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Activity and Communication within the Neuron
The neuron is, in many respects, just a cell. It has a nucleus on the inside and a cell membrane that defines its perimeter. In the middle is a biochemical stew of ions, amino acids, proteins, and DNA along with a collection of smaller structures that provide for the metabolic needs of the cell itself. What makes a neuron distinctive, though, is the peculiarity of its cell membrane. The membrane is highly sensitive to stimulation. Poke it, stimulate it electrically or chemically, and the neuronal membrane actually changes its structure, producing a cascade of changes that can eventually lead to an electrical signal called an action potential. This signal—sent from one end of the neuron to the other—is the neuron’s main response to input as well as the fundamental information carrier of the nervous system.
At its heart, the action potential involves electrical changes. To study these changes, we can begin by inserting one microelectrode into a neuron’s axon and placing a second electrode on the axon’s outer surface (Figure 3.9). This setup allows us to measure elec-trical activity near the cell’s membrane. It also tells us that even when the neuron is not being stimulated in any way, there’s a voltage difference between the inside and the out-side of the cell. Like a miniature battery with a positive and a negative connection, the inside of the axon is electrically negative relative to the outside, with a difference of –70 or so millivolts (a standard AA battery, at 1.5 volts, has over 200 times this voltage). Because this small voltage difference occurs when the neuron is stable, it’s traditionally called the neuron’s resting potential.
How does this situation change when the neuron is stimulated? To find out, we can use a third microelectrode to apply a brief electrical pulse to the outside surface of the cell. This pulse reduces the voltage difference across the membrane. If the pulse is weak, it may reduce the voltage difference a little bit, but the neuron’s mem-brane will maintain its integrity and quickly work to restore the resting potential of –70 millivolts. But if the pulse is strong enough to push the voltage difference past a critical excitation threshold (about –55 millivolts in mammals), something dramatic happens. In that region of the cell, the voltage difference between the inside and outside of the membrane abruptly collapses to zero and, in fact, begins to reverse itself. The inside of the membrane no longer shows a negative voltage compared to the outside; instead, it suddenly swings positive—up to +40 millivolts. This momen-tary change in voltage, the action potential, is crucial for the functioning of the neuron. This change is short-lived, and the resting potential is restored within a millisecond or so.
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