Contacts of pore tubules and sensory dendrites in antennal chemosensilla of a silkmoth: Demonstration of a possible pathway for olfactory molecules

Antennal olfactory hairs of Antheraea polyphemus were investigated by means of transmission electron microscopy. Adequate preservation of dendrites and extracellular pore tubules is obtained by mechanical opening of the hair lumen and subsequent chemical fixation. The dendritic membrane has a cell coat. The dendrites contain microfilamentous structures in addition to their cytoplasmatic microtubules. The extracellular pore tubules traverse the hair cuticle and reach into the hair lumen for maximally 350 nm. Their diameter varies between 20 and 40 nm, depending on the preparation method. They consist of an electron-dense wall and an electron-lucent core. The wall has a helical substructure and is covered with a fuzzy coat. Contacts of pore tubules and dendritic membranes occur wherever dendrites are near the inner surface of the hair cuticle. Some of the pore tubules terminate approximately at right angles on the dendritic membrane, others lie against the membrane. The contact seems to be made via the surface coats of the tubules and the membrane. The structure of pore tubules which had been negatively stained with uranyl acetate is similar to the conventionally thin-sectioned material. The observations provide support for earlier assumptions that pore tubules are the pathways by which odor molecules reach the dendritic membrane.     

Modeling activity-dependent synapse restructuring

The spread of electrical activity in a dendritic tree is shaped, in part, by its morphology. Conversely, experimental evidence is growing that electrical and chemical activity can slowly shape the morphology of the dendrite. In this theoretical study, the dendritic spines are dynamic elements, with biophysical properties that change in response to patterns of electrical activity. Recent experiments and diagrammatic models suggest that activity-dependent processes can regulate structural modifications in dendritic spines as well as their distribution along the dendrite. This study considers how local changes in spine structure (minutes to hours) can influence patterns of electrical activity along the dendrite; and how electrical activity due to synaptic events and excitable membrane dynamics can, over time, influence the morphology of the dendrite. The model presents a slow subsystem for structural synaptic plasticity associated with long-term potentiation. A perturbation problem evolves naturally when the spine stem shortens, since the ratio of spine stem resistance to input resistance is small. Hence, the difference between the spine head and dendritic potentials become negligible. This paper presents an asymptotic expansion of head potential in terms of dendritic potential. This leads to a reduced model for post-synaptic restructuring that captures the dynamics of the full model in a briefer computation period when the spines are well connected to the dendrite.     

Sensitivity of Prestin-Based Membrane Motor to Membrane Thickness

Prestin is the membrane protein in outer hair cells that harnesses electrical energy by changing its membrane area in response to changes in the membrane potential. To examine the effect of membrane thickness on this protein, phosphatidylcholine (PC) with various acyl-chain lengths were incorporated into the plasma membrane by using γ-cyclodextrin. Incorporation of short chain PCs increased the linear capacitance and positively shifted the voltage dependence of prestin, up to 120 mV, in cultured cells. PCs with long acyl chains had the opposite effects. Because the linear capacitance is inversely related to the membrane thickness, these voltage shifts are attributable to membrane thickness. The corresponding voltage shifts of electromotility were observed in outer hair cells. These results demonstrate that electromotility is extremely sensitive to the thickness of the plasma membrane, presumably involving hydrophobic mismatch. These observations indicate that the extended state of the motor molecule, which is associated with the elongation of outer hair cells, has a conformation with a shorter hydrophobic height in the lipid bilayer.     

Motoneuron dendrites: role in synaptic integration.

Dendrites constitute over 80 per cent of the receptive surface area in cat motoneurons. Calculations based on matched electrical and gemoetrical measurements in these neurons indicate that the specific resistance of dendritic membranes in resting motoneurons is at least 2,000 ohm-cm2. When the specific membrane resistance is this high, even the most distal dendritic synapses can contribute significantly to the depolarization of the soma, and hence influence the rate of action potential generation. However, dendritic membrane resistance depends strongly on the level of background synaptic activity. The conductance changes associated with excitatory synaptic activity on a dendrite can be great enough to reduce significantly both the excitatory synaptic driving potential and the effective membrane resistance on that dendrite, and thus greatly reduce the effectiveness of synapses on the dendrite. Inhibitory synaptic activity produces an even greater reduction in dendritic membrane resistance. Thus the relative effectiveness of dendritic synapses depends on the type, distribution, and intensity of background synaptic activity, as well as on dendritic geometry and resting membrane properties.     

