Membranes and Sensory Transduction

The main purpose of this book is to unify approaches and ideas in the field of aneural sensory transduction. This field has recently come to the attention of several research groups in various disciplines, and their number seems to be growing. Unfortunately, because of the diverse scientific backgrounds of the researchers in the field, the apparent heterogeneity of experimental techniques (i. e. , behavioral response analysis, sophisticated biochemical and genetic manipulations, conventional and pulsed laser spectroscopy) and theoretical approaches may be discouraging, for both the experienced worker and the new comer. Actually, this heterogeneity is more apparent than real, and unifying concepts, approaches, and ideas already exist, particularly with respect to all the questions concerning the role of membranes and their properties (such as ion permeability, electric potentials, and active transport) in the various steps of sensory perception and transduction processes. It is currently accepted that most, if not all, the fundamental facts in molecular sensory physiology of aneural organisms, be they chemosensory, photosensory, or geosensory, can ultimately be understood in terms of a few basic ideas. Each chapter of this book emphasizes and clarifies the role of mem brane properties and phenomena in the particular sensory response examined. Of course, in some cases, this task has been rather complex because of the limited amount of experimental data clearly supporting a membrane-based model of sensory transduction.

1 Membranes: Structure and Function.- 1. The Dynamic Structure of Cell Membranes.- 1.1. Introduction.- 1.2. Chemical Components.- 1.3. Membrane Structural Framework.- 1.4. Danielli-Davson and Fluid Mosaic Models.- 1.5. Laboratory Models.- 2. Kinetic and Thermodynamic Approach to Membrane Transport Properties.- 2.1. Equilibrium State.- 2.2. Nonequilibrium States.- 2.3. The Nernst-Planck Flux Equations.- 2.4. Specific Cases.- 2.5. Partition Coefficient at an Oil-Water Interface.- 2.6. Electrostatic Potentials at a Membrane-Solution Interface...- 2.7. Dipoles at a Membrane-Solution Interface.- 2.8. The Eyring Theory.- 3. Transport in Excitable Membranes.- 3.1. Introduction.- 3.2. Description of the Ionic Currents.- 3.3. The Hodgkin-Huxley Model.- 3.4. Ionic Channels.- 3.5. Selective Binding and Selective Permeability.- 3.6. The Sodium Channel.- 3.7. The Potassium Channel.- 3.8. Gating Currents.- 3.9. Noise Analysis.- 3.10. Single-Channel Recordings.- 3.11. Electrically Gated Channels in Planar Lipid Bilayers.- 3.12. Ionic Carriers in Planar Lipid Bilayers.- 4. Model Photosensitive Membranes.- 4.1. Introduction.- 4.2. Chlorophyll-Containing Lipid Bilayers.- 4.3. Bacteriorhodopsin and the Purple Membrane.- 4.4. Rhodopsin in Model Membranes.- References.- 2 Biochemistry of Chemosensory Behavior in Prokaryotes and Unicellular Eukaryotes.- 1. Diversity and Unity in Chemotaxis.- 1.1. Diverse Roles of Chemosensory Responses.- 1.2. Response-Regulator Model.- 2. Perception of Chemicals.- 2.1. Sensing the Gradient.- 2.2. Chemoreceptor Proteins on the Surface of Bacteria.- 2.3. Internal Receptors in Bacteria.- 2.4. Chemoreceptors in Eukaryotes.- 3. Signal Transduction.- 3.1. Focusing the Signals in Bacteria.- 3.2. Mechanism of Excitation in Paramecium.- 3.3. Signal Transduction in Leukocytes and Dictyostelium.- 4. Behavioral Responses.- 4.1. Klinokinesis in Bacteria.- 4.2. Klinokinesis and Orthokinesis in Ciliates.- 4.3. Tactic Behavior.- 5. Adaptation and Signal Processing.- 5.1. Role of Adaptation in Sensory Responses.- 5.2. Biochemistry of Adaptation in Bacteria.- 5.3. Adaptation in Eukaryotes.- 6. Models for Chemotaxis.- 7. Conclusion.- References.- 3 Mechanosensory Transduction in Protozoa.- 1. Introduction.- 2. Depolarizing and Hyperpolarizing Membrane Responses to Mechanical Stimulation.- 3. Ionic Mechanisms of the Mechanoreceptor Responses.- 4. Kinetic Analysis of the Mechanoreceptor Currents.- 5. Topographical Distribution of the Mechanoreceptor Channels.- 5.1. Ciliary Membrane or Somatic Membrane?.- 5.2. Anteroposterior Distribution.- 5.3. Dorsoventral Distribution.- 5.4. Species-Dependent Differences in Mechanosensitivity.- 6. Coupling of the Mechanoreceptor Responses with the Behavioral.- Responses.- References.- 4 Temperature Sensing in Microorganisms.- 1. Introduction.- 1.1. Overview.- 1.2. Difficulties Peculiar to Thermosensing.- 2. Cellular Components or Processes Affected by Temperature.- 2.1. Enzymes.- 2.2. Membranes.- 2.3. Protein-Lipid Interactions.- 2.4. Membrane Lipid Composition.- 3. Biological Examples.- 3.1. Bacteria.- 3.2. Ciliates.- 3.3. Acellular Slime Molds.- 3.4. Cellular Slime Molds.- 3.5. Nematodes.- 4. Summary.- References.- 5 Microbial Geotaxis.- 1. The Phenomena of Geotaxis.- 1.1. Geotaxis and Geotropism.- 1.2. Occurrence of Geotaxis.- 1.3. Orientation Direction Provided by Gravity.- 1.4. Energetics of Geotaxis.- 1.5. Biological Significance of Geotaxis.- 1.6. Geotaxis and Other Behaviors.- 2. Arenas of Debate.- 2.1. Mechanics versus Mechanism.- 2.2. Mechanisms of Response.- 2.3. Unproved Existence of Gravireceptors for Geotaxis.- 2.4. Involvement of Membranes and Sensory Transduction.- 3. Methods of Assessment.- 4 Hypotheses on the Mechanism of Geotaxis.- 4.1. Old Mechanisms and New Disputes.- 4.2. Physical Mechanisms.- 4.3. Physiological Mechanisms.- 5. Discussion.- 5.1. Physical and/or Physiological Mechanisms?.- 5.2. Conjectures on Physiological Mechanisms.- 5.3. Conclusion.- References.- 6 Photosenso