Finding the Location of Axonal Activation by a Miniature Magnetic Coil.

Magnetic stimulation for neural activation is widely used in clinical and lab research. In comparison to electric stimulation using an implanted electrode, stimulation with a large magnetic coil is associated with poor spatial specificity and incapability to stimulate deep brain structures. Recent developments in micromagnetic stimulation (µMS) technology mitigates some of these shortcomings. The sub-millimeter coils can be covered with soft, biocompatible material, and chronically implanted. They can provide highly specific neural stimulation in the deep neural structure. Although the µMS technology is expected to provide a precise location of neural stimulation, the exact site of neural activation is difficult to determine. Furthermore, factors that could cause the shifting of the activation site during µMS have not been fully investigated. To estimate the location of axon activation in µMS, we first derived an analytical expression of the activating function, which predicts the location of membrane depolarization in an unmyelinated axon. Then, we developed a multi-compartment, Hodgkin-Huxley (H-H) type of NEURON model of an unmyelinated axon to test the impact of several important coil parameters on the location of axonal activation. The location of axonal activation was dependent on both the parameters of the stimulus and the biophysics properties of the targeted axon during µMS. The activating function analysis predicted that the location of membrane depolarization and activation could shift due to the reversal of the coil current and the change in the coil-axon distance. The NEURON modeling confirmed these predictions. Interestingly, the NEURON simulation further revealed that the intensity of stimulation played a significant role in the activation location. Moderate or strong coil currents activated the axon at different locations, mediated by two distinct ion channel mechanisms. This study reports several experimental factors that could cause a potential shift in the location of neural activation during µMS, which is essential for further development of this novel technology.

Axonal GABAA stabilizes excitability in unmyelinated sensory axons secondary to NKCC1 activity.

KEY POINTS: GABA depolarized sural nerve axons and increased the electrical excitability of C-fibres via GABAA receptor. Axonal excitability responses to GABA increased monotonically with the rate of action potential firing. Action potential activity in unmyelinated C-fibres is coupled to Na-K-Cl cotransporter type 1 (NKCC1) loading of axonal chloride. Activation of axonal GABAA receptor stabilized C-fibre excitability during prolonged low frequency (2.5 Hz) firing. NKCC1 maintains intra-axonal chloride to provide feed-forward stabilization of C-fibre excitability and thus support sustained firing. ABSTRACT: GABAA receptor (GABAA R)-mediated depolarization of dorsal root ganglia (DRG) axonal projections in the spinal dorsal horn is implicated in pre-synaptic inhibition. Inhibition, in this case, is predicated on an elevated intra-axonal chloride concentration and a depolarizing GABA response. In the present study, we report that the peripheral axons of DRG neurons are also depolarized by GABA and this results in an increase in the electrical excitability of unmyelinated C-fibre axons. GABAA R agonists increased axonal excitability, whereas GABA excitability responses were blocked by GABAA R antagonists and were absent in mice lacking the GABAA R ß3 subunit selectively in DRG neurons (AdvillinCre or snsCre ). Under control conditions, excitability responses to GABA became larger at higher rates of electrical stimulation (0.5-2.5 Hz). However, during Na-K-Cl cotransporter type 1 (NKCC1) blockade, the electrical stimulation rate did not affect GABA response size, suggesting that NKCC1 regulation of axonal chloride is coupled to action potential firing. To examine this, activity-dependent conduction velocity slowing (activity-dependent slowing; ADS) was used to quantify C-fibre excitability loss during a 2.5 Hz challenge. ADS was reduced by GABAA R agonists and exacerbated by either GABAA R antagonists, ß3 deletion or NKCC1 blockade. This illustrates that activation of GABAA R stabilizes C-fibre excitability during sustained firing. We posit that NKCC1 acts in a feed-forward manner to maintain an elevated intra-axonal chloride in C-fibres during ongoing firing. The resulting chloride gradient can be utilized by GABAA R to stabilize axonal excitability. The data imply that therapeutic strategies targeting axonal chloride regulation at peripheral loci of pain and itch may curtail aberrant firing in C-fibres.

