Transfer properties of branching dendrites with tonically activated inputs

In the mathematical model of a neuron, properties of the active membrane during action of tonically activated synaptic inputs of an N-methyl-D-aspartate (NMDA) type were studied. In the dendrites with these properties, the impact of geometry on the somatopetal current transfer was investigated. The conditions were considered when the distal sites are more effective, as compared with the proximal ones, in their contribution to the somatopetal current. Contributions of “sister” branches to the net current transferred to the soma were compared at different levels of depolarization caused by action of homogeneously distributed NMDA synaptic inputs.     

Synthesis of mathematical models of branching axons and dendrites

A synthesis was made of models of branching neuronal cable structures from a full set of standard basic models. The study aimed to produce an instrument of mathematical modelling making it possible to reflect true life morphological and electrophysiological characteristics of axons and dendrites, discarding some of the restrictions and simplifications characterizing existing models of the structures mentioned. Equivalent electrical circuits of branching axons and dendrites were set up with in-series and node connections of standard four-terminal networks corresponding to basic segments with active or passive membrane. Equations were obtained for electrical processes in branching neuronal neurites, generalized in the case of multiple binary branching with arbitrary symmetry and branching structure. A difference scheme common to the whole class of models contemplated was produced and the algorithm of a numerical solution to the difference equations thus obtained was elaborated. The instrument described makes it possible to synthesize diverse models of branching axons and dendrites, offering considerably greater opportunities for modelling the main electrophysiological processes developing in these structures of electrotonus, propagation of excitation, and interaction between these two factors.State University Commemorating Tricentenary of Russo-Ukrainian Union. Dnepropetrovsk. Translated from Neirofiziologiya, Vol. 20, No. 4, pp. 471–479, July–August, 1988.     

Modeling organelle transport in branching dendrites with a variable cross-sectional area

The purpose of this paper is to develop a method for calculating organelle transport in dendrites with a non-uniform cross-sectional area that depends on the distance from the neuron soma. The model is based on modified Smith–Simmons equations governing molecular motor-assisted organelle transport. The developed method is then applied to simulating organelle transport in branching dendrites with two particular microtubule (MT) orientations reported from experiments. It is found that the rate of organelle transport toward a dendrite’s growth cone heavily depends on the MT orientation, and since there is experimental evidence that the MT orientation in a particular region of a dendrite may depend on the dendrite’s developmental stage, the obtained results suggest that a rearrangement of the MT structure may depend on the amount of organelles needed at the growth cone.     

Neural activity and branching of embryonic retinal ganglion cell dendrites

The shape of a neuron's dendritic arbor is critical for its function as it determines the number of inputs the neuron can receive and how those inputs are processed. During development, a neuron initiates primary dendrites that branch to form a simple arbor. Subsequently, growth occurs by a process that combines the extension and retraction of existing dendrites, and the addition of new branches. The loss and addition of the fine terminal branches of retinal ganglion cells (RGCs) is dependent on afferent inputs from its synaptic partners, the amacrine and bipolar cells. It is unknown, however, whether neural activity regulates the initiation of primary dendrites and their initial branching. To investigate this, Xenopus laevis RGCs developing in vivo were made to express either a delayed rectifier type voltage-gated potassium (KV) channel, Xenopus Kv1.1, or a human inward rectifying channel, Kir2.1, shown previously to modulate the electrical activity of Xenopus spinal cord neurons. Misexpression of either potassium channel increased the number of branch points and the total length of all the branches. As a result, the total dendritic arbor was bigger than for control green fluorescent protein-expressing RGCs and those ectopically expressing a highly related mutant non-functional Kv1.1 channel. Our data indicate that membrane excitability regulates the earliest differentiation of RGC dendritic arbors.     

