Intro | Inside Electrode at Rest | Outside Electrode at Rest | Inside Electrode with Action Potential | Outside Electrode with Action Potential
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Figure 2 illustrates the recording of electrical charge inside and outside a neuron during its resting state and during an action potential. An electrical current travels along the cell membrane of a neuron when it is stimulated. The rapid movement or transmission of an action potential along the cell membrane is called the neuronal impulse. The neuronal impulse is an all-or-nothing event that occurs only when stimulation is strong enough.
Studies of this phenomenon in the 1930's using "giant" squid axons measured the electrochemical changes that underlie this neuronal impulse and propagation or transmission of the signal. The charges both inside and outside the cell membrane were recorded using an oscilloscope, an instrument that uses a fluorescent screen to display a visual representation of electrical variations. The swimming mechanism of the squid (an aquatic animal found in the Atlantic Ocean) consists of an enlarged tubular structure or giant axon that is several times larger than the biggest human axon. The large size of the squid axon allowed for easier insertion of the recording electrodes of the oscilloscope. Neuroscience now has the micro-technology to measure directly from microscopic elements of the human neuron. A commonly used microelectrode is made of glass tube that is tapered to a tip diameter of 0.0005 millimeters or less and filled with a solution of a current conducting salt such as potassium chloride.
This line of research has revealed that while at rest the neuron has different concentrations of negative and positive ions within the fluid found on either side of the cell membrane. This difference in concentration creates an electrical condition similar to a battery. Sodium and potassium ions (which are both positively charged) and chloride ions (which are negatively charged) move across the cell membrane at different rates. These differences in ionic permeability underlie the membrane potentials shown in the figure.
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Structural and functional integrity of the membrane of a neuron is essential to maintaining the resting potential, generating an action potential, and propagating a neuronal impulse. The neuronal membrane is composed of two fat layers (phospholipids) with protein molecules embedded between and within. The phospholid molecules have a water-attracting head (which form the outer and inner boundaries of the membrane where contact is made with extra-cellular fluids and cytoplasm) and two water-repelling tails (which form the internal layer of the membrane. The membrane is approximately eight nanometers thick; or less than 0.00001 millimeters. This molecular structure provides the neuron with an adequately firm, yet flexible boundary that can control the movement of substances into and out of the cell.
| Suggestions for further study |
Keynes, R.D. (1979, March). Ion channels in the nerve-cell membrane, Scientific American, 240(3), 126-132, 134-135.
Neher, E., Sakmann, B. (1992, March). The patch clamp technique, Scientific American, 266(3), 28-35.
Regan, D. (1979, December). Electrical responses evoked from the human brain, Scientific American, 241(6), 134-146.
http://www.sfn.org/briefings/nmda.html
(NMDA Receptors)
from Society for Neuroscience -- Brain Briefings, 1994.
NMDA receptor blockers and the prevention of neuronal damage due to
stroke, epilepsy, Huntington's Disease, and AIDS.