Excitation properties of computational models of unmyelinated peripheral axons.

Biophysically based computational models of nerve fibers are important tools for designing electrical stimulation therapies, investigating drugs that affect ion channels, and studying diseases that affect neurons. Although peripheral nerves are primarily composed of unmyelinated axons (i.e., C-fibers), most modeling efforts focused on myelinated axons. We implemented the single-compartment model of vagal afferents from Schild et al. (1994) (Schild JH, Clark JW, Hay M, Mendelowitz D, Andresen MC, Kunze DL. J Neurophysiol 71: 2338-2358, 1994) and extended the model into a multicompartment axon, presenting the first cable model of a C-fiber vagal afferent. We also implemented the updated parameters from the Schild and Kunze (1997) model (Schild JH, Kunze DL. J Neurophysiol 78: 3198-3209, 1997). We compared the responses of these novel models with those of three published models of unmyelinated axons (Rattay F, Aberham M. IEEE Trans Biomed Eng 40: 1201-1209, 1993; Sundt D, Gamper N, Jaffe DB. J Neurophysiol 114: 3140-3153, 2015; Tigerholm J, Petersson ME, Obreja O, Lampert A, Carr R, Schmelz M, Fransén E. J Neurophysiol 111: 1721-1735, 2014) and with experimental data from single-fiber recordings. Comparing the two models by Schild et al. (1994, 1997) revealed that differences in rest potential and action potential shape were driven by changes in maximum conductances rather than changes in sodium channel dynamics. Comparing the five model axons, the conduction speeds and strength-duration responses were largely within expected ranges, but none of the models captured the experimental threshold recovery cycle-including a complete absence of late subnormality in the models-and their action potential shapes varied dramatically. The Tigerholm et al. (2014) model best reproduced the experimental data, but these modeling efforts make clear that additional data are needed to parameterize and validate future models of autonomic C-fibers.NEW & NOTEWORTHY Peripheral nerves are primarily composed of unmyelinated axons, and there is growing interest in electrical stimulation of the autonomic nervous system to treat various diseases. We present the first cable model of an unmyelinated vagal nerve fiber and compare its ion channel isoforms and conduction responses with other published models of unmyelinated axons, establishing important tools for advancing modeling of autonomic nerves.

Spike propagation in axons under stretch growth conditions in cultured neurons from dorsal root ganglion.

Computational software NEURON was used to simulate the stretch growth neurons in order to investigate the ability of dorsal root ganglion neurons to generate and propagate action potentials after a period of rapid axon stretch growth in vitro, and under what stimulating parameters can evoke action potentials. In the simulation, we found the stretch growth neuron had higher spike amplitude than from the static culture neuron in the soma and all axonal branch. In addition, the conduction velocity was also faster in the stretch growth axon. When the stimulating frequency was less than 15 Hz or the stimulating voltage was lower than 15 mV, no spike was evoked. Increasing stimulating frequency from 15 Hz to 5000 Hz or stimulating voltage from 15 mV to 100 mV had almost no effect on the spike amplitude. Interestingly, the first spike time and absolute refractory period (ARP) in different axonal branches and somas decreased stepwise with incremental increase in the stimulating frequency. It is concluded that the stretch growth neuron had higher amplitude and faster conduction velocity than the static culture neuron. In addition, some stimulating parameters had been analyzed in this study, which provided guidelines for electrophysiological experiments in future.

Spike propagation through the dorsal root ganglia in an unmyelinated sensory neuron: a modeling study.

