The Extent of the Dendritic Tree and the Number of Synapses in the Frog Motoneuron

Voltage imaging with genetically encoded indicators

Membrane voltages are ubiquitous throughout cell biology. Voltage is most commonly associated with excitable cells such as neurons and cardiomyocytes, although many other cell types and organelles also support electrical signaling. Voltage imaging in vivo would offer unique capabilities in reporting the spatial pattern and temporal dynamics of electrical signaling at the cellular and circuit levels. Voltage is not directly visible, and so a longstanding challenge has been to develop genetically encoded fluorescent voltage indicator proteins. Recent advances have led to a profusion of new voltage indicators, based on different scaffolds and with different tradeoffs between voltage sensitivity, speed, brightness, and spectrum. In this review, we describe recent advances in design and applications of genetically-encoded voltage indicators (GEVIs). We also highlight the protein engineering strategies employed to improve the dynamic range and kinetics of GEVIs and opportunities for future advances.

Somato-dendritic morphology and dendritic signal transfer properties differentiate between fore- and hindlimb innervating motoneurons in the frog Rana esculenta

Background

The location specific motor pattern generation properties of the spinal cord along its rostro-caudal axis have been demonstrated. However, it is still unclear that these differences are due to the different spinal interneuronal networks underlying locomotions or there are also segmental differences in motoneurons innervating different limbs. Frogs use their fore- and hindlimbs differently during jumping and swimming. Therefore we hypothesized that limb innervating motoneurons, located in the cervical and lumbar spinal cord, are different in their morphology and dendritic signal transfer properties. The test of this hypothesis what we report here.

Results

Discriminant analysis classified segmental origin of the intracellularly labeled and three-dimensionally reconstructed motoneurons 100% correctly based on twelve morphological variables. Somata of lumbar motoneurons were rounder; the dendrites had bigger total length, more branches with higher branching orders and different spatial distributions of branch points. The ventro-medial extent of cervical dendrites was bigger than in lumbar motoneurons. Computational models of the motoneurons showed that dendritic signal transfer properties were also different in the two groups of motoneurons. Whether log attenuations were higher or lower in cervical than in lumbar motoneurons depended on the proximity of dendritic input to the soma. To investigate dendritic voltage and current transfer properties imposed by dendritic architecture rather than by neuronal size we used standardized distributions of transfer variables. We introduced a novel combination of cluster analysis and homogeneity indexes to quantify segmental segregation tendencies of motoneurons based on their dendritic transfer properties. A segregation tendency of cervical and lumbar motoneurons was detected by the rates of steady-state and transient voltage-amplitude transfers from dendrites to soma at all levels of synaptic background activities, modeled by varying the specific dendritic membrane resistance. On the other hand no segregation was observed by the steady-state current transfer except under high background activity.

Conclusions

We found size-dependent and size-independent differences in morphology and electrical structure of the limb moving motoneurons based on their spinal segmental location in frogs. Location specificity of locomotor networks is therefore partly due to segmental differences in motoneurons driving fore-, and hindlimbs.

Spatial heterogeneity of passive electrical transfer properties of neuronal dendrites due to their metrical asymmetry

The complex and diverse geometry of neuronal dendrites determines the different morphological types of neurons and influences the generation of complex and diverse discharge patterns at the cell output. The recent finding that each temporal pattern has its spatial signature in the form of a combination of high- and low-depolarization states of asymmetrical dendritic branches with active membrane properties raises the question of the nature of such characteristic spatial heterogeneity of electrical states. To answer this, we consider passive dendrites as a conventional reference case using the known current transfer functions, which we complete by corresponding parametric sensitivity functions. These functions for metrically asymmetrical bifurcations of different sizes, as the simplest elements constituting arborizations of arbitrary geometry, are analyzed under different membrane conductivity conditions related to the intensity of activation of ion channels. Characteristic relationships are obtained on the one hand among the size (branch lengths), metrical asymmetry (difference between sister branches in length and/or diameter), and membrane conductivity, and on the other hand, for the difference between the branches in their current transfer effectiveness as an indicator of their electrical asymmetry (heterogeneity). These relationships (i) allow the introduction of a biophysically based criterion for the electrical distinction between metrically asymmetrical branches, (ii) show how the difference first increases and then decreases with increasing membrane conductivity, and (iii) show that the greatest electrical heterogeneity occurs in a lower or higher range of conductivity, corresponding to larger or smaller bifurcation size. As a consequence, the characteristic low-, medium-, and high-conductance states are derived such that metrically asymmetrical parts of simple and complex trees are electrically distinct when the membrane conductivity lies in the size-related medium range, and indistinct otherwise.