Theoretical study on electrical properties of dendritic spines

In most parts of mammalian central nervous system the majority of synapses are located on dendritic spines. Several suggestions have been made about the functional significance of the dendritic spines. We investigate electrical properties of dendritic spines in the neurons with arbitrary dendritic geometry. Following Butz & Cowan (1974), all dendritic branches, including spines, are treated as cylinders of uniform passive membrane. We show that the postsynaptic potential due to the synapse on the spine is represented as a convolution integral of the following two functions. The first is the postsynaptic potential caused by the same synapse on the branching point where the spine stalk is attached to the main dendritic trunk. The second function is determined mainly by the morphological and electrical properties of the spine and it represents the attenuation effect of the spine. On the assumption that the diameter of the spine stalk is sufficiently small compared to that of the parent dendrite to which the spine stem is attached, we obtain an approximation of the second function and conclude that morphological change of the spine does not produce an effective change of the postsynaptic potential, hence does not provide the neural basis for learning or memory simply by changing cable properties of dendrites. Moreover, we show that synapses on the dendritic spine are not effectively isolated from other synapses on the same assumption.     

Electrical properties of the dendrite in an insect mechanoreceptor: Effects of antidromic or direct electrical stimulation

A study of the negative phase of the spikes recorded extra cellularly from insect mechanoreceptor has been performed in order to characterize some electrical properties of the dendrite which contains the transducing part of the sensory neuron. These properties have been investigated in mechanoreceptors of the metathoracic leg of the locust Schistocerca gregaria by firing antidromic action potentials both at rest and during mechanical or electrical stimulation. The amplitude of the negative phase of the spike appears to be correlated with the polarization of the dendritic membrane, although when bursts of action potentials are applied, the relation is more complex, including a depressive influence of a given spike on the following spike. The receptor potential and the antidromic dendritic spikes both originate in the same region of the dendrite but they involve different ionic processes. Our results indicate that the dendrite is electrically excitable. The spike which originates in the dendrite has an initial negative phase with a small superimposed positive component. A spike of this shape is never observed under natural stimulation. It is proposed that the negative phase of the antidromic impulse provides a suitable means for studying the variations in electrical polarization of the dendrite which cannot be recorded directly.     

Analysis of an excitable dendritic spine with an activity-dependent stem conductance

 Dendritic spines are the major target for excitatory synaptic inputs in the vertebrate brain. They are tiny evaginations of the dendritic surface consisting of a bulbous head and a tenuous stem. Spines are considered to be an important locus for plastic changes underlying memory and learning processes. The findings that synaptic morphology may be activity-dependent and that spine head membrane may be endowed with voltage-dependent (excitable) channels is the motivation for this study. We first explore the dynamics, when an excitable, yet morphologically fixed spine receives a constant current input. Two parameter Andronov–Hopf bifurcation diagrams are constructed showing stability boundaries between oscillations and steady-states. We show how these boundaries can change as a function of both the spine stem conductance and the conductance load of the attached dendrite. Building on this reference case an idealized model for an activity-dependent spine is formulated and analyzed. Specifically we examine the possibility that the spine stem resistance, the tunable “synaptic weight” parameter identified by Rall and Rinzel, is activity-dependent. In the model the spine stem conductance depends (slowly) on the local electrical interactions between the spine head and the dendritic cable; parameter regimes are found for bursting, steady states, continuous spiking, and more complex oscillatory behavior. We find that conductance load of the dendrite strongly influences the burst pattern as well as other dynamics. When the spine head membrane potential exhibits relaxation oscillations a simple model is formulated that captures the dynamical features of the full model. Received: 10 January 1997/Revised version: 25 March 1997     

