Total Dendritic Area Research Articles

Dynamic Range of Vertebrate Retina Ganglion Cells: Importance of Active Dendrites and Coupling by Electrical Synapses

The vertebrate retina has a very high dynamic range. This is due to the concerted action of its diverse cell types. Ganglion cells, which are the output cells of the retina, have to preserve this high dynamic range to convey it to higher brain areas. Experimental evidence shows that the firing response of ganglion cells is strongly correlated with their total dendritic area and only weakly correlated with their dendritic branching complexity. On the other hand, theoretical studies with simple neuron models claim that active and large dendritic trees enhance the dynamic range of single neurons. Theoretical models also claim that electrical coupling between ganglion cells via gap junctions enhances their collective dynamic range. In this work we use morphologically reconstructed multi-compartmental ganglion cell models to perform two studies. In the first study we investigate the relationship between single ganglion cell dynamic range and number of dendritic branches/total dendritic area for both active and passive dendrites. Our results support the claim that large and active dendrites enhance the dynamic range of a single ganglion cell and show that total dendritic area has stronger correlation with dynamic range than with number of dendritic branches. In the second study we investigate the dynamic range of a square array of ganglion cells with passive or active dendritic trees coupled with each other via dendrodendritic gap junctions. Our results suggest that electrical coupling between active dendritic trees enhances the dynamic range of the ganglion cell array in comparison with both the uncoupled case and the coupled case with cells with passive dendrites. The results from our detailed computational modeling studies suggest that the key properties of the ganglion cells that endow them with a large dynamic range are large and active dendritic trees and electrical coupling via gap junctions.

Dichotomous Anatomical Properties of Adult Striatal Medium Spiny Neurons

Principal medium spiny projection neurons (MSNs) of the striatum have long been thought to be homogeneous in their somatodendritic morphology and physiology. Recent work using transgenic mice, in which the two major classes of MSN are labeled, has challenged this assumption. To explore the basis for this difference, D(1) and D(2) receptor-expressing MSNs (D(1) and D(2) MSNs) in brain slices from adult transgenic mice were characterized electrophysiologically and anatomically. These studies revealed that D(1) MSNs were less excitable than D(2) MSNs over a broad range of developmental time points. Although M(1) muscarinic receptor signaling was a factor, it was not sufficient to explain the dichotomy between D(1) and D(2) MSNs. Reconstructions of biocytin-filled MSNs revealed that the physiological divergence was paralleled by a divergence in total dendritic area. Experimentally grounded simulations suggested that the dichotomy in MSN dendritic area was a major contributor to the dichotomy in electrophysiological properties. Thus, rather than being an intrinsically homogenous population, striatal MSNs have dichotomous somatodendritic properties that mirror differences in their network connections and biochemistry.

Quantitative analysis of the dendritic architectures of cat hypoglossal motoneurons stained intracellularly with horseradish peroxidase

Little is known about the dendritic architecture of cat hypoglossal motoneurons. Thus, the present study was done to provide quantitative descriptions of hypoglossal motoneurons and to determine correlations between dendritic size parameters by using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Twelve hypoglossal motoneurons stained with HRP were antidromically activated by stimulation applied to the medial branch of hypoglossal nerve. Eight (type I) and four (type II) of the 12 motoneurons were located in the ventral and dorsal parts of the ventromedial subnucleus of hypoglossal nucleus, respectively. The somatodendritic morphology of the two types of neurons was remarkably different, especially in the dendritic arborization pattern. The type I neurons established an egg-shaped dendritic tree that was distributed through the nucleus to the reticular formation; the type II dendritic tree was confined within the nucleus and presented a rostrocaudally oriented, mirror-image, fan-shape appearance. The total dendritic area and length and the number of terminations and branch points were significantly larger for type I than for type II neurons. For the two types of neuron, there was a positive correlation between stem dendritic diameter and several dendritic size parameters. Although the slopes of the regression lines were slightly different between the two, these were not statistically significant. The present study provides evidence that hypoglossal motoneurons located in the ventromedial subnucleus could be divided into two types according to the dendritic arborization pattern and quantitative analysis of the dendritic tree and according to neuronal location and suggests that the two types of hypoglossal motoneurons can be viewed as intrinsically distinct cell types: type I and type II, which innervate extrinsic and intrinsic muscles, respectively. In addition, the morphometric analysis made it possible to estimate the size of the dendritic tree by measuring the stem dendritic diameter. J. Comp. Neurol. 405:345–358, 1999. © 1999 Wiley-Liss, Inc.

Quantitative analysis of the dendritic architectures of cat hypoglossal motoneurons stained intracellularly with horseradish peroxidase

Little is known about the dendritic architecture of cat hypoglossal motoneurons. Thus, the present study was done to provide quantitative descriptions of hypoglossal motoneurons and to determine correlations between dendritic size parameters by using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Twelve hypoglossal motoneurons stained with HRP were antidromically activated by stimulation applied to the medial branch of hypoglossal nerve. Eight (type I) and four (type II) of the 12 motoneurons were located in the ventral and dorsal parts of the ventromedial subnucleus of hypoglossal nucleus, respectively. The somatodendritic morphology of the two types of neurons was remarkably different, especially in the dendritic arborization pattern. The type I neurons established an egg-shaped dendritic tree that was distributed through the nucleus to the reticular formation; the type II dendritic tree was confined within the nucleus and presented a rostrocaudally oriented, mirror-image, fan-shape appearance. The total dendritic area and length and the number of terminations and branch points were significantly larger for type I than for type II neurons. For the two types of neuron, there was a positive correlation between stem dendritic diameter and several dendritic size parameters. Although the slopes of the regression lines were slightly different between the two, these were not statistically significant. The present study provides evidence that hypoglossal motoneurons located in the ventromedial subnucleus could be divided into two types according to the dendritic arborization pattern and quantitative analysis of the dendritic tree and according to neuronal location and suggests that the two types of hypoglossal motoneurons can be viewed as intrinsically distinct cell types: type I and type II, which innervate extrinsic and intrinsic muscles, respectively. In addition, the morphometric analysis made it possible to estimate the size of the dendritic tree by measuring the stem dendritic diameter.

