Phobic response

Fig. 15. Specific chemical inhibition of physiological bluelight responses 04) Swimming, inverse phobic response and direct photophobic response of Euglena as a function of iodide concentration118. (B) Phototropic and geotropic curvature of the avena coleoptile as function of azide concentration1S41...

A response is defined as any stimulus-induced alteration in the activity of the organism s motor apparatus which can (but need not always) result in an alteration of the movement or orientation of the organism. Where nature or occurrence of a response depends upon an increase or decrease in stimulus intensity, an increase may be indicated by the use of the term step-up and a decrease by the term step-down . Thus, a step-up photophobic response is a phobic response which occurs upon an increase in light intensity. The responses are classified as follows ... 

These results were interpreted using the electron pool hypothesis There is an electron pool situated in the linear photosynthetic electron transport chain between photosystems II and I (Fig. 9). A phobic response is triggered by a decrease in the flow rate through the pool. This can be accomplished in two ways ... 

When a trichome moves into a photosystem I light trap, electrons are drained out of the pool. In both cases a phobic response is caused, but different patterns result, one being an accumulation in, the other a dispersal from, the light trap l0]). 

Fig. 12. Action spectrum of light-induced potential changes measured with microelectrodes (after Hader46)) and action spectrum of photophobic response. Abscissa wavelength in nm Ordinates amplitude of light-induced potential changes in mV and phobic response in relative units respectively (after Nultsch88))...

Microscopic analysis of the photobehaviour demonstrated that phobic responses are inhibited only in those organisms which move more or less parallel to the electric field lines. In those leaving the light trap perpendicular to the field lines, the phobic responses were not impaired. But since the light trap was not completely closed, no accumulations were formed. 

In general, a phobia can be best defined as an intense, unrealistic fear of an object, event, or feeling (Plaud Vavrovsky, 1998). It is estimated that 18% of the American adult population suffers from some type of phobia (Hall, 1997). Yet exactly how phobias develop and what triggers and sustains the phobic response remains elusive. There are three primary types of phobias (a) agoraphobia, the fear of being unable to escape when leaving a safe place) (b) social phobia, the fear, embarrassment, and avoidance of social situations and (c) specific or simple phobia, the fear of an object or other than social situations. 

Kreimer, G. and Witman, G.B. (1994). Novel touch-induced, Ca -dependent phobic response in a flageUate green alga. Cell Motil. Cytoskeleton 29, 97—109. 

Table 2 summarizes the behavioral mechanisms of chemotaxis in the systems reviewed in this book. If these systems faithfully represent the mechanisms of chemotaxis in nature, it appears that prokaryotes mainly employ trial-and-error chemotaxis mechanisms— the phobic response and klinokinesis, defined in Table 2 and, in more detail, in Chapter 1. In contrast, eukaryotes appear to primarily employ the more direct mechanism—modulation of the direction of movement according to the stimulant gradient. A number of non-mutually-exclusive reasons may account for this difference. First, the mechanisms employed by bacteria... 

Gradient-stimulated modulation of the linear velocity followed by a directional change (phobic response) Bacteria (e.g., Rhodobactersphaeroides, Sinorhizobium melUoti), archaea (e.g., some cells of Halobacterium salmarium) 3... 

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