共查询到20条相似文献,搜索用时 15 毫秒
1.
Anatol Rapoport 《Bulletin of mathematical biology》1952,14(4):351-363
The response time of a random net is defined as the expected time (measured in the number of synaptic delays) required for
the excitation in the net (measured by the fraction of neurons firing per unit time) to reach a certain level. The response
time is calculated in terms of the net parameters as a function of the intensity of the outside stimulation. Two principal
types of cases are studied, 1) an instantaneous initial stimulation, and 2) continuously applied stimulation. It is shown
that for a certain type of net where the required level of excitation is small, the response time-intensity equation reduces
to the one derived on the basis of the “one-factor” theory applied to a neural connection. More general assumptions, however,
give different types of equations.
The concept of the “net threshold” is defined, and its calculation indicated. The net threshold for instantaneous stimulation
is, in general, greater than that for continuous stimulation. The results are discussed with reference to existing theories
of reaction times. 相似文献
2.
A “probabilistic” rather than a “deterministic” approach to the theory of neural nets is developed. Neural nets are characterized
by certain parameters which give the probability distributions of different kinds of synaptic connections throughout the net.
Given a “state” of the net (i.e., the distribution of firing neurons) at a given moment, an equation for the state at the
next moment of quantized time is deduced. Certain very special cases involving constant distributions are solved. A necessary
condition for a steady state is deduced in terms of an integral equation, in general non-linear. 相似文献
3.
J. B. Roberts 《Bulletin of mathematical biology》1948,10(3):123-129
We discuss under the McCulloch and Pitts assumptions for neural nets a circuit consisting ofk cycles such that one cycle is activated by an outside stimulus and sends an impulse to a second cycle which in its turn sends
an impulse to the next cycle, etc., up to thekth cycle, which sends an impulse to a response. We thus have a “series” ofk cycles “interacting”. We give several theorems regarding the response patterns of such circuits under the additional constraint
that the stimulus acts but once, and at the time it acts the circuit is at rest. 相似文献
4.
John Miles Little 《Journal of bioethical inquiry》2010,7(1):91-102
Aesthetics is a vexed topic in philosophy, with a long history. For my purposes, an aesthetic experience is a foundational
affective response to an object, to which terms such as “ugly”, “beautiful”, “pretty” or “harmonious” are applied. These terms
are derived from a Discourse of aesthetics; some remain constant, others change from generation to generation. Aesthetics
and ethics have been linked in Western thought since the days of Plato and Aristotle. This essay examines what is happening
to that link in contemporary experience. It emphasises the ways in which the popular media exploit aesthetic appeal to penetrate
their market, and to exploit and frame intuitive responses to current and past events. Production values, the artfulness of
editors and the financial interests of producers and directors thus do much to determine contemporary aesthetic and ethical
judgements. That which is beautifully presented invites the ethical involvement of the audience. Events whose images are beautifully
presented constitute “hyperevidence”, a pre-judged, reinforced and amplified illusion of reality and participation. Understanding
how aesthetic excellence draws audiences into ethical relationship with what is presented becomes an important part of education
in ethics, including bioethics. 相似文献
5.
6.
Reimut Wette 《Bulletin of mathematical biology》1953,15(3):251-260
A critical examination of the “classical” theories of photoreception in view of more recent experimental findings yields the
result that these theories do not possess the property to describe all the more significant phenomena of photoreception correctly,
and to some extent suffer the lack of more general applicability. The basis for a new and presumably more general theory of
photoreception based on dynamical aspects is laid out. Emphasis is put on the time course of afferent and efferent excitation
in the photoreception model, consisting of a receptor element, an afferent and an efferent neuron of the one-factor Rashevsky-type,
and an effector organ. 相似文献
7.
N. Rashevsky 《Bulletin of mathematical biology》1947,9(1):1-8
We may consider that most of the human behavior is a set of learned responses to certain patterns which recur frequently in
the course of human life. Some “abnormal” events or experiences may result in the learning of abnormal responses, and thus
in abnormal behavior. The “abnormal” responses may begin to be learned after some of the normal response patterns have been
fairly well established. The development of both normal and abnormal behavior may thus be represented by learning curves of
the type studied by H. D. Landahl. Applying some of the results of the theory of learning curves and considering that the
normal and abnormal reactions may reciprocally inhibit each other, a quantitative theory of some psychoses may be developed.
