Chemical, Electrical Pulses Spark Brain Cell Communication
Over the next couple of decades, scientists began to tease out the details of nerve cell communication. They learned that the connections Cajal had hypothesized to be the underpinnings of all human behavior were actually formed through a complex chemical signaling process. In this relay race of life, they learned, one cell squirts out a neurotransmitter, a chemical messenger, that crosses the synaptic gap between nerve cells and latches onto receptors on the surface of a neighboring cell. It wasn't long before dozens of neurotransmitters were discovered and systematically analyzed to determine their roles in cognition, behavior, and disease processes.
By the 1970s, it had become clear that brain function was the result of a complex interplay of chemical transmitters jolted into action by electrical impulses. The pulses were generated by ion channels within the neuron, which acted like the starting gun for the relay race of interneuronal communication. Today, scientists continue to elaborate the processes of cell-to-cell communication in exquisite detail, and a new arm of science has evolved that is now exploring the events that occur "beyond the receptor" within the postsynaptic cell.
The 1970s and 1980s were important decades for brain science. The development of PET (positron emission tomography) during this period enabled scientists to capture anatomical images of the living, functioning human brain and to begin to inventory the neurotransmitters involved in various behaviors or brain disorders. PET takes advantage of the fact that nerve cells metabolize the sugar glucose to derive the energy needed to perform their roles in brain function. By measuring changes in glucose uptake by nerve cells, PET enables scientists to determine which areas of the brain are activated during specific tasks (such as the finger-tapping exercise discussed earlier).
The introduction of PET got people thinking about other strategies for mapping the brain, and MRI (magnetic resonance imaging) soon followed. Rather than measuring how much glucose cells metabolize, MRI uses intensely powerful magnets and radio-wave pulses to capture images of the brain's structure (standard MRI) and function (fMRI). Standard MRI relies on the fact that molecules within cells, when placed in the strong magnetic field of an MRI scanner, "line up" in a certain fashion, much like the needle on a compass lines up with the Earth's magnetic field. When pulses of radio waves are applied to tissues with such alignment, the nuclei of individual molecules resonate the signals back in varying patterns that correspond to the chemical makeup of each area of tissue being studied. Scientists can then reconstruct anatomical images based on the patterns of resonance.
In recent years, sophisticated computer techniques have enabled brain imagers to take MRI to the next level, creating images that depict brain function in addition to anatomical structure. Using a standard MRI scanner, scientists can track which areas of the brain are active. When a specific region of the brain is active, neurons in that area use more oxygen. fMRI takes advantage of the different magnetic properties of oxygenated and deoxygenated blood, blood that has not been used by brain cells and blood that has been used. The relative concentrations of oxygenated and deoxygenated blood are measured and charted onto standard MR images of the brain to show which areas are "working."
Brain Function: The Sum of Many Parts
Imaging techniques such as PET and fMRI have revolutionized the field of brain science, enabling the precise mapping of brain functions and structures and permitting scientists to search out the roots of brain disorders or injuries. In addition, they have helped advance a "systems" view of brain function. According to this view, no one structure or area of the brain acts alone to drive a specific behavior or mental task. While certain brain areas may be specialized for certain tasks, brain function relies on networks of interconnected neurons. These specialized pathways enable the brain to analyze and assimilate information from external (e.g., sensory) as well as internal (e.g., hormonal) cues in order to respond with appropriate physical and psychological behaviors.
Systems neuroscience helps explain how people such as victims of stroke or head trauma, whose brains have been injured in a discrete site, can, over time, redevelop the functions lost as a result of the injury. Nerve cells in their brains in effect forge new pathways, bypassing the injured site and forming new connections, as if finding a new route to get to work after discovering that a bridge is out on the usual route. This ability to adapt, which scientists call plasticity, seems to be particularly strong in young brains, but "old" brains routinely learn new tricks, scientists have found.
Here is an image of a single neuron taken from a rat brain,
isolated in culture. Scientists estimate that the human brain
has more than 100 billion neurons and about one quadrillion
(one with 15 zeros) connections between neurons.