Xenopus vocalizations are controlled by a sexually differentiated hindbrain central pattern generator

Male and female African clawed frogs (Xenopus laevis) produce rhythmic, sexually distinct vocalizations as part of courtship and mating. We found that Xenopus vocal behavior is governed by a sexually dimorphic central pattern generator (CPG) and that fictive vocalizations can be elicited from an in vitro brain preparation by application of serotonin or by electrical stimulation of a premotor nucleus. Male brains produced fictive vocal patterns representing two calls commonly produced by males in vivo (advertisement and amplectant call), as well as one call pattern (release call) that is common for juvenile males and females in vivo but rare for adult males. Female brains also produced fictive release call. The production of male calls is androgen dependent in Xenopus; to test the effects of androgens on the CPG, we examined fictive calling in the brains of testosterone-treated females. Both fictive male advertisement call and release call were produced. This suggests that all Xenopus possess a sexually undifferentiated pattern generator for release call. Androgen exposure leads to a gain-of-function, allowing the production of male-specific call types without prohibiting the production of the undifferentiated call pattern. We also demonstrate that the CPG is located in the brainstem and seems to rely on the same nuclei in both males and females. Finally, we identified endogenous serotonergic inputs to both the premotor and motor nuclei in the brainstem that may regulate vocal activity in vivo.

Quantitative morphological analysis of the motoneurons innervating muscles involved in tongue movements of the frogRana esculenta

We give an account of an effort to make quantitative morphological distinctions between motoneurons of the frog innervating functionally different groups of muscles involved in the movements of the tongue. The protractor, retractor, and inner muscles of the tongue were considered on the basis of their major action during the prey-catching behavior of the frog. Motoneurons were selectively labeled with cobalt lysin through the nerves of the individual muscles, and dendritic trees of successfully labeled neurons were reconstructed. Each motoneuron was characterized by 15 quantitative morphological parameters describing the size of the soma and dendritic tree and 12 orientation variables related to the shape and orientation of the dendritic field. The variables were subjected to multivariate discriminant analysis to find correlations between form and function of these motoneurons. According to the morphological parameters, the motoneurons were classified into three functionally different groups weighted by the shape of the perikaryon, mean diameter of stem dendrites, and mean length of dendritic segments. The most important orientation variables in the separation of three groups were the ellipses describing the shape of dendritic arborization in the horizontal, frontal, and sagittal planes of the brainstem. These findings indicate that characteristic geometry of the dendritic tree may have a preference for one array of fibers over another.

Relationship between morphoelectrotonic properties of motoneuron dendrites and their trajectory

The distribution and geometry of the dendritic trees of spinal motoneurons obey several well-established rules. Some of these rules are based on systematic relationships between quantitative geometrical features (e.g., total dendritic length) and the three-dimensional trajectory followed by dendrites from their origin to their termination. Because dendritic geometry partially determines the transmission of current and voltage signals generated by synapses on the dendritic tree, our goal was to compare the efficacy of signal transmission by dendritic trajectories that followed different directions. To achieve this goal, we constructed detailed compartmental models of the dendritic trees of three intracellularly stained biventer cervicis/complexus (BCCM) motoneurons and calculated the electronic properties of 361 dendritic paths. Each trajectory was classified according to its orientation, e.g., rostral, rostral-dorsal-lateral. The attenuation of current and voltage signals en route to the soma was strongly related to trajectory orientation. Trajectories with similar attenuation factors formed functional subunits that were arranged in distinct domains within the ventral horn. Changes in R(m) or R(i) had little effect on which trajectories belonged to each functional subunit. However, differences in the efficacy of signal transmission between subunits increased during high network activity (mimicked by decreases in R(m)). The most efficient subunit delivered two times more current and four times more voltage to the soma than the least efficient subunit. These results indicate that the input-output properties of motoneurons depend on the direction of the path taken by dendrites from their origin at the cell body to their terminals.

