ScienceMasterIf it's your job to develop the mind,
shouldn't you know how the brain works?
What Are Neurons?
- Written by Kenneth Wesson Kenneth Wesson
- Published: 08 October 2003 08 October 2003
The two most prevalent types of nerve cells in the brain are glial cells and neurons. Glial cells comfortably outnumber neurons by 10 to 1, although some neuroscientists estimate the figure to be as great as fifty to one. Higher ratios of glial cells to neurons appear to indicate greater degrees of cerebral functioning ("smarter") in human brains. Albert Einstein had 40% more glial cells than men of a comparable age. Somewhere between 70-90% of the cells in the cerebral cortex are glial cells, whose main responsibility, is to play "nursemaid" by assisting, pampering and nourishing the talkative neurons. Without glia, neurons could not carry out their operations properly. They would not receive the proper nutrients (neurotrophins). The lack of proper nutrition in the world of neurons, means early neuronal death.
Early psychologists surmised that glial (meaning "glue") cells helped neurons remain in their proper places in the cerebral cortex. Unlike neurons, glial cells actually reproduce inside the brain throughout one’s lifetime. Most brain tumors are gliomas, since glial cells are mitotic, not neuromas. Because neurons are considered postmitotic, they are largely unable to re-enter cell division cycles, and do not divide for the most part. The neurons that you are born with must be carefully nourished and guarded to last your entire life --over 80 years for most Americans. Very few new neurons seem to be generated later.
The Brain Hypothesis
The Brain hypothesis - "The brain is the single source of all human behavior."
The Neuron hypothesis - "The most important unit of brain structure and function is the neuron."
The Brain as Computer theory - "The human brain is like a computer."
In reality, it operates in ways that are nothing at all like the rules governing the operational nature of a computer. The human brain functions in ways that are quite alien to nearly all known engineering principles in use today.
Neuron as Computer theory - Each neuron is comparable to a single computer. With one hundred billion neurons in the brain, we have the neural capacity of approximately one hundred billion networked computers each of which gets modified and updated on an "all day, every day" basis.
Our cumulative experiences determine how the "neurons/computers" get networked together and which neurons get networked more than others.
The human brain is composed of over one trillion nerve cells, and roughly 100 billion of which are neurons. So small in size are the brain’s neurons that if they were lined up single file, one cubic centimeter of them could stretch over 400,000 miles. Neurons serve as the brain's fundamental building blocks and as primary "network communicators" in the cerebral cortex (see figure 2). For decades, it was believed that only the neurons in the human brain communicated with one another. However, recent research has found that some glial cells actually do contribute to neuronal communications.
There are neurons that operate (1) the sensory systems for data intake, (2) the decision-making systems in the frontal lobes for informational processing, and (3) the motor systems that facilitate actions we take or the movements we make.
There are three basic parts of each neuron -- the cell body, the axon, and dendrites. Dendrites are the antenna-like extensions emanating from the neuron. They receive communication signals from other neurons and can make as many as 50,000 vital connections with others in its effort to decipher the outside world. Dendrites increase with use and a stimulating environment, but they will shrink from neglect or from impoverishment, whether that poverty comes from a lack of stimulation, nutrition or other sources.
To maintain neuronal functioning, one of the chief assignments that glial cells have is to produce myelin, a fatty insulating substance comparable to the rubberized insulation found surrounding an electrical cord on a home appliance. Neurons communicate by sending electro-chemical signals to one another. The myelination facilitates sending the electrical component of that message more efficiently. With its protective coating around the signal-sending elongated portion of the neuron, electrical signals can travel more rapidly down the neuron’s axon without its signal scattering and activating nearby cells. The devastating effects of diseases like multiple sclerosis are the result of de-myelination taking place on the axons found in both the brain and the spinal column.
When students experience some problems in reading, those difficulties are in the frontal lobe in the region of the brain that encompasses Broca’s Area. Most important in the cases of some poor readers is the fact that this area is frequently quite low in myelin formations. The abnormalities are in this frontal tract and not in the visual cortex as once thought.
