Biomedical subjects

J P Rospars

Publications and source records attributed to J P Rospars.

At least 19 recordsLinked to original sources

PubMed2001

Membrane potential and its electrode-recorded counterpart in an electrical model of an olfactory sensillum.

Insect receptor neurons are surrounded with auxiliary cells and encased in a hair. Their electrical activity is usually recorded with an electrode located at the tip of the hair. Analytical expressions giving the membrane potential along the sensory dendrite and the tip-recorded potential are derived for a neuron in steady-state conditions. They formally close the gap between theoretical models and experimental measurements, when transduction mechanisms and active membrane properties are not taken into account. It is shown that the tip-recorded potential reflects correctly the relative variations of the dendritic membrane potential as a function of stimulus intensity over a large range of parameters. The geometric and electrical characteristics of the sensillum that need be known to compute the dendritic membrane potential from the tip-recorded potential are given.

Animals↗

PubMed2001

Ligand-receptor interaction under periodic stimulation: a modeling study of concentration chemoreceptors.

The first step of chemosensory transduction consists in the association of ligand molecules with receptor proteins borne by the cell membrane. In this article, the time evolution of ligand-receptor complexes is studied in the presence of a periodically changing ligand concentration. This type of stimulation is a close approximation to some natural situations, for example in olfaction. The transient and steady-state periodic levels of the complexes, resulting from a single-step (binding) or double-step (binding and activation) reaction, are determined. When possible, analytical solutions are given, if not for the complete model, at least for its simplified version at low ligand concentration. Otherwise, solutions are found numerically and both the complete and simplified versions of the model are compared. The results obtained are discussed with respect to actual experimental data based on the moth sex-pheromone receptor. Periodic steady states are achieved very quickly and their amplitude decreases when the stimulation frequency increases. We show that the simplified description is adequate if only a fraction of activated receptors is sufficient to produce the maximum response, as is actually the case in the example treated. The role of the frequency of stimulation is investigated and it is shown to possess an optimal range between 2 and 5 Hz.

Animals↗

PubMed2000

Sexually dimorphic and isomorphic glomeruli in the antennal lobes of the sphinx moth Manduca sexta.

Antennal lobes of adult male and female Manduca sexta were compared in order to investigate the nature and extent of sexual dimorphism of the primary olfactory center of this lepidopteran species. Complete identification of the glomeruli led to the conclusion that all female glomeruli have homologous male counterparts. Thus, there is no sex-specific glomerulus present in one sex and absent in the other. Sexual dimorphism (i.e. glomeruli present but morphologically different in males and females), however, does occur in the three glomeruli composing the male macroglomerular complex. The female homologs of this complex consist of two previously identified 'large female glomeruli' and one newly identified normal-sized glomerulus. The lateral and medial large female glomeruli are interpreted to be homologous to the first two macroglomerular-complex glomeruli-the cumulus and toroid 1. The third male component, the toroid 2, was tentatively identified with a normal-sized spheroidal glomerulus of the female, called here the 'small female glomerulus'. The 60 'ordinary' glomeruli that make up the rest of the glomerular neuropil were found to be homologous in males and females, with the exception of two anomalous (or uncertain) glomeruli. Some variations in relative position and size observed among those glomeruli suggest a diffuse, quantitative kind of sexual dimorphism.

Animals↗

PubMed2000

Perireceptor and receptor events in olfaction. Comparison of concentration and flux detectors: a modeling study.

Transduction in chemosensory cells begins with the association of ligand molecules to receptor proteins borne by the cell membrane. The receptor-ligand complexes formed act as signaling compounds that trigger a G-protein cascade. This receptor-ligand interaction, described here by a single-step or double-step reaction, depends on factors controlling the access of the ligand molecules to the cell membrane. Two basic mechanisms can be distinguished: concentration detectors (CD), in which the ligand can freely diffuse to the membrane, and flux detectors (FD), in which it accumulates irreversibly in a distinct perireceptor space where it is chemically deactivated. These two systems, plus their generalization, are investigated and compared. The transient and steady-state numbers of complexes are studied as a function of the external ligand concentration. The biological significance of the results is shown in a well-studied example of FD, the insect sex-pheromone olfactory receptor neuron. How the number of complexes can code for the intensity of stimulation is analyzed using the size, dynamic range and sensitivity of the steady-state responses, and the time needed to reach a predefined level of the transient responses. It is shown that the FD design affords a large increase in sensitivity (a shift of the threshold response towards low concentration) with respect to the CD design, which is paid for by a lesser ability to follow fast changes in stimulus intensity.

