Science

  • Issue 109 / January -February 2016



    The Intricate Beauty of the Nervous System

    Brian Turk

    The human body is comprised of many systems, but none quite like the nervous system. This enigmatic system runs throughout the whole body, accomplishing millions of tasks every second. It is the system with the largest number and variation of cells, thus making it extremely complex and difficult to understand. Yet the beauty of this system comes from the simplicity within this complexity.



    Let us start with the building blocks of the system, which are the nerve cells (also called neurons). The main function of nerve cells is the transportation of signals through electrical pathways. The morphology of neurons is entirely different from that of other types of cells, and this is what helps the nervous system function effectively.



    The typical nerve cell can be broken down into three parts: the dendrites, the body, and the axon. The signal enters the nerve from the dendrites, runs through the body, and leaves the cell by way of the axon.
    The head of the nervous system is the brain. As only one part of the system, the brain is formed of about 80 billion neurons. Apart from the brain, there are billions more neurons in the nervous system, which runs throughout the body. Other than the neurons, there are neuroglia in the brain, which consist of approximately another 80 billion cells. In the brain alone, there are over 160 billion cells whose only purpose is to transport signals. But where do these signals originate from and where are they taken to?



    The functions of the nervous system are performed through three main categories. The first category is the central nervous system (CNS), which is made up of the brain, the brainstem, and the spinal cord. This part is the center for decision making. The second category includes all the cells in the body apart from the nervous system, such as sensory cells and muscle cells. The final category is the peripheral nervous system (PNS), which seeps through the body and is the messenger between the CNS and the rest of the body. The whole function of the nervous system boils down to the transfer of electrical messages between these two ends.



    An example is when a finger touches a flame. The sensory cells on the tip of the finger produce a “Hot” signal, and pass this electric signal on to the PNS. The PNS then transports this signal to the CNS, where electricity is translated into a meaningful message. The CNS then produces a “Withdraw” signal and sends it via the PNS to the muscles of the finger, which in turn withdraw the finger from the flame. This reflex does not even reach the brain and is processed in the spinal cord due to its simplicity. It is as if the brain cannot be troubled with such petty tasks. Slightly more complex tasks go up a little bit further to the brainstem. The tasks brought to the brainstem are not sudden reflexes, but do not require thinking, either. Some examples are chewing, swallowing, maintaining balance, and eye movement. The brain, however, performs the most complex tasks, such as sight, language, learning, and emotion.



    Now that the signals’ pathways are clear, we must ask how exactly do cell groups in the central nervous system know how to react to various situations? How can a small group of nerve cells in the cerebellum (attached to the brain) decide to make the body lean to the left while falling to the right? How does a tiny spec of neurons in the pons (located in the brainstem) know when to start secreting saliva in the mouth? How are miniscule unconscious cells entrusted with decisions concerning the well-being of the entire body? They cannot see the area they are controlling. The only thing that comes to these cells is electricity. How on Earth do these cells know so much from only electric signals?



    Let’s look at balance, but keep in mind that every mechanism is totally unique and they cannot be categorized into three or four groups. Our journey begins in the depths of the ears. There are three canals called the semicircular canals, which are all perpendicular to each other. The tip of each canal is filled with a thick fluid called the endolymph. On the bases of these tips are hair-like receptors. The thick fluid flows within the canal in accord with gravity, thus tilting the hair-like receptors. If these receptors tilt to one side, they generate a large amount of electricity. If they tilt to the other side they generate a small amount. Each canal represents one axis, and signals from all three canals make up a 3D world. Signals from each canal continuously flow to the small group of neurons in the cerebellum and are combined to make up a map of how the head is positioned. For example:



    • The signal from the X-axis canal is high intensity. (This may mean the head is tilted right.)

    • The signal from the Y-axis canal is low intensity. (This may mean the head is tilted upward.)

    • The signal from the Z-axis canal is high intensity. (This may mean the head is tilted to the front.)


     


    These three signals combine to give the coordinates of the position of the head. In this example, the variation is in the intensity of electricity. But in other signaling pathways, the variation may be in other features of electric signals, such as the frequency, the pattern, or the combinations of all of these. So by “reading” these different inputs of electricity, blind and deaf cells can “comprehend” complex situations and act accordingly.



    It really is unbelievable how such sophisticated information can be simplified. This system may ring a bell to some of you: the main operating principle of the computer is exactly the same. Mere numbers, 0 and 1 (the signals), can combine to form complex information that the processor (the brain) can use to complete tasks satisfactorily. Yes, the brain is effectively a supercomputer capable of processing millions of signals every second in order to keep the body in check. It is capable of increasing its processing speed and can be trained to learn new things. Its memory capacity cannot be filled throughout a lifetime of learning, and it has a supporting system (the neuroglia) which optimizes its performance. It does not require any updates and can work without rest for over 100 years. It is compact and extremely lightweight. Of course, such a wondrous supercomputer requires a lot of resources in order to keep functioning. Although the human brain represents only 2% of the body’s weight, it receives 15% of the blood pumped from the heart, consumes 20% of total body oxygen, and utilizes 25% of total body glucose.



    What we have talked about so far concerns around 80 billion neurons. Well, what about the other 80 billion we mentioned earlier? They are called the neuroglia and their main purpose is to lighten the neurons’ load. These cells form the environment in which nerve cells can operate most efficiently. Sounds simple enough, right? Well, we need to consider the fact that neurons handle delicate cargo. The signal formed by one end of the communication channel has to reach the other end with no change or loss in its features (frequency, intensity, pattern, etc.). A single mishap may generate serious consequences. The environment that the neuroglia are entrusted with includes molecular content, temperature, electrical stability, blood flow, and much more. No wonder there are 80 billion of them assigned to this job!



    Let’s take a look into the types of neuroglia and what they do. First of all, there are the microglia. These guys are the bodyguards of the brain. They are actually specialized macrophages, which are a part of the immune system. In the case of brain damage, they sweep the area clean of any bacteria that may have infected the site.



    Another type of neuroglia is the astrocytes. These cells are in charge of the blood flow to the neurons. They connect the nerve cells to the blood vessels and control the flow of blood by either dilating or constricting the vessels. They also constitute the majority of the “blood-brain barrier”. The blood-brain barrier is the border between the regular blood of the body and the fluid the brain swims in. The astrocytes in this barrier are like the chefs of the brain, selecting what is in the neurons’ menu. They allow only specific molecules through the barrier, meeting the needs of the brain during high activity and preventing waste during low activity. A third type of neuroglia is the ependymal cells. These cells produce the fluid the brain swims in, called the cerebrospinal fluid. Together with the astrocytes, they help form the optimal vital fluid for the brain. One final type of neuroglia is the oligodendrocytes. Oligodendrocytes form the specialized “myelin sheath,” which can be compared to a blanket. This sheath wraps around the nerve cells in the CNS, isolating them electrically. This isolation is key in the preservation and fast transportation of electric signals.



    To conclude, the nervous system is extremely complex. Within this complexity, we find beauty beyond speech. How are these seemingly distant cells and organs in touch with each other through just electricity? With what decision-making mechanism can mere unconscious cells make such critical moves? We only have surface level knowledge of these mechanisms. But one other crucial question is how these cells managed to form such a complex system in the first place. They couldn’t have gone through the process of trial-and-error because error means certain death for such an intricate mechanism. So, did the cells gather around and engineer this perfect system by brainstorming? Were they capable of combining limited organic resources to design a brain that all of mankind could not even come close to after thousands of years of advancement? Ask your brain!


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