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CHAPTER 5 Membrane Potentials and...

CHAPTER 5 Membrane Potentials and UNIT II Action Potentials Electrical potentials exist across the membranes of virtu- creates a membrane potential of opposite polarity to that ally all cells of the body. Some cells, such as nerve and in Figure 5-1A, with negativity outside and positivity muscle cells, generate rapidly changing electrochemical inside. Again, the membrane potential rises high enough impulses at their membranes, and these impulses are used within milliseconds to block further net diffusion of to transmit signals along the nerve or muscle membranes. sodium ions to the inside; however, this time, in the mam- In other types of cells, such as glandular cells, macro- malian nerve fiber, the potential is about 61 millivolts posi- phages, and ciliated cells, local changes in membrane tive inside the fiber. potentials also activate many of the cell’s functions. This Thus, in both parts of Figure 5-1, we see that a con- chapter reviews the basic mechanisms whereby mem- centration difference of ions across a selectively perme- brane potentials are generated at rest and during action able membrane can, under appropriate conditions, create by nerve and muscle cells. See Video 5-1. a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission BASIC PHYSICS OF MEMBRANE result from such rapidly changing diffusion potentials. POTENTIALS The Nernst Equation Describes the Relationship of Membrane Potentials Caused by Ion Diffusion Potential to the Ion Concentration Difference Concentration Differences Across a Across a Membrane. The diffusion potential across a Selectively Permeable Membrane membrane that exactly opposes the net diffusion of a par- In Figure 5-1A, the potassium concentration is great ticular ion through the membrane is called the Nernst po- inside a nerve fiber membrane but very low outside the tential for that ion, a term that was introduced in Chapter membrane. Let us assume that the membrane in this case 4. The magnitude of the Nernst potential is determined is permeable to the potassium ions but not to any other by the ratio of the concentrations of that specific ion on ions. Because of the large potassium concentration gradi- the two sides of the membrane. The greater this ratio, the ent from the inside toward the outside, there is a strong greater the tendency for the ion to diffuse in one direction tendency for potassium ions to diffuse outward through and therefore the greater the Nernst potential required to the membrane. As they do so, they carry positive electri- prevent additional net diffusion. The following equation, cal charges to the outside, thus creating electropositivity called the Nernst equation, can be used to calculate the outside the membrane and electronegativity inside the Nernst potential for any univalent ion at the normal body membrane because of negative anions that remain behind temperature of 98.6°F (37°C): and do not diffuse outward with the potassium. Within 61 Concentration inside about 1 millisecond, the potential difference between the EMF (millivolts ) = ± × log z Concentration outside inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to where EMF is the electromotive force and z is the electri- the exterior, despite the high potassium ion concentra- cal charge of the ion (e.g., +1 for K+). tion gradient. In the normal mammalian nerve fiber, the When using this formula, it is usually assumed that potential difference is about 94 millivolts, with negativity the potential in the extracellular fluid outside the mem- inside the fiber membrane. brane remains at zero potential, and the Nernst potential Figure 5-1B shows the same phenomenon as in Fig- is the potential inside the membrane. Also, the sign of the ure 5-1A, but this time with a high concentration of potential is positive (+) if the ion diffusing from inside to sodium ions outside the membrane and a low concentra- outside is a negative ion, and it is negative (−) if the ion is tion of sodium ions inside. These ions are also positively positive. Thus, when the concentration of positive potas- charged. This time, the membrane is highly permeable to sium ions on the inside is 10 times that on the outside, the the sodium ions but is impermeable to all other ions. Dif- log of 10 is 1, so the Nernst potential calculates to be −61 fusion of the positively charged sodium ions to the inside millivolts inside the membrane.␣ 63 UNIT II Membrane Physiology, Nerve, and Muscle DIFFUSION POTENTIALS Table 5-1 Resting Membrane Potential in Different (Anions)– Nerve fiber (Anions)– Nerve fiber Cell Types + – – – + – + – + – (Anions) – + (Anions) + Cell Type Resting Potential (mV) + – – + – + – – + – + + – Neurons −60 to −70 + + – + – + + – + + + – K K Na – + Na Skeletal muscle −85 to −95 + – – + + – + – – + – + + – Smooth muscle −50 to −60 + – – + – + + – (–94 mV) – + (+61 mV) + – Cardiac muscle −80 to −90 + – – + – + + – Hair (cochlea) −15 to −40 + – – + – + + – Astrocyte −80 to −90 A B Erythrocyte −8 to −12 Figure 5-1 A, Establishment of a diffusion potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside Photoreceptor −40 (dark) to −70 (light) the cell to outside the cell through a membrane that is selectively per- meable only to potassium. B, Establishment of a diffusion potential when the nerve fiber membrane is permeable only to sodium ions. inside the membrane. The reason for this phenomenon Note that the internal membrane potential is negative when potas- sium ions diffuse and positive when sodium ions diffuse because of is that excess positive ions diffuse to the outside when opposite concentration gradients of these two ions. their concentration is higher inside than outside the membrane. This diffusion carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The Goldman Equation Is Used to Calculate the Dif- The opposite effect occurs when there is a gradient for fusion Potential When the Membrane Is Permeable a negative ion. That is, a chloride ion gradient from the to Several Different Ions. When a membrane is per- outside to the inside causes negativity inside the cell meable to several different ions, the diffusion potential because excess negatively charged chloride ions diffuse that develops depends on three factors: (1) the polarity to the inside while leaving the nondiffusible positive ions of the electrical charge of each ion; (2) the permeability on the outside. of the membrane (P) to each ion; and (3) the concentra- Fourth, as explained later, the permeability of the tion (C) of the respective ions on the inside (i) and out- sodium and potassium channels undergoes rapid side (o) of the membrane. Thus, the following formula, changes during transmission of a nerve impulse, called the Goldman equation or the Goldman-Hodgkin- whereas the permeability of the chloride channels does Katz equation, gives the calculated membrane potential not change greatly during this process. Therefore, rapid on the inside of the membrane when two univalent pos- changes in sodium and potassium permeability are pri- itive ions, sodium (Na+) and potassium (K+), and one marily responsible for signal transmission in neurons, univalent negative ion, chloride (Cl−), are involved: which is the subject of most of the remainder of this CNa+ PNa+ + CK+ PK+ + CCIo− PCI− chapter.