Insulin induces cobalt uptake in a subpopulation of rat cultured primary sensory neurons

Previous findings show that both the vanilloid receptor 1 and the insulin receptor are expressed on small primary sensory neurons. As insulin evokes activity in second messengers which could induce opening of the vanilloid receptor 1, we examined, by using the cobalt-uptake technique, whether or not insulin can activate cultured rat primary sensory neurons through activating the vanilloid receptor 1. Capsaicin (50, 100 and 500 nm) induced concentration-dependent labelling in primary sensory neurons. Preincubation of cells in insulin (10 micromoles) for 10 min followed by a 2-min wash did not produce significant change in the capsaicin-induced labelling. Coapplication of insulin (10 micromoles) with capsaicin, however, potentiated the 50 and 100 nm capsaicin-evoked staining. Insulin itself also produced cobalt labelling in a concentration-dependent manner. The size-frequency distributions of neurons showing capsaicin- or insulin-induced cobalt accumulation were similar. The insulin-induced cobalt labelling was significantly reduced by the tyrosine kinase inhibitor, tyrphostin AG1024, the vanilloid receptor 1 antagonists, ruthenium red and capsazepine, the protein kinase inhibitor, staurosporine and the phospholipase C inhibitor neomycin. Double immunostaining of cultured primary sensory neurons and sections from dorsal root ganglia revealed that about one-third of the cells coexpress the insulin receptor and vanilloid receptor 1. These findings suggest that insulin activates a subpopulation of primary sensory neurons, probably through phosphorylation- and/or phosphatidylinositol(4,5)biphosphate hydrolysis-evoked activation of the vanilloid receptor 1. Although the insulin-induced activation of vanilloid receptor 1 seems to be a short-lived effect in vitro, in vivo it might play a role in the development of burning pain sensation in hyperinsulinism.

An approach to the complexity of the brain

The establishment of ordered neuronal connections is supposed to take place under the control of specific cell adhesion molecules (CAM) which guide neuroblasts and axons to their appropriate destination. The extreme complexity of the nervous system does not provide a favorable medium for the development of deterministic connections. Simon's [112] theorems offer a mean to approach the high level of complexity of the nervous system. The basic tenet is that complex systems are hierarchically organized and decomposable. Such systems can arise by selective trial and error mechanisms. Subsystems in complex systems only interact in an aggregate manner, and no significant information is lost if the detail of aggregate interactions is ignored. A number of nervous activities, which qualify for these requirements, are shown. The following sources of selection are considered: internal and external feedbacks, previous experience, plasticity in simple structures, and the characteristic geometry of dendrites. The role played by CAMs and other membrane-associated molecules is discussed in the sense that they are either inductor molecules that turn on different homeobox genes, or downstream products of genes, or both. These molecules control cellular and tissular differentiation in the developing brain creating sources of selection required for the trial and error process in the organization of the nervous tissue.

Structural and physiological properties of connections between individual reticulospinal axons and lumbar motoneurons of the frog

Although the direct, monosynaptic influence of brainstem projections onto motoneurons is well-known, detailed morphological studies on the synaptic contact systems and a correlation with their functional properties are largely lacking. In this work, 43 pairs, each formed by a reticulospinal fiber contacting a lumbar motoneuron, were identified and studied electrophysiologically. Four of these were successfully labeled intracellularly with horseradish peroxidase (HRP) or neurobiotin and reconstructed using a computer-assisted camera lucida with high resolution. The mean amplitude of excitatory post-synaptic potentials (EPSPs) recorded in these four pairs varied from 100 to 730 microV, spanning most of the range obtained for all pairs (70-1,200 microV; mean +/- SD: 400 +/- 250 microV). Between two and four collaterals of reticulospinal axons established 4-19 close appositions with a labeled motoneuron. Mean distance from the origin of each collateral to any bouton on that collateral was 566-817 microm. A presynaptic action potential must pass 11 branch points on average to reach it. Similarly, the boutons presumably contacting motoneurons were on average 558-624 microm (9-11 branch points) from the origin of the collateral. The distributions of diameters of all boutons and those making putative contacts with stained motoneurons were very similar. The dendritic surface of stained motoneurons was symmetrically distributed along the rostrocaudal axis with more than half the surface being more than 500 microm from the soma. However, the contacts from reticulospinal axons were concentrated ventromedially, 262-356 microm (range of average values for four connections) from the motoneuron soma, in some instances on very proximal dendritic segments. Thus, the location and size of putative contacts in relation to axonal collaterals was not distinguishable from location and size of other boutons, but they occupied specific positions on dendrites of lumbar motoneurons. The number of contacts formed by a reticulospinal axon on a motoneuron in a particular location could be described as the product of the available dendritic surface and the total number of presynaptic boutons in this region. Compartmental models of the reconstructed motoneurons were created, and currents with the time course of an alpha function were injected at the sites of these putative contacts. Despite the restricted volume occupied by contacts from a single fiber, a high variability of their contributions to somatic EPSPs owing to electrotonic attenuation was shown: The coefficient of variation of quantal responses was estimated to be between 60% and 120%, comparable to the variability of the path distance between contacts and soma (50-90%).

