Transcranial brain stimulation and evidence of ephaptic coupling have sparked strong interests in understanding the effects of weak electric fields on the dynamics of neuronal populations. While their influence on the subthreshold membrane voltage can be biophysically well explained using spatially extended neuron models, mechanistic analyses of neuronal spiking and network activity have remained a methodological challenge. More generally, this challenge applies to phenomena for which single-compartment point neuron models are oversimplified. Here we employ a pyramidal neuron model that comprises two compartments, allowing to distinguish basal-somatic from apical dendritic inputs and accounting for an extracellular field in a biophysically minimalistic way. Using an analytical approach we fit its parameters to reproduce the response properties of a canonical, spatial model neuron and dissect the stochastic spiking dynamics of single cells and large networks. We show that oscillatory weak fields effectively mimic anti-correlated inputs at the soma and dendrite and strongly modulate neuronal spiking activity in a rather narrow frequency band.
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In recent years, it has been demonstrated an increasing role of astrocytes in information processing and cognitive functions, which depends on the close relationship between neurons and astrocytes into the cerebral cortex. The mammalian cerebral cortex is fundamentally composed by neurons and astrocytes organized in two closely related three-dimensional structures. Neurons are organized in layers and columns and communicate each other through synapses, while astrocytes are connected each other through intercellular channels gap-junctions forming a complex structural and functional syncytium.
In addition to synaptic neurotransmission, other non-synaptic mechanisms called ephaptic interactions have been considered critical for coupling and synchronization of neurons in the cerebral cortex. Schematic representation of the ultrastructure of the neuron-astroglial organization in the neocortex. Astrocytes are stellated white cells with a black nucleus. Neuronal axons A , and dendrites D are surrounded by the astroglial matrix. The extracellular space is near virtual m in physiological conditions.
The addition of synaptic currents, action potential currents and astrocytic ionic currents generate local field potential LFPs in the intercellular space. These ephaptic effects are due to the summation of all sources that contribute to the extracellular local field potential LFP in a point of the cortex, which results from the addition of synaptic currents, action potential currents, and astrocytic ionic currents.
The effects of the action potentials and astrocytic ionic currents have been considered negligible in comparison to the contribution of synaptic currents. Therefore, the role of astroglial electric and magnetic fields in information processing has been scarcely studied. These waves were associated with a reduction in neuronal activity and LFPs in the hippocampus. From an ultrastructural perspective, the extracellular space in the mammalian cerebral cortex is a very narrow space among the cytoplasmic membranes of neurons and astrocytes Fig.
This intercellular space has a low electrical resistance that makes it a preferred pathway over trans-membrane paths for induced currents of endogenous electric and magnetic fields. The electrophysiological correlation between astrocytes and neurons has been attributed mainly to extracellular changes of ionic currents following a complex spatial and temporal fluctuation in which the astroglial syncytium plays a fundamental function regulating the resting potential as well as the excitability of cortical neurons.
In fact, it has been demonstrated a high concordance between the amplitude of the magnetic fields of neurons obtained by experimental and theoretical approaches, supporting the concept that bio-magnetic fields generated into the cerebral cortex can have enough amplitude to be biologically active.
Therefore, the orientation of neurons and their anatomical relationship with astrocytes in the human cerebral cortex seems to be a critical factor determining the effects of bio-electric and bio-magnetic fields produced by neuronal currents, which can modulate adjacent neurons and astrocytes by ephaptic interaction in the adequate intra-cortical topology.
Besides, bio-electro-magnetic fields generated by ionic currents in the astroglial syncytium may likely modulate feedback the activity and coupling of neighboring neurons by a direct magnetic modulation of the intercellular LFPs. Recent interest in this issue comes from the fact that extra-cranial electric and magnetic field stimulations have shown therapeutic actions in the clinical practice. Martinez-Banaclocha M Neuroscience. You must be logged in to post a comment.
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For this reason, for…. Astrocytes modulate spinal pain processing by way of… Astrocytes were long thought to play only a supporting role in the central nervous system.
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Ephaptic coupling is a form of communication within the nervous system and is distinct from direct communication systems like electrical synapses and chemical synapses. It may refer to the coupling of adjacent touching nerve fibers caused by the exchange of ions between the cells, or it may refer to coupling of nerve fibers as a result of local electric fields. Myelination is thought to inhibit ephaptic interactions. The idea that the electrical activity generated by nervous tissue may influence the activity of surrounding nervous tissue is one that dates back to the late 19th century. Early experiments, like those by du Bois-Reymond,  demonstrated that the firing of a primary nerve may induce the firing of an adjacent secondary nerve termed "secondary excitation". This effect was not quantitatively explored, however, until experiments by Katz and Schmitt  in , when the two explored the electric interaction of two adjacent limb nerves of the crab Carcinus maenas. Their work demonstrated that the progression of the action potential in the active axon caused excitability changes in the inactive axon.
Ephaptic coupling in cortical neurons
The electrochemical processes that underlie neural function manifest themselves in ceaseless spatial and temporal fluctuations in the extracellular electric field. The local field potential LFP , used to study neural interactions during various brain states, is regarded as an epiphenomenon of coordinated neural activity. Yet the extracellular field activity feeds back onto the electrical potential across the neuronal membrane via ephaptic coupling Jefferys et al, Physiol Rev, We stimulated in rat somatosensory cortical slices a variety of layer 5 neural types and recorded inside and outside their cell bodies while pharmacologically silencing synaptic transmission. Pyramidal cells couple to the extracellular field distinctly different from interneurons.