Biomedical subjects

Tiaza Bem

Publications and source records attributed to Tiaza Bem.

5 recordsLinked to original sources

PubMed2005

Electrical coupling induces bistability of rhythms in networks of inhibitory spiking neurons.

Information processing in higher brain structures is thought to rely on the synchronization of spiking neurons. Increasing evidence indicates that, within these structures, inhibitory neurons are linked by both chemical and electrical synapses. However, how synchronized states may emerge from such circuits is not fully understood. Using snail neurons interconnected through a dynamic-clamp system, we show that networks of spiking neurons linked by both reciprocal inhibition and electrical coupling can express two coexisting coordination patterns of different rhythms. One of these patterns consists of antiphase firing of the network partners whereas, in the other, neurons fire synchronously. Switching between patterns may be evoked immediately by transient stimuli, demonstrating bistability of the network. Thus electrical coupling can provide a potent way for instantaneous reconfiguration of activity patterns in inhibitory spiking networks without alteration of intrinsic network properties by modulatory processes.

Algorithms↗

PubMed2003

Short duty cycle destabilizes a half-center oscillator, but gap junctions can restabilize the anti-phase pattern.

Mutually inhibitory pacemaker neurons with duty cycle close to 50% operate as a half-center oscillator (anti-phase coordination, i.e., 180 degrees out of phase), even in the presence of weak to modest gap junctional coupling. For electrical coupling strength above a critical value synchronization occurs. But, as shown here with modeling studies, the effects of electrical coupling depend critically on a cell's duty cycle. Instead of oscillating either in-phase or anti-phase, model cells with short duty cycle express additional rhythmic patterns, and different transitions between them, depending on electrical coupling strength. For weak or no electrical coupling, cells do not oscillate in anti-phase but instead exhibit almost in-phase activity. Strengthening this weak coupling leads to stable anti-phase activity. With yet stronger electrical coupling stable inphase (synchrony) emerges but it coexists with the anti-phase pattern. Thus the network shows bistability for an intermediate range of coupling strength. For sufficiently strong electrical coupling synchrony is the network's only attracting rhythmic state. Our results, numerical and analytical (phase plane analysis), are based on a minimal but biophysically motivated pacemaker model for the slowly oscillating envelope of bursting neurons. However, illustrations for an Hodgkin-Huxley model suggest that some of our results for short duty cycle may extend to patterning of repetitive spikes. In particular, electrical coupling of intermediate strength may promote anti-phase activity and provide bistability of anti-phase and in-phase spiking.

Biological Clocks↗

PubMed2003

From swimming to walking: a single basic network for two different behaviors.

In this paper we consider the hypothesis that the spinal locomotor network controlling trunk movements has remained essentially unchanged during the evolutionary transition from aquatic to terrestrial locomotion. The wider repertoire of axial motor patterns expressed by amphibians would then be explained by the influence from separate limb pattern generators, added during this evolution. This study is based on EMG data recorded in vivo from epaxial musculature in the newt Pleurodeles waltl during unrestrained swimming and walking, and on a simplified model of the lamprey spinal pattern generator for swimming. Using computer simulations, we have examined the output generated by the lamprey model network for different input drives. Two distinct inputs were identified which reproduced the main features of the swimming and walking motor patterns in the newt. The swimming pattern is generated when the network receives tonic excitation with local intensity gradients near the neck and girdle regions. To produce the walking pattern, the network must receive (in addition to a tonic excitation at the girdles) a phasic drive which is out of phase in the neck and tail regions in relation to the middle part of the body. To fit the symmetry of the walking pattern, however, the intersegmental connectivity of the network had to be modified by reversing the direction of the crossed inhibitory pathways in the rostral part of the spinal cord. This study suggests that the input drive required for the generation of the distinct walking pattern could, at least partly, be attributed to mechanosensory feedback received by the network directly from the intraspinal stretch-receptor system. Indeed, the input drive required resembles the pattern of activity of stretch receptors sensing the lateral bending of the trunk, as expressed during walking in urodeles. Moreover, our results indicate that a nonuniform distribution of these stretch receptors along the trunk can explain the discontinuities exhibited in the swimming pattern of the newt. Thus, separate limb pattern generators can influence the original network controlling axial movements not only through a direct coupling at the central level but also via a mechanical coupling between trunk and limbs, which in turn influences the sensory signals sent back to the network. Taken together, our findings support the hypothesis of a phylogenetic conservatism of the spinal locomotor networks generating axial motor patterns from agnathans to amphibians.