Unmyelinated C-fibers are a major type of sensory neurons conveying pain information. Action potential conduction is regulated by the bifurcation (T-junction) of sensory neuron axons within the dorsal root ganglia (DRG). Understanding how C-fiber signaling is influenced by the morphology of the T-junction and the local expression of ion channels is important for understanding pain signaling. In this study we used biophysical computer modeling to investigate the influence of axon morphology within the DRG and various membrane conductances on the reliability of spike propagation. As expected, calculated input impedance and the amplitude of propagating action potentials were both lowest at the T-junction. Propagation reliability for single spikes was highly sensitive to the diameter of the stem axon and the density of voltage-gated Na(+) channels. A model containing only fast voltage-gated Na(+) and delayed-rectifier K(+) channels conducted trains of spikes up to frequencies of 110 Hz. The addition of slowly activating KCNQ channels (i.e., KV7 or M-channels) to the model reduced the following frequency to 30 Hz. Hyperpolarization produced by addition of a much slower conductance, such as a Ca(2+)-dependent K(+) current, was needed to reduce the following frequency to 6 Hz. Attenuation of driving force due to ion accumulation or hyperpolarization produced by a Na(+)-K(+) pump had no effect on following frequency but could influence the reliability of spike propagation mutually with the voltage shift generated by a Ca(2+)-dependent K(+) current. These simulations suggest how specific ion channels within the DRG may contribute toward therapeutic treatments for chronic pain.

Immunocytochemical Localization of Monoamine Oxidase Type B in Rat's Peripheral Nervous System.

Immunohistochemistry is used to investigate subcellular localization of monoamine oxidase type B (MAOB) in the axon of the rat's peripheral nervous system. Through light and electron microscopy, the presence of MAOB-immunoreactive structures in the propria lamina of tongue and on the outer membranes of mitochondria in both myelinated and unmyelinated axons can be detected. As a result, MAOB may potentially play a crucial role in the axons of the rat's peripheral nervous system and may be closely associated with both axonal transport and nerve conduction.

Efecto de la conductancia de potasio Kv3.1 en la tasa y frecuencia de disparo de un axón no mielinado / Efect of the Kv3.1 potassium conductance in the firing frequency and firing rate of an unmyelinated axón

Objetivo: Determinar la relación entre la conductancia de potasio Kv3.1 y la tasa de disparo (Td) de un modelo neuronal llamado neurona1 formado por un soma, un cuello y un axón no mielinado durante un estímulo de corriente de 10 ms de duración y a 40°c. Materiales y métodos: A partir del software libre neuron se simuló la propagación de ráfagas de potenciales de acción a través de neuronal, variando la conductancia específica máxima de potasio Kv3.1 (G Kv31) relativa a la conductancia específica máxima de potasio (G K) estudiada por a.l. Hodgkin y a.f. Huxley en 1952, de tal forma que G Kv31+G K=1.6S/ cm². Resultados: En una estructura neuronal con las características biofísicas de neuronal, Td varía en forma sigmoidea para 0 < G Kv31/G K < 0.455 y decae exponencialmente para 0.455 < G Kv31/G K < 15, respectivamente. Para el primer caso, Td aumenta 11 veces más que la frecuencia (f) respecto del número de espigas en cada ráfaga. Conclusión: La observación de la conductancia de potasio del tipo Kv3.1 en algún tipo de neurona no implica necesariamente la propagación de ráfagas de alta tasa de disparo. Su efecto es más pronunciado (11 veces) en la modulación de Td que en el aumento de f.

Objective: To determine the relationship between the Kv3.1 potassium conductance and the firing rate (Td) in a neuronal model called neuron1, consisting of a soma, a hillock and an unmyelinated axon, during a constant current stimulus 10ms long and at 40°c. Methodology: Using the free software neuron, the propagation of action potentials along a neuronal structure called neurona1 was simulated. The maximum Kv3.1 conductance (G Kv31) relative to the maximum potassium conductance (Gk) studied in 1952 by a.l. Hodgkin and a.f. Huxley and in this paper called HH conductance, was varied such that Gk, +G Kv3.1= 1.6s/cm2. Results: In a neuronal structure with the biophysical characteristics of neuronl, Td varies in a sigmoid way for all g kv31such that 0