110 FACTORS AFFECTING CILIARY ACTIVITY 



the preparation of the models. Hoffman-Berling (1955) noted 

 that ATP may be serving a double function, acting specifically 

 as an energy source for contraction, and unspecifically as a 

 plasticizer for relaxation; he attributed the effect of ATP con- 

 centration on rate of beat that he found to its plasticizer action. 

 Pyrophosphate may also act as a plasticizer, for Bishop (1958a) 

 has found that addition of pyrophosphate w^ith ATP may increase 

 the amplitude of contraction. In addition, Bishop (1958e) has 

 demonstrated a Marsh -factor type of relaxing system in sperm 

 tails similar to that known in muscle; this is sensitive to Ca++, 

 since models with ATP and Marsh -factor from rabbit muscle 

 would not contract until Ca++ was added. 



It is important to notice that although contractions occur 

 alternately on the two sides of the cilia or fiagella models, the 

 contractions are not propagated along the organelles as bending 

 waves, and there was no forward movement or spiralling of sperm 

 models. 



10. Hormones and Ciliary Activity 



Several substances known to act as hormones and chemical 

 mediators in other parts of animals have been found in ciliated 

 epithelia, and have been shown to have effects on the activity of 

 cilia. It is not easy to be certain whether these substances are acting 

 directly on the ciliary mechanism, and in no case is the role of the 

 hormones fully understood. A summary of results obtained with 

 hormone substances mentioned is given in Table 9. 



Perhaps the most interesting of these substances is serotonin 

 (5-hydroxytryptamine), which is believed to be released by 

 some motor nerves of molluscs, and has been found to excite the 

 hearts of some crustaceans and some molluscs at very low 

 concentrations (Welsh, 1954: Page, 1958). It is also known to 

 increase the activity of rhythmically contracting smooth muscle 

 fibres of the chick amnion, at a stage when no nerve connexions 

 are present (e.g. see references in Page, 1958), and to increase the 

 rate of beat of isolated mammalian atria (Trendelenberg, 1960). 

 Recently, a fairly wide interest has been taken in this substance, 

 both in vertebrates and invertebrates, but its action does not seem 

 to be understood in any of the many places in which it has been 

 found. 



