The permeability of the axon membrane to Na+ and K+ is regulated by gates, which open in response to stimulation. Net diffusion of these ions occurs in two stages: first Na+ moves into the axon, then K+ moves out. This flow of ions,and the changes in the membrane potential that result, constitute an event called an action potential.
All cells in the body maintain a potential difference (voltage) across the membrane, or resting membrane potential, in which the inside of the cell is negatively charged in comparison to the outside of the cell (for example, in neurons it is -70 mV). As explained in chapter 6, this potential difference is largely the result of the permeability properties of the plasma membrane. The membrane traps large, negatively charged organic molecules within the cell and permits only limited diffusion of positively charged inorganic ions. These properties result in an unequal distribution of these ions across the membrane. The action of the Na+/K+ pumps also helps to maintain a potential difference because they pump out three sodium ions (Na+) for every two potassium ions (K+) that they transport into the cell. Partly as a result of these pumps, Na+ is more highly concentrated in the extracellular fluid than inside the cell, whereas K+ is more highly concentrated within the cell.
Although all cells have a membrane potential, only a few types of cells have been shown to alter their membrane potential in response to stimulation. Such alterations in membrane potential are achieved by varying the membrane permeability to specific ions in response to stimulation. A central aspect of the physiology of neurons and muscle cells is their ability to produce and conduct these changes in membrane potential. Such an ability is termed excitability or irritability.
An increase in membrane permeability to a specific ion results in the diffusion of that ion down its concentration gradient, either into or out of the cell. These ion currents occur only across limited patches of membrane (located fractions of a millimeter apart), where specific ion channels are located. Changes in the potential difference across the membrane at these points can be measured by the voltage developed between two electrodes— one placed inside the cell and the other placed outside the plasma membrane at the region being recorded. The voltage between these two recording electrodes can be visualized by connecting them to an oscilloscope (fig. 7.11).
In an oscilloscope, electrons from a cathode-ray "gun" are sprayed across a fluorescent screen, producing a line of light. This line deflects upward or downward in response to a potential difference between the two electrodes. The oscilloscope can be calibrated in such a way that an upward deflection of the line indicates that the inside of the membrane has become less negative (or more positive) compared to the outside of the membrane. A downward deflection of the line, conversely, indicates that the inside of the cell has become more negative. The oscilloscope can thus function as a voltmeter with an ability to display voltage changes as a function of time.
If both recording electrodes are placed outside of the cell, the potential difference between the two will be zero (because there is no charge separation). When one of the two electrodes penetrates the cell membrane, the oscilloscope will indicate that the intracellular electrode is electrically negative with respect to the extracellular electrode; a membrane potential is recorded. We will call this the resting membrane potential (rmp) to distinguish it from events described in later sections. All cells have a resting membrane potential, but its magnitude can be different in different
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■ Figure 7.11 Observing depolarization and hyperpolarization.
The difference in potential (in millivolts [mV]) between an intracellular and extracellular recording electrode is displayed on an oscilloscope screen. The resting membrane potential (rmp) of the axon may be reduced (depolarization) or increased (hyperpolarization). Depolarization is seen as a line deflecting upward from the rmp, and hyperpolarization by a line deflecting downward from the rmp.
The Nervous System: Neurons and Synapses types of cells. Neurons maintain an average rmp of -70 mV, for example, whereas heart muscle cells may have an rmp of -85 mV.
If appropriate stimulation causes positive charges to flow into the cell, the line will deflect upward. This change is called depolarization, since the potential difference between the two recording electrodes is reduced. A return to the resting membrane potential is known as repolarization. If stimulation causes the inside of the cell to become more negative than the resting membrane potential, the line on the oscilloscope will deflect downward. This change is called hyperpolarization (fig. 7.11). Hyperpolarization can be caused either by positive charges leaving the cell or by negative charges entering the cell.
Depolarization of a dendrite or cell body is excitatory, whereas hyperpolarization is inhibitory, in terms of their effects on the production of nerve impulses. The reasons for this relate to the nature of nerve impulses (action potentials), as will be explained shortly.
centration gradient out of the cell. The K+ gates will then close and the permeability properties of the membrane will return to what they were at rest.
Since opening of the gated Na+ and K+ channels is stimulated by depolarization, these ion channels in the axon membrane are said to be voltage regulated. The channel gates are closed at the resting membrane potential of -70 mV and open in response to depolarization of the membrane to a threshold value.
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