tailieunhanh - PSYCHIATRY, PSYCHOANALYSIS, AND THE NEW BIOLOGY OF MIND - PART 6

Một vài trong số các thụ thể này là methyl-chấp nhận (hóa hướng) protein có mức độ thay đổi kết cộng hóa trị là tỷ lệ thuận với cường độ kích thích kinh tế. Họ tạo ra một kích thích tín hiệu bản chất của mà vẫn không được biết mà quyết định | Neurobiology and Molecular Biology 195 chemotransduction from studies of bacteria Adler Koshland et al. . Bacteria E. coli Salmonella have different chemoreceptors for different attractant and repellent sugars. A few of these receptors are methyl-accepting chemotaxis proteins whose degree of covalent modification is proportional to stimulus intensity. They generate an excitatory signal the nature of which is still not known which determines frequency of tumbling the changes in the direction of rotation of the flagella that move the bacterium. In response to a positive gradient of attractant the tumbling is suppressed the flagella rotate counterclockwise for long periods moving the bacterium in a straight path. For an escape response to a repellent the flagella rotate clockwise causing the bacterium to tumble. The response of the bacterium can adapt over time even though the attractant or repellent is still present. This adaptation results from a change in the methylation of the methyl-accepting chemotaxis proteins. Thus as in the adenylate cyclase and transducin systems chemoreception in bacteria involves more than sensing and recognition of the ligand by the receptor. In each case the receptors are part of a complex of molecules that initiate a cascade of events both in series and in parallel. In the case of the aspartate receptor Koshland et al. the three key functions recogni-tion signal transduction and adaptation can be separated from each other by the techniques of in situ mutagenesis. Recent studies have indicated that in the multicellular nervous systems of invertebrates and vertebrates there is imposed upon the network of nerve cells and interconnections that control a behavior a set of regulatory processes that can alter the excitable properties of nerve cells and modify the strength of their connections. These regulatory processes are activated by experience such as learning and result in the modification of behavior. Learning refers to the modification