age, functional maturity of the cortical areas in general is usually not achieved until later early childhood. According to Tanner (1990), 2 clear gradients of development oceur during the first 2 years after birth. First is the order in which general func- tional areas develop, and second is the order in which bodily localizations advance within the areas. The rate of development among the cerebral lobes is quite varied. Each lobe has its own rate of development, each area in each lobe has its own developmental rate, and each layer of those areas has different rates of development. In order of increasing maturity, the occipital lobe (visual functions) matures first, the parietal lobe (somatosensory functions) matures next, and the temporal (auditory and memory Jfunctions) and frontal (memory and motor functions) lobes are the slowest to reach full maturity. The most advanced part of the cortex is the primary motor area, which is involved in the execution and control of movement. Next is the primary sensory area, then the primary visual area in the occipital lobe, then the primary auditory area in the temporal lobe. The association areas lag behind the corresponding primary areas. 58 Part 2 Biological Growth and Development Head Collisions in Sport and Brain Function in 2001, the Society for Neuroscience %conducted a review of the research concern— ing head collisions that result from contact sports such as football, hockey, boxing, and soccer. Researchers found interesting data for supposedly minor head injuries. New evidence confirms that even minor head collisions can create changes in mental function. For example, most contacts are not hard enough to actually cause an open wound or large bump, but they can cause the brain to reverberate around in the skull. Typical effects of contact caused by brain movement include a brief loss of consciousness, lightheadedness, and dizziness. Many parents and coachesconsider these events part of the game and are unconcerned. However, the evi— dence indicates that Such contact can cause problems with memory and attention. For example, in one study researchers gave a series of written and verbal tests to college football players before their season started. Players who experienced a concussion during play were retested. The players’ performance on tests of verbal learning, memory, and speed of informa— tion processing was noticeably worse for up to 1 week after the blow. Another study examined amateur soccer players who previously experi— enced concussions as part of a match. Similarly, their performance on tests of memoryand information processing was impaired. Further— more, the research indicated that athletes who had the most concussions did the worst. This suggests that hits to the head, even supposedly mihor ones, may lead to lasting, cumulative damage in the brain. Studies of brain activity complement those findings. Athletes who suffered several concus- sions had weaker activity in brain regions that play a role in certain memory functions. The athletes also had problems conducting memory tasks. Are the effects long lasting? Although long—term research was not available, a study of hockey players who had one or more concussions at least 6 months prior to testing revealed significant deficits in memory and attention. This study also indicated that the damage may accumulate; that is, performance was worse for players who had 3 or more con- cussions, compared to 1 concussion. This type of research is quite useful for creating strategies that may better assess the severity of a head injury and strategies that help confirm that the brain is recovered before a player is allowed to return to the game. SOU RCE Society for Neuroscience (February 2001). Knocking nog- gins. 3mm Briefings {Society for Neuroscience website). Following the basic principle of cephalocaudal development within the motor area, control of the legs generally evolves later than the capabilities for upper—body movements. The same is generally true in the sensory area. By about 2 years, the motor and sensory areas are similar in maturity and the association areas add considerably to function. The cerebellum, the area of the brain primarily responsible for controlling coordinated motor responses, lags behind the midbrain, areas of the spinal cord, and the cere— bral cortex in development. Closely linked to its functional maturity is myelination of the tracts between it and the cortex, which reaches potential about age 4. Q iii"; ’ sphere onal terms they are quite different. Each hemisphere has its own specialized functions (i.e., func— y and E . Further- lEtES who i .