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.     

Initiation and propagation of action potentials in gonadotropin-releasing hormone neuron dendrites

Gonadotropin-releasing hormone (GnRH) neurons are the final output neurons in a complex neuronal network that regulates fertility. The morphology of GnRH neuron dendrites is very different to other central neurons in that they are very long, thin, and unbranched. To study the function of these dendrites, we have used Na(+) and Ca(2+) imaging in combination with dual soma and dendrite electrical recordings in brain slices from GnRH-GFP mice. Here, we show that GnRH neurons actively propagate Na(+) spikes throughout their dendrites. Multisite dendritic recordings confirmed that these spikes were observed in one of the dendrites before the soma in the great majority of neurons tested. Na(+) imaging experiments revealed that the initial 150 μm of dendrite has a higher density of functional Na(+) channels than more distal regions, suggesting that this region of dendrite is highly excitable and may be the site of spike initiation. Finally, we show that the depolarization from dendritic spikes opens voltage-gated Ca(2+) channels giving rise to dendritic Ca(2+) transients. Together, these findings suggest that the proximal dendrites of GnRH neurons are highly excitable and are likely to be the site of action potential initiation in these neurons.     

Activity-dependent reconfiguration of the effective dendritic field of motoneurons

A neuron in vivo receives a continuous bombardment of synaptic inputs that modify the integrative properties of dendritic arborizations by changing the specific membrane resistance (R(m)). To address the mechanisms by which the synaptic background activity transforms the charge transfer effectiveness (T(x)) of a dendritic arborization, the authors simulated a neuron at rest and a highly excited neuron. After in vivo identification of the motoneurons recorded and stained intracellularly, the motoneuron arborizations were reconstructed at high spatial resolution. The neuronal model was constrained by the geometric data describing the numerized arborization. The electrotonic structure and T(x) were computed under different R(m) values to mimic a highly excited neuron (1 kOhm x cm(2)) and a neuron at rest (100 kOhm x cm(2)). The authors found that the shape and the size of the effective dendritic fields varied in the function of R(m). In the highly excited neuron, the effective dendritic field was reduced spatially by switching off most of the distal dendritic branches, which were disconnected functionally from the somata. At rest, the entire dendritic field was highly efficient in transferring current to the somata, but there was a lack of spatial discrimination. Because the large motoneurons are more sensitive to variations in the upper range of R(m), they switch off their distal dendrites before the small motoneurons. Thus, the same anatomic structure that shrinks or expands according to the background synaptic activity can select the types of its synaptic inputs. The results of this study demonstrate that these reconfigurations of the effective dendritic field of the motoneurons are activity-dependent and geometry-dependent.     

Quantitative synaptology of functionally different types of cat medial gastrocnemius α-motoneurons

The aim of this ultrastructural investigation was to study quantitatively the synaptology of the cell bodies and dendrites of cat medial gastrocnemius (MG) α-motoneurons of functionally different types. In electrophysiologically classified and intracellularly HRP-labelled MG α-motoneurons of the FF (fast twitch, fatigable), FR (fast twitch, fatigue resistant) and S (slow twitch, very fatigue resistant) types, the synaptic covering of the soma as well as that of dendritic segments located within 100 μm and at 300, 700, and 1,000 μm distance, respectively from the soma, was analyzed. The synaptic boutons were classified into the L-(apposition length > 4 μm) and S-types (<4 μm) with spherical synaptic vesicles, and the F-type with flat or pleomorphic synaptic vesicles. The length of apposition towards the motoneuron membrane was measured for each bouton profile. Approximately 1,000 boutons contacted the soma and a similar number of boutons contacted the proximal dendrites within 50 μm from the soma. The number of dendritic boutons was larger at the 300 μm distance than at the 100 and 700 μm distances. The three types of motoneurons showed similar values for percentage synaptic covering and synaptic packing density in the proximal dendrites, while in the most distal dendritic regions the S motoneurons had more than 50% higher values for percentage covering, packing density and total number of boutons. The S motoneurons also exhibited a larger preponderance of F-type boutons on the soma. The ratio between the F- and S-types of boutons decreased somatofugally along the dendrites in the type FF and FR motoneurons, while in the S motoneurons it remained fairly constant.     

