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Nervous System 2 PDF Print E-mail
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Our nervous system evolved from a simple string of nerve cells that could only bring about the simplest reflex movements such as e.g. in the lancelet. This tiny animal is about the simplest vertebrate and is not much more complicated than a worm. At the other end of this developmental line is the human with the most complicated nervous system. The most important principle underlying this development is that new elements are continually being added to the existing system; they do not displace it. Quite the reverse in fact, as the older elements are, as it were, manipulated by the newly evolved higher elements. Our nervous system consists of superimposed control circuits that continually make more complicated behavior possible.

A distinction is made between two subdivisions: the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), that part of the nervous system outside the CNS which consists mainly of the nerves that extend from the CNS.

Another distinction made is of functional character, namely between the voluntary and the involuntary nervous systems. The first allows us to control our skeletal muscles consciously. The second, also called the autonomic nervous system (ANS), regulates all events not under conscious control: the activity of the internal organs, such as the heart, stomach, etc. The ANS itself has two parts: the sympathetic part that activates, and the parasympathetic part that de-activates.

The central nervous system is built up of the following parts:

Spinal cord

The 'simplest' part of our nervous system is the spinal cord. In theory, it can still be compared with the nervous system of the lancelet. As noted earlier, the kneejerk reflex travels along the spinal cord. All our motor activity happens by manipulation of such reflex centers by the higher nerve centers. All communication with the brain travels through the spinal cord. Two thick tracks of axons run from the brain to these reflex centers for motor control: the pyramidal tracts. In addition, thick bundles of axons travel to the brain from the sensory receptors in the skin, muscles, etc. and control sensory input of the brain.

Brain stem

This is actually the first part of the original brain stem that continued to develop because the most important sensory organs (in order of their evolutionary development: for smell, taste, vision and hearing) are localized at the front side of the body. Information processing from these organs led to continual enlargement of the brain.

Control of the vital functions is organized in the brain stem. This is where breathing and blood pressure are regulated; where the awake/sleep cycle is regulated. All (passive) vital functions necessary to stay alive are controlled from here.

Little brain (Cerebellum)

The cerebellum plays an important role in body movement. While the brain stem can only regulate simple reflexes 'independently', in the cerebellum complicated patterns of movement, 'movement melodies', are initiated and stored as fixed patterns. The cerebellum is present even in primitive fish and its importance increases along the evolutionary scale. The more complicated the motor activity of an animal, the larger the cerebellum.

Limbic system

The limbic system can best be described as the first beginning of the great brain. Research has given us some idea of how this system works. Both the nature and the results of such research can best be illustrated by the following famous example. The Spanish neurophysiologist DEGADO planted electrodes into a specific part of the limbic system of a Spanish fighting bull and fitted it with a receiver so that when a signal was sent by a transmitter a current would flow through one of the electrodes and stimulate the cells concerned. Once the wound from the operation had healed, the bull was taken to the arena and, in the traditional way, provoked fury by the toreadors. Delgado then entered the arena and provoked the bull which then charged him, head down to take Delgado on his horns.
When the bull was only a few meters away from Delgado, he pressed the button of the transmitter in his pocket which stimulated the bull's brain and the bull calmed down immediately. Efforts by the toreadors to enrage the bull again were unsuccessful as long as Delgado kept stimulating the bull's brain. Only by pressing another button which stimulated a different part of the limbic system could the bull again be brought to a state of rage without the help of the toreadors. Evidently, the cells in which Delgado planted his electrodes requlate rage and tranquillity. The cells concerned are part of the limbic system and are found in the amygdaloid body, the amygdala. In a way similar to this rats and other animals can, by selective stimulation of other parts of the limbic system, be motivated to eat, resp. refuse food, to sexual arousal resp. be made immune to the usual sexual stimulations. In short, the limbic system regulates a number of very vital emotions. This effect can also be elicited in humans:

'The first time we were able to demonstrate that systems in the limbic brain that both start and stop attack behaviour was with patient Thomas R. Thomas' chief problem was his violent rage.... NOTE 2 Electrodes were implanted in his amygdala and the daily stimulation of specific parts of it (the lateral) kept him free from attacks of rage for two months. Since it is not possible to continue this regimen throughout a patient's life, those parts of his amygdala which elicited an attack of rage when stimulated (the medial) were destroyed electrically. His attacks of rage subsequently stopped. This operation was, incidentally, thereason for a lawsuit between the two neurosurgeons and the mother of Thomas R. who had fought the operation NOTE 3 . The court's decision is unknown to the author.