␣ EMF (millivolts ) = −61 × log i i CNao+ PNa+ + CKo+ PK+ + CCIi− PCI− Resting Membrane Potential of Different Cell Types. In Several key points become evident from the Goldman some cells, such as the cardiac pacemaker cells discussed in Chapter 10, the membrane potential is continuously equation. First, sodium, potassium, and chloride ions are changing, and the cells are never “resting”. In many other the most important ions involved in the development of cells, even excitable cells, there is a quiescent period in which membrane potentials in nerve and muscle fibers, as well a resting membrane potential can be measured. Table 5-1 as in the neuronal cells. The concentration gradient of shows the approximate resting membrane potentials of some each of these ions across the membrane helps determine different types of cells. The membrane potential is obviously the voltage of the membrane potential. very dynamic in excitable cells such as neurons, in which Second, the quantitative importance of each of the ions action potentials occur. However, even in nonexcitable cells, in determining the voltage is proportional to the membrane the membrane potential (voltage) also changes in response permeability for that particular ion. If the membrane has to various stimuli, which alter activities for the various ion zero permeability to sodium and chloride ions, the mem- transporters, ion channels, and membrane permeability for brane potential becomes entirely dominated by the concen- sodium, potassium, calcium, and chloride ions. The resting membrane potential is, therefore, only a brief transient state tration gradient of potassium ions alone, and the resulting for many cells.␣ potential will be equal to the Nernst potential for potassium. Electrochemical Driving Force. When multiple ions The same holds true for each of the other two ions if the contribute to the membrane potential, the equilibrium membrane should become selectively permeable for either potential for any of the contributing ions will differ from one of them alone. the membrane potential, and there will be an electrochemi- Third, a positive ion concentration gradient from inside cal driving force (Vdf) for each ion that tends to cause net the membrane to the outside causes electronegativity 64  Chapter 5 Membrane Potentials and Action Potentials 0 Nerve fiber –+–+–+–+–+–+–+– — + +–++––+–+––++–+ –+–+–+–+–+–+–+– +–++––+–+––++–+ –+–+–+–+–+–+–+– I +–++––+–+––++–+ KC Silver–silver –+–+–+–+–+–+–+– UNIT II +++++++++++ +++++ chloride +–++––+–+––++–+ –––––––––– ––––– electrode –+–+–+–+–+–+–+– +–++––+–+––++–+ Electrical potential – – – – – – – – – (–70 – – – – – – – 0 + + + + + + + + + mV) + + + + + + + + (millivolts) Figure 5-2 Measurement of the membrane potential of the nerve fiber using a microelectrode. –70 movement of the ion across the membrane. This driving force is equal to the difference between the membrane po- Figure 5-3 Distribution of positively and negatively charged ions in tential (Vm) and the equilibrium potential of the ion (Veq) the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber. Note the alignment of negative charges along the inside Thus, Vdf = Vm – Veq. surface of the membrane and positive charges along the outside The arithmetic sign of Vdf (positive or negative) and the surface. The lower panel displays the abrupt changes in membrane valence of the ion (cation or anion) can be used to predict potential that occur at the membranes on the two sides of the fiber. the direction of ion flow across the membrane, into or out of the cell. For cations such as Na+ and K+, a positive Vdf predicts ion movement out of the cell down its electro- is zero, which is the potential of the extracellular fluid. chemical gradient, and a negative Vdf predicts ion move- Then, as the recording electrode passes through the volt- ment into the cell. For anions, such as Cl−, a positive Vdf age change area at the cell membrane (called the electrical predicts ion movement into the cell, and a negative Vdf pre- dipole layer), the potential decreases abruptly to −70 mil- dicts ion movement out of the cell. When Vm = Veq, there livolts. Moving across the center of the fiber, the potential is no net movement of the ion into or out of the cell. Also, remains at a steady −70-millivolt level but reverses back the direction of ion flux through the membrane reverses as to zero the instant it passes through the membrane on the Vm becomes greater than or less than Veq; hence, the equi- opposite side of the fiber. librium potential (Veq) is also called the reversal potential.␣ To create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be trans- Measuring the Membrane Potential ported outward. The remaining ions inside the nerve The method for measuring the membrane potential is fiber can be both positive and negative, as shown in simple in theory but often difficult in practice because of the upper panel of Figure 5-3. Therefore, transfer of the small size of most of the cells and fibers. Figure 5-2 an incredibly small number of ions through the mem- shows a small micropipette filled with an electrolyte solu- brane can establish the normal resting potential of −70 tion. The micropipette is impaled through the cell mem- millivolts inside the nerve fiber, which means that only brane to the interior of the fiber. Another electrode, called about 1/3,000,000 to 1/100,000,000 of the total posi- the indifferent electrode, is then placed in the extracellu- tive charges inside the fiber must be transferred. Also, lar fluid, and the potential difference between the inside an equally small number of positive ions moving from and outside of the fiber is measured using an appropriate outside to inside the fiber can reverse the potential voltmeter. This voltmeter is a highly sophisticated elec- from −70 millivolts to as much as +35 millivolts within tronic apparatus that is capable of measuring small volt- as little as 1/10,000 of a second. Rapid shifting of ions in ages despite extremely high resistance to electrical flow this manner causes the nerve signals discussed in sub- through the tip of the micropipette, which has a lumen sequent sections of this chapter.