Kinetics↗

PubMed1998

A simple analytical method for determining the steady-state potential in models of geometrically complex neurons.

A method is presented for solving the cable equation for a spiking neuron below firing threshold or a nonspiking neuron of arbitrary geometry under constant stimulation. The neuron structure is considered as a tree composed of a set of cylinder cables of three types (terminal, intermediate and branching) characterized by their lengths, diameters and linear membrane properties. The stimulation can result from either a uniform conductance-change over a whole cable segment or a point injection of a current. Other special segments are considered (synapses, voltage clamp, lumped soma). Equations are given for replacing any segment with its Thévenin equivalent, i.e. resistance and electromotive force. The step by step use of these elementary equations allows one to find the Thévenin equivalent of the whole neuron and to determine the steady-state membrane potential at any point.

Algorithms↗

PubMed1998

Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor intensity.

Response properties of the receptor potential at steady state were analyzed in a biophysical model of an olfactory sensory neuron embedded in a multicell environment. The neuron structure was described as a set of several identical dendrites (or cilia) bearing the transduction mechanisms, joined to a nonsensory part--dendritic knob, soma, and axon. The different ionic compositions of the media surrounding the neuron sensory and nonsensory parts and the extraneuronal voltage sources, which both result from the presence of auxiliary cells, were also taken into account. Analytical solutions were found to describe how the receptor potential at the nonsensory part responds to a uniform change in the odorant-dependent conductance resulting from odorant stimulation of the sensory dendrites. We investigated the influence of various geometrical and electrical parameters on the receptor-potential response in the classical model neuron within a homogeneous environment and in the model neuron surrounded with auxiliary cells. First, it was found that the maximum amplitude of the receptor potential is independent of the neuron structure in the absence of auxiliary cells but not in their presence. In the latter case, the amplitude decreases with the length and number of sensory dendrites and with the input resistance of the nonsensory part. Second, the sensitivity (as measured by the increase in membrane conductance at half-maximum response) of the neuron model in the absence of auxiliary cells is higher, but its dynamic range is narrower than in their presence. The dynamic range is wide and the sensitivity low when the input resistance of the nonsensory part is small and the sensory dendrite is unbranched. Both sensitivity and dynamic range are higher for a longer dendrite. These results help understand the morphology of insect olfactory sensilla and can be generalized to other neuron types.

Animals↗

PubMed1997

A stochastic model for interconnected neurons.

A model is proposed to describe the collective behavior of a biologically plausible neural network, composed of interconnected spiking neurons which separately receive external stationary stimulations. The spiking dynamics of each neuron is represented by an hourglass metaphor. This network model was first studied in a special case where the connections are only inhibitory (Cottrell, 1988, 1992). We study the network dynamics as a function of the parameters which quantify the strengths of both inhibitory and excitatory connections. We show that the model exhibits two kinds of limit states. In the first states (convergent case), the system is ergodic and all neurons have a positive mean firing rate. In the other states (divergent case), some neurons become definitively inactive while the sub-network of the active neurons is ergodic. The patterns which result from these divergent states can be seen as a neural coding of the external stimulation by the network. This property is applied to the olfactory system to produce a code for an odor. The role of inhibitory connections in odor discrimination is studied.

Action Potentials↗

PubMed1997

Coding of odour intensity in a sensory neuron.

A deterministic biophysical model of an olfactory sensory neuron under constant stimulation is presented with the aim of describing the successive conversion steps, including receptor activation, conductance change, receptor potential and firing frequency, that are involved in the coding of odorant concentration. This model is divided in two parts. The odorant-sensitive part (OSP), consisting of one cylindrical dendrite, is connected to the odorant-insensitive part (OIP), corresponding to passive dendrite, soma and axon. Each part exerts a specific effect on the coding properties of the conversion steps, i.e. their magnitude, sensitivity and dynamic range. The maximum conductance of the OSP affects positively all coding properties whereas the input resistance of the OIP, which depends on its size and shape, affects positively the sensitivity and negatively the dynamic range. These findings are helpful for understanding the input-output properties of many types of neurons.

