Components of Conduction and Transmission of Impulse in the Nervous System
The fundamental components of neural impulse conduction and transmission consist of transmembrane currents generated by ionic movements across neuronal membranes, which create electrical signals that propagate through volume conduction in the extracellular space, with the largest contributions coming from neurons with extended dendritic arbors receiving spatially organized synaptic inputs. 1
Cellular and Molecular Components
Transmembrane Current Generation
Transmembrane currents are the primary source of all electrical and magnetic signals in the nervous system, generated through both active mechanisms (voltage-gated ion channels) and passive mechanisms (capacitive coupling between conductive elements). 1
The neuronal membrane functions as a capacitor while ion channels act as resistors, with the lipid bilayer separating charged ions between intracellular and extracellular compartments. 2
At rest, potassium permeability dominates, establishing the resting membrane potential close to the potassium equilibrium potential through differential ion channel conductances. 2
Action Potential Mechanism
Action potential generation requires a large, brief sodium influx followed by voltage-dependent potassium efflux, representing the core signaling mechanism for activating synaptic transmission at axon terminals. 2
The pattern of action potential firing depends on the interaction of multiple voltage-dependent ion conductances, with the neuron integrating synaptic potentials to generate either subthreshold potentials or suprathreshold depolarizations. 2
High-frequency extracellular signals (above several hundred Hz) are dominated by spiking activity, while low-frequency signals (below tens of Hz) primarily reflect subthreshold synaptic activation, though spikes contribute to both frequency ranges. 1
Spatial Organization and Field Generation
Neuronal Morphology Contributions
Neurons with extended, oriented dendritic arbors generate the largest current dipoles and contribute most significantly to field potentials, particularly pyramidal neurons in hippocampus and cortex with their open-field geometry. 1
Temporally correlated synaptic inputs at restricted dendritic sites contribute strongly to field potentials, with spatially organized inputs onto basal or apical dendrites generating the largest extracellular potentials. 1
Neurons with symmetric dendritic arbors (stellate cells, basket cells) form "closed fields" and contribute relatively little when receiving spatially uniform synaptic input, as their transmembrane currents tend to cancel. 1
Volume Conduction
Volume conduction allows electromagnetic fields to propagate through biological tissues, enabling distant sources to contribute directly to field potentials through the spread of electrical activity. 1
The extracellular medium is essentially resistive (ohmic) with negligible capacitive component across the frequency range relevant for neuronal recordings, based on experimental evidence showing weak frequency dependencies. 1
Field potentials sum activity from all electrically active membranes in space and time, from axon terminals to soma, encompassing action potentials to very slow conductances. 1
Biophysical Modeling Framework
Cable Theory Application
Cable theory simplifies three-dimensional neuronal complexity to one-dimensional core conductors, allowing calculation of transmembrane currents across compartmentalized neuronal structures. 1
The sum of transmembrane ionic and capacitive currents across the entire cellular surface must equal zero, meaning currents entering the cell are balanced by currents leaving at other locations. 1
Dipole Approximation
For distant recordings (MEG, EEG), current sources can be represented using dipole approximation, summarizing all microcurrent sources within a tissue volume by an effective dipole moment per unit volume. 1
Geometric factors often dominate over synaptic strength in determining effective dipole moment, with spatial separation of positive and negative sources creating larger signals than symmetrically distributed sources. 1
Synaptic Transmission Components
Synaptic transmission efficacy is graded based on calcium influx into presynaptic terminals, with the number and shape of action potentials determining calcium entry and transmission strength. 2
Presynaptic ion channels can be modulated by neurotransmitters, affecting synaptic transmission by altering calcium influx independent of action potential characteristics. 2
Excitation conduction occurs without noteworthy energy consumption, as ion current flow through membranes follows concentration gradients. 3
Critical Caveats
Myelinated axonal fibers, nonmyelinated axon compartments, and presynaptic terminals contribute relatively little to local field potentials due to their small membrane areas, despite their importance in signal propagation. 1
The crossover frequency between spike-dominated and synapse-dominated signals varies by brain region and state, ranging from ~50 Hz in visual cortex to 100 Hz in hippocampus. 1
Small-amplitude current sources arranged in particular spatial configurations (sheets) can generate large-amplitude signals, while interference by return currents in other neurons can cancel active synaptic currents. 1