Asymmetric electrotonic coupling between the soma and dendrites alters the bistable firing behaviour of reduced models

The goal of the study was to investigate the influence of asymmetric coupling, between the soma and dendrites, on the nonlinear dynamic behaviour of a two-compartment model. We used a recently published method for generating reduced two-compartment models that retain the asymmetric coupling of anatomically reconstructed motor neurons. The passive input-output relationship of the asymmetrically coupled model was analytically compared to the symmetrically coupled case. Predictions based on the analytic comparison were tested using numerical simulations. The simulations evaluated the nonlinear dynamics of the models as a function of coupling parameters. Analytical results showed that the input resistance at the dendrite of the asymmetric model was directly related to the degree of coupling asymmetry. In contrast, a comparable symmetric model had identical input resistances at both the soma and dendrite regardless of coupling strength. These findings lead to predictions that variations in dendritic excitability, subsequent to changes in input resistance, might change the current threshold and onset timing of the plateau potential generated in the dendrite. Since the plateau potential underlies bistable firing, these results further predicted that asymmetric coupling might alter nonlinear (i.e. bistable) firing patterns. The numerical simulations supported analytical predictions, showing that the fully bistable firing pattern of the asymmetric model depended on the degree of coupling asymmetry and its correlated dendritic excitability. The physiological property of asymmetric coupling plays an important role in generating and stabilizing the bistability of motor neurons by interacting with the excitability of dendritic branches.     

Axonal branching pattern and coupling mechanisms of the cerebral giant neurones in the snail,lymnaea stagnalis

The axonal branching pattern of the two cerebral giant neurones (CGCs) of Lymnaea stagnalis was studied with intrasomatically applied horseradish peroxidase. The cells are symmetrical. Each CGC projects to the ipsilateral n. labialis medius and n. arteriae labialis, the subcerebral commissure, and to all ipsi- and contralateral buccal nerves. The contralateral buccal nerves are reached via the ipsilateral cerebro-buccal connective and the buccal commissure. The CGC fire action potentials 1:1 in a driver-follower relationship. Each cell is capable of both driving and following. The relationship depends on the membrane potentials of the somata. In driving CGC spikes are initiated in a cerebral spike trigger zone located near the soma. In following cells spikes are initiated in a distal zone located in the buccal ganglia. The buccal zone is only affected by the partner CGC. CGC are synchronized by three coupling mechanisms: mutual excitatory chemical synapses, electrotonic coupling, and common input. The chemical and electrotonic connections are located in the buccal ganglia. All spikes are relayed to the partner cell via the chemical synapses. The electrotonic coupling improves the efficiency of the chemical synapses. The dual connection selectively synchronizes the CGC-axonal spikes from each side of the buccal mass. Common excitatory input affects the cerebral spike trigger zones and can initiate simultaneous spikes in both cells. This results in bilateral synchrony of spikes in the CGC-axons in both the buccal and the lip nerves.     

Calcium microdomains in aspiny dendrites

Dendritic spines receive excitatory synapses and serve as calcium compartments, which appear to be necessary for input-specific synaptic plasticity. Dendrites of GABAergic interneurons have few or no spines and thus do not possess a clear morphological basis for synapse-specific compartmentalization. We demonstrate using two-photon calcium imaging that activation of single synapses on aspiny dendrites of neocortical fast spiking (FS) interneurons creates highly localized calcium microdomains, often restricted to less than 1 microm of dendritic space. We confirm using ultrastructural reconstruction of imaged dendrites the absence of any morphological basis for this compartmentalization and show that it is dependent on the fast kinetics of calcium-permeable (CP) AMPA receptors and fast local extrusion via the Na+/Ca2+ exchanger. Because aspiny dendrites throughout the CNS express CP-AMPA receptors, we propose that CP-AMPA receptors mediate a spine-free mechanism of input-specific calcium compartmentalization.     

Secretory trafficking in neuronal dendrites

The neuronal secretory pathway represents the intracellular route for proteins involved in synaptic transmission and plasticity, as well as lipids required for outgrowth and remodelling of dendrites and axons. Although neurons use the same secretory compartments as other eukaryotic cells, the enormous distances involved, as well as the unique morphology of the neuron and its signalling requirements, challenge canonical models of secretory pathway organization. Here, we review evidence for a distributed secretory pathway in neurons, suggest mechanisms that may regulate secretory compartment distribution, and discuss the implications of a distributed secretory pathway for neuronal morphogenesis and neural-circuit plasticity.     

