scholarly journals Dendritic Action Potential Initiation in Hypothalamic Gonadotropin-Releasing Hormone Neurons

Endocrinology ◽  
2008 ◽  
Vol 149 (7) ◽  
pp. 3355-3360 ◽  
Author(s):  
Carson B. Roberts ◽  
Rebecca E. Campbell ◽  
Allan E. Herbison ◽  
Kelly J. Suter
Keyword(s):  
Action Potentials ◽  
Excitatory Input ◽  
Environmental Cues ◽  
Spike Generation ◽  
Neuronal Phenotype ◽  
Gnrh Neurons ◽  
Action Potential Initiation ◽  
Afferent Inputs ◽  
Spike Initiation ◽  
Potential Initiation

It is dogma that action potentials are initiated at the soma/axon hillock of neurons. However, dendrites often exhibit conductances necessary for spike generation and represent functionally independent processing compartments within neurons. GnRH neurons provide an interesting neuronal phenotype with simple, relatively unbranched, unipolar or bipolar dendrites of extensive lengths (>1000 μm) covered in spines. These neurons control fertility and must integrate a variety of internal homeostatic and external environmental cues. We used imaging, electrophysiological, and modeling studies to understand how they integrate and process information along dendrites. Simultaneous recordings from distal dendrites and somata of individual GnRH neurons indicate distal dendrites are the primary site of spike initiation in these cells. Compartmental modeling indicates that sites of spike initiation depend upon location of excitatory input and dendrite geometry. Together, these studies demonstrate a novel pattern of spike generation in mammalian neurons and indicate that afferent inputs within distal dendritic microdomains directly initiate action potentials.

Biophysical studies of mechanoreceptors

1986 ◽  
Vol 60 (4) ◽  
pp. 1107-1115 ◽  
Author(s):  
P. Grigg
Keyword(s):  
Action Potentials ◽  
Dynamic Responses ◽  
Initiation Zone ◽  
Biophysical Studies ◽  
Electrogenic Na Pump ◽  
Action Potential Initiation ◽  
Branching Point ◽  
The Many ◽  
Spike Initiation ◽  
Potential Initiation

Mechanoreception can be viewed as a series of sequential mechanical and ionic processes that take place in mechanosensitive end organs and in the terminals of the nerves that innervate them. Stimuli act on a transducer after being transmitted through some material having a combination of elastic and viscoelastic properties. Channels that open under membrane loading have recently been described in muscle cells and are presented as a model for transduction. When open these channels are cation specific. Ions passing through transducer channels depolarize a spike-initiating zone on the cell. These currents may also activate other conductances in the cell, so that the total generator current may have many components. In many mechanoreceptors, action potential initiation results in activation of an electrogenic Na+ pump at the spike-initiation zone, which modifies the threshold for subsequent action potentials. Action potentials initiated in the many branches of a single sensory axon interact at the branching point of the axon. The rules governing this interaction are complex. The above factors, together or separately, are responsible for the dynamic responses and adaptation observed in mechanoreceptors.

scholarly journals Modeling Action Potential Initiation and Back-Propagation in Dendrites of Cultured Rat Motoneurons

1998 ◽  
Vol 80 (2) ◽  
pp. 715-729 ◽  
Author(s):  
Hans-R. Lüscher ◽  
Matthew E. Larkum
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Initial Segment ◽  
Back Propagation ◽  
Channel Density ◽  
Axon Hillock ◽  
Dendritic Membrane ◽  
Action Potential Initiation ◽  
Spike Initiation ◽  
Potential Initiation

Lüscher, Hans-R. and Matthew E. Larkum. Modeling action potential initiation and back-propagation in dendrites of cultured rat motoneurons. J. Neurophysiol. 80: 715–729, 1998. Regardless of the site of current injection, action potentials usually originate at or near the soma and propagate decrementally back into the dendrites. This phenomenon has been observed in neocortical pyramidal cells as well as in cultured motoneurons. Here we show that action potentials in motoneurons can be initiated in the dendrite as well, resulting in a biphasic dendritic action potential. We present a model of spinal motoneurons that is consistent with observed physiological properties of spike initiation in the initial segment/axon hillock region and action potential back-propagation into the dendritic tree. It accurately reproduces the results presented by Larkum et al. on motoneurons in organotypic rat spinal cord slice cultures. A high Na+-channel density of ḡ Na = 700 mS/cm2 at the axon hillock/initial segment region was required to secure antidromic invasion of the somato-dendritic membrane, whereas for the orthodromic direction, a Na+-channel density of ḡ Na = 1,200 mS/cm2 was required. A “weakly” excitable ( ḡ Na = 3 mS/cm2) dendritic membrane most accurately describes the experimentally observed attenuation of the back-propagated action potential. Careful analysis of the threshold conditions for action potential initiation at the initial segment or the dendrites revealed that, despite the lower voltage threshold for spike initiation in the initial segment, an action potential can be initiated in the dendrite before the initial segment fires a spike. Spike initiation in the dendrite depends on the passive cable properties of the dendritic membrane, its Na+-channel density, and local structural properties, mainly the diameter of the dendrites. Action potentials are initiated more easily in distal than in proximal dendrites. Whether or not such a dendritic action potential invades the soma with a subsequent initiation of a second action potential in the initial segment depends on the actual current source-load relation between the action potential approaching the soma and the electrical load of the soma together with the attached dendrites.

