Neurons are surrounded by a plasma memebrane (a fluid, organic membrane made largely of a phospholipid bilayer). The inside of a neuron contains the cytoplasm, various organelles, and many other things that a neuron needs to survive. It also contains a large amount of potassium ions (K⁺). Even though they contain a lot of positively charged potassium ions, neurons contain a large number of negatively charged proteins to balance this out(to some extent). The outside of a neuron is generally a neutral environment containing large concentrations of sodium (Na⁺)and chlorine(Cl^-), and smaller amounts of potassium. It is important to note two facts, one, ions tend to move down their concentration gradients to a point of equilibrium. This means that the sodium, chlorine, and potassium are constantly trying to get across the neural membrane. Equilibrium is reached when the concentration on the inside of the membrane equals the concentration out side, and is different for each ion. This kind of equilibrium is called a chemical equilibrium, and a difference in concentration between two areas is called a chemical gradient. When something is said to flow down or along a gradient, it moves from an area of high concentration to an area of lower concentration. The second fact that is important to note is that ions also tend to move down an electrical gradient. This means that positevely charged ions, want to go to areas of high negative charge, and negatively charged ions try to go to areas of high positive charge. Thus ions in solution will move to favor a more neutral environment, rather than segregating by charge. The best way of looking at electrical gradients is similar to chemical gradients; whenever you have a large concentration of a charge in an area, those charges seek to move away from each other, to areas which have fewer of the charges in question. This statement is reinforced by the basic scientific principle that like charges repel, and unlike charges attract. It is evident then that there are multiple forces acting on the ions in a system, and it is important to keep track of and understand their effects on the neuron. The total forces acting on the system due to the chemical and electrical gradients is called an electrochemical gradient. *It is important to note that movement of ions along their gradient requires no energy, and indeed is actually a source of potential energy; however to move ions against their gradients, from low to high concentration or toward an area of similar charge, requires the input of energy. This can be equated to rolling a boulder down a hill. To move a boulder from the top of a mountain (an area of high concetration) to the bottom of the mountain (an area of low concentration), one just lets the boulder go. However to move the boulder from the bottom to the top (from low to high concentration) requires considerable energy. The height of the mountain in question can also be equated to the magnitude of the difference in concentration. The larger the difference, the higher the "mountain" the boulder must climb, the more energy that must be exerted to move the ion*.
Whenever there is a difference in charge set up across a barrier there is the potential to release energy. This is known as voltage and is measured in volts. It is the same principle for which a battery and all electrical devices rely on. In the neuron there is a slighty negative inside (approximately -70 mV) and a neutral outside (O V), seperated by a membrane. It is this difference that is the basis of the electrical nerve impulse. The cell membrane maintains this energy potential (also known as the membrane potential) by keeping the charges (ions) seperated. Any change in the delicate balance of ions will change a neurons ability to fire (send a nerve impulse). Cell membranes contain active ion pumps that help to maintain the appropriate ion concentrations across the membrane. These ion pumps are special trans-membrane proteins. They have contact with both the inside and outside portions of the neural membrane. These ion pumps can be equated to a bicycle pump in that they take something from somewhere, and put it somewher else. In this case they take ions form one side of the membrane and transport them to the other side. They are also akin to bicycle pumps by their need for the input of energy in order to operate. A bicycle pump uses mechanical energy (given by your arms as you push the pump handle up and down), while the ion pumps utilize energy stored in the chemical bonds of an organic molecule called ATP, which is a product of cellular metabolism (metabolism is a process that involves the breakdown of glucose). One of the most important active ion pumps is called the sodium-potassium pump. These proteins transport three sodium ions from inside the neuron to the outside, and two potassium ions from the outside to the inside.