Electrical Bistability of the Motoneuronal Dendritic Membrane Determined by Non-inactivating Inward Currents: a Model Study

We investigated features of the spatial pattern of electrical bistable states of dendrites using a computer model of an abducens motoneuron with the dendritic branching reconstructed in detail. The dendritic membrane has an N-shaped current-voltage relation (I-V curve) determined mainly by the presence of L-type calcium channels. Such channels, according to indirect experimental data, are present in the dendrites of these cells together with glutamatergic NMDA-type channels also capable of determining electrical bistability of the membrane and the corresponding specific patterns of electrical activity generated by such neurons. For our model, we obtained steady-state local I-V curves and transferred spatial distribution maps of the membrane potential difference (surface density of transmembrane currents), as well as increments of the axial dendritic current, to three-dimensional images of the reconstructed branching dendrites. The latter increments determine the contribution of a dendritic site in general axial current delivering the charge to the trigger zone of a neuron. The simulation results showed that incorporation of non-inactivating calcium channels into dendritic membrane leads to the origination of a pattern of spatial distribution of bistable electrical states in the dendrites, which were not described earlier. Such features are most important under conditions of a stable state of high depolarization of the relevant parts of the dendrites. In this case, the respective feature was the existence of a zone of maximum density of the inward transmembrane current, which covers areas of first-order branching of all dendrites. Since the greatest relative contribution to the total current belongs to the inward calcium current, the above zone of first branchings can be considered a “hot spot” zone characterized by increased entry of Ca²⁺. This may have important functional consequences for local intracellular calcium signaling.     

The tectorial membrane of the lizard ear: types of structure

This study is concerned with the forms of the tectorial membrane in the lizard ear and its manner of attachment to the ciliary tufts of the hair cells. These structures and their variations were observed in 20 species representing eight families of lizards. Three forms of tectorial membrane were found, a continuous form that extends throughout the length of the auditory papilla, an abbreviated form that reaches the papilla only in one region, and a dendritic form that is particularly narrow at first and then branches extensively to supply all the hair cells. Occasionally the lower edge of the tectorial membrane makes direct connections with the hair tufts. More often there are special connecting structures between the membrane and the hair tufts. Seven types of these structures were identified, as follows: (1) simple fibers, (2) open network, (3) heavy network, (4) fiber plate, (5) finger processes, (6) sallets, and (7) remote connections. These types of tectorial connections are described and illustrated.     

The mechanism of sensory transduction in the sensilla of the trochanteral hair plate of the cockroach,Periplaneta americana

Summary The trochanteral hair plate of the cockroach leg contains approximately 60 hair sensilla that are deflected by a joint membrane during flexion of the leg. Previous work has shown that the organ is a mechanoreceptor which limits leg flexion during walking by reflex connections to flexor and extensor motoneurons. Functional analysis of the largest sensilla has shown that their behaviour may be well approximated by a velocity detector followed by a unidirectional rectifier.We report here the results of an examination of the largest sensilla by scanning and transmission electron microscopy in an attempt to correlate the structure with the known functional elements. Each hair is innervated by a single sensory dendrite which is surrounded by an electron dense dendritic sheath. The dendrite terminates below the hair shaft in a tubular body containing a parallel array of microtubules embedded in an electron dense matrix, while the dendritic sheath extends beyond the tubular body to form the walls of the ecdysial canal. At the proximal end of the tubular body the dendritic sheath and sensory dendrite are anchored to the cuticular socket by a fibrous dome which seems to form a fulcrum around which the tubular body can be deflected by movements of the hair. We suggest that the basis for the detection of velocity may be mechanical differentiation by a fluid space between the dendritic sheath and the tubular body. The structure is also discussed with relation to the mechanism of sensory transduction and the possible causes of the unidirectional sensitivity.Supported by the Canadian Medical Research Council. The authors gratefully acknowledge the expert technical assistance of Sita Prasad     

Ion channels for the mechano-electrical transduction and efferent synapse of the hair cell

Auditory and vestibular information is applied to the hair cell hair bundle as mechanical energy, and is transduced into electrical energy by gating ion channels. The m-e.t. channel has a unitary conductance of 50 pS and a broad selectivity to monovalent cations and to divalent cations. Ca ions are the most permeable through the channel. The angular displacement of the hair bundle is the primary gating factor. Circumstantial evidence indicates the possibility of the direct gating of channels by the membrane deformation itself. The transduction potential activates voltage gated Ca channel and leads to the release of neurotransmitters which activate afferent neurones. Cholinergic muscarinic receptors likely mediate the inhibitory efferent innervation to the hair cell.     

