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A U Larkman

Publications and source records attributed to A U Larkman.

At least 19 recordsLinked to original sources
PubMed1998

Rapid report: the reliability of excitatory synaptic transmission in slices of rat visual cortex in vitro is temperature dependent.

1. A total of twelve synaptic connections between pairs of pyramidal neurones in layer 2/3 of slices of rat visual cortex maintained in vitro was investigated using whole-cell voltage recordings under visual control. The connections varied widely in strength, with the mean peak amplitudes of the resulting excitatory postsynaptic potentials (EPSPs) ranging between approximately 40 microV and 2 mV at 23 degrees C. The smaller mean amplitudes included a substantial proportion of apparent failures of transmission. 2. The properties of these EPSPs were examined over a range of temperatures between 13 and 36 degrees C. All the connections became more reliable, in that they showed fewer apparent failures of transmission, and showed less trial-to-trial variability at the higher temperatures. These changes appeared to be due primarily to an increase in the mean number of transmitter quanta released per presynaptic action potential. 3. At 36 degrees C most connections were relatively reliable, with a mean failure rate of only 16 %. Five connections showed virtually no failures (1 % or fewer) at this temperature. 4. We conclude that quantal transmitter release is temperature dependent at these synapses, and that experiments performed at room temperature could lead to an exaggerated impression of the unreliability of transmission at central excitatory synapses.

Animals↗
PubMed1997

Calibration of an autocorrelation-based method for determining amplitude histogram reliability and quantal size.

1. We describe a method, based on autocorrelation and Monte Carlo simulation, for determining the likelihood that peaks in synaptic amplitude frequency histograms could have been a result of finite sampling from parent distributions that were unimodal. 2. The first step was to calculate an 'autocorrelation score' for the histogram to be tested. A unimodal distribution was fitted to the test histogram and subtracted from it. The resulting difference function was smoothed and its autocorrelation function calculated. The amplitude of the first (non-zero lag) peak in this autocorrelation function was taken as the autocorrelation score for that histogram. The score depends on the sharpness of the histogram peaks, the equality of their spacing and the number of trials. 3. The second stage was to generate large numbers of random samples, each of the same number of trials as the histogram, from a unimodal generator distribution of similar shape. The autocorrelation score was calculated for each sample and the proportion of samples with scores greater than the histogram gave the likelihood that the histogram peaks could have arisen by sampling artifact. 4. The method was calibrated using simulated non-quantal and quantal histograms with different signal-to-noise ratios and numbers of trials. For a quantal distribution with four peaks and a signal-to-noise ratio of 3, a sample size of about 500 trials was needed for 95% of samples to be distinguished from a non-quantal distribution. 5. The ability of the autocorrelation method to distinguish quantal from non-quantal distributions was compared against two conventional statistical tests, the chi 2 and the Kolmogorov-Smirnov goodness of fit tests. The autocorrelation method was more specific in extracting quantized responses. The Kolmogorov-Smirnov test in particular could not distinguish quantal distributions with multiple peaks even if the peaks were very sharp. 6. The improved discrimination of the autocorrelation method proved important when applied to experimental data recorded from hippocampal synapses. Of thirty-three histograms that were significantly different from smooth distributions by the autocorrelation method (P < 0.05), only seventeen were significantly different using the chi 2 test and only two when using the Kolmogorov-Smirnov test. 7. The autocorrelation method also gave an estimate of histogram peak spacing or quantal size. Using simulated quantal distributions, we showed that this estimate was likely to be correct within a few per cent for distributions that gave P < 0.01 by autocorrelation scoring.

Animals↗
PubMed1997

Assessment of the reliability or amplitude histograms from excitatory synapses in rat hippocampal CA1 in vitro.