In particular, the effects of shock may be deduced from the assumption that they cause the more recently learned abnormal
reactions to be “unlearned” more readily, than the earlier learned “normal” reactions. The effectiveness of shock treatments
as a function of the duraction of psychosis is discussed from this point of view. 相似文献
8.
Anatol Rapoport 《Bulletin of mathematical biology》1952,14(1):35-44
The spread of excitation in a “random net” is investigated. It is shown that if the thresholds of individual neurons in the
net are equal to unity, a positive steady state of excitation will be reached equal to γ, which previously had been computed
as the weak connectivity of the net. If, however, the individual thresholds are greater than unity, either no positive steady
state exists, or two such states depending on the magnitude of the axone density. In the latter case the smaller of the two
steady states is unstable and hence resembles an “ignition point” of the net. If the initial stimulation (assumed instantaneous)
exceeds the “ignition point,” the excitation of the net eventually assumes the greater steady state.
Possible connections between this model and the phenomenon of the “preset” response are discussed. 相似文献
9.
We conducted a detail study of the photosynthetic apparatus in assimilating organs of three introduced evergreen conifer species:
Taxus cuspidate S. et Z. ex E. (Far-Eastern yew), Thuja occidentalis L. (arbovitae “green”), and Th. occidentalis f. “Reingold” (arbovitae “yellow”) at various times in their life cycle. We studied the potential photosynthesis rate; composition
and ratios of pigments, including primary carotenoids; the violaxanthin cycle (VC) activity, the synthesis of a secondary
carotenoid, rhodoxanthin; and chloroplast ultrastructure. In winter and spring, β-carotene and lutein (primary carotenoids)
contents were relatively constant in yew and arbovitae “yellow”. In December, the VC in yew was balanced and in arbovitae
“yellow” unbalanced. In arbovitae “yellow”, the zeaxanthin pool was heterogeneous, and only part of it took part in the VC.
It can be assumed that the other part of the pool can be oxidized to form a secondary carotenoid, rhodoxanthin. This secondary
carotenoid was also accumulated in arbovitae “green”; its synthesis took place during the season, when the photosynthesis
rate of plants was the lowest, and a significant chloroplast reorganization occurred (the number of thylakoids in grana decreased
and plastoglobules appeared). We suppose that rhodoxanthin forms a filter for the light under the conditions of high insolation
in winter. Thus, the evergreen conifer plants studied, which are adapted to growing at high latitudes where temperature is
low and insolation is high in winter and spring, have a system for protecting the photosynthetic apparatus against photodestruction.
In the basis of this system, the primary and secondary carotenoids lie, whose content changes during the year. 相似文献
10.
The relational structure of RNA, DNA, and protein bears an interesting similarity to the determination problem in category
theory. In this paper, we present this deep-structure similarity and use it as a springboard for discussing some abstract
properties of coding in various systems. These abstract properties, in turn, may shed light on the evolution of the DNA world
from a semiotic perspective. According to the perspective adopted in this paper, living systems are not information processing
systems but “meaning-making” systems. Therefore, what flows in the genetic system is not “information” but “value.” We define
meaning, meaning-making, and value and then use these terms to explain the abstract dynamics of coding, which can illuminate many forms of sign-mediated activities
in biosystems. 相似文献
11.
Seventy-five years ago, the geneticist Richard Goldschmidt hypothesized that single mutations affecting development could
result in major phenotypic changes in a single generation to produce unique organisms within animal populations that he called
“hopeful monsters”. Three decades ago, Sarah P. Gibbs proposed that photosynthetic unicellular micro-organisms like euglenoids
and dinoflagellates are the products of a process now called “secondary endosymbiosis” (i.e., the evolution of a chloroplast
surrounded by three or four membranes resulting from the incorporation of a eukaryotic alga by a eukaryotic heterotrophic
host cell). In this article, we explore the evidence for Goldschmidt’s “hopeful monster” concept and expand the scope of this
theory to include the macroevolutionary emergence of organisms like Euglena and Chlorarachnion from secondary endosymbiotic events. We argue that a Neo-Goldschmidtian perspective leads to the conclusion that cell chimeras
such as euglenids and dinoflagellates, which are important groups of phytoplankton in freshwater and marine ecosystems, should
be interpreted as “successful monsters”. In addition, we argue that Charles Darwin had euglenoids (infusoria) in mind when
he speculated on the “primordial intermediate form”, although his Proto-Euglena-hypothesis for the origin of the last common
ancestor of all forms of life is no longer acceptable. 相似文献
12.