Motoneurons of the axial swimming muscles in hatchling Xenopus tadpoles: features, distribution, and central synapses

Xenopus tadpole motoneurons make cholinergic synapses within the spinal cord. This excitation changes with longitudinal position and contributes to the excitation that controls motor activity and its longitudinal spread during swimming. To explore the anatomic constraints on this excitation, backfilling has been used to examine the anatomy and distribution of the whole population of spinal motoneurons, to define the extent of their central axons and to find where they make synapses. Motoneuron features show considerable variation but do not allow their separation into primary and secondary. Most motoneurons have descending central axons and it is likely that central synapses are made from these axons as longitudinal dendritic extent is very limited. Motoneuron density reaches a broad plateau over the mid-trunk region at 12-13 per 100 microm. Soma size does not change with longitudinal position, but the dorsoventral extent of the dendrites decreases caudally, whereas the central axon length increases. Motoneuron distribution data were used to estimate the longitudinal distribution of central motoneuron axons. This has a broad plateau at 12-14 per 100 microm over much of the trunk and only decreases significantly caudal to the anus. This distribution correlates with cholinergic excitation during swimming. Transmission electron microscopy of motoneurons backfilled with horseradish peroxidase was used to show that central motoneuron axons make en passant synapses with motoneuron dendrites and the dendrites of other unstained neurons. By using measures of synapse frequency and total dendrite length, trunk motoneurons are estimated to each receive 100-200 synapses.

Localization of last-order premotor interneurons in the lumbar spinal cord of rats

There is strong evidence that neural circuits underlying certain rhythmic motor behaviors are located in the spinal cord. Such local central pattern generators are thought to coordinate the activity of motoneurons through specific sets of last-order premotor interneurons that establish monosynaptic contacts with motoneurons. After injections of biotinylated dextran amine into the lateral and medial motor columns as well as the ventrolateral white matter at the level of the upper and lower segments of the lumbar spinal cord, we intended to identify and localize retrogradely labelled spinal interneurons that can likely be regarded as last-order premotor interneurons in rats. Regardless of the location of the injection site, labelled interneurons were revealed in laminae V-VIII along a three- or four-segment-long section of the spinal gray matter. Although most of the stained cells were confined to laminae V-VIII in all cases, the distribution of neurons within the confines of this area varied according to the site of injection. After injections into the lateral motor column at the level of the L4-L5 segments, the labelled neurons were located almost exclusively in laminae V-VII ipsilateral to the injection site, and the perikarya were distributed throughout the entire mediolateral extent of this area. Interneurons projecting to the lateral motor column at the level of the L1-L2 segments were also located in laminae V-VII, but most of them were concentrated in the middle one-third or in the lateral half of this area. Following injections into the medial motor column at the level of the L1-L2 segments, the majority of labelled neurons were confined to the medial aspect of laminae V-VII and lamina VIII, and the proportion of neurons that were found contralateral to the injection site was strikingly higher than in the other experimental groups. The results suggest that the organization of last-order premotor interneurons projecting to motoneurons, which are located at different areas of the lateral and medial motor columns and innervate different muscle groups, may present distinct features in the rat spinal cord.

