Book 1 Reading and Speaking (1108795), страница 23
Текст из файла (страница 23)
Subtle changes in the chemistry and structure of the brain begin at midlife in most people. During alifetime, the brain is at risk for losing some of its neurons, but neuron loss is not a normal process of aging. Braintissue can respond to damage or loss of neurons in Alzheimer’s disease or after stroke by expanding dendrites andrefining connections between neurons. A damaged brain neuron can readjust to damage only if its cell bodyremains intact.
If it does, regrowth can occur in dendrites and axons. When neurons are completely destroyed,nearby surviving neurons can compensate, in part, by growing new dendrites and connections.Intellectual capacityIn the first large studies to follow the same group of normal healthy humans for many years, scientists haveuncovered unexpected results. They report declines in some mental functions and improvements in others.
In onestudy, the speed of carrying out certain tasks became slower, but vocabulary improved. Several studies found lesssevere declines in the type of intelligence relying on learned or stored information, compared with the type thatuses the ability to deal with new information.This research is supported by animal studies in which scientists found that changes in mental function aresubtle. For example, in rodents and primates in which only minor brain abnormalities can be detected, certainspatial tasks, such as navigating to find food, tend to become more difficult with age.The aging brain is only as resilient as its circuitry.
Scientists debate whether this circuitry is changed only byneuron atrophy or whether some neuron loss over time also is inevitable. In any event, when the circuitry begins tobreak down, remaining neurons can respond by expanding their roles.Learning conditions may dictate what happens to brain cells. Studies of rats shed light on some of thechanges that occur in brain cells when the animals live in challenging and stimulating environments. In tests ofmiddle-aged rats exposed to these environments, researchers found that dendrites in the cerebral cortex, which isresponsible for all conscious activity, developed more and longer branches when compared with rats housed inisolated conditions. Another study showed that brain cells in rats given acrobatic training had greater numbers ofsynapses per cell than rats given only physical exercise or rats who were inactive.
The scientists concluded thatmotor learning generates new synapses. Physical exercise, however, improved blood circulation in the brain.Other scientists report that rats reared in a stimulating environment made significantly fewer errors on amaze test than did similar rats kept in an isolated environment. Moreover, the stimulated rats showed an increasein brain weight and cortical thickness when compared with the control animals.Older rats tend to form new dendrites and synapses as do younger animals in response to enrichedenvironments. But the response is more sluggish and not as large. Compared to younger rats, the older rats haveless growth of the new blood vessels that nourish neurons.While much has been learned about the aging brain, many questions remain to be answered.
For instance,does the production of proteins decline with age in all brain neurons? In a given neuron, does atrophy cause ahigher likelihood of death? How does aging affect gene expression in the brain—the organ with the greatestnumber of active genes? Are there gender differences in brain aging that may be due to hormonal changes atmenopause?Neuroscientists speculate that certain genes may be linked to events leading to death in the nervous system.By understanding the biology of the proteins produced by genes, scientists hope to be able to influence the survivaland degeneration of neurons. (From the book Brain Facts: a Primer on the Brain and Nervous System, 2002)Exercise 7.
Are the following statements true of false, according to the text? Explain your opinion, correctthe false statements.1. Mental decline with age is caused by loss of neurons.2. Healthy aging is accompanied by a considerable loss of memory.3. Nerve cells can both age and regenerate with time.4. Neurons respond very slowly to the change of environment and lifestyle.5. Age can bring some improvement of mental activity.Exercise 8. Summarize all the information about age-related transformations in the human organismdiscussed in this unit and speak on aging and the current research in this sphere.53Unit 15. FoodThen I commended mirth, because a man hathno better thing under the sun, than to eat, andto drink, and to be merry.Ecclesiastes 8:15:One should eat to live, not live to eat.CiceroThere is no love sincerer than the love of food.George Bernard ShawExercise 1.
What do you know about food?1. What nutrients are necessary for human life?2. Why are vitamins and minerals necessary?3. How much food should people consume? What does the daily norm of food consumption depend on?4. What is generally understood as “healthy diet”?5. What food products are considered healthy and unhealthy? Why?6. What factors can lead to overweight and obesity?7.
What role have dietary habits played in human evolution?8. How did cooking food affect human physiology?Exercise 2. The article below deals with the role of dietary habits in human evolution. As you read the text,find answers to the questions:1. What were the main dietary shifts in human evolution?2. What changes have they led to?Food for ThoughtDietary change was a driving force in human evolutionBy William R. LeonardWe humans are strange primates. We walk on two legs, carry around enormous brains and have colonizedevery corner of the globe. Anthropologists and biologists have long sought to understand how our lineage came todiffer so profoundly from the primate norm in these ways, and a growing body of evidence indicates that thesemiscellaneous quirks of humanity in fact have a common thread: they are largely the result of natural selectionacting to maximize dietary quality and foraging efficiency.
Changes in food availability over time, it seems, stronglyinfluenced our hominid ancestors. Thus, in an evolutionary sense, we are very much what we ate.So when and how did our ancestors’ eating habits diverge from those of other primates? Further, to whatextent have modern humans departed from the ancestral dietary pattern?To appreciate the role of diet in human evolution, we must remember that the search for food, itsconsumption and, ultimately, how it is used for biological processes are all critical aspects of an organism’secology. The energy dynamic between organisms and their environments—that is, energy expended in relation toenergy acquired—has important adaptive consequences for survival and reproduction. The type of environment acreature inhabits will influence the distribution of energy into maintenance energy (which keeps an animal alive ona day-to-day basis) and productive energy (which, on the other hand, is associated with producing and raisingoffspring covering the increased costs that mothers incur during pregnancy and lactation).
Thus, by looking at theway animals go about obtaining and then allocating food energy, we can better discern how natural selectionproduces evolutionary change.Becoming BipedsWithout exception, living nonhuman primates habitually move around on all fours, or quadrupedally, whenthey are on the ground. Scientists generally assume therefore that the last common ancestor of humans andchimpanzees (our closest living relative) was also a quadruped. Exactly when the last common ancestor lived isunknown, but clear indications of bipedalism—the trait that distinguished ancient humans from other apes—areevident in the oldest known species of Australopithecus, which lived in Africa roughly four million years ago.
Ideasabout why bipedalism evolved abound in the paleoanthropological literature. C. Owen Lovejoy of Kent StateUniversity proposed in 1981 that two-legged locomotion freed the arms to carry children and foraged goods. Morerecently, Kevin D. Hunt of Indiana University has posited that bipedalism emerged as a feeding posture thatenabled access to foods that had previously been out of reach.
Peter Wheeler of Liverpool John Moores Universitysubmits that moving upright allowed early humans to better regulate their body temperature by exposing lesssurface area to the blazing African sun.The list goes on. In reality, a number of factors probably selected for this type of locomotion. My ownresearch, conducted in collaboration with my wife, Marcia L. Robertson, suggests that bipedalism evolved in ourancestors at least in part because it is less energetically expensive than quadrupedalism. Our analyses of the54energy costs of movement in living animals of all sizes have shown that, in general, the strongest predictors of costare the weight of the animal and the speed at which it travels. What is striking about human bipedal movement isthat it is notably more economical than quadrupedal locomotion at walking rates.For hominids living between five million and 1.8 million years ago, during the Pliocene epoch, climate changespurred this morphological revolution.
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