‘st. This ipposedly ulative ; :nt those 31 concus— ‘ions that ms. The g memory :hough , a study of 0 testing ty and ; hat the Formance more con— This type I g strategies a head m that the illowed to locking nog- znce website). ant within the 1e capabilities : sensory area. maturity and :llum, the area inated motor and the cere— al maturity is ches potential the two hemi— they are quite ns (i.e., func— The concept of lateral dominance of limb control Chapter 2 Heredity and Neurological Changes id 59 Corpus callosum \x ., $1 Left hemisphere I ll Left hand Right hand tional asymmetries), a characteristic lmown as brain (hemispheric) lateral- ization. In most humans, the left hemisphere is associated with the gover— nance of language, logic, and sequential processing, whereas the right hemisphere is specialized for nonverbal, Visuospatial functions (e.g., music awareness, map reading, and figure drawing). Of more direct relevance to motor behavior is that the cortex of the right hemisphere controls muscu~ lar activity in and receives sensory input from the left half of the body (Figure 2.9); Whereas the left hemisphere has a complementary role in con— scious movement on the right side of the body. Dividing and connecting the two hemispheres of the brain is a tough band of myelinated tissue called the corpus rallamm. One of its primary functions is to provide the link for shared information between the two hemispheres, in essence allowing the right hand to know what the left hand is doing. Thus, the corpus callosum is important for the functional integration of the two cerebral hemispheres and possibly for the mani— festation of functional asymmetries. The callosum undergoes marked growth in overall size and myelination during postnatal development, and by 5 years, its development is fairly advanced (Katz, 1995; Witelson & Kigar, 1988), However, behavioral studies of the development of inter— hemispheric communication linked to corpus callosum function suggest that development continues over the first 10 years of life (Fagard et al., 2001). Further discussion of this topic will be provided in Chapters 8 and 9 in relation to theoretical and applied considerations of functional (motor) asymmetries (i.e., the deveiopment of handedness, footedness, and eye preference). 60 Part 2 Biological Growth and Development bi l} Qbfieetive gait? it There are considerable individual differences in aging of the brain. In addi— tion, the many components of the brain itself vary in degree of‘age—related change. The loss of neurons in various regions is accepted as a significant part of the aging process. It is estimated that this loss is about 5 percent to 30 percent by old age. The brain loses about 7 percent of its weight from the time when we are younger adults. This amount could be substantially more if some form of organic brain disease or other secondary aging ele— ment (e.g., drugs, diet, alcohol, or health of the cardiovascular system) is present. It is generally believed that the brain has such remarkable recov— ery capabilities (plasticity) that, even though significant neuron loss may occur, the brain may only lose a small portion of its ability to function. In fact, it has been reported that to compensate for losses, older brains liter— ally rewire themselves for a given task (Rapaport, 1994). In addition to a significant loss of neurons, other important changes appear to be occurring within the neurons themselves. One of the most critical of these changes involves the gradual shrinking and withering away of the dendrites (density) and axons (i.e., interconnectivity among neurons), while other mutations may include the cell body. Process deficits normally associated with advanced aging include a decrease in neuron connectivity, delay in new axon growth and myelination, decrease in neuron excitability, and estimated two fold increase in synaptic delay (Scheibel, 1996). Several of the characteristics listed have been associated with the older adult phe— nomenon known as psychomotor slowing (to be discussed in greater detail in Chapters 7 and 11). In general, aging may be constituted by a proliferation of abnormal material and functions referred to as vacuoles, plaques, and tangles. Vacaales are thick granules surrounded by fluid. Although they are most common in diseased brains, vacuoles show up in three fourths of the brains checked from people over 80 years of age (Adams, 1980). Smile plaques, collections of debris consisting of cells, silicone, and macrophagic elements, are fre— quently found in the frontal and occipital lobes of the brain. Nem’ofibrillmy tangles, tangled clumps of double—helical strands of protein, appear in older brains, but no one clearly understands their effect. Tangles tend to be par» ticularly prominent in the cerebral cortex as a whole and in the thalamus, basal ganglia, and spinal cord. Studies have also noted that as neurons and dendrites deteriorate, neuroglia (the connective tissue that fills the spaces between neurons) increase in number. Bodies of arranges; star—shaped neu— roglia, get larger in the aging brain and may prevent neurotransmitters from building up between nerve cells. One of the most dramatic decreases in basic neurological function related to motor behavior is the deterioration of vestibular awareness (bal— ance). Neurons in one layer of the cerebellum die off fairly rapidly after the age of 60. With this decrease is an estimated 40 percent loss of vestibu- lar hair and nerve cells by 70 years. As previously noted, the cerebellum ‘ain. In addi— f age—related a significant 5 percent to Weight from substantially ty aging ele— ar system) is rkable recov— :on loss may function. In ‘ brains liter— ‘tant changes of the most ,thering away )ng neurons), cits normally connectivity, n excitability, 996). Several er adult phe— zater detail in of abnormal rigles. Wrcuoles at common in rains