Electrical activity in the chemoreceptors of the blowfly. II. Responses to electrical stimulation

An analysis of the various parts of the electrical responses to the chemical and electrical stimulation of a single labellar chemosensory hair of the blowfly, Phormia regina, indicates that the recording conditions for the spike potentials approximate the intracellular recordings made in other types of sense cells. The large positive resting potential probably arises from the basement membrane of the hypodermal cells and neurilemma rather than from the neurons at the base of the chemosensory hair. The responses to polarizing currents passed through single chemosensory hairs support this analysis. The behavioral responses to similar polarizing currents are shown to result from the action of the current on the neurons at the bases of the adjacent chemosensory hairs. The reported neural interaction of the two chemosensory neurons associated with the chemosensory hair is probably due to the physical-chemical attributes of the stimulating solution rather than to any real neural interaction. Observations on the latency of the initial nerve impulse in response to chemical stimulation indicate that the chemosensory neurons are normally free from spontaneous spike activity.     

Optimal Electrical Properties of Outer Hair Cells Ensure Cochlear Amplification

The organ of Corti (OC) is the auditory epithelium of the mammalian cochlea comprising sensory hair cells and supporting cells riding on the basilar membrane. The outer hair cells (OHCs) are cellular actuators that amplify small sound-induced vibrations for transmission to the inner hair cells. We developed a finite element model of the OC that incorporates the complex OC geometry and force generation by OHCs originating from active hair bundle motion due to gating of the transducer channels and somatic contractility due to the membrane protein prestin. The model also incorporates realistic OHC electrical properties. It explains the complex vibration modes of the OC and reproduces recent measurements of the phase difference between the top and the bottom surface vibrations of the OC. Simulations of an individual OHC show that the OHC somatic motility lags the hair bundle displacement by ∼90 degrees. Prestin-driven contractions of the OHCs cause the top and bottom surfaces of the OC to move in opposite directions. Combined with the OC mechanics, this results in ∼90 degrees phase difference between the OC top and bottom surface vibration. An appropriate electrical time constant for the OHC membrane is necessary to achieve the phase relationship between OC vibrations and OHC actuations. When the OHC electrical frequency characteristics are too high or too low, the OHCs do not exert force with the correct phase to the OC mechanics so that they cannot amplify. We conclude that the components of OHC forward and reverse transduction are crucial for setting the phase relations needed for amplification.     

AII (Rod) amacrine cells form a network of electrically coupled interneurons in the mammalian retina

AII (rod) amacrine cells in the mammalian retina are reciprocally connected via gap junctions, but there is no physiological evidence that demonstrates a proposed function as electrical synapses. In whole-cell recordings from pairs of AII amacrine cells in a slice preparation of the rat retina, bidirectional, nonrectifying electrical coupling was observed in all pairs with overlapping dendritic trees (average conductance approximately 700 pS). Coupling displayed characteristics of a low-pass filter, with no evidence for amplification of spike-evoked electrical postsynaptic potentials by active conductances. Coincidence detection, as well as precise temporal synchronization of subthreshold membrane potential oscillations and TTX-sensitive spiking, was commonly observed. These results indicate a unique mode of operation and integrative capability of the network of AII amacrine cells.     