Organization of the ambiguus nucleus in the frog (Rana esculenta)

The common root of the glossopharyngeal, vagal, and accessory nerves and the individual branches of the vagus complex were labeled with cobalt, and the organization of the ambiguus nucleus was studied. The cell column labeled through the common root extended from the upper part of the medulla to the rostral spinal cord over a distance of about 3,500 microns. The labeling of individual branches revealed four subdivisions. 1) The pharyngomotor subdivision occupied the rostral 800 microns of the cell column. It gave origin to the innervation of the pharyngeal muscles. 2) The visceromotor subdivision, consisting of small and medium-sized cells labeled by way of the visceral branches of the vagus, was found in the rostrocaudal extent of the medulla. 3) the laryngomotor subdivision extended in the obex region over a distance of more than 1,000 microns. It supplied the sphincter muscles of the larynx. The dilator laryngeal muscle was represented in the rostral part of the visceromotor subdivision. 4) The accessory nerve subdivision was located in the lower medulla and the rostral spinal cord. From the results, the following conclusions are drawn. 1) The basic organization of the frog ambiguus nucleus is comparable to that of the rat, differences in nuclear organization reflecting differences in peripheral structures. 2) The cytoarchitectonic structure of the four subdivisions innervating different peripheral targets characteristically differ from each other. 3) On the basis of its characteristic neuronal morphology, the accessory nerve nucleus is regarded as an independent structure.

Comparison of the topology and growth rules of motoneuronal dendrites

The complexity, shape, and branching modes of the dendrites of spinal motoneurons were compared in cat, rat, and frog using topological analysis and growth models. The complexity of motoneuronal dendrites, measured as the mean number of terminal segments, varied significantly among samples and was related to contractile properties of innervated motor units. Despite this variation, all mature motoneurons having a mean number of terminal segments per dendrite greater than ten (up to 24.3) exhibited a narrow range of values of coefficients describing the symmetry of tree shapes (0.42-0.47). This implies low variability in the topological shape of motoneuronal dendrites of different animals. This similarity of tree shapes proved to be a result of the similarity of growth rules. The growth of the dendrites could be described to a first approximation by a two-parameter (Q and S) model called the QS model and by a multitype Markovian model. The estimation of parameters of the QS model, in which parameter Q is related to the probability of branching of intermediate segments, revealed that Q was equal or close to 0, implying that branching of dendrites is restricted to terminal segments. The estimates of the parameter S, which describes whether the probability of branching increases (S < 0) or decreases (S > 0) exponentially with segment order, were positive. This was in agreement with the results of estimation of probabilities of branching provided by the Markovian model, which showed that the branching probabilities decreased with segment order in an exponential manner in most of the neurons studied. The QS and Markovian models involve different assumptions about the sequence and timing of branching events, and selection of the best model can provide insight into details of dendritic outgrowth. Extensive simulation of tree outgrowth using a Markovian model revealed significant differences between stimulated trees and real dendrites, particularly with regard to variability of the number of terminals and to symmetry. In contrast, the QS model provided a good fit to the mean values and standard deviations of basic topological parameters. This model is adequate to describe the shape of mature motoneuronal dendrites. It implies that dendritic branches have many opportunities to bifurcate during the whole time of development and that bifurcating potency of a branch is a function of the number and position of other branches of that dendrite. Combined with analysis of metrical properties such as lengths of segments, the QS model can assist in a quantitative analysis of development and plasticity.

A fast 3-dimensional neuronal tree reconstruction system that uses cubic polynomials to estimate dendritic curvature

The main goal of this work was to develop and test the accuracy of our 3DARBOR neuronal tree reconstruction system by comparing it with a very precise but time-consuming method of reconstruction (NEUTRACE). Comparison was performed by reconstructing 18 dendritic trees of frog spinal motoneurons from serial sections with both methods and comparing several morphological summaries of the two reconstructions. In 3DARBOR the planar projection of the dendritic trees was drawn and fed into an IBM-compatible PC through a graphic tablet. Dendritic coordinates along the perpendicular (focus) axis on the plane of drawing were estimated by an interpolation method. The interpolation was based on the lengths of projected dendrites and the coordinates of points where dendrites entered the next section. Focus axis coordinates of these points could automatically be calculated from the serial numbers and thicknesses of sections. 3DARBOR was tested by comparison of the distributions of characteristic points of dendritic trees, segment lengths and branching angles. Product moment analysis on dendritic trees was also performed. It was concluded that 3DARBOR is a fast enough reconstruction system without any systematical error of interpolation that can correctly supply the most morphological parameters and visualize the natural arborizations.