checked es, collections rents, are free Nearofibriilary spear in older 2nd to be par— the thalamus, s neurons and ills the spaces rushaped neu~ 'otransmitters gical function wareness (bal— r rapidly after oss of vestibu— he cerebellum summary Chapter 2 Heredity and Neuroiogicai Changes tit 61 Neurological Structure Function Brain weight ' Decreases Number of neurons Decreases Dendritic density Decreases Nerve conduction velocity Decreases Connectivity Decreases functions to coordinate voluntary motor behavior and vestibular awareness. Balance as measured by speedwof—return—to~equilibrimn and body sway deteriorates significantly during oid age. (e.g., Shumway—Cook 8t Woolla~ cott, 2001). The topic of balance in older persons will be discussed in Chapter 11. Another basic neural process crucial to motor behavior is nerve con— duction velocity. This function also declines with increasing age. The loss appears to be more prominent in the distal segments of the body and in the lumbosacral regions. The results of such a loss have been evidenced by several studies showing how reaction and movement time slow with increased age. In general, the decline of both fimctions tends to occur most rapidly in the lower parts of the body and in areas of most frequent use, such as the fingers. The general rate of decline appears to be affected by the complexity of the task. Table 2.3 presents a summary list of selected neurological functions that normally show signs of deterioration with aging. The blueprint for our heredity is contained in the genes found within the 23 pairs of chromosomes of a human cell nucleus. Heredity influences numerous developmental factors such as aging, puberty, general body type, height, muscle fiber type, and skeletal age. These factors can also be affected by the environment as evidenced by the documented effects of diet and exercise on body weight and aging. The nervous system serves three primary functions: (a) sensory, (b) integrative, and (c) motor. The basic function of the CNS is to trans— mit information about the environment and the body to the brain, and to carry information from the brain to muscles and glands, thus producing motor responses and bodily adaptations to environmental demands. The PNS is a branching network of nerves, including the somatic system that controls all the skeletal muscles. The basic parts of a neuron are the cell body, dendrites, and axon. Dendrites are the receiving part of the neuron, whereas the function of the 62 n Part 2 Biological Growth and Development fhink m. ak- abou axon is to carry information away from the cell body to other cells. The functional connection between the axon and another neuron is called a synapse. The ascending and descending motor pathway systems provide the channels through which nerve impulses travel. How fast the information travels depends to some degree on the presence of myelin, which forms an insulating sheath around many axons. Development of the CNS can be described in a sequence of six inte— grated biological events: cell proliferation, migration, integration, differen— tiation, myelination, and cell death. The process of proliferation is virtually completed by birth. As proliferation occurs, cells migrate to their final loca— tion and elaborate, which is evidenced by the developmental changes that take place in the axon and dendrite structures. Neurons are also involved in an intricate interweaving process to target cells (integration) and, in turn, become more specialized (differentiation). A significant influence in the wiring process is stimulation provided by the environment. During this general process, the development of myelin (myelination) also transpires. A normal part of the developmental process is cell death. The brain undergoes several rapid structural and developmental (level of functioning) changes during the first few years. During the course of development, each hemisphere establishes its own specialized functions; this is referred to as lateralization. Related to this concept is the notion that hand and foot motor control on one side of the body is controlled by areas of the brain in the opposite—side hemisphere. Several neurological changes occur during the latter stages of aging. Along with the loss of neurons and brain weight, important functional changes are also evident. Among the most affected are vestibular awareness (balance) and nerve conduction velocity (a slowing of movement). "9 a” - .4. ...-,;i u H". a v»... ‘ U t 1. List some of the phenotype characteristics you have and from which parent you inherited them. Can you think of ways that the environment affects the phenotype over time? Given that the environment (what we do) can change the number of neurons, what are some behavioral implications? . From this discussion, describe the role of biology/ environment in early brain development. If you were designing a motor development (physi— cal activity) program for 3— and 4—year—olds, what would it look like based on this information? . What scientific measurement techniques provide us with information about development of the brain? . Name some reasons why there are large individual differences in brain function of the elderly. ...
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