Presynaptic action potential amplification by voltage-gated Na+ channels in hippocampal mossy fiber boutons

Action potentials in central neurons are initiated near the axon initial segment, propagate into the axon, and finally invade the presynaptic terminals, where they trigger transmitter release. Voltage-gated Na(+) channels are key determinants of excitability, but Na(+) channel density and properties in axons and presynaptic terminals of cortical neurons have not been examined yet. In hippocampal mossy fiber boutons, which emerge from parent axons en passant, Na(+) channels are very abundant, with an estimated number of approximately 2000 channels per bouton. Presynaptic Na(+) channels show faster inactivation kinetics than somatic channels, suggesting differences between subcellular compartments of the same cell. Computational analysis of action potential propagation in axon-multibouton structures reveals that Na(+) channels in boutons preferentially amplify the presynaptic action potential and enhance Ca(2+) inflow, whereas Na(+) channels in axons control the reliability and speed of propagation. Thus, presynaptic and axonal Na(+) channels contribute differentially to mossy fiber synaptic transmission.     

Sharpness of Spike Initiation in Neurons Explained by Compartmentalization

In cortical neurons, spikes are initiated in the axon initial segment. Seen at the soma, they appear surprisingly sharp. A standard explanation is that the current coming from the axon becomes sharp as the spike is actively backpropagated to the soma. However, sharp initiation of spikes is also seen in the input–output properties of neurons, and not only in the somatic shape of spikes; for example, cortical neurons can transmit high frequency signals. An alternative hypothesis is that Na channels cooperate, but it is not currently supported by direct experimental evidence. I propose a simple explanation based on the compartmentalization of spike initiation. When Na channels are placed in the axon, the soma acts as a current sink for the Na current. I show that there is a critical distance to the soma above which an instability occurs, so that Na channels open abruptly rather than gradually as a function of somatic voltage.     

Signal processing in the axon initial segment

The axon initial segment (AIS) is a specialized membrane region in the axon of neurons where action potentials are initiated. Crucial to the function of the AIS is the presence of specific voltage-gated channels clustered at high densities, giving the AIS unique electrical properties. Here we review recent data on the physiology of the AIS. These data indicate that the role of the AIS is far richer than originally thought, leading to the idea that it represents a dynamic signal processing unit within neurons, regulating the integration of synaptic inputs, intrinsic excitability, and transmitter release. Furthermore, these observations point to?a critical role of the AIS in disease.     

Rats with Malformations of Cortical Development Exhibit Decreased Length of AIS and Hypersensitivity to Pilocarpine-Induced Status Epilepticus

Malformations of cortical development (MCD) are critical brain development disorders associated with varied abnormalities in both anatomic structures and neural functioning. It is also a very common etiology to the epilepsy, in which the alteration on excitability of cortical neurons is hypothesized as one of important causes to the epileptic seizures. Due to the key role in regulating neuron firing properties, the plasticity of axon initial segment (AIS) was investigated in present study to further determine the relation between MCD and epilepsy. Our results showed a prolonged decrease in the length of AIS occurred in MCD animal models. Besides, the AIS was also found greatly shortened in MCD models during the acute, but not chronic phase of status epileptics compared with intact controls. Our findings of identification of AIS plasticity in MCD animal models and its hypersensitivity to status epilepsy are significant in furthering our understanding of the pathophysiological mechanisms involved in this disorder.     

Ultrastructure of the synapses of the initial axonal segment of the pyramidal neuron

Ultrastructure of the proximal part of the axon in the neurons, identified according to a number of morphological signs as pyramidal, has been studied in the layer III of the cat cerebral hemisphere sensomotor cortex. In sections, tangential to the cortical surface, in the initial axonal segment, a submembranous osmophilic layer and fasciculi of microtubules are revealed. On the initial segment spines are found, they contain cysterns resembling by their structure the spine system of the dendritic spines. Axonal terminals revealed along the axonal distribution are in contact both with the axonal trunk and with the spines. Regarding the initial segment, they are presynaptic, contain oval synaptic vesicles and form symmetric axo-axonal synapses only. In transversal sections axonal terminals are detected, arranging on the surface of the initial segment mostly as single ones, in longitudinal sections they are seen as clusters. Analysing the author's data and those from the literature, a conclusion is made that in intact animals the synaptic contacts at the initial segment of the axon are the only form of axo-axonal synapses in the neocortex.     