Morphology and ultrastructure of the sensory spine,a presumed mechanoreceptor of Sphaeroma hookeri (Crustacea,Isopoda), and remarks on similar spines in other peracarids

The isopod Sphaeroma hookeri and many other isopods and peracarids have a sensory spine with laterally inserting sensory hair, positioned in the apical region of the propodal palm of pereopod 1. This spine is innervated by five to eight sensory cells (each giving rise to one cilium) the dendrites of which can be divided into an inner and outer dendritic segment. The cilia are surrounded by an extracellular, electron-dense dendritic sheath. Thirteen enveloping cells are present. The outer dendritic segment (structure beyond the basal bodies) contains two receptor lymph cavities; the inner one lying within the dendritic sheath is homologous with the inner receptor lymph cavity of insects. Scolopales, or tubular bodies, are lacking; their function is probably accomplished by the dendritic sheath. Apically the sensory hair does not have a pore, and the spine is heavily sclerotized. The inner dendritic segment begins with a basal body from which rootlets of different length and thickness extend into the dendrite. In the latter is an accumulation of vesicles. The dendrites keep close contact with other dendrites and the enveloping cells by desmosomal membrane structures. The possible importance of the sensory spine for phylogenetic studies is discussed.     

Calcium-Induced Calcium Release during Action Potential Firing in Developing Inner Hair Cells

In the mature auditory system, inner hair cells (IHCs) convert sound-induced vibrations into electrical signals that are relayed to the central nervous system via auditory afferents. Before the cochlea can respond to normal sound levels, developing IHCs fire calcium-based action potentials that disappear close to the onset of hearing. Action potential firing triggers transmitter release from the immature IHC that in turn generates experience-independent firing in auditory neurons. These early signaling events are thought to be essential for the organization and development of the auditory system and hair cells. A critical component of the action potential is the rise in intracellular calcium that activates both small conductance potassium channels essential during membrane repolarization, and triggers transmitter release from the cell. Whether this calcium signal is generated by calcium influx or requires calcium-induced calcium release (CICR) is not yet known. IHCs can generate CICR, but to date its physiological role has remained unclear. Here, we used high and low concentrations of ryanodine to block or enhance CICR to determine whether calcium release from intracellular stores affected action potential waveform, interspike interval, or changes in membrane capacitance during development of mouse IHCs. Blocking CICR resulted in mixed action potential waveforms with both brief and prolonged oscillations in membrane potential and intracellular calcium. This mixed behavior is captured well by our mathematical model of IHC electrical activity. We perform two-parameter bifurcation analysis of the model that predicts the dependence of IHCs firing patterns on the level of activation of two parameters, the SK2 channels activation and CICR rate. Our data show that CICR forms an important component of the calcium signal that shapes action potentials and regulates firing patterns, but is not involved directly in triggering exocytosis. These data provide important insights into the calcium signaling mechanisms involved in early developmental processes.     

Modeling the electrical behavior of anatomically complex neurons using a network analysis program: Passive membrane

We describe the application of a popular and widely available electrical circuit simulation program called SPICE to modeling the electrical behavior of neurons with passive membrane properties and arbitrarily complex dendritic trees. Transient responses may be calculated at any location in the cell model following current, voltage or conductance perturbations at any point. A numbering method is described for binary trees which is helpful in transforming complex dendritic structures into a coded list of short cylindrical dendritic segments suitable for input to SPICE. Individual segments are modeled as isopotential compartments comprised of a parallel resistor and capacitor, representing the transmembrane impedance, in series with one or two core resistors. Synaptic current is modeled by a current source controlled by the local membrane potential and an alpha-shaped voltage, thus simulating a conductance change in series with a driving potential. Extensively branched test cell circuits were constructed which satisfied the equivalent cylinder constraints (Rall 1959). These model neurons were perturbed by independent current sources and by synaptic currents. Responses calculated by SPICE are compared with analytical results. With appropriately chosen model parameters, extremely accurate transient calculations may be obtained. Details of the SPICE circuit elements are presented, along with illustrative examples sufficient to allow implementation of passive nerve cell models on a number of common computers. Methods for modeling excitable membrane are presented in the companion paper (Bunow et al. 1985).     