1. Excitatory postsynaptic potentials (EPSPs) were evoked using minimal extracellular stimulation and recorded from pyramidal cells from the CA1 region of slices taken from adult rats and maintained in vitro. 2. Segments of data were selected that gave EPSP amplitude frequency histograms that showed approximately equally spaced peaks. Selection was performed either on the basis of stationarity of the EPSP mean and standard deviation, or on the trajectory of a graph of the (coefficient of variation)-2 against mean for the EPSP, or, in some cases, by trial and error. 3. For each histogram, we determined the likelihood that peaks of similar sharpness and equality of spacing could have arisen by sampling artifact from a smooth distribution, using a method based on autocorrelation and Monte Carlo simulation. 4. Thirty-three histograms were analysed. For twenty-six of these, the likelihood of sampling artifact was estimated at 1 in 100 or less, and for eleven histograms the likelihood was less than 1 in 1000. For the histogram with the clearest peaks, the likelihood was less than 1 in 350,000. Histograms judged to be reliable by this method could occur when the EPSP mean amplitude was changing. 5. We conclude that random sampling artifact is very unlikely to be the explanation for the peaks in our data histograms. It seems more likely that they are due to a quantal synaptic transmission mechanism with low quantal variability. 6. The autocorrelation method also gives a measure of the mean peak spacing, and hence the mean quantal size, for each histogram. Quantal sizes ranged from 93 to 285 microV, with a mean +/- S.D. of 172 +/- 47 microV. 7. From these quantal sizes and the EPSP mean amplitudes we calculated the mean number of quanta released per trial for each histogram. This ranged from 0.36 to 6.9, with a mean of 3.3 +/- 1.67.

Animals↗
PubMed1997

Quantal analysis of excitatory synapses in rat hippocampal CA1 in vitro during low-frequency depression.

1. We have performed a detailed quantal analysis of excitatory postsynaptic potentials (EPSPs) evoked by minimal extracellular stimulation in the CA1 region of slices of adult rat hippocampus maintained in vitro. 2. EPSPs were evoked at 2-5 Hz, and the eight that were analysed all showed at least a 50% depression of mean peak amplitude during recording. 3. EPSP amplitude fluctuations were analysed by three methods: the use of amplitude frequency histograms with clear and reliable peaks where available, graphs of the EPSP (coefficient of variation)-2 against EPSP mean, and analysis of EPSP mean and standard deviation assuming simple binomial statistics with the number of release sites (N) kept constant but the quantal size (Q) and the release probability (Pr) allowed to vary over time. 4. The results of the three analysis procedures were in good agreement. Seven EPSPs showed a substantial reduction in the mean number of quanta released per trial, and in three cases this was the predominant mechanism of the depression. Five EPSPs showed a substantial decrease in Q. Values for N ranged between 3 and 18, with a median of 6; Pr ranged between 0.14 and 0.81 and Q between 66 and 275 microV. 5. We used the Q estimates from the binomial method to correct the recorded EPSP amplitudes for changes in quantal size over time. For seven out of the eight EPSPs, this rescaling procedure allowed histograms with clear peaks to be obtained from longer runs of data, or improved the sharpness of the peaks in histograms from all the recorded data. The improvement in peak sharpness was assessed using an autocorrelation-based method. The correction was much less successful if the Q estimates were obtained with a variant of the binomial method in which Pr was held constant and N was allowed to vary. 6. The only simple explanation for the success of the correction procedure is that changes in quantal size were a major factor in obscuring peaks in histograms based on large numbers of trials, and that the quantal size estimates from the binomial method with N held constant were reasonably accurate. 7. We conclude that transmission at these synapses was quantal with relatively low quantal variance, but repetitive stimulation often induced substantial changes in the quantal parameters that might prevent the success of conventional quantal analysis approaches.

Animals↗
PubMed1997

The effects of synaptic noise on measurements of evoked excitatory postsynaptic response amplitudes.

