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ScienceWeek
2004 18 June B1 NEUROBIOLOGY: ON THE STRUCTURE OF THE HUMAN BRAIN
The following points are made by J.S. Allen et al (American Scientist 2004 92:246):
1) Because many factors influence neural structures, the study of brain volume, or volumetrics, has the potential to offer insights from many perspectives. In an evolutionary context, studies of brain volume across species can link anatomical, behavioral and ecological data. Species that have unpredictably large or small brains are useful for studying the forces of evolution that influence brain size. For example, it has been suggested that fruit-eating primates have a higher brain-to-body mass ratio than leaf-eating primates because locating widely dispersed, seasonally available fruit makes greater cognitive demands than finding more convenient foods, such as leaves.
2) Volumetrics can also illuminate developmental patterns within and across species, which in turn suggest how evolution might be constrained by implicit rules of neurological growth. The study of neurological diseases also depends on a systematic analysis of brain size and shape. For instance, some children with autism have atypically large brains, and Alzheimer's disease causes progressive brain atrophy. In both cases, the pathological processes that underlie these conditions manifest as changes in brain volume. So volumetric studies are both a means to understanding brain function and an end in themselves.
3) Neuroanatomy has undergone a revolution in the past 30 years. The leap became possible with the introduction of new imaging technologies such as x-ray computed tomography (CT, also called CAT scanning), magnetic resonance imaging (MRI) and positron emission tomography (PET). With these tools, researchers can view the structure and activity of the living human brain in unprecedented detail. For the structural and volumetric study of the brain, CT and MRI have been of critical importance.
4) Computed tomography is the older technology. It uses the variable absorption of x rays by different brain components to visualize structures inside the skulls of living subjects. A single CT image is the product of thousands of individual measurements, which are made as the x-ray source swivels in a full circle around the head.
5) Unlike CT, MRI does not use x rays, relying instead on powerful magnets to momentarily align the nuclei of hydrogen atoms in body tissues, most of which are within water molecules. When the magnet is turned off, the infinitesimal spinning (or resonating) nuclei fall back to a normal state, releasing energy in the form of radio waves. The frequency of these waves provides a measure of local hydrogen concentration, which varies according to tissue type, such as bone or fat. This produces a very fine-grained map -- often as good as a postmortem analysis. The technique clearly distinguishes gray matter (mostly neuronal cell bodies), white matter (mostly nerve fibers insulated by fatty myelin, plus supporting cells) and cerebrospinal fluid or CSF (the liquid that fills the spaces within and around the brain). In addition, individual MR scans can be stacked to form a virtual three-dimensional model, then resliced along any plane or angle.(1-5)
References (abridged):
1. Allen, J. S., H. Damasio and T. J. Grabowski. 2002. Normal neuroanatomical variation in the human brain: An MRI-volumetric study. American Journal of Physical Anthropology 118:341-358.
2. Allen, J. S., H. Damasio, T. J. Grabowski, J. Bruss and W. Zhang. 2003. Sexual dimorphism and asymmetries in the gray-white composition of the human cerebrum. NeuroImage 18:880-894.
3. Baar‚, W. F. C., H. E. Hulshoff Pol, D. I. Boomsma, D. Posthuma, E. J. C. de Geus, H. G. Schnack, N. E. M. van Haren, C. J. van Oel and R. S. Kahn. 2001. Quantitative genetic modeling of variation in human brain morphology. Cerebral Cortex 11:816-824.
4. Bartley, A. J., D. W. Jones and D. R. Weinberger. 1997. Genetic variability of human brain size and cortical gyral patterns. Brain 120:257-269.
5. Bookstein, F., K. Sch„fer, H. Prossinger, H. Seid-ler, M. Fieder, C. Stringer, G. W. Weber, J.-L. Arsuaga, D. E. Slice, F. J. Rohlf, W. Recheis, A. J. Mariam and L. F. Marcus. 1999. Comparing frontal cranial profiles in archaic and modern Homo by morphometric analysis. Anatomical Record (New Anatomist) 257:217-224.