Sensitivity of an insect mechanoreceptor during moulting

ABSTRACT. The fine structure and the behavioural threshold for vibration sensitivity of the eight thoracic filiform hairs of Barathra brassicae caterpillars were investigated through an intermoult/moult cycle. Associated with each filiform hair is one bipolar sensory cell and three enveloping cells. The outer dendritic segment terminates in an ecdysial canal in the hair base and a tubular body lies at its distal end. Shortly before apolysis the dendrite elongates. By this means the connection between the sensory cell and the old cuticular apparatus is maintained while the epithelium and the old thoracic cuticle are separating. The new cuticular apparatus of the filiform hair is formed in the second half of the larval stage by the three enveloping cells. A second tubular body in the elongated outer dendritic segment is formed at the base of the replacement hair 10 h before next ecdysis, so that the new hair functions as soon as ecdysis is completed, the old cuticular apparatus with the old tubular bodies being torn away with the exuvia during ecdysis. Sensitivity to a 300 Hz tone was tested in the standing wave of a Kundt's tube. Throughout most of the larval instar the threshold was 2.0 ± 0.3 μm particle displacement amplitude until 1–2h before ecdysis when it rose to 6.8 ± 1.3 μm and at 10–30 min before the beginning of ecdysis no reaction to sound could be detected. Once the old cuticle was shed maximum sensitivity returned as soon as the replacement hairs were erect. The sensilla are therefore physiologically functional at all developmental stages except for 30–60 min during actual ecdysis.     

The electro-dynamics of the dendritic space in Purkinje cells of the cerebellum

The functional geometry of the reconstructed dendritic arborization of Purkinje neurons is the object of this work. The combined effects of the local geometry of the dendritic branches and of the membrane mechanisms are computed in passive configuration to obtain the electrotonic structure of the arborization. Steady-currents applied to the soma and expressed as a function of the path distance from the soma form different clusters of profiles in which dendritic branches are similar in voltages and current transfer effectiveness. The locations of the different clusters are mapped on the dendrograms and 3D representations of the arborization. It reveals the presence of different spatial dendritic sectors clearly separated in 3D space that shape the arborization in ordered electrical domains, each with similar passive charge transfer efficiencies. Further simulations are performed in active configuration with a realistic cocktail of conductances to find out whether similar spatial domains found in the passive model also characterize the active dendritic arborization. During tonic activation of excitatory synaptic inputs homogeneously distributed over the whole arborization, the Purkinje cell generates regular oscillatory potentials. The temporal patterns of the electrical oscillations induce similar spatial sectors in the arborization as those observed in the passive electrotonic structure. By taking a video of the dendritic maps of the membrane potentials during a single oscillation, we demonstrate that the functional dendritic field of a Purkinje neuron displays dynamic changes which occur in the spatial distribution of membrane potentials in the course of the oscillation. We conclude that the branching pattern of the arborization explains such continuous reconfiguration and discuss its functional implications.     

Initiation of Sodium Spikelets in Basal Dendrites of Neocortical Pyramidal Neurons

Cortical information processing relies critically on the processing of electrical signals in pyramidal neurons. Electrical transients mainly arise when excitatory synaptic inputs impinge upon distal dendritic regions. To study the dendritic aspect of synaptic integration one must record electrical signals in distal dendrites. Since thin dendritic branches, such as oblique and basal dendrites, do not support routine glass electrode measurements, we turned our effort towards voltage-sensitive dye recordings. Using the optical imaging approach we found and reported previously that basal dendrites of neocortical pyramidal neurons show an elaborate repertoire of electrical signals, including backpropagating action potentials and glutamate-evoked plateau potentials. Here we report a novel form of electrical signal, qualitatively and quantitatively different from backpropagating action potentials and dendritic plateau potentials. Strong glutamatergic stimulation of an individual basal dendrite is capable of triggering a fast spike, which precedes the dendritic plateau potential. The amplitude of the fast initial spikelet was actually smaller that the amplitude of the backpropagating action potential in the same dendritic segment. Therefore, the fast initial spike was dubbed “spikelet”. Both the basal spikelet and plateau potential propagate decrementally towards the cell body, where they are reflected in the somatic whole-cell recordings. The low incidence of basal spikelets in the somatic intracellular recordings and the impact of basal spikelets on soma-axon action potential initiation are discussed.