Animals↗

PubMed2003

Maturation of rhythmic neural network: role of central modulatory inputs.

Modulatory systems are well known for their roles in tuning the cellular and synaptic properties in the adult neuronal networks, and play a major role in the control of the flexibility of functional outputs. However far less is known concerning their role in the maturation of neural networks during the development. In this review, using the stomatogastric nervous system of lobster, we will show that the neuromodulatory system exerts a powerful influence on developing neural networks. In the adult the number of both motor target neurons and their modulatory neurons is restricted to tens of identifiable cells. They are therefore well characterized in terms of cellular, synaptic and morphological properties. In the embryo, these target cells and their neuromodulatory population are already present from mid-embryonic life. However, the motor output generated by the system is quite different: while in the embryo all the target neurons are organized into a single network generating unique motor pattern, in the adult this population splits into two distinct networks generating separate patterns. This ontogenetic partitioning does not rely on progressive acquisition of adult properties but rather on a switch between two possible network operations. Indeed, adult networks are present early in the embryonic life but their expression is repressed by central modulatory neurons. Moreover, embryonic networks can be revealed in the adult system again by altering modulatory influences. Therefore, independently of the developmental age, two potential network phenotypes co-exist within the same neuronal architecture: when one is expressed, the other one is hidden and vice versa. These transitions do not necessarily need dramatic changes such as growth/retraction of processes, acquisition of new intra-membrane proteins etc. but rather, as shown by modelling studies, it may simply rely on a subtle tuning of pre-existing intercellular electrical coupling. This in turn suggests that progressive ontogenetic alteration may not take place at the level of the target network but rather at the level of modulatory input neurons.

Action Potentials↗

PubMed2002

Electrical coupling can prevent expression of adult-like properties in an embryonic neural circuit.

Electrical coupling is widespread in developing nervous systems and plays a major role in circuit formation and patterning of activity. In most reported cases, such coupling between rhythmogenic neurons tends to synchronize and enhance their oscillatory behavior, thereby producing monophasic rhythmic output. However, in many adult networks, such as those responsible for rhythmic motor behavior, oscillatory neurons are linked by synaptic inhibition to produce rhythmic output with multiple phases. The question then arises whether such networks are still able to generate multiphasic output in the early stage of development when electrical coupling is abundant. A suitable model for addressing this issue is the lobster stomatogastric nervous system (STNS). In the adult animal, the STNS consists of three discrete neural networks that are comprised of oscillatory neurons interconnected by reciprocal inhibition. These networks generate three distinct rhythmic motor patterns with large amplitude neuronal oscillations. By contrast, in the embryo the same neuronal population expresses a single multiphasic rhythm with small-amplitude oscillations. Recent findings have revealed that adult-like network properties are already present early in the embryonic system but are masked by an as yet unknown mechanism. Here we use computer simulation to test whether extensive electrical coupling may be involved in masking adult-like properties in the embryonic STNS. Our basic model consists of three different adult-like STNS networks that are built of relaxation oscillators interconnected by reciprocal synaptic inhibition. Individual model cells generate slow membrane potential oscillations without action potentials. The introduction of widespread electrical coupling between members of these networks dampens oscillation amplitudes and, at moderate coupling strengths, may coordinate neuronal activity into a single rhythm with different phases, which is strongly reminiscent of embryonic STNS output. With a further increase in coupling strength, the system reaches one of two final states depending on the relative contribution of inhibition and inherent oscillatory properties within the networks: either fully synchronized and dampened oscillations, or a complete collapse of activity. Our simulations indicate that, beginning from either of these two states, the emergence of distinct adult networks during maturation may arise from a developmental decrease in electrical coupling that unmasks preexisting adult-like network properties.

Action Potentials↗