Spontaneously occurring synaptic events (synaptic noise) recorded intracellularly are usually assumed to be independent of evoked postsynaptic responses and to contaminate measures of postsynaptic response amplitude in a roughly Gaussian manner. Here we derive analytically the expected noise distribution for excitatory synaptic noise and investigate its effects on amplitude histograms. We propose that some fraction of this excitatory noise is initiated at the same release sites that contribute to the evoked synaptic event and develop an analytical model of the interaction between this fraction of the noise and the evoked postsynaptic response amplitude. Recording intracellularly with sharp microelectrodes in the in vitro hippocampal slice preparation, we find that excitatory synaptic noise accounts for up to 70% of the intracellular recording noise, when inhibition is blocked pharmacologically. Up to 20% of this noise shows a significant correlation with the evoked event amplitude, and the behavior of this component of the noise is consistent with a model which assumes that each release site experiences a refractory period of approximately 60 ms after release. In contrast with classical models of quantal variance, our models predict that excitatory synaptic noise can cause the apparent variance of successive peaks in an excitatory synaptic amplitude histogram to decrease from left to right, and in some cases to be less than the variance of the measured noise.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1995

The variance of successive peaks in synaptic amplitude histograms: effects of inter-site differences in quantal size.

Variability in the measured amplitude of evoked synaptic events can arise from several factors, including: measurement noise, trial-to-trial variation in the amplitude of the response at a single release site, or variation between different release sites (inter-site variation) in the mean amplitude of the single quantal response. Classic (linear) models of variability include only the first two of these factors, although differences in the number of postsynaptic receptors or the degree of electrotonic attenuation for different release sites could cause substantial inter-site variations in quantal size. In this paper, the effect of inter-site variation on the variance of successive histogram peaks has been determined analytically and verified by computational studies. This effect is minimal at the edges of the histogram and contributes maximally to central peaks. Linear approximations to the variance of successive histogram peaks may therefore result in very poor fits to measured data if substantial inter-site variation in quantal size is involved. Our computational results indicate that for synaptic contacts with high release probabilities and substantial inter-site variation, the variance of histogram peaks will decrease with increasing quantal content.

Animals↗
PubMed1995

Dendritic morphology of CA1 pyramidal neurones from the rat hippocampus: I. Branching patterns.

The aim of this study was to provide quantitative descriptions of the dendritic branching patterns of pyramidal neurones in the CA1 region of the rat hippocampus. Thirteen adult cells were filled with biocytin and reconstructed by using the light microscope. The number of basal trees arising from the soma of each cell ranged from two to eight. There was wide variation in the number of terminal segments per tree. Six cells had single apical trunks, and seven had trunks that bifurcated in stratum radiatum. The number of apical oblique trees ranged from nine to 30, with each tree usually showing a lower degree of branching than basal trees. Basal and oblique trees had similar branching patterns, with the majority of branch points occurring close to the origin of the tree. Both basal and oblique terminal segments were generally much longer than intermediate segments and constituted up to 90% of the combined dendritic length of the tree. The branching pattern of the apical tuft was different, with many relatively long intermediate segments; terminal segments contributed only some 66% of the combined dendritic length of these trees. The mean total combined dendritic length for six adult cells reconstructed and measured completely was 11,900 +/- 1,000 microns (standard deviation). The relative proportions of the different parts of the dendritic system, although not the total dendritic length, were correlated with the location of the soma relative to the cell body layer. Cells with somata close to the stratum pyramidale/stratum radiatum border had more dendrites terminating in stratum radiatum and fewer in stratum oriens than cells with somata further from it.

Animals↗
PubMed1995

Dendritic morphology of CA1 pyramidal neurones from the rat hippocampus: II. Spine distributions.