Cell-pool tryptophan phases in ergot alkaloid fermentation 总被引:2,自引:0,他引:2
Three cell-pool tryptophan phases are recorded as characteristics of the alkaloid fermentation byClaviceps paspali grown on a simple defined medium without tryptophan. Within the early phase designated “tryptophan down” the alkaloid-biosynthetic
activity of the mycelium attains the maximum, protein synthesis is reduced and extracellular proteases are formed. Cell-pool
tryptophan level (b) drops, tryptophan synthetase activity (c) intensifies and sums of logb+logc after different time intervals remain constant. In the subsequent “tryptophan up” phase tryptophan level (b) increases, alkaloid yield (a) becomes a function of time and reaches the top level still tolerable by tryptophan synthetase. The difference of the logb—logc is constant. The tryptophan synthetase diminishes its activity simultaneously with the alkaloid-biosynthetic activity of
the mycelium. The district between the “tryptophan down” and “tryptophan up” phase is an especially promising target for the
investigation of the regulation of alkaloid formation and continuous fermentation of these compounds. During the third, i.e.
“tryptophan over” phase, cell-pool tryptophan accumulates and attains a concentration exerting a negative effect on the alkaloid
biosynthesis. 相似文献
13.
Dominic Boyer 《Dialectical Anthropology》2010,34(3):305-324
Using an approach derived from the anthropology and sociology of knowledge, this article explores the historical emergence
of European social theory and its contemporary place in the human sciences. I direct ethnographic attention to a sense of
crisis or impasse in social theory’s capacity to frame and to analyze the complexity of contemporary relations in the world.
By reanalyzing this crisis talk as a phenomenological reaction to the growing (sub)specialization of social theory, I offer
a new way of thinking about social theory in terms of specialized analytical attentions. I also suggest how we can move from
crisis talk to a new ethics of theoretical complementarity, inspired by Dilthey, which I term “multiattentional method.” 相似文献
14.
C. C. Walker 《Bulletin of mathematical biology》1984,46(5-6):845-857
Experiments on random binary, ternary, etc. (P=2, 3,…, 10) switching nets are reported. Behavioral cycle lengths are examined as functions of output variety,P, input connectance,K, and net size,N. Overall, output variety appears an influential, well-behaved net property. Strong, but well-behaved interactions appear
among net variables. In high connectance nets, median cycle length grows approx. asP
N/2. Other factors constant, one-connected nets show the shortest cycles, and connectance effects appear to converge asymptotically
aroundN. Data for cycle length as a function of net size suggest a concavity not compatible with the Kauffman “square root law” (Kauffman,
1969). Evidence of a positive relationship between cycle length and run-in length is found in two-input nets; weaker evidence
is obtained that in higher connectance nets this relationship becomes negative in sign. The “modular complexity” ofP>2 nets is examined briefly. 相似文献
15.
This paper reviews, up to their recent developments, two types of models of the cell cycle: those considering the size controls
over the cycle events and the transition probability models. The distribution of inter-mitotic time and the sister-sister
and motherdaughter correlations implied by the two approaches are discussed in view of some relevant experimental data.
This work was supported by Progetto Finalizzato “Controllo della Crescita Neoplastica” of the National Research Council. 相似文献
16.