Neurons that are linked to critical brain circuits, which handle our survival skills, are the first candidates for myelination inside the womb. Otherwise we would not survive our first year of life. Further myelination continues gradually in the brain, and completely different regions of the brain are myelinated according to genetic instructions directing all developmental activities at their designated times. A careful analysis of the exact order in which various areas of the brain are myelinated also provides us with information regarding when a child is ready for learning and/or mastering different kinds of concepts or assignments. There is an interesting correlation between the synchronous development of particular competencies and the simultaneous myelination of the brain areas localized for carrying out that specific responsibility.
Dr. Sally Shaywitz at Yale University is currently tracking the cortical changes that are seen in the brains of 5 and 6-year old novice readers before they learn to read and afterwards. This data is being compared to normal structural and functional changes that occur in early cortical development of children of similar ages, as well as comparisons with the brains of experienced readers.
The brain processes an average of approximately 40,000 pieces of neuro-stimuli per second. Each element gets dissected and key aspects of an experience are broken down into the specific traits, elements, and parts composing it. All informational contacts inside the brain occur through the transmission of neurons, where each neuron pitches in a single element that goes into the making of any given experience or single idea. Each of the deconstructed "pieces" is forwarded to a region of the brain that has neurons that specialize in processing elements of a specific nature.
When a yellow ball is watched as it rolls across a table, the color yellow goes to one part of the brain, while the shape of the ball goes to another. The movement is processed else where and the association cortex links the object "table" with the event, while line orientation is sent to still another area of the cerebral cortex, and so forth, each element receiving a completely different type of analysis and processing. This data dissection and storage method allows the brain to process a limitless number of experiences using elements of many similar network systems.
Once the information arrives for analysis, an attempt is made to match that component with previously stored related memory elements residing on an existing neural network. If a match is made, the constituent elements and the larger event are "recognized." If not, the search continues, although we are oftentimes unaware of the on-going processing. This is why many "ah-hah" experiences occur at 3:00 in the morning.
As memory elements are encoded by the cerebral cortex and the sub-cortical structures involved (e.g., the amygdala for traumatic/emotional memories), particular series of neurons are activated --they "fire." The explicit elements or traces of any memory are actually a collection of those representative neurons that are distributed throughout several regions of the brain, but that operate together to re-assemble a distinct memory. When neurons representing some of the pieces of that same memory begin to fire together, they will all fire together eventually (whether they do so accurately, prematurely, or even erroneously). A coordinated and repetitive activity among any combination of neurons strengthens their synaptic connections. Neurons that actively fire together (a signal travels at approximately 270 miles per hour) retain strong synaptic connections. ("Neurons that fire together wire together.") Each time you think and re-think about an event that strengthens the memory, which is why obsessions and "depression" can be so inescapable or debilitating. It is the "wiring" of neurons that cements memories together along with the facts or details related to a given memory.
When the representative cells composing a memory are damaged or destroyed, their branch-like connections (dendrites) can retract and the remaining neurons will need to be "re-wired" around the damaged area, which is how "physical therapy" or "re-training" operates, neurologically re-establishing connections for one’s most valuable memories and important physical functions.
Like the recently discovered mirror neurons that fire when the brain anticipates an action by someone else, there are sensory-specific regions of the cortex that are re-activated when a particular sound or object is recalled. Using the technique of monitoring cerebral blood flow, which is detected on a moment-by-moment basis with functional MRI, researchers have shown that depending on the specific memory, different areas of the auditory or visual cortex were reactivated. Interestingly, they were exactly the same regions that were active when the initial exposure to the sight or was experienced proving that memories are truly reconstructions made by the same neural networks.
Incoming information is generally moved from the posterior (back) area of the brain forwards to the anterior areas in the front of the brain (immediately behind the forehead) as increasingly more sophisticated analysis of any data takes place. Since 4/5 of the information entering the brain does so via the eyes, information is sent forward from the visual cortex ("I saw something") to the association cortex ("What was it?" or "Have we recorded anything like it previously?"). The association cortices send the information to the appropriate regions of the cortex for further processing. Once the incoming data has been analyzed by the pertinent processing systems, a "thoughtful/rational" decision is made by the frontal cortex prompting the most reasonable response based on the information that was assessed and the conclusions that were drawn. Finally, the commands are sent to the motor cortex ("What should I do?"), which executes the directives transferred from the decision-making frontal cortex.