␣ diameter usually less than 1 micrometer and a resistance of more than 1 million ohms. For recording rapid changes RESTING MEMBRANE POTENTIAL OF in the membrane potential during transmission of nerve NEURONS impulses, the microelectrode is connected to an oscillo- scope, as explained later in the chapter. The resting membrane potential of large nerve fibers when The lower part of Figure 5-3 shows the electrical they are not transmitting nerve signals is about −70 mil- potential that is measured at each point in or near the livolts. That is, the potential inside the fiber is 70 millivolts nerve fiber membrane, beginning at the left side of the more negative than the potential in the extracellular fluid figure and passing to the right. As long as the electrode is on the outside of the fiber. In the next few paragraphs, the outside the neuronal membrane, the recorded potential transport properties of the resting nerve membrane for 65 UNIT II Membrane Physiology, Nerve, and Muscle Outside K+ 3Na+ 2K+ Selectivity K+ 4 mEq/L filter K+ 140 mEq/L (–94 mV) (–94 mV) A 3Na+ Na+ K+ Na+ K+ ATP ADP 2K+ K+ "leak" 142 mEq/L 4 mEq/L Na+-K+ pump channels Figure 5-4 Functional characteristics of the Na+-K+ pump and the K+ Na+ K+ “leak” channels. The K+ leak channels also leak Na+ ions into the cell 14 mEq/L 140 mEq/L (–86 mV) slightly but are much more permeable to K+. ADP, Adenosine diphos- (+61 mV) (–94 mV) phate; ATP, adenosine triphosphate. B sodium and potassium and the factors that determine the + – – + level of this resting potential are explained. + – – + Diffusion – + Active Transport of Sodium and Potassium Ions + – – + Through the Membrane—the Sodium-Potassium Na+ Na+ – + pump (Na+-K+) Pump. Recall from Chapter 4 that all cell mem- + – – + branes of the body have a powerful Na+-K+ pump that 142 mEq/L + – 14 mEq/L – + continually transports sodium ions to the outside of the + – – + + – – + cell and potassium ions to the inside, as illustrated on the – + left side in Figure 5-4. Note that this is an electrogenic Diffusion – + pump because three Na+ ions are pumped to the outside + – – + K+ K+ for each two K+ ions to the inside, leaving a net deficit of pump – + + – – + positive ions on the inside and causing a negative poten- 4 mEq/L + – 140 mEq/L – + tial inside the cell membrane. + – – + The Na+-K+ pump also causes large concentration gra- + – (–90 mV) – + dients for sodium and potassium across the resting nerve + – – + (Anions)– + – (Anions)– membrane. These gradients are as follows: C – + Na+ (outside): 142 mEq/L Figure 5-5 Establishment of resting membrane potentials under three conditions. A, When the membrane potential is caused entirely by potassium diffusion alone. B, When the membrane potential is Na+ (inside): 14 mEq/L caused by diffusion of both sodium and potassium ions. C, When the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. K + (outside): 4 mEq/L channels may also leak sodium ions slightly but are far K + (inside): 140 mEq/L more permeable to potassium than to sodium, normally The ratios of these two respective ions from the inside about 100 times as permeable. As discussed later, this dif- to the outside are as follows: ferential in permeability is a key factor in determining the level of the normal resting membrane potential.␣ Na+ inside /Na+ outside = 0.1 Origin of the Normal Resting Membrane Potential K + inside /K + outside = 35.0 Figure 5-5 shows the important factors in the establish- ␣ ment of the normal resting membrane potential. They are Leakage of Potassium Through the Nerve Cell Mem- as follows. brane. The right side of Figure 5-4 shows a channel pro- tein (sometimes called a tandem pore domain, potassium Contribution of the Potassium Diffusion Potential. channel, or potassium [K+] “leak” channel) in the nerve In Figure 5-5A, we assume that the only movement of membrane through which potassium ions can leak, even ions through the membrane is diffusion of potassium in a resting cell. The basic structure of potassium channels ions, as demonstrated by the open channels between was described in Chapter 4 (Figure 4-4). These K+ leak the potassium symbol (K+) inside and outside the mem- 66  Chapter 5 Membrane Potentials and Action Potentials brane. Because of the high ratio of potassium ions inside 0 to outside, 35:1, the Nernst potential corresponding to this ratio is −94 millivolts because the logarithm of 35 is — + 1.54, and this, multiplied by −61 millivolts, is −94 mil- livolts. Therefore, if potassium ions were the only factor I KC Silver–silver causing the resting potential, the resting potential inside chloride UNIT II the fiber would be equal to −94 millivolts, as shown in ++++ –––– +++++ electrode –––– ++++ ––––– the figure.␣ –––– ++++ –––––– Contribution of Sodium Diffusion Through the ++++ –––– ++++++ Nerve Membrane. Figure 5-5B shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions Overshoot through the K+-Na+ leak channels. The ratio of sodium +35 ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the 0 membrane of +61 millivolts. Also shown in Figure 5-5B Millivolts tion Re p is the Nernst potential for potassium diffusion of −94 olariza olar millivolts. How do these interact with each other, and iza what will be the summated potential? This question can Dep tio n be answered by using the Goldman equation described –70 previously. Intuitively, one can see that if the membrane Resting is highly permeable to potassium but only slightly per- meable to sodium, the diffusion of potassium contributes 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 far more to the membrane potential than the diffusion of Milliseconds sodium. In the normal nerve fiber, the permeability of Figure 5-6 Typical action potential recorded by the method shown the membrane to potassium is about 100 times as great in the upper panel. as its permeability to sodium. Using this value in the Goldman equation, and considering only sodium and varies in different cells from as low as around −10 mil- potassium, gives a potential inside the membrane of −86 livolts in erythrocytes to as high as −90 millivolts in skel- millivolts, which is near the potassium potential shown etal muscle cells.␣ in the figure.␣ NEURON ACTION POTENTIAL Contribution of the Na+-K+ Pump. In Figure 5-5C, the Na+-K+ pump is shown to provide an additional contribu- Nerve signals are transmitted by action potentials, which tion to the resting potential. This figure shows that con- are rapid changes in the membrane potential that spread tinuous pumping of three sodium ions to the outside oc- rapidly along the nerve fiber membrane. Each action poten- curs for each two potassium ions pumped to the inside of tial begins with a sudden change from the normal resting the membrane. The pumping of more sodium ions to the negative membrane potential to a positive potential and outside than the potassium ions being pumped to the in- ends with an almost equally rapid change back to the nega- side causes a continual loss of positive charges from inside tive potential. To conduct a nerve signal, the action potential the membrane, creating an additional degree of negativity moves along the nerve fiber until it comes to the fiber’s end. (about −4 millivolts additional) on the inside, beyond that The upper panel of Figure 5-6 shows the changes which can be accounted for by diffusion alone. that occur at the membrane during the action potential, Therefore, as shown in Figure 5-5C, the net mem- with the transfer of positive charges to the interior of the brane potential when all these factors are operative at the fiber at its onset and the return of positive charges to the same time is about −90 millivolts. However, additional exterior at its end. The lower panel shows graphically the ions, such as chloride, must also be considered in calcu- successive changes in membrane potential over a few lating the membrane potential. 