Theoretical analysis of the spike activity of a neuron with an N-shape stationary volt-ampere characteristic of the dendritic membrane

Neuronal discharge frequency as a function of depolarizing current strength was analyzed using a mathematical model. The dendritic membrane of the model neuron was characterized by an N-shaped stationary current-voltage characteristic which is now confirmed experimentally. The potential of distal dendrites with N-shaped current voltage characteristic changes from rest to stable depolarization as current rises. This change causes discharge frequency augmentation. Two smooth ranges of frequency-current characteristics are explained as follows: in the first low frequency-low current range all dendrites are at rest, and in the second high frequency-strong current range there appears stable depolarization. If stable depolarization of dendrites is a synchronous event, then there is a frequency jump between the ranges and if dendrites depolarize steadily one-by-one, then there is only a nonsmooth point. The steady depolarization causes hysteresis of the frequency-current characteristic, including spontaneous discharge after cessation of strong depolarization.     

Dendritic integration in a recurrent network

Classical nonlinear cable theory is appropriate for the unmyelinated axonal membrane because voltage-dependent ion channels are densly distributed, but dendrites with a sparse density distribution of voltage-dependent ion channels show "weakly" excitable membrane properties. Therefore, a model for "weakly" active dendrites is presented by introducing voltage-dependent ion channels at discrete locations along the dendritic cable. This provides an alternative representation for the investigation of regenerative potentials in dendrites in order to explore how active dendrites influence synaptic integration. As an example, we consider a two-neuron recurrent network of biophysically distinct conductance-based model neurons with discrete clusters of persistent sodium channels. Analytical solutions, expressed in terms of a Volterra series expansion for the voltage in response to a suprathreshold input current at the soma of one neuron, are obtained to investigate dendritic spikes, and the effect of backpropagation on distal dendritic spike-like potentials.     

Influence of the hyperpolarization-activated cation current, I(h), on the electrotonic properties of the distal apical dendrites of hippocampal CA1 pyramidal neurones

The electrical field application technique has revealed that the electrotonic length of the distal apical dendrites of hippocampal CA1 pyramidal neurones is long compared to the rest of the cell. This difference may be due to an asymmetrical distribution of channels responsible for the leak conductance in distal and proximal membrane segments. One such conductance, the hyperpolarization-activated cation current, I_h, is reported to display an increasing density with distance from the soma along the apical dendrite. Such asymmetry of I_h could be a major cause of the increased electrotonic length of the distal apical dendrite. In the present study we found that blockade of I_h, by bath application of Cs⁺ (3 mM) or ZD7288 (20 μM), reduced the electrical field-induced transmembrane polarization (TMP) in the distal apical dendrites. In some neurones the polarization reversed polarity, reflecting a movement of the indifference point (site of zero polarization) from the distal dendrites, across the recording site to a more proximal position. These effects were more pronounced when Cs⁺ and ZD7288 were applied locally to the distal apical dendrites. Bath application of another antagonist of leak conductance, Ba²⁺ (1 mM), also decreased the average field-induced polarization. This latter effect, however, did not reach statistical significance. These data suggest that I_h is partly responsible for the distal location of the indifference point, and indicate that an elevated activity of I_h contributes to the relatively increased electrotonic length of the most distal part of the apical dendrites.     

Morphology and frequency of axon terminals on the somata, proximal dendrites, and distal dendrites of dorsal neck motoneurons in the cat.

