How many neurons in the brain
Among other things, it can explain well-known birth defects of the brain or the nervous system in terms of the stage at which development was disturbed. This model of the sequence of brain development has led toward many fruitful lines of investigation in neuroscience. But the pruning of excess connections-clearly a process of great importance for the shape of the mature brain-continues for years. After about 18 months of age, no more neurons are added, and the aggregation of cell types into distinct regions is roughly complete. These events do not occur in rigid sequence but overlap in time, from about 5 weeks after conception onward. The physical bases for perception are beginning (more.)Īccording to this scheme, the essential stages are (1) proliferation of a vast number of undifferentiated brain cells (2) migration of the cells toward a predetermined location in the brain and the beginning of their differentiation into the specific type of cell appropriate to that location (3) aggregation of similar types of cells into distinct regions (4) formation of innumerable connections among neurons, both within and across regions and (5) competition among these connections, which results in the selective elimination of many and the stabilization of the 100 trillion or so that remain. In this 12- to 14-week-old embryo, nerve cells are proliferating at the rate of about 15 million per hour. The development of the human brain during gestation is a highly complex project on a tight schedule.
HOW MANY NEURONS IN THE BRAIN SERIES
The majority of developmental neuroscientists today respond to this challenge by proposing a series of stages in which built-in instructions and the effects of arbitrary external events are mingled to an intriguing degree. For such an account must not only explain a sequence of development of great orderliness and efficacy but also allow room for the creative effects of chance-in the form of random mutations and the ensuing natural selection-that have led to the propagation of this particular form of brain in the first place.
HOW MANY NEURONS IN THE BRAIN FULL
But how is such an intricate network constructed in the first place? Does the genetic material of the fertilized egg already contain a full set of building specifications for the human brain, in which every cell is created as a minute increment in the overall design? And if the set of instructions is indeed so closed and specific, how could chance or random mutations or the influence of the environment have played a role-as they so evidently have done and continue to do-in the emergence of the first human brains?įrom these questions, it is easy to see that any scientific account of the development of the human brain has to meet a formidable challenge. The brain's 100 trillion or so interconnections provide the physical basis for its speed and sophistication. The great number of functions that the brain reliably carries out and the specificity with which these are assigned to one or another type of cell or small location in the whole assembly are stunning in their complexity yet the feat of growing a human brain occurs in hundreds of millions of individuals each year. But it is not the volume of growth alone that makes the production of a human brain staggering to consider. To arrive at the more than 100 billion neurons that are the normal complement of a newborn baby, the brain must grow at the rate of about 250,000 nerve cells per minute, on average, throughout the course of pregnancy. The making of the human brain from the tip of a 3 millimeter neural tube is a marvel of biological engineering.