Investigation of the dendritic geometry of brain stem motoneurons with different functions using multivariant statistical techniques in the frog

We give an account of an effort to make quantitative morphological distinctions between motoneurons innervating functionally different muscles in the trigeminal and facial motor nuclei of the frog. Six groups of neurons were considered in the two nuclei on the basis of their peripheral targets. One group consisted of neurons (n = 7) innervating the levator bulbi muscle, which separates the orbital cavity from the oral cavity. In the second, third and fourth groups, motoneurons (n = 27) innervating jaw closer muscles (temporalis, masseter, pterygoideus) were studied. Neurons (n = 6) innervating the submaxillary muscle comprised the fifth group. This muscle forms the muscular floor of the mouth. It is active in deglutition and contributes to the opening of the mouth. The sixth group is formed by neurons of the facial nucleus (n = 7), which innervate the depressor mandibulae muscle. This is the main opener of the mouth. Neurons were selectively stained by cobalt labelling through the muscle nerves and the morphometric values of successfully labelled neurons were fed into a IBM AT 386 computer through a digitizing tablet for three-dimensional reconstruction. Four neurons labelled directly through the motor root of the trigeminal nerve but innervating unidentified muscles were added to the investigation. Two sets of quantitative measurements were taken from the neurons. In the first set (neurometric data), 17 quantitative variables were measured in the perikaryon and the dendritic arbor. In the second set, 15 variables concerned with the orientation and shape of the dendritic tree, the relation of the perikaryon to the dendritic tree and the spatial expansion of dendrites were measured in the three dimensions of Cartesian space (product-moment data). The data were subjected to multivariant statistical analysis. First, they were partitioned with cluster analysis. The average linkage between groups algorithm and the cosine of vectors of variables, or the Pearson correlation similarity coefficients were used. Neurometric data and product-moment data were analysed separately and in combination, and six to seven clusters were considered. In each case, the majority of neurons innervating jaw closer muscles were grouped into clusters different from neurons innervating jaw opener muscles. The best separation of functionally different neurons was achieved with the neurometric data set. The groups of neurons obtained from cluster analysis were subjected to non-parametric discriminant analysis with the eight nearest-neighbour classification criterion, and the results were checked with a cross-validation technique.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphology of developing rat genioglossal motoneurons studied in vitro: relative changes in diameter and surface area of somata and dendrites

This study describes the postnatal change in size of motoneurons in the hypoglossal nucleus that innervate the genioglossus muscle. Such anatomical information is essential for determining the cellular mechanisms responsible for the changes observed in the electrical properties of these motoneurons during postnatal development. The cells analyzed here are part of an earlier study (Núñez-Abades et al. [1994] J. Comp. Neurol. 339:401-420) where 40 genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labeled by intracellular injection of neurobiotin in an in vitro slice preparation of the rat brainstem and their cellular morphology was reconstructed in three-dimensional space. The sequence of postnatal dendritic growth can be described in two phases. The first phase, between birth (1-2 days) and 13-15 days, was characterized by no change in either dendritic diameter (any branch order) or dendritic surface area of GG motoneurons. However, maturation of the dendritic tree produced more surface area at greater distances from the soma by redistributing existing membrane (retracting some terminal branches). During the second phase, between 13-15 days and 19-30 days, the dendritic surface area doubled as a result of an increase in the dendritic diameter across all branch orders and a generation of new terminal branches. In contrast to the growth exhibited by the dendrites, there was little discernible postnatal growth of somata. At all ages, dendrites of GG motoneurons show the largest amount of tapering in the first-and second-order dendrites. The calculated dendritic trunk parameter deviated from a value 1.0, indicating that the dendritic tree of developing GG motoneurons cannot be modeled accurately as an equivalent cylinder. However, the value of this parameter increased with age. Strong correlations were found between the diameter of the first-order dendrite and the dendritic surface area, dendritic volume, combined dendritic length, and, to a lesser extent, the number of terminal dendrites in GG motoneurons. Correlations were also found between somal and dendritic geometry but only when data were pooled across all age groups. These data support earlier studies on kitten phrenic motoneurons, which concluded that postnatal growth of motoneurons was not a continuous process. Based on the fact that there was no growth in the first 2 weeks, the changes in the membrane properties described during this phase of postnatal development (e.g., decrease in input resistance) cannot be attributed to increases in the total membrane surface area of these motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending propriospinal axons in the hindlimb enlargement of the red-eared turtle: cells of origin and funicular courses