The purpose of the present study was to compare the frequency of different classes of axon terminals on selected regions of the somatodendritic surface of dorsal neck motoneurons. Single motoneurons supplying neck extensor muscles were antidromically identified and intracellularly stained with horseradish peroxidase. By using light microscopic reconstructions as a guide, axon terminals on the somata, proximal dendrites (within 250 microns of the soma), and distal dendrites (more than 540 microns from the soma) were examined at the electron microscopic level. Axon terminals were divided into several classes based on the shape, density, and distribution of their synaptic vesicles. The proportion of axon terminals belonging to each axon terminal class was similar on the somata and proximal dendrites. However, there were major shifts in the relative frequency of most classes of axon terminals on the distal dendrites. The most common classes of axon terminals on the somata and proximal dendrites contained clumps of either spherical or pleomorphic vesicles. These types of axon terminals accounted for more than 60% of the axon terminals on these regions. In contrast, only 11% of the axon terminals found on distal dendrites belonged to these types of axon terminals. The most commonly encountered axon terminal on distal dendrites contained a dense collection of uniformly distributed spherical vesicles. These types of axon terminals accounted for 40% of all terminals on the distal dendrites, but only 5-7% of the axon terminals on the somata and proximal dendrites. Total synaptic density on each of the three regions examined was similar. However, the percentage of membrane in contract with axon terminals was approximately four times smaller on distal dendrites than somata or proximal dendrites. Axon terminals (regardless of type) were usually larger on somata and proximal dendrites than distal dendrites. These results indicate that there are major differences in the types and arrangement of axon terminals on the proximal and distal regions of dorsal neck motoneurons and suggest that afferents from different sources may preferentially contact proximal or distal regions of the dendritic trees of these cells.     

Electrotonic parameters of cat spinal alpha-motoneurons evaluated with an equivalent cylinder model that incorporates non-uniform membrane resistivity

The membrane voltage transients in response to constant intracellular current steps were analyzed in cat spinal alpha-motoneurons, using an equivalent cylinder model extended to include spatial non-uniformity in membrane resistivity. The main hypothesis was that the soma membrane resistivity did not differ significantly from the dendritic membrane resistivity. Evidence was found that the uniform-resistance cylinder model cannot be applied to motoneurons on the basis of inconsistencies in calculations of electrotonic length. Assuming a constant membrane capacitance, the membrane resistivity of the dendrites was found to be 500 times that of the soma. We conclude that there is a considerable difference in the membrane resistivity between the soma and dendrites of motoneurons, and that uniform-resistance models should not be applied to spinal alpha-motoneurons.     

Calcium-activated potassium conductances contribute to action potential repolarization at the soma but not the dendrites of hippocampal CA1 pyramidal neurons.

Evidence is accumulating that voltage-gated channels are distributed nonuniformly throughout neurons and that this nonuniformity underlies regional differences in excitability within the single neuron. Previous reports have shown that Ca2+, Na+, A-type K+, and hyperpolarization-activated, mixed cation conductances have varying distributions in hippocampal CA1 pyramidal neurons, with significantly different densities in the apical dendrites compared with the soma. Another important channel mediates the large-conductance Ca2+-activated K+ current (IC), which is responsible in part for repolarization of the action potential (AP) and generation of the afterhyperpolarization that follows the AP recorded at the soma. We have investigated whether this current is activated by APs retrogradely propagating in the dendrites of hippocampal pyramidal neurons using whole-cell dendritic patch-clamp recording techniques. We found no IC activation by back-propagating APs in distal dendritic recordings. Dendritic APs activated IC only in the proximal dendrites, and this activation decayed within the first 100-150 micrometer of distance from the soma. The decay of IC in the proximal dendrites occurred despite AP amplitude, plus presumably AP-induced Ca2+ influx, that was comparable with that at the soma. Thus we conclude that IC activation by action potentials is nonuniform in the hippocampal pyramidal neuron, which may represent a further example of regional differences in neuronal excitability that are determined by the nonuniform distribution of voltage-gated channels in dendrites.     

Synaptic control of excitability in isolated dendrites of hippocampal neurons

The apical dendrites of CA1 pyramidal cells were isolated from their cell bodies by making cuts through proximal stratum radiatum of transverse hippocampal slices from the guinea pig. This lesion separated the distal apical dendritic elements from the somata, basal dendrites, and 50 to 100 microns of the proximal apical dendritic tree. Orthodromic stimuli in stratum radiatum evoked excitatory synaptic responses in isolated dendrites, but no phasic inhibitory components could be detected. In spite of this surgically produced disinhibition, orthodromic stimuli did not elicit burst activity at the resting membrane potential. However, isolated dendrites and intact dendrites could generate multiple slow spike activity when directly stimulated with depolarizing current pulses. When isolated dendrites were depolarized by DC current, excitatory postsynaptic potentials could evoke subthreshold intrinsic slow depolarizations, or repetitive slow spikes, similar to responses elicited by depolarizing current pulses alone. After exposure to bicuculline (5 microns), both intact and isolated dendrites generated bursts of activity following synaptic activation. A possible mechanism for this action of bicuculline is blockade of a residual GABA-mediated inhibition which was not expressed as a postsynaptic hyperpolarization in isolated dendrites. This bicuculline-sensitive event was capable of depressing dendritic excitability in the absence of the recurrent inhibitory synaptic input and was very effective in controlling burst activity. Our results indicate that the dendritic electrical behavior is dependent on a complex interaction between synaptic and voltage-sensitive events.     

