The nervous system

The central nervous system consists of three main parts (see figure 1). The oldest part, looking at it from an evolutionary angle, is the spinal cord. This bundle of nerves stretches between the head and the tailbone; it is the part of the nervous system needed for reflexes. For a reflex, two nerves are needed; one to register that something is happening and signal this to the spinal cord; the other to transmit the response from the spinal cord to the area concerned.

Figure-1

Take for instance the patella or "knee-jerk" reflex (see figure 2). When you knock the tendon below the kneecap, your upper leg muscles will be stretched. Sensors in the muscles will relay this to the spinal cord (a). From there a reflex signal will be sent back to the muscles (b). This gives the muscles in the upper leg the signal to contract and stretch the knee, moving the lower leg forward.

We need these reflexes to keep upright, because when a person is standing and the tendon is suddenly stretched, then he or she might bend the knee and fall over. So these muscle reflexes are constantly guarding our upright posture. This reflex is an inbuilt response which happens automatically, without requiring any thought. In other words, this part of the nervous system regulates the automatic movements.

Figure-2

The middle part of the central nervous system is the brain stem and an area above it called the limbic system. It is situated at the top of the spine, inside the skull. This part is connected to the emotions and memory.

The last part to be added during evolution is what we actually call the brain. The brain is situated inside the skull and wraps itself around its base, the brain stem. When comparing these three separate parts of the brain with the animal kingdom, we can observe how mammals have supposedly developed from sea-creatures into amphibians into apes. For example a sea-anemone is mainly a reflex body. When it is touched, it recoils immediately. This little creature has only got a primitive automatic nervous system like a spinal cord. Frogs have developed the middle part of the nervous system (the brain stem) and a little bit of a brain. We notice that these animals have rudimentary senses: sight, hearing, taste and smell. In more developed animals such as mammals, the actual brain increases in size compared with the other two older parts, the middle and the lower. So, the more developed the animal, the bigger the brain. We humans have exceptionally large brains compared to animals.

When studying the function of the brain, it is helpful to keep in mind the following view of nature: looking at the immediate world around us, we could say that there are four main realms. The first is the mineral world. This is what we can touch. The earth provides substance. The next step up the ladder is the plant realm. On this step, life is added to substance. In other words it becomes living matter. Plants live in a world of rhythms, such as day and night, summer and fall. They exist in cycles of growing and wilting. The next realm is the animal world.

What is the added extra here? Try to answer for yourself the question: "what can animals do that plants cannot?" Well, for example, animals can move and breathe with their lungs. They can use their senses. They have eyes, a sense of smell and taste. They have instincts, drives and perhaps passions. Animals are aware of their surroundings. The added extra on this step of the ladder is consciousness. Now, what realm do you think we will enter with the next step? This is the human realm. What can we do that animals cannot? We can speak and walk upright. We can think and be creative and use our intelligence. We can judge. Above all, we have a choice in what we do. Therefore, we can behave differently from our fellow human beings. Each of us has a name. We are individuals, whereas animals belong to a group, like cows or tigers. Each cow has not got an individual name, but is part of the herd. Animals do not think. For example, contrary to the popular saying, a cow will graze in a meadow and is unlikely to look over the fence to see whether the grass is greener elsewhere. It just follows its instincts and has no choice. A tiger has to kill for its food, but we do not say that every tiger is bad!

By being human, we have a choice in what we can or will do. This freedom comes with responsibilities. In the human realm, because of our free will, ethical judgments are made and we can talk about good or bad behaviour. So, in addition to being alive like plants and conscious of our surroundings like animals, we are actually self-aware. The quintessence added to our realm is self-consciousness (see the diagram).

It will come as a surprise, perhaps, to know that the main function of the brain is to inhibit the two lower parts of the central nervous system! It is difficult to imagine that through evolution we have developed this enormous brain to suppress the functions of the brain stem and spinal cord. However, in a person with cerebral palsy, the reflexes of the spinal cord are not inhibited by the large brain, because of brain damage at birth. That is why these people have exaggerated movements. With reference to inhibition of the middle part of the brain, you can imagine a person suppressing a smile (an emotion) when he feels it would be inappropriate to show it. On the other hand, a person who has suffered a stroke (which causes damage to the large brain) can become emotionally changeable if a particular part of the brain is damaged. When he hears a sad story he can't stop himself crying; but if somebody tells him a joke straight away, he could start laughing.

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