The human brain is considered as the most important and complex part of the body consisting of about 180 billion cells (Kolb and Whishaw 84). Of those 180 billion cells, 60 billion neurons are actively involved in thought processing, and each of these may synapses with as many as 15,000 neighboring neurons. Because of this complexity, for many years researchers in neuroscience have been hesitant to take on the difficult task of explaining the intricacies of the human brain. Prenatal Development The brain is among the first body parts to specialize and function in the embryo.
It originates as a flat sheet of cells on the upper surface, called the neural plate. The brain begins to develop between the second and third week after fertilization and continue to develop rapidly throughout gestation (Spear 406-407). At 3 weeks of embryonic development, a tube appears along the back of the embryo. This is the neural tube, from which the entire nervous system develops. At the top of the tube, three bulges develop to form the three main divisions of the brain- the forebrain, the midbrain, and the hindbrain, and, behind them, the spinal cord.
By the time the embryo is 13 mm (y in) in length the three swellings have become five, as the forebrain itself separates into the region to become the cerebral hemispheres and below this the diencephalons. The swellings are so large that to accommodate them the tube must begin to kink. At 7 weeks, the parts of the developing neural tube initially form a straight line, but the tube soon bends so that the forebrain and hindbrain are at right angles to each other.
The hindbrain develops rapidly at this stage and begins to sprout a series of nerves (cranial nerves). The forebrain also begins to enlarge, forming two bulges. These will become the large, folded cerebrum and underlying structures, such as the thalamus. At 11 weeks, most features of the adult brain appear in rudimentary form. The hindbrain differentiates into the cerebellum (largely concerned with balance and coordination) and the pons and medulla (which control vital functions such as breathing and heartbeat).
Meanwhile, the forebrain continues to grow, and the bulk of it – the cerebrum – begins to overlap the underlying structures. By the fifth month, the wrinkles on the cortical surface of the cerebrum have appeared, and simple but recognizable EEG patterns have developed. Once the tube has been closed off, the cells within it divide, going through a number of precursor stages before their daughters eventually differentiate to give rise to the populations of neurons and glia that will form the adult brain.
The rate of cell proliferation is extraordinary: an average of more than 250,000 neurons per minute must be formed during the nine months of pregnancy—a rate dramatically surpassed by that of synapse formation: More than 30,000 synapses must be formed per second under every square centimeters of cortex to generate the complement present in the early post-natal period. During pregnancy the fetal brain grows dramatically in size and complexity, and the neurons and glia which constitute it must find their appropriate positions and make their ordered connections, for instance within the six layers of the cerebral cortex.
Because the cells are generated from a single initial sheet, it is necessary for them to migrate considerable distances to their final location. The cerebral hemispheres develop from the front portion of the neural tube, and, as their progenitor cells are formed, by the fifth week of pregnancy, the wall of the tube bulges to form the cerebral vesicles. Over the next hundred days, cells close to the ventricle are destined to give rise to the neurons which will form the cortex, while the glia are born in a second layer slightly further from the ventricles.
The newborn neurons migrate from the ventricular zone towards the surface of the cerebral vesicles, where they meet axons growing in from the region of the developing brainstem, through which the later-born neurons must migrate. Before birth, massive enlargement of the cerebrum continues. Its most impressive development occurs in the cerebral cortex (the outer layers of the cerebrum) – the site of all higher conscious activity. At birth, the cerebrum makes up the bulk of the brain (The American Medical Association 12-13).
Hence, by the time of birth, virtually all of the approximately 100 billion neurons in the human brain already are present (Cowan 113-115). Infancy But even this phenomenal rate of development may be an understatement (Clarke 345). Further Myers stated that in humans, though the brain tissue from the cerebral cortex has increased in complexity of the neural networks, the number of nerve cells don’t increase, but their interconnections do (63) Research in animals indicates that early in development, about twice as many neurons are produced than will be present in the adult brain.
In addition, many neurons initially grow axons that connect to the wrong targets. During the normal course of development, the excess neurons die and the inappropriate connections degenerate, leaving the appropriate connections in place (Cowan, Fawcett, OLeary, and Stanfield 1258-1260). Scientists believe that this overproduction and, later, death of neurons and their connections is an important mechanism for forming and fine-tuning the developing nervous system. The brain is not completely developed even in full-term newborn infants.
A great deal of brain development takes place in the first few months of postnatal life; and, in fact, brain growth continues at least until adolescence (Benjamin, Hopkins, and Nation 313). They further added at birth, the human brain is immature: The neural networks that enable infants to walk, talk, and remember are still forming. This helps explain why infants’ memories do not predate during their third or fourth birthdays. In infancy, the brain also grows rapidly specifically during the first two years after birth (Spear 170).
Unlike all other cells in the body, however, the neurons are not usually replaced when they die, and from early infancy onwards there is indeed a small but steady loss of neurons. The growth is accounted for by increases in the number of glial cells, but above all in the massive development of dendritic processes and synaptic connections, as the brain “wires itself up” in a spectacular interplay of epigenetic specificity and experience-dependent plasticity—that is, the way that neural pathways are modified as a result of experience and most notably, learning and memory.
Although all of a person’s neurons are present at birth, the number and complexity of the connections among neurons increase substantially after birth (Parmelee and Sigman 2:95-98), and this increase is partly responsible for the growth in brain size. Thus, both the increased neural connections and the development of myelin after birth make possible more and more complex behavior and thought as the child grows. In some areas of the brain, these developmental changes continue until adolescence (Yakovieve & Lecours 5-7).
The human brain, and its functions, thus develop at first rapidly and then more steadily over the first few years of infancy, across puberty, and even into late adolescence. Works Cited Benjamin, Ludy, Hopkins, Roy, and Jack Nation. Psychology. 2nd ed. New York: Macmillan Publishing Company, 1997. Clarke, P. G. H. Neuronal Death in the development of the vertebrate nervous system. Trends in Neuroscience. Cambridge: Harvard University Press, 1995. Cowan,W. M. “The development of the brain”. Scientific America,241(1989):113-120. Cown,W. M. ,Fawcett,. j. w. , O`Leary,. D.
D. M. ,& Stanfield,B. B. “Regressive Events in Neurogenesis”. Science,225(1991):1258-1260. Clayman,C. B. ,M. D. “The Brain and Nervous System. ” The American Medical Association. 2nd ed. 1997. Kolb,B. , AND Whishaw,I. O. Fundamentals of human neuropsychology. New York: Freeman,1995. Parmelee,A. H. , and Sigman,M. D. Prenatal brain development and behavior. In P. H. Mussen (Ed). Handbook of Child Psychology,Vol II. Infancy and development psychology. New York: John Wiley &Sons, 1984. Spear, Peter D. Psychology: perspective on behavior, New York: John Wiley &Sons, 1998.