Lab #6: Neurophysiology Simulation

Neurons (Fig 6.1) are cells in the nervous system that are used conduct signals at high speed from one part of the body to another. This enables rapid, precise responses to occur in order to compensate for changes in the environment. Neurons are able to send signals at high speed due to their ability to generate and conduct an electrical signal called an action potential down the length of their axons. An action potential is a brief reversal of the membrane potential, so that for a brief interval at a segment of the axon the intracellular fluid just inside of the plasma membrane is more positive than is the extracellular fluid just outside the plasma membrane. This signal is typically generated at the axon hillock of the neuron, and requires the opening of voltage-gated ion channels — specialized pore-like transmembrane proteins that open to allow ion passage in response to changes in the relative charge difference across the plasma membrane. There are two different types of voltage-gated ion channels important for the generation action potentials: those specific for sodium ion (Na), and those specific for potassium ion (K). In the intervals between action potentials (i.e., when the neuron is “resting”) the two types of ions are kept at different concentrations across the plasma membrane (Fig 6.2). Na is maintained at higher concentrations outside the cell than inside the cell. Conversely, K tends to be accumulated at higher concentrations inside the cell than outside the cell. The potential for movement of these ions across the cell membrane is thus influenced by the concentration gradients for each ion. Moreover, charge differences across the cell membrane affect the potential for diffusion of these ions. The interior of cells is typically more negatively charged than is the outside of the cell, due to negative charges on certain side-chains of the amino acids of proteins inside the cell, phosphorylated compounds (e.g., ATP), etc. As a result, under resting conditions, there is a strong electrochemical gradient favoring the flow of Na into the cell, and a weak electrochemical gradient favoring the flow of K out of the cell. Ion concentrations are maintained at relatively constant levels, however, due to the normally low permeability of the plasma membrane to Na and low-level activity of the Na/K pump, which pumps Na back out into the extracellular fluid and K back into the intracellular fluid. The distribution of charged particles across the cell membrane at rest generates the resting potential of the cell membrane, which is variable among different neurons, but typically around -70 mV. The membrane potential (the difference in overall charge across the plasma membrane) of the neuron can change if the relative difference in charges across the membrane is changed. The action potential is generated by just such a redistribution of charged particles across the membrane. By opening large numbers of voltage-gated channels, the permeability of the membrane to Na and K is increased markedly, allowing the ions to flow along their respective electrochemical gradients from one side of the membrane to the other. Figure 6.2. Distribution of ions across the plasma membrane during resting potential. Different font sizes for Na and K indicate differences in relative concentration.