In this connection it is striking that disorders in these regulating mechanisms lead to behavioral disorders which, though not necessarily seen by all as addictions, are nevertheless seen as 'cravings': bulimia, anorexia nervosa, gambling fever, etc. We may assume that the addictive effect of some drugs is connected with their influence on the receptors in the limbic system.

Furthermore, the limbic system influences the hormonal system by means of the hypophysis (pituitary gland), a small gland attached to the base of the brain. Finally, the limbic system, especially that part to do with the hippocampus, plays a very important role in memory.

The limbic system is already somewhat developed in fish and reaches full development in the reptiles. In rudimentary form its main function is processing olfactory stimuli; later, this function shifts to regulating emotions. An echo of this old function can be seen in the fact that no sensory stimulation can elicit stronger emotions than a smell. The human limbic system is virtually no different than that of the reptiles. It represents the 'crocodile' in us.

Subcortical centers

These are the communication stations between the brain stem and the limbic system on the one hand and the cerebral cortex on the other. The complicated behavior patterns are organized here. In this connection an important part of these centers, which some think are a part of the limbic system, is the nucleus accumbens.

The role these centers play becomes clear with experiments in which laboratory animals with a permanently implanted electrode or a permanent infusion stimulate themselves by pressing a button. When such an electrode is implanted in the nucleus accumbens, and the laboratory animal realizes that the sensation it is getting is caused by pressing the button, the feeling is so pleasurable that the animal will lie down on the button. Other locations in this center have the reverse effect. The theory now is that this center functions as a kind of punishment/reward center in the sense that eating when you are hungry also stimulates the reward center, while with satiation the punishment center is stimulated if you then continue to eat. Stimulation of this center could, then, give a feeling of reward, without there necessarily being any 'rewardable' behavior. Why should you eat, drink, have sex, etc. if you can also elicit the ultimate feeling of reward by electrode (or by a needle in your arm)?

These subcortical centers, together with some parts of the cerebral cortex which other mammals also have, can be referred to as the 'horse ' in us.

Cortex cerebri (the cerebral cortex)

The cerebral cortex is the most complicated part of the nervous system. It is divided into four parts: the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe. All sensory perceptions are 'projected' onto the cerebral cortex and the outside world is represented by these projection fields. For example, each point in the visual field is represented by a point in the visual projection field in the occipital lobe. This is, incidently, where the story comes from that you can go blind if you fall on your tailbone. The shock is conducted along the spinal cord to the back of the head and the skull knocks against the visual projection field. This if damaged, causes blindness although nothing is wrong with the eyes. In the same way, all sounds, from high to low, are represented in the auditory projection field in the temporal lobe, the entire body surface is represented in the sensory cortex in the parietal lobe and all muscles in the frontal lobe. In addition to these projection fields, we also have association fields where the connections between this sensory input and motor output are made. That part of the brain which 'seats' language is particularly large even compared with our closest relative, the chimpanzee. In this association field the different aspects of language - hearing, seeing (both objects and letters) and speaking (larynx muscles) coordinate. The slightest damage to this area can lead to abnormalities such as aphasia, dyslexia, etc.

The cerebral cortex is, mainly because of the latter mentioned association field, the 'knight' in us. Every psychosocial worker should realize that we are not people, but rather knights sitting on a horse, with the horse standing on a crocodile. We pay close attention to the horse, but if the crocodile in us suddenly turns left or right, we tumble to the ground: we call it a life crisis then.