Quantitative morphology of physiologically identified and intracellularly labeled neurons from the guinea-pig laterodorsal tegmental nucleus in vitro

Mesopontine cholinergic neurons have been implicated in the initiation and maintenance of rapid eye movement sleep via their efferent connections to the thalamus and the medial pontine reticular formation. As a first step toward understanding how these modulatory neurons integrate synaptic input, we have investigated the dendritic architecture of laterodorsal tegmental nucleus neurons. The principal cells of the guinea-pig laterodorsal tegmental nucleus were identified electrophysiologically in a brain slice preparation, then were intracellularly injected with biocytin and reconstructed using a computer-aided tracing system. The somata were large (27 ± 3 μm; n = 11) and gave rise to an average of 4.8 primary dendrites which, in most cases, emerged from the soma in a pattern that was radially symmetric in the plane of the slice. Primary dendrites had an average of 3.7 endings. A single axon arose from either the soma or a proximal dendrite and exited the nucleus with a medial and/or lateral trajectory. Some axons also gave rise to a local terminal plexus composed of fine fibers bearing numerous punctate swellings that ramified profusely within the neuron's dendritic field. Total dendritic area averaged about 10 5 μm 2, and therefore the average contribution of the soma to the total surface area (20%) was significantly larger than the values reported for many other cell types. Dendritic diameters were non-uniform in three respects. Some processes were sparsely spiny. Most processes were varicose, with the degree of varicosity increasing substantially in secondary and tertiary dendritic segments. There was also a large degree of taper in dendritic processes; those processes with a non-negative taper had an average diameter decrease of 40 ± 25%. Dendritic processes deviated from the criteria necessary for a Rall equivalent cylinder approximation due to non-uniformity in morphotonic path length, failure to conform to the Rall 3/2 branching rule and non-uniformity of dendritic diameter. An analysis was done to assess the impact of dendritic varicosities on the extraction of cable parameters for these cells. Voltage traces were simulated by solving the cable equation for a varicose dendrite and then membrane parameters were recovered using an equivalent cylinder model. Errors in the extracted values of specific membrane conductance and specific membrane capacitance were quite small (≤5%), while larger errors were seen for electrotonic length (≤21%) and intracellular resistivity (≤50%). These data indicate that the principal cells of the laterodorsal tegmental nucleus, while possessing a relatively simple dendritic structure in terms of number and branchiness of dendrites, display a heterogeneity of dendritic process types. Processes range from smooth to markedly varicose, and can be aspiny or sparsely spiny. The possibility that the dendritic varicosities function as sites of either electrical or chemical compartmentalization is discussed. The degree of error resulting from a Rall equivalent cylinder approximation in light of these varicosities indicated that a generalized cable model approach may prove more effective in estimating their cable parameters.

Size and remoteness: two relatively independent parameters of dendrites, as studied for spinal motoneurones of the cat.

1. The spatial extent of motoneuronal dendrites was analysed using data from the fifty-two dendritic trees of four completely reconstructed cat motoneurones that had been labelled with intracellularly injected horseradish peroxidase. The cells belonged to m. triceps surae, and their physiological properties covered much of the known range for this muscle. 2. The percentage of total dendritic area extending beyond a radial somatofugal distance of 750 microns (PA750) was selected as an index of 'dendritic remoteness'. 3. The value of our standard remoteness index PA750 was well correlated with a number of other possible remoteness measures, such as the percentage of total dendritic area beyond 500 or 1000 microns or the percentage of cumulative dendritic length beyond 750 or 1000 microns. 4. Most of the dendritic parameters could be classified as belonging either to a cluster of remoteness-related properties or to a cluster of size-related properties. Remoteness-related properties were significantly correlated with PA750 but not with stem diameter. Size-related properties were significantly correlated with stem diameter but not with the remoteness index PA750. The remoteness-related properties included the mean somatofugal distance to branch points, the mean and maximum distance to terminal ending and, somewhat less distinctly, the mean length of terminal branches. The size-related properties included the total dendritic area, the cumulative dendritic length, and the total number of branch points and terminal endings. 5. The extent of area expansion at branch points and the overall extent of tapering were not systematically related to differences in remoteness. 6. The relative degree of remoteness was greater for medial than for lateral dendrites. 7. Even in the present limited sample of cells, there were marked and systematic differences between different neurones with respect to the relative remoteness of their dendrites. The slowest and most high-resistance motoneurone was the one with the most remote dendrites. On the whole there was, however, a considerable degree of independence between the variation in dendritic remoteness and the variation in physiologically determined properties (axonal conduction velocity, input resistance, after-hyperpolarization). 8. In the discussion it is suggested that differences in dendritic remoteness might lead to a selectivity between the various members of a given motoneurone pool with respect to their reception of topographically distributed systems of synapses. It is proposed that such differences might be of interest in relation to, for instance, the organization of innervation to motoneurones belonging to different task groups within the same muscle.