10,000ths of a second, illustrating the explosive onset of In summary, the diffusion potentials alone caused the action potential and the almost equally rapid recovery. by potassium and sodium diffusion would give a mem- The successive stages of the action potential are as follows. brane potential of about −86 millivolts, with almost all of this being determined by potassium diffusion. An addi- Resting Stage. The resting stage is the resting membrane tional −4 millivolts is then contributed to the membrane potential before the action potential begins. The mem- potential by the continuously acting electrogenic Na+- brane is said to be “polarized” during this stage because K+ pump, and there is a contribution of chloride ions. As of the −70 millivolts negative membrane potential that is mentioned previously, the resting membrane potential present.␣ 67 UNIT II Membrane Physiology, Nerve, and Muscle Depolarization Stage. At this time, the membrane sud- Activation Selectivity denly becomes permeable to sodium ions, allowing rapid gate Na+ Na+ filter Na+ diffusion of positively charged sodium ions to the interior of the axon. The normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction—a process called depolarization. In large nerve fibers, the great excess of positive sodium Inactivation gate ions moving to the inside causes the membrane poten- Resting Activated Inactivated tial to actually overshoot beyond the zero level and to be- (−70 mV) (−70 to +35 mV) (+35 to −70 mV, come somewhat positive. In some smaller fibers, as well delayed) as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state.␣ Repolarization Stage. Within a few 10,000ths of a sec- ond after the membrane becomes highly permeable to K+ K+ sodium ions, the sodium channels begin to close, and the Resting Slow activation (−70 mV) (+35 to −70 mV) potassium channels open to a greater degree than normal. Inside Then, rapid diffusion of potassium ions to the exterior re- Figure 5-7 Characteristics of the voltage-gated sodium (top) and establishes the normal negative resting membrane poten- potassium (bottom) channels, showing successive activation and in- tial, which is called repolarization of the membrane. activation of the sodium channels and delayed activation of the po- To explain more fully the factors that cause both depo- tassium channels when the membrane potential is changed from the larization and repolarization, we will describe the special normal resting negative value to a positive value. characteristics of two other types of transport channels through the nerve membrane, the voltage-gated sodium change in the activation gate, flipping it all the way to and potassium channels.␣ the open position. During this activated state, sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as VOLTAGE-GATED SODIUM AND 500- to 5000-fold.␣ POTASSIUM CHANNELS The necessary factor in causing both depolarization and Inactivation of the Sodium Channel. The upper right repolarization of the nerve membrane during the action panel of Figure 5-7 shows a third state of the sodium potential is the voltage-gated sodium channel. A voltage- channel. The same increase in voltage that opens the ac- gated potassium channel also plays an important role in tivation gate also closes the inactivation gate. The inacti- increasing the rapidity of repolarization of the membrane. vation gate, however, closes a few 10,000ths of a second These two voltage-gated channels are in addition to the after the activation gate opens. That is, the conforma- Na+-K+ pump and the K+ leak channels. tional change that flips the inactivation gate to the closed state is a slower process than the conformational change Activation and Inactivation of the that opens the activation gate. Therefore, after the sodi- Voltage-Gated Sodium Channel um channel has remained open for a few 10,000ths of a The upper panel of Figure 5-7 shows the voltage-gated second, the inactivation gate closes, and sodium ions no sodium channel in three separate states. This channel has longer can pour to the inside of the membrane. At this two gates—one near the outside of the channel called the point, the membrane potential begins to return toward activation gate, and another near the inside called the the resting membrane state, which is the repolarization inactivation gate. The upper left of the figure depicts the process. state of these two gates in the normal resting membrane Another important characteristic of the sodium chan- when the membrane potential is −70 millivolts. In this nel inactivation process is that the inactivation gate will state, the activation gate is closed, which prevents any not reopen until the membrane potential returns to or entry of sodium ions to the interior of the fiber through near the original resting membrane potential level. There- these sodium channels. fore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber.␣ Activation of the Sodium Channel. When the mem- brane potential becomes less negative than during the Voltage-Gated Potassium Channel and Its resting state, rising from −70 millivolts toward zero, it Activation finally reaches a voltage—usually somewhere around The lower panel of Figure 5-7 shows the voltage-gated −55 millivolts—that causes a sudden conformational potassium channel in two states—during the resting state 68  Chapter 5 Membrane Potentials and Action Potentials Signal generator with Feedback Na+ channel command voltage amplifier 30 Conductance (mmho/cm2) Activation 20 K+ channel In 10 ct iva a Membrane t io n potential Ampere amplifier 0 UNIT II meter –70 mV +10 mV –70 mV Membrane potential Reference electrode in fluid 0 1 2 3 Time (milliseconds) Voltage Current Figure 5-9 Typical changes in conductance of sodium and potassium electrode electrode ion channels when the membrane potential is suddenly increased from the normal resting value of −70 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium Axon channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (acti- vate), and the rate of opening is much slower than that of the sodium channels. Figure 5-8 Voltage clamp method for studying flow of ions through specific channels. by the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by (left) and toward the end of the action potential (right). this voltage clamp from −70 millivolts to zero, the voltage- During the resting state, the gate of the potassium chan- gated sodium and potassium channels open, and sodium nel is closed, and potassium ions are prevented from and potassium ions begin to pour through the channels. To passing through this channel to the exterior. When the counterbalance the effect of these ion movements on the membrane potential rises from −70 millivolts toward desired setting of the intracellular voltage, electrical cur- zero, this voltage change causes a conformational open- rent is injected automatically through the current electrode ing of the gate and allows increased potassium diffusion of the voltage clamp to maintain the intracellular voltage outward through the channel. However, because of the at the required steady zero level. To achieve this level, the slight delay in opening of the potassium channels, they current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. open, for the most part, at about the same time that the To measure how much current flow is occurring at each sodium channels are beginning to close because of inac- instant, the current electrode is connected to an ampere tivation. Thus, the decrease in sodium entry to the cell meter that records the current flow, as demonstrated in and the simultaneous increase in potassium exit from the Figure 