Action Potentials↗

PubMed1996

Time-dependent solutions for a cable model of an olfactory receptor neuron.

A mathematical model for an olfactory receptor neuron is investigated. The physiological and anatomical background required for the construction of a mathematical model are explained. The model, which has been described previously, has three components, including the sensory dendrite on which are found the receptor proteins themselves, and others consisting of a passive cable leading to a trigger zone and axon. In the present paper, we pursue an analytical approach for determining the change in time of the receptor potential in the important case of a subthreshold square pulse of odorant stimulation delivered uniformly at the sensory dendrite. Then, the input current increases in time to its asymptotic value. This latter condition means that we can use a Green's function approach in order to obtain accurate representations for the solution for the entire length of the nerve cell. In the case of finite cables the solution is obtained as an infinite series which is shown to converge and can be easily used to find the depolarization at all space and time points of interest. A steady-state result is obtained directly by solving the relevant ordinary differential equation. For a semi-infinite cable an explicit expression is found for the voltage as a function of time and space variables involving a single integral. However, the exact expression follows from this for the steady-state result. The analytical results obtained are compared to numerical solutions and employed to investigate the effect of varying the position of the trigger zone and the electronic length of the neuron.

Humans↗

PubMed1996

Coding of odor intensity in a steady-state deterministic model of an olfactory receptor neuron.

The coding of odor intensity by an olfactory receptor neuron model was studied under steady-state stimulation. Our model neuron is an elongated cylinder consisting of the following three components: a sensory dendritic region bearing odorant receptors, a passive region consisting of proximal dendrite and cell body, and an axon. First, analytical solutions are given for the three main physiological responses: (1) odorant-dependent conductance change at the sensory dendrite based on the Michaelis-Menten model, (2) generation and spreading of the receptor potential based on a new solution of the cable equation, and (3) firing frequency based on a Lapicque model. Second, the magnitudes of these responses are analyzed as a function of odorant concentration. Their dependence on chemical, electrical, and geometrical parameters is examined. The only evident gain in magnitude results from the activation-to-conductance conversion. An optimal encoder neuron is presented that suggests that increasing the length of the sensory dendrite beyond about 0.3 space constant does not increase the magnitude of the receptor potential. Third, the sensitivities of the responses are examined as functions of (1) the concentration at half-maximum response, (2) the lower and upper concentrations actually discriminated, and (3) the width of the dynamic range. The overall gain in sensitivity results entirely from the conductance-to-voltage conversion. The maximum conductance at the sensory dendrite appears to be the main tuning constant of the neuron because it determines the shift toward low concentrations and the increase in dynamic range. The dynamic range of the model cannot exceed 5.7 log units, for a sensitivity increase at low odor concentration is compensated by a sensitivity decrease at high odor concentration.

Models, Neurological↗

PubMed1996

Coding of stimulus intensity in an olfactory receptor neuron: role of neuron spatial extent and passive dendritic backpropagation of action potentials.

The olfactory receptor neuron provides a good opportunity to analyze a biophysical model of a single neuron because its dendritic structure is simple and even close to a cylinder in the case of the moth sex-pheromone receptor cell. We have considered this cylindrical case and studied two main problems. First, we were concerned with the effect of the neuron's length on the receptor potential for a constant stimulus-induced conductance change. An analytical solution for the receptor potential was determined by using input resistances. It was shown that the longer the neuron, the greater its ability to code over a wide range of values of the intensity of the stimulus. Second, we studied numerically the passive backpropagation of action potentials into the dendrite and its influence on firing frequency. While propagating along the dendrite, the action potential decreases in amplitude and its shape becomes rounded. The firing frequency in the model with backpropagation was found to be greater than that obtained analytically in the absence of backpropagation. However, for any given conductance change, when normalized with respect to their maxima, both firing frequencies were found to be very similar over a wide range of parameter values. Therefore, the actual firing rate (with backpropagation) may be approximated by the analytical solution without backpropagation if the actual firing rate for a large conductance change is known.

Action Potentials↗

PubMed1995

Modeling insect olfactory neuron signaling by a network utilizing disinhibition.