Intrinsic and extrinsic determinants of ion channel localization in neurons

Neurons are an extremely diverse group of excitable cells with a wide variety of morphologies including complex dendritic trees and very long axons. The electrical properties of neurons depend not only on the types of ion channels and receptors expressed, but also on where these channels are located in the cell. Two extreme examples that illustrate the subcellular polarized nature of neurons and the tight regulation of ion channel localization can be seen at the axon initial segment and the node of Ranvier. The axon initial segment is important for initiation of action potentials in the axon, whereas the node of Ranvier is required for the rapid, faithful and efficient propagation of action potentials along the axon. Given the similarity of their functions it is not surprising that nearly every protein component of the axon initial segment is also found at the node. However, there is one very important difference between these two sites: nodes require extrinsic, glial-derived factors in order to form, whereas the axon initial segment is intrinsically determined by the neuron. This mini-review discusses recent results that have begun to clarify the intrinsic and extrinsic mechanisms underlying formation of nodes and axon initial segments, and poses several important unanswered questions regarding their unique mechanisms of formation.     

The chandelier neuron in schizophrenia

Markers of GABA neurotransmission between chandelier neurons and their synaptic targets, the axon initial segment (AIS) of pyramidal neurons, are altered in the dorsolateral prefrontal cortex (DLPFC) of subjects with schizophrenia. For example, immunoreactivity for the GABA membrane transporter (GAT1) is decreased in presynaptic chandelier neuron axon terminals, whereas immunoreactivity for the GABA(A) receptor α2 subunit is increased in postsynaptic AIS. These alterations are most marked in cortical layers 2-3. In addition, other determinants of the function of chandelier cell-pyramidal neuron synapses, such as ankyrin-G (which regulates the recruitment of sodium channels to the AIS), are also selectively altered in superficial layer pyramidal neurons in subjects with schizophrenia. Each of these components of chandelier cell-pyramidal neuron connectivity exhibits distinctive developmental trajectories in the primate DLPFC, suggesting that disturbances in these trajectories could contribute to the pathogenesis of schizophrenia. Recent findings that inputs from neocortical chandelier neurons are excitatory provide new ideas about the role of this circuitry in the pathophysiology of cortical dysfunction in schizophrenia.     

Electrical excitability of the soma of sensory neurons is required for spike invasion of the soma,but not for through-conduction

The cell soma of primary sensory neurons is electrically excitable, and is invaded by action potentials as they pass from the peripheral nerve, past the dorsal root ganglion (DRG) and toward the spinal cord. However, there are virtually no synapses in the DRG, and no signal processing is known to occur there. Why, then, are DRG cell somata excitable? We have constructed and validated an explicit model of the primary sensory neuron and used it to explore the role of electrical excitability of the cell soma in afferent signaling. Reduction and even elimination of soma excitability proved to have no detectable effect on the reliability of spike conduction past the DRG and into the spinal cord. Through-conduction is affected, however, by major changes in neuronal geometry in the region of the t-junction. In contrast to through-conduction, excitability of the soma and initial segment is essential for the invasion of afferent spikes into the cell soma. This implies that soma invasion has a previously unrecognized role in the physiology of afferent neurons, perhaps in the realm of metabolic coupling of the biosynthesis of signaling molecules required at the axon ends to functional demand, or in cell-cell interaction within sensory ganglia. Spike invasion of the soma in central nervous system neurons may play similar roles.     