5-8. cell combine to speed the repolarization process, leading Finally, the investigator adjusts the concentrations to full recovery of the resting membrane potential within of the ions to other than normal levels both inside and another few 10,000ths of a second. outside the nerve fiber and repeats the study. This ex- periment can be performed easily when using large nerve fibers removed from some invertebrates, especially the The Voltage Clamp Method for Measuring the Effect giant squid axon, which in some cases is as large as 1 mil- of Voltage on Opening and Closing of Voltage-Gated limeter in diameter. When sodium is the only permeant Channels. The original research that led to quantitative ion in the solutions inside and outside the squid axon, the understanding of the sodium and potassium channels was voltage clamp measures current flow only through the so ingenious that it led to Nobel Prizes for the scientists sodium channels. When potassium is the only permeant responsible, Hodgkin and Huxley, in 1963. The essence of ion, current flow only through the potassium channels is these studies is shown in Figures. 5-8 and 5-9. measured. Figure 5-8 shows the voltage clamp method, which Another means for studying the flow of ions through is used to measure the flow of ions through the different an individual type of channel is to block one type of chan- channels. In using this apparatus, two electrodes are in- nel at a time. For example, the sodium channels can be serted into the nerve fiber. One of these electrodes is used blocked by a toxin called tetrodotoxin when it is applied to to measure the voltage of the membrane potential, and the the outside of the cell membrane where the sodium acti- other is used to conduct electrical current into or out of the vation gates are located. Conversely, tetraethylammonium nerve fiber. ion blocks the potassium channels when it is applied to the This apparatus is used in the following way. The inves- interior of the nerve fiber. tigator decides which voltage to establish inside the nerve Figure 5-9 shows typical changes in conductance of fiber. The electronic portion of the apparatus is then adjust- the voltage-gated sodium and potassium channels when ed to the desired voltage, automatically injecting either pos- the membrane potential is suddenly changed through use itive or negative electricity through the current electrode at of the voltage clamp, from −70 millivolts to +10 millivolts whatever rate is required to hold the voltage, as measured and then, 2 milliseconds later, back to −70 millivolts. Note 69 UNIT II Membrane Physiology, Nerve, and Muscle the sudden opening of the sodium channels (the activa- within another fraction of a millisecond. The onset of tion stage) within a small fraction of a millisecond after the action potential also initiates voltage gating of the the membrane potential is increased to the positive value. potassium channels, causing them to begin opening However, during the next millisecond or so, the sodium more slowly, a fraction of a millisecond after the sodium channels automatically close (the inactivation stage). channels open. At the end of the action potential, the Note the opening (activation) of the potassium chan- return of the membrane potential to the negative state nels, which open less rapidly and reach their full open state only after the sodium channels have almost completely causes the potassium channels to close back to their closed. Furthermore, once the potassium channels open, original status but, again, only after an additional mil- they remain open for the entire duration of the positive lisecond or more delay. membrane potential and do not close again until after the The middle portion of Figure 5-10 shows the ratio of membrane potential is decreased back to a negative value.␣ sodium to potassium conductance at each instant dur- ing the action potential, and above this depiction is the action potential itself. During the early portion of the SUMMARY OF EVENTS THAT CAUSE THE action potential, the ratio of sodium to potassium con- ACTION POTENTIAL ductance increases more than 1000-fold. Therefore, far Figure 5-10 summarizes the sequential events that more sodium ions flow to the interior of the fiber than occur during and shortly after the action potential. The potassium ions to the exterior. This is what causes the bottom of the figure shows the changes in membrane membrane potential to become positive at the action conductance for sodium and potassium ions. During potential onset. Then, the sodium channels begin to close, the resting state, before the action potential begins, the and the potassium channels begin to open; thus, the ratio conductance for potassium ions is 50 to 100 times as of conductance shifts far in favor of high potassium con- great as the conductance for sodium ions. This dispar- ductance but low sodium conductance. This shift allows ity is caused by much greater leakage of potassium ions for a very rapid loss of potassium ions to the exterior but than sodium ions through the leak channels. However, virtually zero flow of sodium ions to the interior. Conse- at the onset of the action potential, the sodium chan- quently, the action potential quickly returns to its baseline nels almost instantaneously become activated and allow level. up to a 5000-fold increase in sodium conductance. The inactivation process then closes the sodium channels Roles of Other Ions During the Action Potential Thus far, we have considered only the roles of sodium and potassium ions in generating the action potential. At least Membrane potential (mV) Overshoot two other types of ions must be considered, negative anions +40 and calcium ions. +20 Action potential Impermeant Negatively Charged Ions (Anions) Inside 100 0 –20 the Nerve Axon. Inside the axon are many negatively 10 –40 charged ions that cannot go through the membrane chan- Na+ conductance K+ conductance –60 nels. They include the anions of protein molecules and 1 of many organic phosphate compounds and sulfate com- –80 0.1 Positive pounds, among others. Because these ions cannot leave afterpotential the interior of the axon, any deficit of positive ions inside 0.01 the membrane leaves an excess of these impermeant nega- Ratio of conductances tive anions. Therefore, these impermeant negative ions are 0.001 100 responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions 10 and other positive ions.