A male moth locates a conspecific female by detecting her sexual-pheromone blend. This detection is carried out in the antennal lobe, the first stage of olfactory information processing, where local inhibitory neurons and projection (relay) neurons interact. Antennal-lobe neurons exhibit low-frequency (< 10 Hz) background activity and bursting (> 100 Hz) activity in response to pheromone stimulation. We describe this behavior by a realistic biophysical neuron model. The bursting behavior of the model is the result of both intrinsic cellular properties and network interaction. A slowly activating and inactivating calcium channel provides a depolarizing current for bursting and disinhibition is shown to be a feasible network mechanism for triggering this calcium channel. Small neural networks utilizing disinhibition are presented with local neurons intercalated between receptor and projection neurons. The firing behaviors of projection neurons in response to stimulation by the pheromone blend or its components are in accordance with experimental results. This network architecture offers an alternative view of olfactory processing from the classical architecture derived from vertebrate studies.

Animals↗

PubMed1994

Spontaneous activity of first- and second-order neurons in the frog olfactory system.

The spontaneous activity of first-order neurons (neuroreceptors of the mucosa) and second-order neurons (mitral cells of the bulb) was recorded extracellularly in the frog olfactory system. To assess the influence of peripheral inputs upon mitral cells, the bulb was either normally connected or partially deafferented. Our first set of findings concern the firing behavior. We found that most neurons generated interspike intervals (ISIs) that were stationary in mean and variance, and were not serially correlated at first and second order. Individual spikes in mitral cells and bursts of spikes in neuroreceptors were found to be generated by a Poisson process. Stochastic modeling suggests that the Poissonian behavior depends on the mean value of the membrane potential at the axon hillock. In these models, the mean potential in mitral cells would be far below the firing threshold and in neuroreceptors it would fluctuate at random between two states, one close to resting potential (between bursts) and the other close to the firing threshold with occasional crossings (within bursts). Secondly, partially deafferented mitral cells had significantly higher activity and lower variance than mitral cells receiving normal afferent input. This effect gives evidence that peripheral inputs influence mitral cells at rest not only through direct excitation but also through indirect inhibition exerted by local neurons. Thus, the unstimulated state of the olfactory bulb would not be qualitatively different from its stimulated state in the sense that both states involve the same types of synaptic interactions. Consequently, understanding the synaptic relationships that take place in the bulb network can benefit from studies of its spontaneous activity.

Action Potentials↗

PubMed1993

Stochastic model neuron without resetting of dendritic potential: application to the olfactory system.

A two-dimensional neuronal model, in which the membrane potential of the dendrite evolves independently from that at the trigger zone of the axon, is proposed and studied. In classical one-dimensional neuronal models the dendritic and axonal potentials cannot be distinguished, and thus they are reset to resting level after firing of an action potential, whereas in the present model the dendritic potential is not reset. The trigger zone is modelled by a simplified leaky integrator (RC circuit) and the dendritic compartment can be described by any of the classical one-dimensional neuronal models. The new model simulates observed features of the firing dynamics which are not displayed by classical models, namely positive correlation between interspike intervals and endogenous bursting. It gives a more natural account of features already accounted for in previous models, such as the absence of an upper limit for the coefficient of variation of intervals (i.e. irregular firing). It allows the first- and second-order neurons of the olfactory system to be described with the same basic assumptions, which was not the case in one-point models. Nevertheless it keeps the main qualitative properties found previously, such as the existence of three regimens of firing with increasing stimulus concentration and the sigmoid shape of the firing frequency of first-order neurons as a function of the logarithm of stimulus concentration.

Action Potentials↗

PubMed1993

Coding of odor intensity.

A model for coding of odor intensity in the first two neuronal layers of olfactory systems is proposed. First, the occupation and activation by odorant molecules of receptor proteins of different types borne by the first order neurons are described as birth and death processes. The occupation (birth) rate depends on the concentration of the odorant, whereas the probability of activation of an occupied receptor depends on the type of the odorant. Second, the spike generation mechanism proposed for the first order neuron depends on the level of the generator potential evoked by the activated receptors and on a time-decaying threshold which is reset to infinity after each spike. The various resulting stochastic regimes of firing activity at different concentrations are described. Third, each second order neuron is influenced by excitation coming from numerous first order neurons, lateral inhibition from other second order neurons, and self-inhibition. All these incoming signals are integrated at the second order neuron. The firing activity of the first and second order neurons is modeled by a first passage time scheme. For both types of neuron the shapes of the curves predicted by the model for the mean firing frequency as a function of stimulus concentration are shown to be in accordance with available experimental results.