Reactive motility of dendrites

The neurohistological data on the nonspecific reactive phenomenon the dendrite neuroplasm excrescences were compared with intravital data on reactive mobility of the processes of the surviving neurons and nerve cells in tissue cultures. It was demonstrated that static pictures on histological preparations correspond to the stages of reactive reconstruction of live dendrites, such as retraction, extrusion and autotomy. The neuroplasmic excrescences reflect the reverse displacement of the neuroplasm associated with the process retraction rather than enhancement of its proximodistal flow.     

Con-nectin axons and dendrites

Unlike adherens junctions, synapses are asymmetric connections, usually between axons and dendrites, that rely on various cell adhesion molecules for structural stability and function. Two cell types of adhesion molecules found at adherens junctions, cadherins and nectins, are thought to mediate homophilic interaction between neighboring cells. In this issue, Togashi et al. (see p. 141) demonstrate that the differential localization of two heterophilic interacting nectins mediates the selective attraction of axons and dendrites in cooperation with cadherins.     

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.     

Computational modeling of dendrites

Computational methods have been part of neuroscience for many years. For example, models developed with these methods have provided a theory that helps explain the action potential. More recently, as experimental patch-electrode techniques have revealed new biophysics related to dendritic function and synaptic integration, computational models of dendrites have been developed to explain and further illuminate these results, and to predict possible additional behavior. Here, a collection of computational models of dendrites is reviewed. The goal is to help explain how such computational techniques work, some of their limitations, and what one can hope to learn about dendrites by modeling them.     

C. elegans bicd-1, homolog of the Drosophila dynein accessory factor Bicaudal D, regulates the branching of PVD sensory neuron dendrites

The establishment of cell type-specific dendritic arborization patterns is a key phase in the assembly of neuronal circuitry that facilitates the integration and processing of synaptic and sensory input. Although studies in Drosophila and vertebrate systems have identified a variety of factors that regulate dendrite branch formation, the molecular mechanisms that control this process remain poorly defined. Here, we introduce the use of the Caenorhabditis elegans PVD neurons, a pair of putative nociceptors that elaborate complex dendritic arbors, as a tractable model for conducting high-throughput RNAi screens aimed at identifying key regulators of dendritic branch formation. By carrying out two separate RNAi screens, a small-scale candidate-based screen and a large-scale screen of the ~3000 genes on chromosome IV, we retrieved 11 genes that either promote or suppress the formation of PVD-associated dendrites. We present a detailed functional characterization of one of the genes, bicd-1, which encodes a microtubule-associated protein previously shown to modulate the transport of mRNAs and organelles in a variety of organisms. Specifically, we describe a novel role for bicd-1 in regulating dendrite branch formation and show that bicd-1 is likely to be expressed, and primarily required, in PVD neurons to control dendritic branching. We also present evidence that bicd-1 operates in a conserved pathway with dhc-1 and unc-116, components of the dynein minus-end-directed and kinesin-1 plus-end-directed microtubule-based motor complexes, respectively, and interacts genetically with the repulsive guidance receptor unc-5.     

Shoot branching

The mature form of a plant shoot system is an expression of several genetically controlled traits, many of which are also environmentally regulated. A major component of this architectural variation is the degree of shoot branching. Recent results indicate conserved mechanisms for shoot branch development across the monocots and eudicots. The existence of a novel long-range branch-inhibiting signal has been inferred from studies of branching mutants in pea and Arabidopsis.     

Synaptic plasticity: regulated translation in dendrites

Synaptic activity can induce neurons to synthesize proteins important for cognition and brain development. Recent results suggest this activity-induced protein synthesis is partially mediated by regulated translation within neuronal dendrites.     

Ultrastructural changes in the dendrites during aging

Dendrite ultrastructure was studied in different brain areas of old rats, aged 28-30 months. Different ultrastructural forms of the reactive and destructive processes in the dendrites, irregularity of their pathomorphological changes with ageing and greater destruction of large dendrites, as compared to small ones are demonstrated.