The numbers and distributions of dendritic spines were estimated for six adult and three juvenile biocytin-injected neurones from the CA1 region of the hippocampus of the albino rat. For each cell, a sample of long dendritic segments that lay favourably in the plane of focus was drawn at high magnification and the visible spines counted. Correction was made for spines obscured by dendritic shafts. Within individual cells, dendrites of similar type and diameter had similar spine densities. For adults, long basal segments averaged 2.4 spines/microns and obliques averaged 3.2 spines/microns. In juveniles, basals averaged 2.3 spines/microns and obliques, 2.5 spines/microns. Apical tuft segments were less spiny, averaging 1.4 spines/microns in adult cells and 1.8 spines/microns in juveniles. There was a positive correlation between spine density and dendrite diameter. Values from this sample were used to assign spine densities to the other segments, and so the total number of spines was estimated for each cell. Adult cells averaged 30,500 +/- 3,900 (S.D.) spines and juveniles, 23,800 +/- 7,100 spines. Adult cells had roughly 50% of their spines in stratum radiatum, 40% in s. oriens, and 10% in s. lacunosum-moleculare. Juvenile cells had a rather higher proportion (20%) in s. lacunosum-moleculare. In general, some 50% of all spines were located within a path length of 200 microns from the soma. These total numbers of spines were much higher than earlier values from Golgi-impregnated cells but align well with estimates of the numbers of axonal boutons supplied to CA1 by CA3 pyramidal cells.

Animals↗
PubMed1995

Synaptic plasticity: hippocampal LTP.

One of the most intensively studied forms of synaptic plasticity is long-term potentiation (LTP). The past year has seen further evidence advanced on both sides of the presynaptic/postsynaptic locus of expression debate, without an obvious path to reconcile the two views. Real progress has been made, however, in clarifying the possible role of nitric oxide as a retrograde messenger and the cellular location of its synthetic enzyme. Intriguing glimpses of the complex involvement of metabotropic glutamate receptors in the induction of LTP have also appeared.

Animals↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. I. Correlation among cell morphology, intrinsic electrophysiological properties, and axon targets.

Previous work has established two structure/function correlations for pyramidal neurons of layer 5 of the primary visual cortex of the rat. First, cells projecting to the superior colliculus have thick apical dendrites with a florid terminal arborization in layer 1, whereas those projecting to the visual cortex of the opposite hemisphere have thinner apical dendrites that terminate below layer 1, without a terminal tuft (e.g., Hallman et al.: J Comp Neurol 272:149, '90). Second, intracellular recording combined with dye injection has revealed two classes of cells: the first has a thick, tufted apical dendrite and fires a distinctive initial burst of two or more impulses, of virtually fixed, short interspike interval, in response to current injection; and the other, with a slender apical dendrite lacking a terminal tuft, tends to have a longer membrane time constant and higher input resistance, and does not fire characteristic bursts (e.g., Larkman and Mason: J Neurosci 10:1407, '90). The present study combined intracellular recording in isolated slices of rat visual cortex and injection of carboxyfluorescein, to reveal soma-dendritic morphology, with prior injection of rhodamine-conjugated microspheres into the superior colliculus or contralateral visual cortex to label neurons according to the target of their axons. This permitted a complete correlation of morphology, intrinsic electrophysiological properties, and identity of the projection target for individual pyramidal cells. Neurons retrogradely labeled from the opposite visual cortex were found in all layers except layer 1 while those labeled from the superior colliculus lay exclusively in layer 5. Within layer 5 interhemispheric cells were more concentrated in the lower half of the layer but extensively overlapped the distribution of corticotectal cells. Every cell studied that projected to the superior colliculus was of the bursting type and had a thick apical dendrite with a terminal tuft. Every cell in this study projecting to the opposite visual cortex was a "nonburster" and had a slender apical dendrite with fewer oblique branches that ended without a terminal tuft, usually in the upper part of layer 2/3. Interhemispheric cells also had rounder, less conical somata and generally had fewer basal dendrites than corticotectal neurons. Many cells with the physiological and morphological characteristics of interhemispheric cells were not back-labeled from the opposite visual cortex, implying that pyramidal cells of this type can have other projection targets (e.g., other cortical sites in the ipsilateral hemisphere).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties.