␣ Conductance (mmho/cm2) Calcium Ions. The membranes of almost all cells of the 1 K+ body have a calcium pump similar to the sodium pump, 0.1 and calcium serves along with (or instead of ) sodium in Na+ some cells to cause most of the action potential. Like the 0.01 sodium pump, the calcium pump transports calcium ions 0.005 from the interior to the exterior of the cell membrane (or 0 0.5 1.0 1.5 Milliseconds into the endoplasmic reticulum of the cell), creating a cal- cium ion gradient of about 10,000-fold. This process leaves Figure 5-10 Changes in sodium and potassium conductance dur- an internal cell concentration of calcium ions of about 10−7 ing the course of the action potential. Sodium conductance increases molar, in contrast to an external concentration of about several thousand–fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during 10−3 molar. the latter stages of the action potential and for a short period thereaf- In addition, there are voltage-gated calcium channels. ter. (These curves were constructed from theory presented in papers Because the calcium ion concentration is more than 10,000 by Hodgkin and Huxley but transposed from a squid axon to apply to times greater in the extracellular fluid than in the intracellular the membrane potentials of large mammalian nerve fibers.) fluid, there is a tremendous diffusion gradient and elec- 70  Chapter 5 Membrane Potentials and Action Potentials trochemical driving force for the passive flow of calcium rise in the membrane potential, thus opening still more ions into the cells. These channels are slightly permeable voltage-gated sodium channels and allowing more stream- to sodium ions and calcium ions, but their permeability to ing of sodium ions to the interior of the fiber. This process calcium is about 1000-fold greater than to sodium under normal physiological conditions. When the channels open is a positive feedback cycle that, once the feedback is strong in response to a stimulus that depolarizes the cell mem- enough, continues until all the voltage-gated sodium chan- nels have become activated (opened). Then, within another UNIT II brane, calcium ions flow to the interior of the cell. A major function of the voltage-gated calcium ion fraction of a millisecond, the rising membrane potential channels is to contribute to the depolarizing phase on the causes closure of the sodium channels and opening of po- action potential in some cells. The gating of calcium chan- tassium channels, and the action potential soon terminates.␣ nels, however, is relatively slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this Initiation of the Action Potential Occurs Only After reason, they are often called slow channels, in contrast the Threshold Potential is Reached. An action poten- to the sodium channels, which are called fast channels. tial will not occur until the initial rise in membrane po- Therefore, the opening of calcium channels provides a tential is great enough to create the positive feedback de- more sustained depolarization, whereas the sodium chan- nels play a key role in initiating action potentials. scribed in the preceding paragraph. This occurs when the Calcium channels are numerous in cardiac muscle and number of sodium ions entering the fiber is greater than smooth muscle. In fact, in some types of smooth muscle, the number of potassium ions leaving the fiber. A sudden the fast sodium channels are hardly present; therefore, the rise in membrane potential of 15 to 30 millivolts is usually action potentials are caused almost entirely by the activa- required. Therefore, a sudden increase in the membrane tion of slow calcium channels.␣ potential in a large nerve fiber, from −70 millivolts up to Increased Permeability of the Sodium Channels When about −55 millivolts, usually causes the explosive devel- There Is a Deficit of Calcium Ions. The concentration of opment of an action potential. This level of −55 millivolts calcium ions in the extracellular fluid also has a profound is said to be the threshold for stimulation.␣ effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very PROPAGATION OF THE ACTION negative level. Therefore, the nerve fiber becomes highly POTENTIAL excitable, sometimes discharging repetitively without prov- ocation, rather than remaining in the resting state. In fact, In the preceding paragraphs, we discussed the action the calcium ion concentration needs to fall only 50% be- potential as though it occurs at one spot on the mem- low normal before spontaneous discharge occurs in some brane. However, an action potential elicited at any one peripheral nerves, often causing muscle “tetany.” Muscle point on an excitable membrane usually excites adjacent tetany is sometimes lethal because of tetanic contraction of portions of the membrane, resulting in propagation of the the respiratory muscles. action potential along the membrane. This mechanism is The probable way in which calcium ions affect the so- dium channels is as follows. These ions appear to bind to demonstrated in Figure 5-11. the exterior surfaces of the sodium channel protein. The Figure 5-11A shows a normal resting nerve fiber, and positive charges of these calcium ions, in turn, alter the Figure 5-11B shows a nerve fiber that has been excited in electrical state of the sodium channel protein, thus altering its midportion, which suddenly develops increased perme- the voltage level required to open the sodium gate.␣ ability to sodium. The arrows show a local circuit of cur- rent flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, posi- INITIATION OF THE ACTION POTENTIAL tive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then Thus far, we have explained the changing sodium and for several millimeters in both directions along the core of potassium permeability of the membrane, as well as the the axon. These positive charges increase the voltage for a development of the action potential, but we have not distance of 1 to 3 millimeters inside the large myelinated explained what initiates the action potential. fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these A Positive-Feedback Cycle Opens the Sodium Chan- new areas immediately open, as shown in Figure 5-11C nels. As long as the membrane of the nerve fiber remains and D, and the explosive action potential spreads. These undisturbed, no action potential occurs in the normal newly depolarized areas produce still more local circuits of nerve. However, if any event causes enough initial rise in current flow farther along the membrane, causing progres- the membrane potential from −70 millivolts toward the sively more and more depolarization. Thus, the depolariza- zero level, the rising voltage will cause many voltage-gated tion process travels along the entire length of the fiber. This sodium channels to begin opening. This occurrence allows transmission of the depolarization process along a nerve or for the rapid inflow of sodium ions, which causes a further muscle fiber is called a nerve or muscle impulse. 71 UNIT II Membrane Physiology, Nerve, and Muscle Direction of Propagation. As demonstrated in Figure 5- repolarization. For a single action potential, this effect is 11, an excitable membrane has no single direction of prop- so minute that it cannot be measured. Indeed, 100,000 agation, but the action potential travels in all directions to 50 million impulses can be transmitted by large nerve away from the stimulus—even along all branches of a nerve fibers before the concentration differences reach the point fiber—until the entire membrane has become depolarized.