Artificial pacemakers

In Britain about 10000 people a year receive an artificial pacemaker. The operation takes less than an hour and is performed under local anaesthetic.

Pacemakers are made up of a pulse generator and two electrodes. The pulse generator is about the size of a thin matchbox and weighs 20 - 60 g. It is powered by a lithium battery and is implanted under the patient's skin. The electrodes are placed intravenously into the right atrium and the right ventricle. Disease or ageing can damage the heart's natural pacemaker and the conduction of impulses through the heart, causing an abnormally slow heart beat. The artificial pacemaker overcomes this by generating electrical impulses artificially and conducting them to the muscles of the heart on demand or when the heart misses a beat.

Unit 7. Control systems

Translation

7.1 How control systems developed

The ability to respond to stimuli is a characteristic of all living organisms. While many stimuli originate from outside, it is also necessary to respond to internal changes. In a single-celled organism, such responses are relatively simple. No part of it is far from the medium in which it lives and so it can respond directly to environmental changes. The inside of a single cell does not vary considerably from one part to another and so there are few internal differences to respond to.

With the development of multicellular organisms came the differentiation of cells which specialized in particular functions. With specialization in one function came the loss of the ability to perform others. This division of labour, made the cells dependent upon one another. Cells specializing in reproduction, for example, depend on other cells to obtain oxygen for their respiration, to provide glucose and to remove waste products. These different functional systems must be coordinated in order to perform efficiently. No body system can work in isolation, but all must be integrated in a coordinated fashion.

There are two forms of integration in most multicellular animals: nervous and hormonal. The nervous system permits rapid communication between one part of an organism and another. The hormonal system provides a slower form of communication and can be compared with the postal system. Both systems need to work together.

All organisms, plant and animal, must respond to environmental changes if they are to survive. To detect these changes sense organs are required. Those detecting external changes are located on the surface of the body and act as a vital link between the internal and external environments. Many other sense cells are located internally to provide information on a constantly changing internal environment. In responding to stimuli, an organism usually modifies some aspect of its functioning. It may need to produce enzymes in response to the presence of food or move away from an unpleasant stimulus. In most cases the organ effecting the change is some distance away from the sense cell detecting the stimulus. A rapid means of communication between the sense cell and the corresponding organ is essential. In animals the nerves perform this function.

The stimuli received by many sense organs are very complex and require different responses. The sense organs must therefore be connected by nerves to all effector organs. One method is to have an individual nerve running from the sense organ to all effectors. Clearly this is only possible in very simple organisms where the sense organs respond to a limited number of stimuli and the number of effectors is small. Large, complex organisms require a different system, because the number of sense organs and effectors is so great that individual links between all of them is not possible. The central nervous system (brain and spinal cord) acts like a switchboard in connecting each incoming stimulus to the appropriate effector.

Summary