Second, the electrical gradient, due to the negative charge within the neuron, tends to pull the positively charged ion inside in the cell. First, the high concentration of Na+ outside the cell pushes it into the cell down the concentration gradient. Na+ is attracted to the inside of neurons at rest by two forces. It is composed mainly of water with dissolved salts and many other chemical substances. The extracellular fluid is the medium in which neurons and glia are embedded. It composes the bulk of cellular material, and provides a suspension medium for organelles and free-floating molecules. The intracellular fluid is the clear viscous fluid within a neuron. This membrane is composed of two layers of lipid (or fat) molecules (phospholipids, in particular). The cell membrane forms the border of a neuron and acts to control the movement of substances into and out of the cell. While K+ is positively charged and more abundant on the inside, there exists a great amount of negatively charged particles (the anions), accounting for the negative charge inside the membrane. As you can see, there are more K+ and anions in the intracellular fluid, and more Na+ and Cl- in the extracellular fluid. The charged particles mentioned above are unequally distributed across the membrane. This charge across the membrane is a store of potential energy and can be used at a future time. The inside of the membrane is -70 millivolts (mV) relative to the extracellular side. The resting potential refers to an inactive axon's difference in electrical charge across its membrane, as measured by a voltmeter. As ions move in and out of the cell, they bring their respective charges (either positive or negative) with them, thus influencing the voltage of cellular membranes. The movement of such ions across neural membranes creates electrical activity. Additionally, these fluids contain many negatively charged protein molecules called anions (A-). These include sodium (Na+), potassium (K+), and chloride (Cl-). Intracellular and extracellular fluid of neurons contain various kinds of charged ions. Electrical Properties of Neurons The Resting Potential
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