One important goal of the growing young brain is to learn how to effectively attend to relevant environmental information and to simultaneously screen out unimportant data --distinguishing the relevant from the superfluous. When new stimuli are connected to a prior experience, particularly one with a very strong emotional connection, the information suddenly moves to a priority position for immediate and preferred processing. In psychology, this is known as "motivation," in life, it is sometimes called desperation.
However, as we all know, everyone does not process the same incoming information in exactly the same way. Testimony to this fact is given daily in the classroom. When students submit their written materials to a teacher, their work serves as hard evidence that information experienced by several individuals is frequently recalled in ways unrecognizable by one another, and even the original author. Human beings all process stimuli based on the ways in which our 100 billion neurons have been idiosyncratically networked. Research has revealed that each of our individual brain configurations is not only personally "tailor-made," but it is, in many ways, even more unique than our fingerprints. Thus, our perceptions are filtered through our well thought-out biases. Those biases all have neuronal representations and person-specific internal wiring, which has molded each individualized brain into a distinctly different organ.
In addition to human beings seeing and recalling events in immensely different ways (as evidenced by the reconstruction of events by students, spouses, witnesses in court, etc.), various living organisms also analyze the same physical input in completely divergent ways. Insects process infrared rays as light, while a human neurobiological system will process this same stimuli as heat. Dogs will detect, dissect, and act upon sounds to which their human owners were completely oblivious. The dog’s owner is not uncaring or negligent; people simply were not bestowed with the neural mechanisms necessary to decipher sounds at a certain pitch in ways that the brains of dogs and other organisms find most "natural" to them.
If any brain is capable of processing the elements of an idea, an object, or an incident, then that capability itself is evident that a corresponding set of neurons has made the analysis possible. The series of events that allowed for the transmission, analysis and interpretation of the information inside the brain came by way of a specific kind of neuronal processing.
For centuries, scientists and children watched with amazement, as spiders would ensnare, what appeared to humans as pathetically errant insects, whose grave mistake was to accidentally fly into a spider’s cobweb. Human beings viewed the web a being nearly transparent, which, to humans at least, accounted for the bug’s "flight plan error" resulting in its fatal capture. However, the ways in which human beings process this event is distinctly different from the manner in which the spider and the insect "see" things.
First, a spider is exclusively interested in attracting only specific insects (ants, wasps, bees, and his fellow spiders are not the wished-for victims). Second, insects do not fail to see the web at all. In fact, they see the spider’s cobweb quite vividly. Insects are captured because they actually do observe the web directly in front of them, but in ways that the spider had intended. It was recently discovered that, in ultraviolet light, a spider’s web consists of visual patterns (seen by the insects) that mimic particular flowers. Those images are of just the sort of flowers that a specific family of insect searches for when buzzing about. The insect gets trapped because its neurons process the spider’s web, not as the menacing mesh net that humans clearly see, but as the kind of flower for which the insect is usually seeking.
The insect is not cognizant that its preferences are the kinds of which the spider is clandestinely well aware. The spider’s handiwork, as it turns out, is a product that impersonates a low budget, deceptive art piece. It is a life-sized mural of flowers, but only to an insect, which of course, is all that matters to the spider. We have been baffled by this spider-insect conundrum for eons, primarily due to a complete neuronal disconnect between the neuronal processing of an insect and our own.
Similarly, neurons in a frog’s brain fire incessantly when a horizontal matchstick is placed directly in front of him mimicking a worm. However, if the exact same matchstick is presented to the frog in a vertical position, the frog’s brain does not fire or respond at all.
We also have learned that there are comparable state of affairs found in the neural systems of insect-eating monkeys, leaf-eating primates, and their fruit-eating cousins. In each group, only the "food of choice" stimulates their brain circuitry, when that particular kind of food is observed. Foods preferred by other primates with different palates do not register on the retina of others, nor inside their brain, nor on any of their neural networks as something that falls into the category befitting the definition "food."
Any incoming data that does not match a human brain’s prior experiences simply does not register in the brain (neurons do not respond in any recognizable or meaningful way). This is why a foreign language might sound like incomprehensible chatter to one individual, while another finds it the best and only meaningful way of communicating.
Next Time We Will Continue with a discussion of Mirror Neurons