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. III. Differential maturation of axon targeting, dendritic morphology, and electrophysiological properties.

This paper describes the early morphological and physiological development of pyramidal neurons in layer 5 of the rat visual cortex in relation to the targets chosen by their axons. Cells were prelabeled by retrograde transport from the superior colliculus or the contralateral visual cortex and intracellularly injected either in fixed slices or after recording in living slices. In the adult, corticotectal cells have thick apical dendrites with an extensive terminal arborization extending into layer 1, and fire characteristic bursts of action potentials when injected with a depolarizing current; interhemispheric cells have slender apical dendrites that terminate without a terminal tuft, usually in layer 2/3, and they display a more regular firing pattern (Kasper et al.: J Comp Neurol, this issue, 339:459-474). At embryonic day E18 (when axons of the two classes of cells are already taking different routes towards their targets) and E21, pyramidal-like cells throughout the cortical plate all have similar soma-dendritic morphology, with spindle-shaped cell bodies and few, short basal dendrites but apical dendrites that all end in distinct tufts in the marginal zone. At postnatal day P3, after the axons of both cell classes have reached their targets, all pyramidal neurons in layer 5 still have distinct terminal arborizations in layer 1, though they vary in complexity and extent. The somata are now more mature (round to ovoid in shape), and the basal dendritic tree has extended. As early as P5, all cells studied could be clearly classified as tufted or untufted (considerably earlier than previously reported; Koester and O'Leary: J Neurosci 12:1382, '92), and these features correlated precisely with the projection target, as in the adult. Measurement showed that although interhemispheric cells lose their terminal tufts, in general the trunks of their apical dendrites do not withdraw but continue to grow, at roughly the same rate as those of corticotectal cells. The two classes of layer 5 pyramidal neurons differentiate from each other in three distinct phases: pathway selection by axons precedes the loss of the apical tuft by interhemispheric cells, and these morphological characteristics are established 10 days before the onset of burst-firing in corticotectal cells. These three steps may be guided by different molecular signals.

Action Potentials↗
PubMed1994

Detailed passive cable models of whole-cell recorded CA3 pyramidal neurons in rat hippocampal slices.

Tight-seal whole-cell recordings were made from cleaned somata of CA3 pyramidal cells deep in hippocampal slices from 19-21-d-old rats. The cells were filled with biocytin, and their voltage responses to short current pulses were recorded. After washout of initial sag, responses scaled linearly with injected current and were stable over time. The dendritic and axonal arbors of four cells were reconstructed and measured using light microscopy. Dendritic spines and axonal boutons were counted and the additional membrane area was incorporated into the relevant segments. The morphology of each neuron was converted into a detailed branching cable model by assuming values for specific membrane capacitance Cm and resistance Rm, and cytoplasmic resistivity Ri. These parameters were optimized for each cell by directly matching the model's response to that of the real cell by means of a modified weighted least-squares fitting procedure. By comparing the deviations between model and experimental responses to control noise recordings, approximate 95% confidence intervals were established for each parameter. If a somatic shunt was allowed, a wide range of possible Rm values produced acceptable fits. With zero shunt, Cm was 0.7-0.8 microFcm-2, Ri was 170-340 omega cm, and Rm ranged between 120 and 200 k omega cm2. The electrotonic lengths of the basal and oblique dendrites were 0.2-0.3 space constants, and those of the apical tufts were 0.4-0.7 space constants. The steady-state electrical geometry of these cells was therefore compact; average dendritic tip/soma relative synaptic efficacies were > 93% for the basal and oblique dendrites, and > 81% for the tufts. With fast transient synaptic inputs, however, the models produced a wide range of postsynaptic potential shapes and marked filtering of voltage-clamp currents.

Animals↗
PubMed1992

Dendritic morphology of pyramidal neurones of the visual cortex of the rat. IV: Electrical geometry.