␣ that action potential conduction ceases. With time, how- ever, it becomes necessary to re-establish the sodium and All-or-Nothing Principle. Once an action potential has potassium membrane concentration differences, which is been elicited at any point on the membrane of a normal achieved by action of the Na+-K+ pump in the same way fiber, the depolarization process travels over the entire as described previously for the original establishment of membrane if conditions are right, but it does not travel at the resting potential. That is, sodium ions that have dif- all if conditions are not right. This principle is called the all- fused to the interior of the cell during the action poten- or-nothing principle, and it applies to all normal excitable tials and potassium ions that have diffused to the exterior tissues. Occasionally, the action potential reaches a point must be returned to their original state by the Na+-K+ on the membrane at which it does not generate sufficient pump. Because this pump requires energy for operation, voltage to stimulate the next area of the membrane. When this “recharging” of the nerve fiber is an active metabolic this situation occurs, the spread of depolarization stops. process, using energy derived from the adenosine tri- Therefore, for continued propagation of an impulse to oc- phosphate (ATP) energy system of the cell. Figure 5-12 cur, the ratio of action potential to threshold for excitation shows that the nerve fiber produces increased heat dur- must at all times be greater than 1. This “greater than 1” ing recharging, which is a measure of energy expenditure requirement is called the safety factor for propagation.␣ when the nerve impulse frequency increases. A special feature of the Na+-K+ ATP pump is that its degree of activity is strongly stimulated when excess RE-ESTABLISHING SODIUM AND sodium ions accumulate inside the cell membrane. In fact, POTASSIUM IONIC GRADIENTS the pumping activity increases approximately in propor- AFTER ACTION POTENTIALS ARE tion to the third power of this intracellular sodium con- COMPLETED—IMPORTANCE OF centration. As the internal sodium concentration rises ENERGY METABOLISM from 10 to 20 mEq/L, the activity of the pump does not Transmission of each action potential along a nerve fiber merely double but increases about eightfold. Therefore, it slightly reduces the concentration differences of sodium is easy to understand how the recharging process of the and potassium inside and outside the membrane because nerve fiber can be set rapidly into motion whenever the sodium ions diffuse to the inside during depolariza- concentration differences of sodium and potassium ions tion, and potassium ions diffuse to the outside during across the membrane begin to run down.␣ +++++++++++++++++++++++ PLATEAU IN SOME ACTION ––––––––––––––––––––––– POTENTIALS ––––––––––––––––––––––– In some cases, the excited membrane does not repolarize A +++++++++++++++++++++++ immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential ++++++++++++––+++++++++ for many milliseconds before repolarization begin. Such a ––––––––––––++––––––––– plateau is shown in Figure 5-13; one can readily see that ––––––––––––++––––––––– B ++++++++++++––+++++++++ ++++++++++––––++++++++ ––––––––––++++–––––––– Heat production ––––––––––++++–––––––– C ++++++++++––––++++++++ ++––––––––––––––––––++ ––++++++++++++++++++–– At rest ––++++++++++++++++++–– 0 100 200 300 ++––––––––––––––––––++ D Impulses per second Figure 5-11 A–D, Propagation of action potentials in both directions Figure 5-12 Heat production in a nerve fiber at rest and at progres- along a conductive fiber. sively increasing rates of stimulation. 72  Chapter 5 Membrane Potentials and Action Potentials the plateau greatly prolongs the period of depolarization. are placed in a solution that contains the drug veratridine, This type of action potential occurs in heart muscle fibers, which activates sodium ion channels, or when the calcium where the plateau lasts for as long as 0.2 to 0.3 second and ion concentration decreases below a critical value, which causes contraction of heart muscle to last for this same increases the sodium permeability of the membrane. long period. The cause of the plateau is a combination of several fac- Re-Excitation Process Necessary for Spontaneous UNIT II tors. First, in heart muscle, two types of channels contribute Rhythmicity. For spontaneous rhythmicity to occur, the to the depolarization process: (1) the usual voltage-activated membrane—even in its natural state—must be permeable sodium channels, called fast channels; and (2) voltage- enough to sodium ions (or to calcium and sodium ions activated calcium-sodium channels (L-type calcium chan- through the slow calcium-sodium channels) to allow auto- nels), which are slow to open and therefore are called slow matic membrane depolarization. Thus, Figure 5-14 shows channels. Opening of fast channels causes the spike portion that the resting membrane potential in the rhythmical con- of the action potential, whereas the prolonged opening of trol center of the heart is only −60 to −70 millivolts, which is the slow calcium-sodium channels mainly allows calcium not enough negative voltage to keep the sodium and calcium ions to enter the fiber, which is largely responsible for the channels totally closed. Therefore, the following sequence plateau portion of the action potential. occurs: (1) some sodium and calcium ions flow inward; (2) Another factor that may be partly responsible for the this activity increases the membrane voltage in the positive plateau is that the voltage-gated potassium channels are direction, which further increases membrane permeability; slower to open than usual, often not opening much until (3) still more ions flow inward; and (4) the permeability in- the end of the plateau. This factor delays the return of the creases more, and so on, until an action potential is gener- membrane potential toward its normal negative value ated. Then, at the end of the action potential, the membrane of −70 millivolts. The plateau ends when the calcium- repolarizes. After another delay of milliseconds or seconds, sodium channels close, and permeability to potassium spontaneous excitability causes depolarization again, and a ions increases.␣ new action potential occurs spontaneously. This cycle con- tinues over and over and causes self-induced rhythmical excitation of the excitable tissue. RHYTHMICITY OF SOME EXCITABLE Why does the membrane of the heart control center not TISSUES—REPETITIVE DISCHARGE depolarize immediately after it has become repolarized, Repetitive self-induced discharges occur normally in rather than delaying for nearly 1 second before the onset the heart, in most smooth muscle, and in many of the of the next action potential? The answer can be found by neurons of the central nervous system. These rhythmi- observing the curve labeled “potassium conductance” in cal discharges cause the following: (1) rhythmical beat Figure 5-14. This curve shows that toward the end of of the heart; (2) rhythmical peristalsis of the intestines; each action potential, and continuing for a short period and (3) neuronal events such as the rhythmical control of thereafter, the membrane becomes more permeable to breathing. potassium ions. The increased outflow of potassium ions In addition, almost all other excitable tissues can dis- carries tremendous numbers of positive charges to the charge repetitively if the threshold for stimulation of the outside of the membrane, leaving considerably more neg- tissue cells is reduced to a low enough level. For example, ativity inside the fiber than would otherwise occur. This even large nerve fibers and skeletal muscle fibers, which continues for nearly 1 second after the preceding action normally are highly stable, discharge repetitively when they potential is over, thus drawing the membrane potential Rhythmical +60 Potassium action Plateau +60 conductance potentials Threshold +40 +40 +20 +20 Millivolts 0 0 Millivolts –20 –20 –40 –40 –60 –60 –80 0 1 2 3 –100 Seconds Hyperpolarization 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure 5-14 Rhythmical action potentials (in millivolts) similar to Seconds those recorded in the rhythmical control center of the heart. Note Figure 5-13 Action potential (in millivolts) from a Purkinje fiber of their relationship to potassium conductance and to the state of hy- the heart, showing a plateau. perpolarization. 