O. A. Zhigalskii 《Biology Bulletin》2011,38(6):629-641
The results of long-term studies of two bank vole (Myodes glareolus) populations in stationary sites in the central part and periphery of its species area are described. Four phases of a multiannual
population cycle and two of its structural parts have been detected for both populations. The first part of the cycle is “determined,”
with the “peak” phase passing into a “depression” (population collapse). This transition is mainly determined by intrapopulation
processes and is weakly dependent on the external conditions of each individual year. The second part is “stochastic,” starting
from a stable point in the cycle in the depression phase. The duration of the second part is determined by the state of the
population and its ability to increase its size, as well as by the weather and food factors, predation pressure, and location
of the population within the species area. The transition from the peak phase to the depression phase (the determined part)
for both populations takes place during one fall-winter-spring season and has no effect on the cycle duration. The duration
of the stochastic part in the core of the species area (the period from depression phase to peak phase) is 1–3 years and in
the periphery, 2–4 years. 相似文献
17.
N
eoturris breviconis (Anthomedusae) has a nerve plexus in the walls of its endodermal canals. The plexus is distinct from the ectodermal nerve
plexuses supplying the radial and circular muscles in the ectoderm and no connections have been observed between them. Stimulation
of the endodermal plexus evokes electrical events recorded extracellularly as “E” potentials. These propagate through all
areas where the plexus has been shown by immunohistology to exist and nowhere else. When Neoturris is ingesting food, trains of “E” potentials propagate down the radial canals to the margin and cause inhibition of swimming.
This response is distinct from the inhibition of swimming associated with contractions of the radial muscles but both may
play a part in feeding and involve chemoreceptors. Preliminary observations suggest that the “E” system occurs in other medusae
including Aglantha digitale (Trachymedusae) where the conduction pathway was previously thought to be an excitable epithelium. 相似文献
18.
Ion C. Baianu 《Bulletin of mathematical biology》1980,42(3):431-446
The mathematical structures underlying the theories of organismic sets, (M, R)-systems and molecular sets are shown to be transformed naturally within the theory of categories and functors. Their natural
transformations allow the comparison of distinct entities, as well as the modelling of dynamics in “organismic” structures. 相似文献
19.
Trog D Moenkemann H Breipohl W Schueller H Schild H Golubnitschaja O 《Amino acids》2007,32(3):373-379
Summary.
Objectives. Human gliomas have a catastrophic prognosis with a median survival in the range of one year even after therapeutic treatment.
Relatively high resistance towards apoptotic stimuli is the characteristic feature of malignant gliomas. Since cell cycle
control has been shown to be the key mechanism controlling both apoptosis and proliferation, this study focuses on DNA damage
analysis and protein expression patterns of essential cell cycle regulators P53 and P21waf1/cip1 in glioma under clinically relevant therapeutic conditions.
Material and methods. U87MG cell line, characterised by wild p53-phenotype relevant for the majority of primary malignant glioblastomas, was used. Glioma cells underwent either irradiation
or temozolomide treatment alone, or combined radio/chemo treatment. DNA damage was analysed by the “Comet Assay”. Expression
rates of target proteins were analysed using “Western-Blot” technique.
Results and conclusions. “Comet Assay” demonstrated extensive DNA damage caused by temozolomide treatment alone and in combination with irradiation,
correlating well with the low survival rate observed under these treatment conditions. In contrast, irradiation alone resulted
in a relatively low DNA damage, correlating well with a high survival rate and indicating a poor therapeutic efficiency of
irradiation alone. Unusually low up-regulation of P53 and P21waf1/cip1 expression patterns was produced by the hereby tested stressful conditions. A deficit in cell cycle control might be the
clue to the high resistance of malignant glioma cells to established therapeutic approaches. 相似文献
20.
I. Opatowski 《Bulletin of mathematical biology》1946,8(3):101-119
The calculation of the size of the “sensitive volume” or “control center” in biological effects of radiations is discussed
from the viewpoint of the probabilistic theory of these phenomena based on the concept of random “effective events”. On the
bases of that theory, the resistivity of a microorganism to radiation is defined as its “mean life” under a radiation of one
roentgen per minute. This mean is calculated for processes with and without recovery. The case of variable sensitivity, as
it occurs for instance during mitosis, is discussed in detail. Methods are given to calculate this variability from survival
curves or similar experimental data. The theory is applied to experiments of A. Zuppinger on irradiation ofAscaris eggs with X-rays. 相似文献