Excitability of the soma in central nervous system neurons

The ability of the soma of a spinal dorsal horn neuron, a spinal ventral horn neuron (presumably a motoneuron), and a hippocampal pyramidal neuron to generate action potentials was studied using patch-clamp recordings from rat spinal cord slices, the "entire soma isolation" method, and computer simulations. By comparing original recordings from an isolated soma of a dorsal horn neuron with simulated responses, it was shown that computer models can be adequate for the study of somatic excitability. The modeled somata of both spinal neurons were unable to generate action potentials, showing only passive and local responses to current injections. A four- to eightfold increase in the original density of Na(+) channels was necessary to make the modeled somata of both spinal neurons excitable. In contrast to spinal neurons, the modeled soma of the hippocampal pyramidal neuron generated spikes with an overshoot of +9 mV. It is concluded that the somata of spinal neurons cannot generate action potentials and seem to resist their propagation from the axon to dendrites. In contrast, the soma of the hippocampal pyramidal neuron is able to generate spikes. It cannot initiate action potentials in the intact neurons, but it can support their back-propagation from the axon initial segment to dendrites.     

Local field potential modeling predicts dense activation in cerebellar granule cells clusters under LTP and LTD control

Local field-potentials (LFPs) are generated by neuronal ensembles and contain information about the activity of single neurons. Here, the LFPs of the cerebellar granular layer and their changes during long-term synaptic plasticity (LTP and LTD) were recorded in response to punctate facial stimulation in the rat in vivo. The LFP comprised a trigeminal (T) and a cortical (C) wave. T and C, which derived from independent granule cell clusters, co-varied during LTP and LTD. To extract information about the underlying cellular activities, the LFP was reconstructed using a repetitive convolution (ReConv) of the extracellular potential generated by a detailed multicompartmental model of the granule cell. The mossy fiber input patterns were determined using a Blind Source Separation (BSS) algorithm. The major component of the LFP was generated by the granule cell spike Na(+) current, which caused a powerful sink in the axon initial segment with the source located in the soma and dendrites. Reproducing the LFP changes observed during LTP and LTD required modifications in both release probability and intrinsic excitability at the mossy fiber-granule cells relay. Synaptic plasticity and Golgi cell feed-forward inhibition proved critical for controlling the percentage of active granule cells, which was 11% in standard conditions but ranged from 3% during LTD to 21% during LTP and raised over 50% when inhibition was reduced. The emerging picture is that of independent (but neighboring) trigeminal and cortical channels, in which synaptic plasticity and feed-forward inhibition effectively regulate the number of discharging granule cells and emitted spikes generating "dense" activity clusters in the cerebellar granular layer.     

Resealing of the proximal and distal cut ends of transected axons: Electrophysiological and ultrastructural analysis

The fates of the proximal and distal segments of transected axons differ. Whereas the proximal segment usually recovers from injury and regenerates, the distal segment degenerates. In the present report we studied the kinetics of the recovery processes of both proximal and distal axonal segment following axotomy and its temporal relations to the alterations in the cytoarchitecture of the injured neuron. The experiments were performed on primary cultured metacerebral neurons (MCn) isolated from Aplysia. We transected axons while monitoring the changes in transmembrane potential and input resistance (Rn) by inserting intracellular microelectrodes into the soma and axon. Correlation between the electrophysiological status of the injured axon and its ultrastructure was provided by rapid fixation of the neuron at selected times postaxotomy. Axotomy leads to membrane depolarization from a mean of ?55.7 S.D. 12.8 mV to ?12.7 S.D. 3.3 mV and decreased Rn from tens of MΩ to 1–3 MΩ. The transected axons remained depolarized for a period of 10–260 s for as long as the axoplasm was in direct contact with the bathing solution. Rapid repolarization and partial recovery of Rn was associated with the formation of a membrane seal over the cut ends by the constriction and subsequent fusion of the axolema. Prior to the formation of a membraneous barrier, electron-dense deposits aggregate at the tip of the cut axon and appear to form an axoplasmic “plug.” Electrophysiological analysis revealed that this “plug” does not provide resistance for current flow and that the axoplasmic resistance is homogenously distributed. The kinetics of injury and recovery processes as well as the ultrastructural changes of the proximal and distal segments are cannot be attributed to differences in the immediated response of the segments to axotomy. © 1993 John Wiley & Sons, Inc.     

