Origins: Brain and Self Organization

By Karl Pribram | Go to book overview
that it is not the case for neuroreceptors. First, it has been shown that highly sensitive "specialist" (unimodal) neuroreceptors such as the neuroreceptors detecting sexual pheromones in insects can trigger an action potential with only one activated receptor (see reviews [22, 50]). More recently, intracellular and patch-clamp recordings of "generalist" (multimodal) neuroreceptors have shown that the opening of only one or very few ionic channels were sufficient to initiate an action potential [13, 18, 38, 55]. If this is confirmed more importance should be given to small numbers of occupied sites (0, 1 and 2), and to the degree of maximum depolarization. The occupation of receptor sites, whether activated or not, is assumed to be an alternating Poisson process with rates λ and μ. This assumption yields an exponential distribution for all the sojourn periods. To generalize it, a distribution with density go(t) and mean μo for the occupation time, and another distribution with density ga(t) and mean μa for the activation time, could be considered. A suitable approach to follow for identifying the parameters of the model will depend on the type of experimental data available. If only ISIs are available then standard methods (for review see [58]) have to be used. If the trajectories of membrane potential are also available the methods introduced by Lánský [29] would be easily adaptable to this problem.Several factors, presently under intensive experimental investigations, have not been explicitly taken into account in the present model. The hormonal and centrifugal influences, the turnover of neuroreceptors and the long term changes in synaptic plasticity have been neglected, so that the model applies only in short term conditions. These limitations are of no consequence because the concentration- response curves to be explained were also established in neurophysiological experiments of short duration. The presence of odorant transporter proteins [60] fits easily into the model because it acts only on the relationship (not specified in the model) between parameter λ (our main variable) and concentration of the odorant. The same remark applies for the mechanisms that eliminate the odorant (they act on μ). The detailed biochemical mechanisms of transduction have not been analyzed. We have assumed that the limiting step of the whole process is the odorant-receptor interaction. As discussed above this assumption leads to a basically correct account of the neuroreceptor behavior, which suggests that an essential feature of the real transduction process has been retained. Likewise, the fine structure of neuroreceptors, the complex neuronal circuitry of glomeruli [19, 46] and the distinction of different kinds of lateral inhibition acting at the input (periglomerular cells) and output (granule cells) levels of the second-order neurons [52] have not been considered in detail. However, these mechanisms do not call for a new model but for extensions and specifications of the present one.
Acknowledgements
This work has been partly supported by a grant #511101 from the Academy of Sciences of the Czech Republic, a fellowship from the Institut National de la Recherche Agronomique (to PL) and a grant "Sciences cognitives" from the Ministère de l'Enseignement Supérieur et de la Recherche (to JPR). The authors thank J. King for revising the manuscript.
References
[1] R. R.H. Anholt, "Odor recognition and olfactory transduction: the new frontier", Chem. Senses, vol. 16, pp. 421-427, 1991.
[2] C. Ascoli, M. Barbi, S. Chillemi and D. Petracchi, "Phase-locked responses in the Limulus lateral eye", Biophys. J., vol. 19, pp. 219-240, 1977.
[3] M. Barbi, V. Carelli, C. Frediani and D. Petracchi, "The self-inhibited leaky integrator: Transfer function and steady state relations", Biol. Cybern., vol. 20, pp. 51-59, 1975.
[4] M. Barbi and E. M. Ferdeghini, "Relevance of the single ommatidium performance in determining the oscillatory response of the Limulus retina:", Biol. Cybern., vol. 39, pp. 45-51, 1980.
[5] P. O. Bishop, W. R. Levick and W. O. Williams, "Statistical analysis of the dark discharge of lateral geniculate neurons", J. Physiol., vol. 170, pp. 598-612, 1964.
[6] J. Boeckh, K. D. Ernst, H. Sass and U. Waldow, "Anatomical and physiological characteristics of individual neurones in the central antennal pathway of insects", J. Insect Physiol., vol. 30, pp. 15- 26, 1984.

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