American Scientist http://www.americanscientist.org
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Related Material:
NEUROSCIENCE: A TOUR OF THE BRAIN
The following points are made by Ian Glynn (citation below):
1) The easiest way to make sense of the structure and organization of the human brain is to look at the way the brain develops in the embryo, and at its evolutionary history. In vertebrates, including ourselves, the nervous system starts as a midline groove in the surface layer of cells on the back of the embryo. This groove becomes deeper, and soon forms a thick-walled tube which separates from the surface, and is destined to form the brain and spinal cord. As the embryo develops, the front end of the tube, which is closed, swells into three connected vesicles which will form the forebrain, the midbrain and the hindbrain, respectively. Later the forebrain divides into an expanded endbrain, and a between-brain that lies between the endbrain and the midbrain. The way these different regions develop, and the functions they have is different in the different classes of vertebrate but there is a common overall pattern.
2) You can get some idea of the evolution of the brain -- from fish, through amphibia and reptiles, to mammals -- by comparing the brains of animals living today. The striking thing in this evolutionary series is the progressive enlargement of the endbrain. In all four classes it develops to form two cerebral hemispheres, but these are small and fused in the fish, larger in amphibia and reptiles, and very large in mammals, particularly in primates. In humans, the cerebral hemispheres are so large that they fill most of the space in the skull. This great increase in size is accompanied by the takeover of roles that in lower vertebrates are performed by other parts of the brain, and by the appearance of behavior of a complexity not seen in lower vertebrates.
3) The surface of the cerebral hemispheres is a crumpled sheet of neurons and supporting cells from 2 to 5 mm thick. This sheet is the cerebral cortex, and the many folds and fissures increase the effective area nearly threefold. Underlying the cortex are masses of axons, which, being mainly myelinated, look white in contrast to the "grey matter" of the cortex. A very large bundle of axons -- the corpus callosum -- crosses from one hemisphere to the other, and provides the main pathway for the transfer of information between the two hemispheres.
4) Deep within the white matter of each hemisphere are three further collections of neurons and supporting cells, the basal ganglia, the hippocampus (from a fanciful resemblance of its shape, in cross-section, to a "sea horse" -- hippokampos in Greek) and the almond-shaped amygdala (from the Greek word for almond). The basal ganglia are largely involved in the control of movement --it is their malfunctioning that causes the rigidity and tremor in Parkinson's disease. The hippocampus and amygdala, together with other structures play a vital part in memory and emotion.
5) The total surface area of the cortex is about a quarter of a square meter -- a little larger than a large pocket handkerchief -- and it contains something like 100 billion neurons. It is almost certainly to this extraordinary structure, more than to any other part of the brain, that we as a species, owe our remarkable intellectual abilities.
6) The between-brain shows nothing like the same expansion in the course of evolution. In all vertebrates, during the embryological development of the between-brain, an outgrowth on each side develops into the retina of the eye and the optic nerve. A conspicuous feature of the mammalian between-brain is the presence, in each side wall, of a large mass of neurons called the thalamus -- the Latin form of a Greek word meaning "inner room".
7) A consequence of the takeover of functions by the cerebral cortex is that, in mammals, information about all sensations has to be carried to the cortex. Some information about smell passes directly from the olfactory organs to a part of the cortex, but information from all the other sense organs (and also information from other parts of the brain) reaches the cortex almost exclusively via one or other thalamus. Each thalamus therefore acts as a great relay station, but this cannot be its sole function as there are even more nerve fibers carrying information from the cortex to the thalamus than there are carrying information from the thalamus to the cortex. The role of these back connections is not known, but a fashionable hypothesis is that they make it possible for the cortex to use representations of information it has just received, to select signals from the thalamus that are most likely to be useful for subsequent cortical processing.