Features of the dendritic morphology of pyramidal neurones of the visual cortex of the rat that are relevant to the development of models of their passive electrical geometry were investigated. The sample of 39 neurones that was used came from layers 2/3 and 5. They had been recorded from and injected intracellularly with horseradish peroxidase (HRP) in vitro as part of a previous study (Larkman and Mason, J. Neurosci 10:1407, 1990). These cells had been reconstructed and measured previously by light microscopy. The relationship between the diameters of parent and daughter dendrites during branching was examined. It was found that most dendrites did not closely obey the "3/2 branch power relationship" required for representation of the dendrites as single equivalent cylinders. Estimates of total neuronal membrane area ranged from 27,100 +/- 7,900 microns2 for layer 2/3 cells to 52,200 +/- 11,800 microns2 for thick layer 5 cells. Dendritic spines contributed approximately half the total membrane area. Both neuronal input resistance and the ratio of membrane time constant to input resistance were correlated with neuronal membrane area as measured anatomically. The relative electrical lengths of the different dendrites of individual neurones were investigated, by using simple transformations to take account of the differences in diameter and spine density between dendritic segments. A novel "morphotonic" transformation is described that represents the purely morphological component of electrotonic length. Morphotonic lengths can be converted into electrotonic lengths by division by a "morphoelectric factor" ([Rm/Ri]1/2). This procedure has the advantage of separating the steps involving anatomical and electrical parameters. These transformations indicated that the dendrites of the apical terminal arbor were much longer electrically than the basal or apical oblique dendrites. In relative electrical terms, most apical oblique trees arose extremely close to the soma, and terminated at similar distances to the basals. These results indicate that the dendrites of these pyramidal cells cannot be represented as single equivalent cylinders. The electrotonic lengths of the dendrites were calculated by using the electrical parameters specific membrane capacitance (Cm), intracellular resistivity (Ri), and specific membrane resistivity (Rm). Conventional values were assumed for Cm (1.0 muFcm-2) and Ri (100 omega cm), but three different Rm values were used for each cell. Two of these were within the conventionally accepted range (10,000-20,000 omega cm2), while the third value was an order of magnitude higher, in line with some recent evidence from modeling and whole-cell recording studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Dendritic morphology of pyramidal neurones of the visual cortex of the rat: I. Branching patterns.

The aim of this study was to provide quantitative descriptions of the branching patterns of basal and apical dendrites of pyramidal neurones from the visual cortex of the rat. Thirty-nine neurones from cortical layers 2/3 and 5, that had been injected with horseradish peroxidase, reconstructed, and measured with the light microscope as part of an earlier study (Larkman and Mason, '90; J. Neurosci. 10:1407-1414), were used. The cells had previously been divided into three classes, layer 2/3 cells and thick and slender layer 5 cells, on the basis of their dendritic morphology. The branching pattern of the basal and apical oblique dendrites was similar for all the cells. Between 3 and 9 basal trees arose from the soma and the number of tips in each tree varied widely, between 1 and 13. The path lengths of all the basal dendrites of a given cell were relatively constant, however. Most basal dendritic branching occurred close to the soma, such that terminal segments were much longer than intermediate segments and contributed approximately 90% of the total dendritic length of each tree. Terminal segments showed only a narrow range of diameters. Most apical oblique trees arose from the proximal part of the apical trunk. They tended to be less highly branched but were otherwise extremely similar to basal trees. Distal oblique trees were unbranched or branched only once, and their terminal segments tended to be shorter and thinner than those of basal trees. The branching pattern of the apical terminal arbors was different, with many longer intermediate segments. The terminal segments tended to be thinner than those of basal or proximal oblique trees. Slender layer 5 cells were without obvious terminal arbors. The basal and proximal oblique dendrites jointly sampled a roughly spherical volume of cortex centred about the soma, and together they accounted for the substantial majority of the cell's total dendritic shaft membrane area. Comparisons with previous studies suggest that intracellular HRP injection can yield a more complete visualization of dendritic morphology than is obtained using Golgi-based methods (unless cells are reconstructed across tissue slabs), and can therefore result in a different view of the relative importance of the various components that make up the cell's dendritic system.

