Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 64

The most important ions that flow through neuronal membrane channels are sodium, potassium, chloride, and calcium. A fifth ion, magnesium, is also important because it controls conduction through the NMDA receptor, a glutamate receptor. The following list tells you more about the roles of these ions inside neurons:

 Sodium: The concentration of sodium is much higher outside the cell than inside the cell. Sodium entering into cells can trigger action potentials and lead to synaptic release, two crucial neural functions.

 Potassium: This ion has a high concentration inside cells compared to fluid outside cells. So, opening potassium channels tends to hyperpolarize cells (make the inside more negative) by the exit of the positively charged potassium ions.

 Chloride: The concentration of chloride is low inside the cell compared to the outside. Although the tendency exists for diffusion to dissipate this concentration gradient, chloride ions feel a repulsive force from the negative membrane potential inside the cell. Chloride flux tends to occur mostly when the cell is depolarized by an excitatory event. Chloride currents are usually inhibitory.

 Calcium: This ion is found in very low (nanomolar to micromolar) concentrations inside neurons because it is heavily buffered by calcium chelating (binding) molecules. Calcium can enter cells through calcium-specific channels and some neurotransmitter channels such as NMDA glutamate receptors and alpha-7 nicotinic acetylcholine receptors. Calcium modulates second messenger cascades inside cells that regulate ion channels from the inside, and may also affect gene expression. The flux of calcium through voltage-dependent calcium channels results in the release of neurotransmitters from axon (the part of the neuron that conducts impulses away from the cell) terminals.

 Magnesium: Normally magnesium does not pass through neuronal membranes. However, when the neuron is close to the resting potential, magnesium, attracted to the negative charge inside, binds in the extracellular “mouth” of the NMDA glutamate receptor. This receptor will not open unless, in addition to binding glutamate released from a pre-synaptic axon terminal, the membrane is partially depolarized. This depolarization (which is often provided by nearby non-NMDA excitatory receptors) reduces the potential across the membrane and thereby favors the release of the magnesium ion, allowing sodium and some calcium to flow through the channel.