Three-dimensional architecture of dendritic trees in type-identified alpha-motoneurons

We have studied the spatial distribution of dendrites of type-identified triceps surae alpha-motoneurons, labeled intracellularly with HRP, using a variety of analytical approaches that were designed to quantify the ways in which dendrites occupy three-dimensional space. All of the methods indicated a strong tendency for motoneuron dendrites to project radially. However, regions dorsal and ventral to the somata contained fewer dendritic elements, and less membrane area, than expected for complete radial symmetry. Individual dendrites projecting into these regions tended to be smaller than those projecting rostrocaudally or mediolaterally. Nevertheless, the center of mass of membrane area for five of six fully analyzed cells was within 100 micron of the soma and, in all six cells, was located in the same dorsoventral plane as the cell soma. Maps of the projection of dendritic branches onto concentric shells at various radial distances from the soma showed that some regions have high concentrations of branches, sometimes with considerable overlap between branches arising from different stem dendrites, while other regions have relatively few branches, or none at all. Each motoneuron exhibited a different pattern of projection and there were no systematic differences between fast-twitch (type F, including both types FF and FR units) and slow-twitch (type S) motoneurons evident in the patterns of dendritic concentration. Assessment of the three-dimensional territories of individual dendrites showed that dendrites with larger numbers of terminal branches tended to have larger spatial territories. Despite considerable scatter, the results suggest that the density of branches tends to be approximately the same in large and small dendrites, and in F and S cell groups. The results are discussed in relation to the spatial location of synaptic input to motoneurons.     

Morphology of retinal ganglion cells in lungless salamanders (fam. Plethodontidae): an HRP and Golgi study

The family Plethodontidae consists of nearly two-thirds of all living urodeles; most of them possess highly developed visual abilities. We investigated the morphology of retinal ganglion cells (RGCs) in four representative species by means of the horseradish peroxidase method in flatmounts and in transverse sections and with the Golgi method in transverse sections. In flatmount preparations, four classes of RGCs were found, differing in dendritic arborization, dendritic field size, and stratification pattern of dendrites in the inner plexiform layer (IPL). Class-1 cells had small dendritic fields (29-44 microns 2) and arborized throughout the entire depth of the IPL. Class-2 cells had medium to large dendritic fields (75-206 microns 2) and mostly arborized in two or three laminae or in a diffuse fashion in the IPL. Class-3 cells had medium to large dendritic fields (72-200 microns 2) but sparse dendritic arborization. They only arborized in the proximal lamina of the IPL. Class-4 cells had large dendritic fields (273-626 microns 2) and branched in the most sclerad stratum of the IPL. No large differences in intraspecific soma size of the different RGC classes were detected (although interspecific soma size varied to a considerable degree) and no "giant" cells typically found in other vertebrate retinas were present. The results suggest that, with respect to the pattern of arborization and stratification of dendrites, lungless salamanders possess morphological classes of RGC similar to those found in frogs, but the morphology of RGCs in lungless salamanders seems to be simplified in comparison to frog RGCs. This simplification might be a consequence of paedomorphosis.     

