Responsiveness in Single Cells - 1 95 



MICRONEEDLE. 

 STIMULUS. 



Fig. 1 1-4. Response of an amoeba, pricked by a 

 microneedle. The specimen moves away in a co- 

 ordinated manner, indicating that the excitation 

 is conducted from the site of stimulation to all 

 parts of the cell. 



not act in a concerted fashion as the amoeba 

 alters the direction of its locomotion. In the 

 amoeba, apparently, any part of the cell may 

 serve temporarily as receptor, conductor, or 

 effector, and scarcely any definite differentia- 

 tion of these parts can be recognized. In the 

 amoeba, however, it has recently been shown 

 that the tip of an advancing pseudopodium 

 is consistently negative in relation to other 

 parts of the cell surface, which indicates that 

 the action potential has its origin here. 



Perhaps the commonest specialized recep- 

 tors in unicellular organisms are the red- 

 colored chromatophores, or "eyespots," that 

 are found in many green flagellates (Figs. 

 9-7 and 10-4). Each chromatophore is a small 

 pigmented granule, which is generally re- 

 garded as a photoreceptor. Probably many of 

 the light-seeking responses that are char- 

 acteristic of the phytoflagellates are initiated 

 in these receptor structures. 



The cilia and flagella of the Protozoa and 

 free-swimming algae serve as effectors of 

 mechanical response, and probably also as 

 receptors of tactile stimuli, since frequently 

 these parts are first to make contact with ex- 

 ternal objects. Many protozoans, especially 

 ciliates, also possess internally placed con- 

 tractile fibrils, called myonemes (Figs. 11-5 

 and 11-6). These intracellular effectors act 

 more or less like the muscle fibers of higher 

 animals. By shortening quickly and forcibly, 

 the myonemes enable a one-celled organism 



RETREATING 



to change its shape, sometimes in very drastic 

 fashion. 



Specialized conductors are relatively rare 

 among unicellular organisms. The undiffer- 

 entiated protoplasm of these cells propagates 

 an excitation at a comparatively slow rate 

 (see p. 447), but within the single cell this is 

 fast enough. Certain large ciliated proto- 

 zoans, however, display a delicate network 

 of intracellular threads, called neurofibrils. 

 The neurofibrillar network forms an inter- 

 connection between the cilia all over the 

 body, and it is thought that the neurofibrils 

 are a conductor system, which coordinates 

 ciliary activity. 



Ciliary Movement. Broadly speaking, the 

 term ciliary movement includes the beating 

 of flagella as well as of cilia, although cilia 

 differ from flagella in being shorter, stouter, 

 and more numerous. Flagella are character- 

 istic of many protozoans and algae, and of the 

 sperm cells of higher plants and animals gen- 

 erally. Cilia are found in a large group of 

 the Protozoa (p. 625) and in the ciliated 

 epithelia of the respiratory and genital tracts 

 of higher animals. 



In action, a cilium swings stiffly and rap- 

 idly in one direction, then limply and slowly 

 returns to its original position (Fig. 11-7). In 

 most ciliated epithelia the stiff effective 

 stroke is always in one direction. For exam- 

 ple, in man's bronchial passages, the ciliary 

 beat is always upward, so that dust and other 