Spinal neurons with descending axons are important components of spinal sensorimotor networks. We used an anatomical tracing technique to study the distribution of descending propriospinal axons and cell bodies in red-eared turtles. We injected horseradish peroxidase into a portion of one funiculus in the middle of the hindlimb enlargement and examined six spinal segments rostral to the injection site (dorsal 3 through dorsal 8) for labeled neuronal cell bodies. Injections into each region of the white matter labeled substantial numbers of descending propriospinal neurons. Each injection labeled cell bodies over most of the six spinal segments examined. Each injection also labeled cell bodies in the ipsilateral dorsal horn, intermediate zone, and ventral horn as well as the contralateral intermediate zone and ventral horn. Injections into each of four regions of the white matter, the dorsal funiculus, the medial part of the lateral funiculus, the lateral part of the lateral funiculus, and the ventral funiculus reliably gave rise to a distinct distribution of labeled cell bodies. These experiments establish that descending propriospinal axons in red-eared turtles are found in all regions of the spinal white matter. This finding contrasts with a popular contemporary view of the organization of descending propriospinal axons in mammals. These experiments also demonstrate that neurons in each region of the gray matter give rise to a different distribution of descending, funicular axons, although these distributions are widely overlapping. Different funicular axon distributions could be associated with different sets of synaptic contacts with the white-matter dendrites of spinal neurons.

Morphology of two classes of target-specific bullfrog sympathetic preganglionic neurons

These experiments took advantage of the unique ability to define target-specific sympathetic preganglionic neurons in the bullfrog spinal cord in order to examine the morphologies of different classes of preganglionic neurons. Sympathetic preganglionic neurons were identified by retrograde transport of fast blue from the sympathetic chain. Subsequently, fast blue-labelled sympathetic preganglionic neurons in fixed spinal cord slices were filled with lucifer yellow and processed for visualization with lucifer yellow antiserum, biotinylated secondary antiserum, and avidin peroxidase. Target specificity of sympathetic preganglionic neurons was determined by anatomical position; sympathetic preganglionic neurons that control the vasculature (C-type sympathetic preganglionic neurons) lie in a position caudal to those that control nonvascular targets [B-type sympathetic preganglionic neurons; Horn and Stofer (1988) J. Comp. Neurol. 268:71]. These two classes of sympathetic preganglionic neurons have qualitatively similar morphologies. However, they exhibit significant quantitative differences in total dendritic length and the rostrocaudal extent of dendrites. These differences are likely to be associated with differences in the number of synapses received by these two classes of sympathetic preganglionic neurons. Moreover, the segmental control of sympathetic preganglionic neurons by descending brainstem projections is likely to be finer for those involved in vascular control than for those that influence other targets.

Simulation of the Effect of Synapses: the Significance of the Dendritic Diameter in Impulse Propagation

The effectiveness of synapses at various sites of the dendritic tree was studied using a segmental cable model with a program developed by Hines (Int. J. Biomed. Comput., 24, 55 - 68, 1989). The model rendered possible a high-fidelity simulation of the dendritic geometry of a frog motoneuron described in the accompanying paper (Birinyi et al., Eur. J. Neurosci., 1003 - 1012, 1992). The model was used in the passive membrane mode and the synaptic activity was simulated with current injections into large and small diameter dendrites at proximal and distal locations. Synaptic efficiency was defined by the charge transfer ratio expressed as the proportion of the injected current which appeared at the soma. The charge transfer ratio was determined with uniform and non-uniform distribution of specific membrane resistance over the soma - dendrite surface while the diameter of selected dendrite segments changed. The best charge transfer ratio was found with the largest dendrite membrane resistance, and the maximum efficiency of synaptic activity appeared at the original size of the dendrite segment stimulated. The amount of current that flowed in the proximal and distal directions from the segment stimulated depended on the diameter of that segment. The increase in diameter of proximal dendrites increased synaptic efficiency on distal dendrites, whereas the reverse caused a decline in synaptic efficiency on proximal dendrites. In addition to the diameter of dendrites, the arborization pattern also played a significant role in this mechanism. It is concluded that the cellulipetal increase in dendrite diameter greatly increases synaptic efficiency.