NMDA-receptors on Purkinje cell dendrites in guinea pig cerebellar slices

The Mg2+-dependent depolarizing action of N-methyl-D-aspartate (NMDA) was intrasomatically investigated, in comparison with quisqualate (QA), in Purkinje cells in cerebellar slices from adult guinea pigs. NMDA applied iontophoretically to the proximal dendritic region (about 100 micron from the Purkinje cell soma) induced depolarizations and spike firings in about the half of the Purkinje cells tested in nominal Mg2+-free medium (contaminated with 4-11 microM Mg2+), and 1 mM Mg2+ almost completely blocked this NMDA action. Application of NMDA onto the distal dendritic region (about 200 micron from the soma) caused no depolarization at all even in the Mg2+-free medium. QA applied onto either the proximal or distal dendritic region consistently showed Mg2+-independent depolarizations. The amplitude of NMDA-induced depolarization in the Mg2+-free medium was non-linearly related to the membrane potential, i.e. smaller at a hyperpolarized potential level. 2-Amino-5-phosphonovalerate blocked the NMDA action partially but more selectively than the QA action, while the reverse was the case for glutamic acid diethylester. These results suggest that the Mg2+-dependent, NMDA-sensitive receptor, which is distinct from the QA receptor and probably similar to the well-known NMDA receptor, is present on the proximal dendrite of the cerebellar Purkinje cell of the guinea pig.     

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice.

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Distribution of 5-hydroxytryptamine-immunoreactive boutons on α-motoneurons in the lumbar spinal cord of adult cats

Recent studies have shown that at least some of the functional effects of serotonin (5-HT) on motoneuron excitability are direct and are mediated via postsynaptic 5-HT receptors on motoneurons. To determine the spatial distribution of direct inputs from the serotonin system on the proximal and distal dendrites of individual motoneurons, we examined identified motoneurons in vivo with a combination of immunohistochemical localization of 5-HT-immunoreactive boutons and intracellular staining with horseradish peroxidase. Seventeen intracellularly stained motoneurons from 12 adult cats were analyzed with light microscopy. Quantitative analysis of 5-HT boutons apposed to dendrites of five representative motoneurons that were entirely reconstructed in three dimensions (each from the lumbosacral spinal cord of a different animal) revealed a total of 7,848 contacts (1,570 ± 487 contacts/postsynaptic neuron; mean ± SD) over the dendrites of these cells. Analysis of contacts on the soma of two of these cells, and on the somas of an additional 12 intracellularly stained motoneurons, revealed a wide range of somatic contacts (11–211 contacts/cell) on motoneuron cell bodies, with an average of 52 contacts/cell. These results indicate that the vast majority of 5-HT-immunoreactive boutons are apposed to dendritic branches rather than to the somatic surface of motoneurons. The spatial distribution of contacts essentially matched the distribution of surface membrane area of the postsynaptic neuron, resulting in a relatively uniform density of contacts (<1/100 μm²) on proximal and distal dendrites. Consequently, the frequency of contacts was higher on the proximal dendritic compartments where available membrane area is greater. There was no preferential distribution of contacts to particular dendrites. Light/electron microscopic correlations were performed on 21 boutons that contacted dendrites (n = 7) of three motoneurons from different animals. At the electron microscope level, most appositions (18/21; 85.7%) selected by our light microscopic criteria were confirmed as direct contacts when the 5-HT boutons were examined through serial sections. Synaptic junctions, generally small and symmetric, were positively identified in only a subset of these cases (n = 6; 28.6%), in part due to the obscuring effects of the peroxidase histochemical precipitate present in both pre- and postsynaptic profiles. A few 5-HT boutons (3/21; 14.3%) selected as contacts by our light microscopic criteria were in fact separated from the adjacent labeled dendrites; in two of these three cases, the separation was due to intrusion of very thin glial lamellae (<0.3 μm in cross section). These results indicate that the bulbospinal serotonergic system(s) provide a significant, direct synaptic input to spinal motoneurons that innervate hindlimb muscles. The nature of the modulatory actions exerted by such widespread synaptic inputs will affect all regions ofthe somatodendritic membrane and will ultimately depend on the nature of the 5-HT receptors present over different parts of the postsynaptic neuron's dendritic tree. J. Comp. Neurol. 393:69–83, 1998. © 1998 Wiley-Liss, Inc.     