Plasticity, in fact, plays a critical role in the entire life cycle of the brain, from its development in infancy, to its continual reshaping as learning occurs, to its ability to adapt to age-related changes that can lead to mental deterioration in later life. Now, new evidence suggests the brain may be even more plastic than previously thought. Turning one of the oldest tenets of neuroscience on its head, scientists recently discovered that nerve cells can regenerate, making the idea of brain repair following trauma or disease thinkable. Revealed at the end of the 20th century, this scientific breakthrough is sure to influence brain science for at least the next century.
Constructing the World's Most Sophisticated Machine
There is perhaps no time in the human life cycle during which plasticity is more important than in the period of nervous system development. A newborn baby's brain, scientists have learned, is not just a miniature version of an adult's. Instead, it is a work in progress, the world's most sophisticated machine in construction phase. Like the scaffolding that shapes the framework of a building, an initial framework of interneuronal "wiring" is present at birth, pre-set by nature via the genetic blueprints provided by the mother and father. The materials are also there: babies are born with virtually all of their lifetime store of nerve cells. (See developments in stem cell research) What remains is the "finish work" of the brain's communications architecture, the fine-tuning of a quadrillion cell-to-cell connections.
In humans, the fine-tuning phase unfolds over several developmental years. "Nurture" largely directs the completion of the wiring process, literally shaping the structure of the brain according to a child's early sensory experiences. During critical periods (or stages) of brain development, these early experiences stimulate neural activity in certain synaptic connections, which in turn become stronger and thrive. A "pruning" process ruled by a philosophy of "use it or lose it" ensues, during which synapses that are not routinely stimulated may wither and die. Within that period, "windows" of opportunity, during which the brain may be specially "primed" for learning certain skills such as language, open according to the developmental schedule of the brain regions underlying those skills. Since it's well known that humans can continue to learn and modify behavior throughout life, it's clear that the windows never really slam shut, even though they may become a bit sticky.
Numerous studies have also shown that babies who are held and caressed regularly do better developmentally and may reap the benefits throughout life.
Children who fail to get the stimulation they need for proper brain development can become tragedies. In the 1990s, studies of Romanian orphans whose cries for comfort were never answered or whose smiles were never encouraged, found lingering impairments in the children's basic social and thinking abilities and in their physical development. Numerous studies have also shown that babies who are held and caressed regularly do better developmentally and may reap the benefits throughout life.
The first few years of life are especially important, as they are periods of rapid change in the synapses. But new understandings about the developing brain indicate that the process of finetuning connections among neurons continues, to varying degrees, into adolescence. In fact, "brain development" probably never really ends-older adults are also capable of forming new synaptic connections and do when they learn new things. But the rapid-paced period during which external stimuli are critical to "normal" brain-building generally begins to dwindle around the mid-teen years.
Growing Pains in the Teenage Brain
Adolescence marks a turning point of sorts for the brain, as some of its structures are nearing maturity, while others are not yet fully developed. The prefrontal cortex, for example-the brain's center for reason, advance planning, and other higher functions-does not reach maturity until the early 20s. Since this part of the brain seems to act as a kind of cerebral "brake" to halt inappropriate or risky behaviors, some scientists believe sluggish development may explain difficulties in resisting impulsive behavior that some adolescents exhibit at times. The brain also has ultimate control over the ebb and flow of powerful hormones such as adrenaline, testosterone, and estrogen, which themselves play critical roles in the changing adolescent body.
The teenage brain is also struggling to adapt to a shift in the circadian rhythm, the brain's internal biological clock, which drives the sleep-wake cycle. The secretion of melatonin sets the timing for this internal clock, a hormone the brain produces in response to the daily onset of darkness. In one study, researchers found that the further along in puberty teens were, the later at night their melatonin was secreted. In practice, that means teens' natural biological clock is telling them to go to sleep later, and to stay asleep longer.
الإثنين يناير 12, 2015 2:09 am من طرف نبراس جميل
موضوع: brain cells communication 