Transduction mechanisms in airway sensory nerves

2006 ◽  
Vol 101 (3) ◽  
pp. 950-959 ◽  
Author(s):  
Thomas Taylor-Clark ◽  
Bradley J. Undem
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Membrane Depolarization ◽  
Sensory Nerves ◽  
Metabotropic Receptors ◽  
Metabotropic Receptor ◽  
Transduction Mechanisms ◽  
The Central Nervous System ◽  
Action Potential Initiation ◽  
Potential Initiation

The induction of action potentials in airway sensory nerves relies on events leading to the opening of cation channels in the nerve terminal membrane and subsequent membrane depolarization. If the membrane depolarization is of sufficient rate and amplitude, action potential initiation will occur. The action potentials are then conducted to the central nervous system, leading to the initiation of various sensations and cardiorespiratory reflexes. Triggering events in airway sensory nerves include mechanical perturbation, inflammatory mediators, pH, temperature, and osmolarity acting through a variety of ionotropic and metabotropic receptors. Action potential initiation can be modulated (positively or negatively) through independent mechanisms caused mainly by autacoids and other metabotropic receptor ligands. Finally, gene expression of sensory nerves can be altered in adult mammals. This neuroplasticity can change the function of sensory nerves and likely involve both neurotrophin and use-dependent mechanisms. Here we provide a brief overview of some of the transduction mechanisms underlying these events.

Properties of Action-Potential Initiation in Neocortical Pyramidal Cells: Evidence From Whole Cell Axon Recordings

2007 ◽  
Vol 97 (1) ◽  
pp. 746-760 ◽  
Author(s):  
Yousheng Shu ◽  
Alvaro Duque ◽  
Yuguo Yu ◽  
Bilal Haider ◽  
David A. McCormick
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Initial Segment ◽  
Pyramidal Cells ◽  
Synaptic Activity ◽  
Up States ◽  
Action Potential Initiation ◽  
Potential Initiation

Cortical pyramidal cells are constantly bombarded by synaptic activity, much of which arises from other cortical neurons, both in normal conditions and during epileptic seizures. The action potentials generated by barrages of synaptic activity may exhibit a variable site of origin. Here we performed simultaneous whole cell recordings from the soma and axon or soma and apical dendrite of layer 5 pyramidal neurons during normal recurrent network activity (up states), the intrasomatic or intradendritic injection of artificial synaptic barrages, and during epileptiform discharges in vitro. We demonstrate that under all of these conditions, the real or artificial synaptic bombardments propagate through the dendrosomatic-axonal arbor and consistently initiate action potentials in the axon initial segment that then propagate to other parts of the cell. Action potentials recorded intracellularly in vivo during up states and in response to visual stimulation exhibit properties indicating that they are typically initiated in the axon. Intracortical axons were particularly well suited to faithfully follow the generation of action potentials by the axon initial segment. Action-potential generation was more reliable in the distal axon than at the soma during epileptiform activity. These results indicate that the axon is the preferred site of action-potential initiation in cortical pyramidal cells, both in vivo and in vitro, with state-dependent back propagation through the somatic and dendritic compartments.

scholarly journals Morphological Characterization of the Action Potential Initiation Segment in GnRH Neuron Dendrites and Axons of Male Mice

Endocrinology ◽  
2015 ◽  
Vol 156 (11) ◽  
pp. 4174-4186 ◽  
Author(s):  
Michel K. Herde ◽  
Allan E. Herbison
Keyword(s):  
Action Potential ◽  
Cell Body ◽  
Brain Slices ◽  
Gnrh Neuron ◽  
Male Mice ◽  
Single Action ◽  
Ankyrin G ◽  
Gnrh Neurons ◽  
Action Potential Initiation ◽  
Potential Initiation

