Final answer:
Charge screening in neurons results from the balanced distribution of ions across the neuron's selectively permeable membrane, establishing a resting membrane potential. Cable theory assumes passive electrical properties and does not directly account for active biological processes like ion channel dynamics or charge screening. Myelination in axons allows for saltatory conduction which minimizes charge screening, speeding up nerve impulse transmission.
Step-by-step explanation:
Charge screening in neurons occurs due to the distribution of ions across the neuronal membrane. In a resting neuron, the cell membrane is selectively permeable to ions like K+ and Cl-, which gives rise to a voltage difference or membrane potential across the membrane when K+ diffuses out due to a concentration gradient but is restricted from totally equalizing across the membrane by the Coulomb force. This electrochemical gradient results in areas of positive and negative charges that remain because like charges repel and unlike charges attract, preventing further ion movement. This process is outlined in Figure 20.25 and is essential for maintaining the resting membrane potential. During an action potential, voltage-gated sodium channels open in response to a depolarization above a certain threshold, allowing Na+ ions to flow into the neuron, which temporarily disrupts the charge distribution, creating the nerve impulse.
In the context of cable theory and charge screening, cable theory assumes that electrical properties of an axon can be described by passive spread of potential changes, which, at nanometer scales and in the time frame of neuronal signaling, would not involve significant charge screening effects. Cable theory does not take into account the complex interactions of ions, channels, and the membrane at physiological conditions. Therefore, it seems to lack a direct account for charge screening, which is a more nuanced and dynamic process influenced by active properties of neurons such as the opening of ion channels and the effects of myelination.
Myelinated axons allow saltatory conduction, where the action potential jumps from node to node (Nodes of Ranvier), minimizing the need for charge screening as there are fewer ion exchanges across the membrane. This myelination and node clustering of ion channels thereby speed up signal transmission and largely prevent charge dissipation that might necessitate screening.