Extra spike formation in sensory neurons and the disruption of afferent spike patterning

The peculiar pseudounipolar geometry of primary sensory neurons can lead to ectopic generation of "extra spikes" in the region of the dorsal root ganglion potentially disrupting the fidelity of afferent signaling. We have used an explicit model of myelinated vertebrate sensory neurons to investigate the location and mechanism of extra spike formation, and its consequences for distortion of afferent impulse patterning. Extra spikes originate in the initial segment axon under conditions in which the soma spike becomes delayed and broadened. The broadened soma spike then re-excites membrane it has just passed over, initiating an extra spike which propagates outwards into the main conducting axon. Extra spike formation depends on cell geometry, electrical excitability, and the recent history of impulse activity. Extra spikes add to the impulse barrage traveling toward the spinal cord, but they also travel antidromically in the peripheral nerve colliding with and occluding normal orthodromic spikes. As a result there is no net increase in afferent spike number. However, extra spikes render firing more staccato by increasing the number of short and long interspike intervals in the train at the expense of intermediate intervals. There may also be more complex changes in the pattern of afferent spike trains, and hence in afferent signaling.     

The functional organization and assembly of the axon initial segment

Action potential initiation, modulation, and duration in neurons depend on a variety of Na+ and K+ channels that are highly enriched at the axon initial segment (AIS). The AIS also has high densities of cell adhesion molecules (CAMs), modulatory proteins, and a unique extracellular matrix (ECM). In contrast to other functional domains of axons (e.g. the nodes of Ranvier and axon terminals) whose development depends on the interactions with different cells (e.g. myelinating glia and postsynaptic cells), the recruitment and retention of AIS proteins is intrinsically specified through axonal cytoskeletal and scaffolding proteins. We speculate that the AIS has previously unappreciated forms of plasticity that influence neuronal excitability, and that AIS plasticity is regulated by the developmental or activity-dependent modulation of scaffolding protein levels rather than directly altering ion channel expression.     

Cortical axons branch to multiple subcortical targets by interstitial axon budding: implications for target recognition and "waiting periods"

We are studying how axons branch in vivo. Individual cortical neurons send axons to both the spinal cord and the basilar pons. Here we show that the corticopontine projection develops by an interstitial budding of collaterals from parent axons rather than a reported mechanism of axon branching, growth cone bifurcation. This mechanism is used regardless of whether the parent axon's postpontine segment, which forms the corticospinal projection, is permanent (motor cortex) or transient (visual cortex). Budding occurs days after the parent axons grow spinally past the pons, accounting for the "waiting period" reported in this system in contrast to an alternative explanation that the growth cones pause outside of their target. Timing and location of pontine collateral budding vary with cortical origin of the parent axon and are correlated with the temporal ordering of axon arrival.     

Epileptic seizures in a heterogeneous excitatory network with short-term plasticity

Epilepsy involves a diverse group of abnormalities, including molecular and cellular disorders. These abnormalities prove to be associated with the changes in local excitability and synaptic dynamics. Correspondingly, the epileptic processes including onset, propagation and generalized seizure may be related with the alterations of excitability and synapse. In this paper, three regions, epileptogenic zone (EZ), propagation area and normal region, were defined and represented by neuronal population model with heterogeneous excitability, respectively. In order to describe the synaptic behavior that the strength was enhanced and maintained at a high level for a short term under a high frequency spike train, a novel activity-dependent short-term plasticity model was proposed. Bifurcation analysis showed that the presence of hyperexcitability could increase the seizure susceptibility of local area, leading to epileptic discharges first seen in the EZ. Meanwhile, recurrent epileptic activities might result in the transition of synaptic strength from weak state to high level, augmenting synaptic depolarizations in non-epileptic neurons as the experimental findings. Numerical simulation based on a full-connected weighted network could qualitatively demonstrate the epileptic process that the propagation area and normal region were successively recruited by the EZ. Furthermore, cross recurrence plot was used to explore the synchronization between neuronal populations, and the global synchronization index was introduced to measure the global synchronization. Results suggested that the synchronization between the EZ and other region was significantly enhanced with the occurrence of seizure. Interestingly, the desynchronization phenomenon was also observed during seizure initiation and propagation as reported before. Therefore, heterogeneous excitability and short-term plasticity are believed to play an important role in the epileptic process. This study may provide novel insights into the mechanism of epileptogenesis.     