Animals↗
PubMed1991

Dendritic morphology of pyramidal neurones of the visual cortex of the rat: II. Parameter correlations.

This study concerns the correlations between the various morphometric parameters obtained for the dendrites of neocortical pyramidal cells. The primary aims were to uncover underlying design principles in dendritic morphology, to see if these differed between different types of dendrite, and to see if estimates of parameters such as total dendritic shaft membrane area could be obtained from a limited number of measurements, avoiding the need to measure every dendritic segment. The data were from a sample of 39 pyramidal neurones, from layers 2/3 and 5 of the visual cortex of the rat, that had been injected with horseradish peroxidase, reconstructed, and measured with the light microscope as part of an earlier study (Larkman and Mason, '90: J. Neurosci. 10:1407-1414). Correlations between the somal area or the combined diameters of the stem segments and measures of the overall size of the dendrites were generally weak. For basal dendrites, the size of a tree was correlated with both its number of tips and the diameter of its stem segment, but these correlations were weaker for apical dendrites. Within individual cells, the diameter of any basal segment was closely related to the size of the tree arising from it, and quantitatively similar relations applied to apical oblique trees from the same cell. Terminal arbor trees showed relations that were similar in pattern but differed quantitatively, whereas apical trunk segment diameter correlations were often weak. In all cases, the number of tips in a tree was closely related to its size. Segment lengths, however, were not closely related to the size of the trees arising from them. It appears that at least some aspects of pyramidal dendritic morphology obey simple design rules. There was heterogeneity between trees of different types, although basal and oblique trees were very similar in most respects. It should prove possible to make use of correlations to estimate the sizes of basal, oblique, and terminal arbor trees from a limited number of measurements, but this does not seem to be possible for apical trunks.

Animals↗
PubMed1991

Dendritic morphology of pyramidal neurones of the visual cortex of the rat: III. Spine distributions.

The vast majority of excitatory synaptic inputs to neocortical pyramidal cells terminate on dendritic spines, which can thus serve as markers, visible by light microscopy, for the locations of these synapses. The aim of this study was to provide estimates of the total numbers and distributions of spines on the dendrites of individual pyramidal neurones from layers 2/3 and 5 of the visual cortex of the rat. High magnification camera lucida drawings were made of dendritic segments lying close to the plane of section and the number of spines per unit length of dendrite calculated for each. These spine densities were used to estimate the numbers of spines on the other dendritic segments and the results were entered to a computer program that calculated various statistics. Mean total numbers of spines per cell were 7,965 +/- 2,723 (S.D.) for layer 2/3 cells, 8,647 +/- 3,097 for slender layer 5 cells, and 14,932 +/- 3,371 for thick layer 5 cells; these figures are in good agreement with previous stereological estimates. For all cell classes, 70% or more of spines were located on the basal and apical oblique dendrites. The distribution of spines with respect to cortical layers was also explored. Most cells had most of their spines in the layer containing the soma, but there were differences within and between cell classes. Layer 2/3 cells showed a progressive reduction in the proportion of their spines in layers 1 and 2 with increasing depth of their soma in the cortex. Thick layer 5 cells had substantial contributions from layers 4, 3, 2, and especially layer 1. Slender layer 5 cells had small contributions from layers 6 and 4, but relatively few spines in layers 3 and 2. The distribution of spines with path distance from the soma was explored by estimating the numbers of spines contained within a series of concentric shells centred on the soma. All cells showed a rapid increase in the number of spines per shell for the proximal 100 micrograms or so, followed by a sharp decline to approximately 250 micrograms, beyond which the number remained relatively constant until the end of the terminal arbor. In each case, the majority of spines were located within a path length of 150 micrograms from the soma.

Animals↗