114 Inside the Living Cell 



the other by a centipede kind of motion (Fig. 20). The atp would be 

 involved either in the making or the breaking of attachments and the 

 energy required would be provided in some way by the energy rich 

 bonds of the atp. Fig. 20 is a rather fanciful picture of such a process. 

 The final question we must ask is how the muscle is activated, i.e. 

 what causes it to begin to contract. As a general rule, as we shall see, 

 muscle action is excited or initiated by electrical disturbances arriving 

 down a nerve. This excitation causes the movement of inorganic ions 

 across the membrane in which the muscle fibres are enclosed. There 

 is a special region of the muscle fibre surface which is in contact with 

 the nerve and it is here that the processes leading to excitement take 

 place. It was shown by Sir Henry Dale and Prof G. L. Brown that, 

 when the nerve signal arrives in this region, the substance acetyl- 

 choline is liberated. It was also found that an enzyme capable of de- 

 stroying acetylcholine is also present; so that the acetylcholine 

 formed only lasts for a short time. The idea that the actual process of 

 muscle excitation is brought about by acetylcholine has been con- 

 firmed by two facts: (1) substances which inhibit acetylclioliiie action 

 interfere with the excitation of the muscle, (2) acetylcholine, when 

 applied artificially, does give rise to muscle contractions. However, 

 very little is known about how the electrical disturbance arriving 

 along the muscle causes the release of acetylcholine, and not a great 

 deal is known of the changes which acetylcholine brings about, ex- 

 cept that it causes marked changes in the ease with which sodium and 

 potassium ions can penetrate the muscle membrane. 



SENSE ORGANS 



Sensitiveness to the environment is probably a fundamental pro- 

 perty of living cells. Even the most primitive forms of life respond to 

 some degree to heat or cold, to light and to chemical substances. Sen- 

 sitiveness to light is probably a part of the original character of life, 

 since as we have seen, the original living complex must have been 

 photosensitive and able to use light energy. Sensitiveness to chemical 

 compounds must also be a very primitive feature of life, since many 

 chemicals upset the delicate balance of the life processes and, in fact, 

 many products of enzyme action are themselves inhibitors and pre- 

 vent enzymes working properly. Organisms often cannot live unless 

 the substances they produce are removed. They also need a fresh 

 supply of the substances used for food and can often detect them at 

 appreciable distances, e.g. the whelk can detect the compounds 

 formed by decaying animal matter at a considerable distance. 



Many primitive animals are light sensitive to some extent over the 