Axon initial segment Kv1 channels control axonal action potential waveform and synaptic efficacy

Action potentials are binary signals that transmit information via their rate and temporal pattern. In this context, the axon is thought of as a transmission line, devoid of a role in neuronal computation. Here, we show a highly localized role of axonal Kv1 potassium channels in shaping the action potential waveform in the axon initial segment (AIS) of layer 5 pyramidal neurons independent of the soma. Cell-attached recordings revealed a 10-fold increase in Kv1 channel density over the first 50 microm of the AIS. Inactivation of AIS and proximal axonal Kv1 channels, as occurs during slow subthreshold somatodendritic depolarizations, led to a distance-dependent broadening of axonal action potentials, as well as an increase in synaptic strength at proximal axonal terminals. Thus, Kv1 channels are strategically positioned to integrate slow subthreshold signals, providing control of the presynaptic action potential waveform and synaptic coupling in local cortical circuits.     

Axons of sacral preganglionic neurons in the cat: I. Origin,initial segment,and myelination

Parasympathetic preganglionic neurons in the cat sacral spinal cord innervate intraspinal neurons and pelvic target organs. Retrograde tracing studies have revealed little of the morphology of their axons including their origin, initial segments, or their myelin, due to methodological limitations. Intracellular labeling of single neurons with neurobiotin or HRP has overcome these problems. Axons were studied in 24 preganglionic neurons. In 21 neurons the axon originated as a branch of a dendrite, without a detectable axon hillock, at distances from the soma ranging from 10 to 110 μm (average 34.1 μm ). In 3 neurons the axon was derived from the soma. Initial segments, present in all cells, ranged from 15 to 40 μm (average 26.8 μm). Nearly all axons followed the initial segment with unmyelinated segments that varied between 59 to 630 μm, followed by myelin and nodes of Ranvier. Internodal distances were variable and relatively short (average 93 μm). Axonal diameters measured over the intraspinal course in 18 axons averaged 1.3 μm (range 0.6–2.4 μm) and were relatively constant compared with other neurons. Spine-like protrusions were observed on the initial segments of 12 cells. Axon collaterals originated from unmyelinated sections and nodes of Ranvier. Antidromic action potentials showing initial segment, soma-dendritic inflections, did not differentiate between soma-derived and dendrite-derived axons. The data suggest that axons originating from a dendrite are the normal structure of preganglionic neurons in the lateral sacral parasympathetic nucleus. It is proposed that the particular structure of these axons may be part of a timing mechanism that coordinates preganglionic neurons with other spinal neurons involved in target organ reflexes.     