8) In the floor of the between-brain are several collections of neurons that together forms the hypothalamus -- hypo being Greek for below. The hypothalamus is tiny, but by controlling the pituitary gland, which secretes hormones that influence other hormone-secreting glands, it dominates the entire hormonal system in the body, and has important effects on metabolism, growth and various processes involved in reproduction. It also acts through the autonomic nervous system -- a discrete part of the nervous system that, as its name suggests, controls events in the body that occur more or less automatically, though not necessarily unconsciously. Of this part of the nervous system, one division (the para-sympathetic nervous system) is concerned with "housekeeping" functions such as appetite, thirst, salt and water balance, body temperature, the movements of the gut and the emptying of the bladder. The other division (the sympathetic nervous system) is continuously concerned with the control of blood pressure, but it is particularly active when the body has to be prepared for vigorous action. As generations of medical students have been taught, it is the system for "fright, flight and fight". Yet another role of the hypothalamus is to act with other parts of the brain in controlling sleep and wakefulness, and in producing some of the physical changes in the body that are normally associated with emotions such as fear, anger or pleasure.
Adapted from: Ian Glynn: An Anatomy of Thought: The Origin and Machinery of the Mind. Oxford University Press 1999, p.164. More information at: http://www.amazon.com/exec/obidos/ASIN/0195158032/scienceweek
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Related Material:
ON BRAIN SIZE
The following points are made by R.M. Sayfarth and D.L. Cheney (Proc. Nat. Acad. Sci. 2002 99:4141):
1) An intriguing question in neurobiology is: "Why do primates have such big brains?" Across the animal kingdom, brain size increases with increasing body size. Despite this common scaling principle, however, brain size to body weight ratios differ from one taxonomic group to another (2). In primates, for example, the brains of apes are generally larger relative to body weight than the brains of monkeys, whereas the brains of monkeys are larger than those of prosimians (2). Structural differences are also apparent. In chimpanzees, a larger proportion of the brain is devoted to neocortex than in monkeys, who in turn have proportionately more neocortex than prosimians (3, 4). Within the neocortex, ape (and especially human) brains have a particularly enlarged prefrontal cortex, an area known to be involved in many forms of abstract thought and rule learning (5, 6).
2) Increases in the size of primate brains have come despite the fact that brain tissue is metabolically very costly. What selective pressures have overcome these costs? When the question is applied to humans, answers typically refer to the adaptive advantages of technology (initially, stone tools) and language. But monkeys and apes use only rudimentary tools and lack language entirely, yet their brains are significantly larger than those of similar-sized mammals. Some other selective pressures must be at work.
3) Among primates, relative brain size (corrected for body weight) is greater in species with larger home ranges and greater in species that are fruit-eating or omnivorous than in species that eat leaves. Species that feed on fruit may face special problems in learning and memory because they depend on widely spaced food that is ephemeral in both space and time. In contrast to this "ecological" explanation of brain evolution, others suggest that primate brains have evolved primarily to deal with social problems. Primates, they argue, live in relatively large groups where an individual's survival and reproductive success depends on its ability to manipulate others within a complex web of kinship and dominance relations. In recent years this "social intelligence" hypothesis has received some of empirical support. The purported link between brain size and ecological or social intelligence is, however, entirely conjectural. We may assume that memorizing the location of ripe fruit or remembering the kin relations of ones' opponents demand considerable brainpower, but this assumption is neither supported nor refuted by any widely accepted evidence. Perhaps more important, the "intelligence" of different species is notoriously difficult to compare. Different species manifest their intelligence in different ways, making it almost impossible to find an objective measure of intelligent performance that can be used across many taxa.
References (abridged):
2. Jerison, H. (1973) The Evolution of the Brain and Intelligence (Academic, New York).
3. Martin, R. D. (1990) Primate Origins and Evolution: A Phylogenetic Reconstruction (Princeton Univ. Press, Princeton).
4. Passingham, R. E. (1982) The Human Primate (Freeman, Oxford).
5. Deacon, T. (1992) The Symbolic Species (Norton, New York).
6. Miller, E. (1999) Neuron 22, 15-17.
Proc. Nat. Acad. Sci. http://www.pnas.org
ScienceWeek http://scienceweek.com
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