Animals↗

PubMed1992

Anatomical identification of glomeruli in the antennal lobes of the male sphinx moth Manduca sexta.

Computer-assisted neuroanatomical methods have been used to demonstrate unique identities of the glomeruli of the antennal lobes (ALs) in males of the sphinx moth Manduca sexta. The glomerular neuropil consists of the male-specific macroglomerular complex, which comprises two closely apposed bulky subunits, and 64 +/- 1 "ordinary" glomeruli arrayed in a shell around a central region of coarse neuropil. Computer-generated maps show the exact locations of all glomeruli and adjacent groups of neuronal somata in a constant Cartesian coordinate system, such that these can be accurately identified in any individual. The glomeruli belong to three classes according to the number and type of identification criteria they satisfy. The larger class comprises glomeruli (n = 44) identified only in the computer-generated maps on the basis of their relative positions. The other two classes include glomeruli that were also identified in sections, either directly from their proximity to readily identifiable structures and their shape and size (n = 10, including the labial-palp-pit-organ (LPO) glomerulus), or indirectly from their positions relative to the former (n = 9). Two very small glomeruli were present in only one AL, demonstrating the existence of anomalous glomeruli, whereas another glomerulus had no homologue in both ALs of one individual. The true number of ordinary glomeruli (per male AL) was thus estimated to be 64. The uncertainty in delineating some glomeruli might affect this number without implying modification of the homologies proposed. The locations of tracts and cell groups, both within and near the AL, are also invariant with respect to glomeruli, as shown in the computer maps. The methods employed are general and might be useful to researchers in related fields. The results obtained call for more attention to the precise geometry of neural structures.

Animals↗

PubMed1992

Modelling of the qualitative discrimination of odours in the first two layers of olfactory system by Jutten and Hérault algorithm.

The olfactory system discriminates many different odours although membrane receptors, and neuroreceptors that bear them, are only broadly tuned to volatile molecules. We investigated how mixed and redundant information carried by neuroreceptors could be efficiently coded by output neurons in the second neuronal layer in the brain. We showed that a good discrimination is yielded if neuroreceptors that terminate on the same output neuron belong to several types, if these types are in different proportions for each output neuron, and if each output neuron exerts an action on all others so that their independence is maximized, according to the Jutten and Hérault algorithm for separating linear mixtures of independent sources. The proposed model details the computations made by neurons and leads to a better understanding of several observed properties of the olfactory system, among which the inhibitory actions of local neurons of the second layer and the identifiability of insect glomeruli.

Algorithms↗

PubMed1983

Invariance and sex-specific variations of the glomerular organization in the antennal lobes of a moth, Mamestra brassicae, and a butterfly, Pieris brassicae.

The glomerular organization of the antennal lobes was analyzed in the moth Mamestra brassicae and comparatively in the butterfly Pieris brassicae. The invariance of the lobes in number, position, and size of the glomeruli was verified quantitatively in the moth for all the glomeruli in individuals of the same sex (67 in males, 68 in females) and for 56 sex-invariant glomeruli which can be identified in all individuals whatever their sex. In the butterfly, the positional variability is greater than in the moth and hinders identification. The most conspicuous sex-variant glomeruli are two adjoining macroglomeruli in the male moth which have homologs of very small size in the female. No such dimorphism was observed in Pieris, a species in which males are attracted to female by visual stimuli and not by a sex pheromone as in Mamestra. There are other sex-variant glomeruli in Mamestra: four varying in location, three subdivided in the female, two found only in the male, and three found only in the female. Consequently, differing olfactory sensitivity related to species and sex could correlate with detectable modifications in the glomerular organization. A hypothesis on the function of glomeruli is discussed in which most of the glomeruli are viewed as "olfactory generalists" giving rise collectively to proposed "across-glomeruli patterns."

Animals↗