GnRH neurons are the final output neurons of the hypothalamic network controlling fertility in mammals. In the present study, we used ankyrin G immunohistochemistry and neurobiotin filling of live GnRH neurons in brain slices from GnRH-green fluorescent protein transgenic male mice to examine in detail the location of action potential initiation in GnRH neurons with somata residing at different locations in the basal forebrain. We found that the vast majority of GnRH neurons are bipolar in morphology, elaborating a thick (primary) and thinner (secondary) dendrite from opposite poles of the soma. In addition, an axon-like process arising predominantly from a proximal dendrite was observed in a subpopulation of GnRH neurons. Ankyrin G immunohistochemistry revealed the presence of a single action potential initiation zone ∼27 μm in length primarily in the secondary dendrite of GnRH neurons and located 30 to 140 μm distant from the cell soma, depending on the type of process and location of the cell body. In addition to dendrites, the GnRH neurons with cell bodies located close to hypothalamic circumventricular organs often elaborated ankyrin G–positive axon-like structures. Almost all GnRH neurons (>90%) had their action potential initiation site in a process that initially, or ultimately after a hairpin loop, was coursing in the direction of the median eminence. These studies indicate that action potentials are initiated in different dendritic and axonal compartments of the GnRH neuron in a manner that is dependent partly on the neuroanatomical location of the cell body.

scholarly journals Etomidate Reduces Initiation of Backpropagating Dendritic Action Potentials: Implications for Sensory Processing and Synaptic Plasticity During Anesthesia

2007 ◽  
Vol 97 (3) ◽  
pp. 2373-2384 ◽  
Author(s):  
Erwin H. van den Burg ◽  
Jacob Engelmann ◽  
João Bacelo ◽  
Leonel Gómez ◽  
Kirsty Grant
Keyword(s):  
Action Potential ◽  
Sensory Processing ◽  
Action Potentials ◽  
Molecular Layer ◽  
Current Source ◽  
Spike Timing ◽  
Stimulus Strength ◽  
Action Potential Initiation ◽  
Potential Initiation

Anesthetics may induce specific changes that alter the balance of activity within neural networks. Here we describe the effects of the GABAA receptor potentiating anesthetic etomidate on sensory processing, studied in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid fish, in vitro. Previous studies have shown that the ELL integrates sensory input and removes predictable features by comparing reafferent sensory signals with a descending electromotor command-driven corollary signal that arrives in part through parallel fiber synapses with the apical dendrites of GABAergic interneurons. These synapses show spike timing–dependent depression when presynaptic activation is associated with postsynaptic backpropagating dendritic action potentials. Under etomidate, almost all neurons become tonically hyperpolarized. The threshold for action potential initiation increased for both synaptic activation and direct intracellular depolarization. Synaptically evoked inhibitory postsynaptic potentials (IPSPs) were also strongly potentiated and prolonged. Current source density analysis showed that backpropagation of action potentials through the apical dendritic arborization in the molecular layer was reduced but could be restored by increasing stimulus strength. These effects of etomidate were blocked by bicuculline or picrotoxin. It is concluded that etomidate affects both tonic and phasic inhibitory conductances at GABAA receptors and that increased shunting inhibition at the level of the proximal dendrites also contributes to increasing the threshold for action potential backpropagation. When stimulus strength is sufficient to evoke backpropagation, repetitive association of synaptic excitation with postsynaptic action potential initiation still results in synaptic depression, showing that etomidate does not interfere with the molecular mechanism underlying plastic modulation.

scholarly journals High Threshold, Proximal Initiation, and Slow Conduction Velocity of Action Potentials in Dentate Granule Neuron Mossy Fibers

2008 ◽  
Vol 100 (1) ◽  
pp. 281-291 ◽  
Author(s):  
Geraldine J. Kress ◽  
Margaret J. Dowling ◽  
Julian P. Meeks ◽  
Steven Mennerick
Keyword(s):  
Action Potential ◽  
Conduction Velocity ◽  
Action Potentials ◽  
Mossy Fiber ◽  
Hippocampal Slices ◽  
Mossy Fibers ◽  
Comparative Approach ◽  
Granule Neurons ◽  
Action Potential Initiation ◽  
Potential Initiation

Dentate granule neurons give rise to some of the smallest unmyelinated fibers in the mammalian CNS, the hippocampal mossy fibers. These neurons are also key regulators of physiological and pathophysiological information flow through the hippocampus. We took a comparative approach to studying mossy fiber action potential initiation and propagation in hippocampal slices from juvenile rats. Dentate granule neurons exhibited axonal action potential initiation significantly more proximal than CA3 pyramidal neurons. This conclusion was suggested by phase plot analysis of somatic action potentials and by local tetrodotoxin application to the axon and somatodendritic compartments. This conclusion was also verified by immunostaining for voltage-gated sodium channel alpha subunits and by direct dual soma/axonal recordings. Dentate neurons exhibited a significantly higher action potential threshold and slower axonal conduction velocity than CA3 neurons. We conclude that while the electrotonically proximal axon location of action potential initiation allows granule neurons to sensitively detect and integrate synaptic inputs, the neurons are sluggish to initiate and propagate an action potential.

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