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| The Neuron as a Battery |
The neuron is an extraordinarily specialized cell. Most neurons are referred to as "bipolar"; they have a cell body and many small extensions, called dendrites, at one end which receive information (Fig. 1). At the other end is its most striking feature: a long axon that ends in "synaptic terminals," which send signals to the dendrites of an adjacent neuron. The longest axon in the human body, the one that goes from the base of the spinal cord to the big toe, is about one meter long. Early studies on the physiology of neurons examined those from the giant axon of the squid, which is so big that it is visible with the naked eye. Note that the neuron, in addition to its specialized functions, carries out nearly all of the functions of a normal cell, except for division.
The neuron is an electric battery and works by changes in its voltage. Compared with its surroundings, the inside of a "resting neuron" has a lower concentration of sodium ions and a higher concentration of potassium ions. Because of this imbalance of positively charged ions across the membrane the inside of the resting neuron is negative relative to the outside. This difference in voltage is called the membrane potential. A typical membrane potential for a neuron at rest, the resting potential, is -0.07 volts, or -70 mV. Although this is a rather modest voltage (about five percent of that of an AA battery), consider that this voltage occurs across a miniscule length - that of the cell membrane. If this were an electric field, the charge separation would be about 100,000 volts per centimeter.
Note that the term "resting neuron" refers only to its electrical state. The cell is really not at rest because, in addition to carrying out all of the normal functions of the cell, the neuron has to maintain this ionic imbalance. This is achieved by the sodium-potassium pump, which actively transports potassium in and sodium out. The pump maintains a negative voltage because it actually pumps three sodium ions out for every two potassium ions it pumps in. The membrane potential of a neuron at any given time is the product of many variables, including the imbalance of ions across the membrane, and the membrane's permeability to each ion. In addition to sodium and potassium, chloride is an important ion in "setting" a neuron's rest potential because negatively charged chloride ions can pass through open "leak channels" at rest. Another ion crucial for neural communication is calcium, which acts as a powerful intracellular signaling molecule once it enters through its ion channels.