颅内电极在感觉运动区皮质发育不良癫痫手术中的应用

目的:探讨长程颅内电极监测及电刺激方法,在感觉运动区皮质发育不良的难治性癫痫外科手术评估中的意义。方法:筛选MRI提示的皮质发育不良区域与重要功能区-感觉运动区位置关系密切的11例难治性癫痫患者,且头皮长程视频脑电监测及PET检查也初步提示癫痫发作与皮质发育不良所在脑区有关,在可疑脑区放置颅内电极,然后进行颅内电极长程视频脑电监测及电刺激检测,对癫痫起源位置及功能区定位,明确癫痫发作起源区域与感觉运动功能区的解剖学关系,在定位结果指导下进行切除术。结果:11例中3例位于左侧半球,8例位于右侧半球,11例感觉运动功能区皮质分布均存在不同程度变异,7例癫痫发作起源区域与感觉运动功能区一定范围重叠,其中5例与感觉区重叠,该5例切除了起源区域与发作有关的部分感觉区,2例部分致痫灶与运动区重叠,该2例仅切除了除与发作有关的运动区以外的癫痫起源区域,4例癫痫发作起源区域与感觉运动功能区相对独立,该4例完全切除癫痫发作起源区域;手术后6例患者发作消失,2例患者发作频率减少90%以上,1例癫痫发作控制无效,2例患者发生部分感觉缺失,但对生活无明显影响。结论:在皮质发育不良的癫痫患者中,有较高比例的病人伴有功能区皮层分布的变异,长程颅内电极监测及电刺激能够实现癫痫起源区域及功能区精确定位,明确功能区变异情况,对于指导病灶切除,避免损伤功能区皮质,减少术后并发症具有重要意义。     

颅内电极在感觉运动区皮质发育不良癫痫手术中的应用

目的：探讨长程颅内电极监测及电刺激方法,在感觉运动区皮质发育不良的难治性癫痫外科手术评估中的意义。方法：筛选MRI提示的皮质发育不良区域与重要功能区-感觉运动区位置关系密切的11例难治性癫痫患者,且头皮长程视频脑电监测及PET检查也初步提示癫痫发作与皮质发育不良所在脑区有关,在可疑脑区放置颅内电极,然后进行颅内电极长程视频脑电监测及电刺激检测,对癫痫起源位置及功能区定位,明确癫痫发作起源区域与感觉运动功能区的解剖学关系,在定位结果指导下进行切除术。结果：11例中3例位于左侧半球,8例位于右侧半球,11例感觉运动功能区皮质分布均存在不同程度变异,7例癫痫发作起源区域与感觉运动功能区一定范围重叠,其中5例与感觉区重叠,该5例切除了起源区域与发作有关的部分感觉区,2例部分致痫灶与运动区重叠,该2例仅切除了除与发作有关的运动区以外的癫痫起源区域,4例癫痫发作起源区域与感觉运动功能区相对独立,该4例完全切除癫痫发作起源区域;手术后6例患者发作消失,2例患者发作频率减少90%以上,1例癫痫发作控制无效,2例患者发生部分感觉缺失,但对生活无明显影响。结论：在皮质发育不良的癫痫患者中,有较高比例的病人伴有功能区皮层分布的变异,长程颅内电极监测及电刺激能够实现癫痫起源区域及功能区精确定位,明确功能区变异情况,对于指导病灶切除,避免损伤功能区皮质,减少术后并发症具有重要意义。     

Bayesian Computation Emerges in Generic Cortical Microcircuits through Spike-Timing-Dependent Plasticity

The principles by which networks of neurons compute, and how spike-timing dependent plasticity (STDP) of synaptic weights generates and maintains their computational function, are unknown. Preceding work has shown that soft winner-take-all (WTA) circuits, where pyramidal neurons inhibit each other via interneurons, are a common motif of cortical microcircuits. We show through theoretical analysis and computer simulations that Bayesian computation is induced in these network motifs through STDP in combination with activity-dependent changes in the excitability of neurons. The fundamental components of this emergent Bayesian computation are priors that result from adaptation of neuronal excitability and implicit generative models for hidden causes that are created in the synaptic weights through STDP. In fact, a surprising result is that STDP is able to approximate a powerful principle for fitting such implicit generative models to high-dimensional spike inputs: Expectation Maximization. Our results suggest that the experimentally observed spontaneous activity and trial-to-trial variability of cortical neurons are essential features of their information processing capability, since their functional role is to represent probability distributions rather than static neural codes. Furthermore it suggests networks of Bayesian computation modules as a new model for distributed information processing in the cortex.