Mitochondrial fission protein Drp1 regulates mitochondrial transport and dendritic arborization in cerebellar Purkinje cells

Mitochondria dynamically change their shape by repeated fission and fusion in response to physiological and pathological conditions. Recent studies have uncovered significant roles of mitochondrial fission and fusion in neuronal functions, such as neurotransmission and spine formation. However, the contribution of mitochondrial fission to the development of dendrites remains controversial. We analyzed the function of the mitochondrial fission GTPase Drp1 in dendritic arborization in cerebellar Purkinje cells. Overexpression of a dominant-negative mutant of Drp1 in postmitotic Purkinje cells enlarged and clustered mitochondria, which failed to exit from the soma into the dendrites. The emerging dendrites lacking mitochondrial transport remained short and unstable in culture and in vivo. The dominant-negative Drp1 affected neither the basal respiratory function of mitochondria nor the survival of Purkinje cells. Enhanced ATP supply by creatine treatment, but not reduced ROS production by antioxidant treatment, restored the hypomorphic dendrites caused by inhibition of Drp1 function. Collectively, our results suggest that Drp1 is required for dendritic distribution of mitochondria and thereby regulates energy supply in growing dendritic branches in developing Purkinje cells.     

Heterogeneity in low voltage-activated Ca2+ channel-evoked Ca2+ responses within neurons of the thalamic paraventricular nucleus

Low voltage-activated Ca2+ channels (LVA or T-type Ca2+ channels) are crucial to burst firing and oscillations in thalamocortical relay cells and are exhibited by neurons in the paraventricular nucleus of thalamus (PVT), a dorsal midline nucleus deemed important in the neural representation of motivational behaviours. We used a functional approach (whole-cell patch-clamp electrophysiology combined with confocal laser scanning microscopy) to analyse the spatial distribution of LVA Ca2+ channel-evoked Ca2+ transients in PVT neurons. We observed that the magnitude of LVA Ca2+ channel-evoked Ca2+ transients was significantly greater in proximal dendrites (located up to 20 microm from the soma) than in the soma. In addition, the magnitudes of these Ca2+ transients varied significantly not only among different dendrites of the same cell but also within individual dendrites. These findings suggest that LVA Ca2+ channels are expressed (i) predominantly on the proximal dendrites and (ii) heterogeneously within individual dendrites of PVT neurons. The spatial characteristics of dendritic LVA Ca2+ channels in PVT neurons suggest that these channels may regulate burst firing by modulating dendritic afferent inputs.     

Structural and functional changes in an identified cricket neuron after separation from the soma. II. Functional changes.

Physiological and behavioural effects of separation from the soma were examined in isolated arborization and isolated axon segments of an identified motor neuron in the Polynesian field cricket, Teleogryllus oceanicus. The identified neuron, the contralateral dorsal longitudinal motor neuron of the metathoracic ganglion (CDLM), has an arborization most of which lies contralateral to its soma within the ganglion. Midline lesions in the ganglion separated CDLM into a distal segment composed of the axon and most of the arborization, and a proximal segment made up of the remaining arborization, neurite and soma. Isolated axonal segments were produced by cutting the nerve containing the CDLM axon. The function of the neuron-muscle system composed of CDLM, its pre-synaptic inputs, and its innervated muscle bundle was examined in contrl and experimentally operated animals. Extracellular recording assessed function in the axon. Electrical or tactil stimulation was used to excite pre-synaptic inputs to the CDLM arborization. Intracellular recording determined changes in post-synaptic potentials and miniature end-plate potentials in the muscle bundle innervated by CDLM. Normal axonal conduction, competence to respond to pre-synaptic input, neuron-muscle transmission, and miniature end-plate potential appearance can remain in the isolated arborization preparation. Physiological viability is longer in the cricket isolated arborization than in other insect distal segments described. Survival times of axonal conduction and the competence of the isolated arborization to respond to pre-synaptic input are roughly correlated with disappearance of the whole distal segment at 100 or more postoperative days. A naturally-occurring breakdown of the metathoracic dorsal longitudinal muscles in Teleogryllus eventually prevents measurements of post-synaptic potentials and miniature end-plate potentials. Normal post-synaptic function mediated by the distal arborization is maintained up to this breakdown, to a maximum of 44 days postoperative. The distal axonal segment of CDLM degenerates physiologically within four days postoperative, a time course approximating that of degeneration in vertebrate peripheral nerve distal axons.