73 UNIT II Membrane Physiology, Nerve, and Muscle nearer to the potassium Nernst potential. This state, called electrical insulator that decreases ion flow through the hyperpolarization, is also shown in Figure 5-14. As long membrane about 5000-fold. At the juncture between each as this state exists, self–re-excitation will not occur. How- two successive Schwann cells along the axon, a small unin- ever, the increased potassium conductance (and the state sulated area only 2 to 3 micrometers in length remains of hyperpolarization) gradually disappears, as shown after where ions still can flow with ease through the axon mem- each action potential is completed in the figure, thereby brane between the extracellular fluid and intracellular fluid again allowing the membrane potential to increase up to inside the axon. This area is called the node of Ranvier.␣ the threshold for excitation. Then, suddenly, a new action potential results and the process occurs again and again.␣ Saltatory Conduction in Myelinated Fibers from Node to Node. Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can SPECIAL CHARACTERISTICS OF SIGNAL flow with ease through the nodes of Ranvier. Therefore, TRANSMISSION IN NERVE TRUNKS action potentials occur only at the nodes. Yet, the action potentials are conducted from node to node by saltatory Myelinated and Unmyelinated Nerve Fibers. Figure conduction, as shown in Figure 5-17. That is, electrical 5-15 shows a cross section of a typical small nerve, re- current flows through the surrounding extracellular fluid vealing many large nerve fibers that constitute most of the outside the myelin sheath, as well as through the axoplasm cross-sectional area. However, a more careful look reveals inside the axon from node to node, exciting successive many more small fibers lying between the large ones. The nodes one after another. Thus, the nerve impulse jumps large fibers are myelinated, and the small ones are unmy- along the fiber, which is the origin of the term saltatory. elinated. The average nerve trunk contains about twice as Saltatory conduction is of value for two reasons: many unmyelinated fibers as myelinated fibers. 1. First, by causing the depolarization process to jump Figure 5-16 illustrates schematically the features of a long intervals along the axis of the nerve fiber, this typical myelinated fiber. The central core of the fiber is the mechanism increases the velocity of nerve transmis- axon, and the membrane of the axon is the membrane that sion in myelinated fibers as much as 5- to 50-fold. actually conducts the action potential. The axon is filled in its center with axoplasm, which is a viscid intracellular fluid. Surrounding the axon is a myelin sheath that is often much thicker than the axon itself. About once every 1 to 3 millime- ters along the length of the myelin sheath is a node of Ranvier. The myelin sheath is deposited around the axon by Schwann cells in the following manner. The membrane of a Schwann cell first envelops the axon. The Schwann cell then rotates around the axon many times, laying down mul- Axon tiple layers of Schwann cell membrane containing the lipid substance sphingomyelin. This substance is an excellent Myelin sheath Schwann cell cytoplasm Schwann cell nucleus Node of Ranvier A Unmyelinated axons Schwann cell nucleus Schwann cell cytoplasm B Figure 5-16 Function of the Schwann cell to insulate nerve fibers. A, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. B, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple Figure 5-15 Cross section of a small nerve trunk containing both unmyelinated nerve fibers (shown in cross section). (A, Modified from myelinated and unmyelinated fibers. Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.) 74  Chapter 5 Membrane Potentials and Action Potentials 2. Second, saltatory conduction conserves energy for action potentials: mechanical pressure to excite sensory the axon because only the nodes depolarize, allowing nerve endings in the skin, chemical neurotransmitters to perhaps 100 times less loss of ions than would other- transmit signals from one neuron to the next in the brain, wise be necessary, and therefore requiring much less and electrical current to transmit signals between succes- energy expenditure for re-establishing the sodium sive muscle cells in the heart and intestine. and potassium concentration differences across the UNIT II membrane after a series of nerve impulses. Excitation of a Nerve Fiber by a Negatively Charged The excellent insulation afforded by the myelin mem- Metal Electrode. The usual means for exciting a nerve or brane and the 50-fold decrease in membrane capacitance muscle in the experimental laboratory is to apply electricity also allow repolarization to occur with little transfer of ions.␣ to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively Velocity of Conduction in Nerve Fibers. The velocity charged. When electricity is applied in this manner, the of action potential conduction in nerve fibers varies from excitable membrane becomes stimulated at the negative as little as 0.25 m/sec in small unmyelinated fibers to as electrode. much as 100 m/sec—more than the length of a football Remember that the action potential is initiated by the field in 1 second—in large myelinated fibers.␣ opening of voltage-gated sodium channels. Furthermore, these channels are opened by a decrease in the normal rest- ing electrical voltage across the membrane—that is, nega- EXCITATION—THE PROCESS OF tive current from the electrode decreases the voltage on the ELICITING THE ACTION POTENTIAL outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect Basically, any factor that causes sodium ions to begin to decreases the electrical voltage across the membrane and diffuse inward through the membrane in sufficient num- allows the sodium channels to open, resulting in an action bers can set off automatic regenerative opening of the potential. Conversely, at the positive electrode, the injec- sodium channels. This automatic regenerative opening tion of positive charges on the outside of the nerve mem- can result from mechanical disturbance of the membrane, brane heightens the voltage difference across the mem- chemical effects on the membrane, or passage of electric- brane, rather than lessening it. This effect causes a state of ity through the membrane. All these approaches are used hyperpolarization, which actually decreases the excitability at different points in the body to elicit nerve or muscle of the fiber rather than causing an action potential.␣ Saltatory conduction Action potential starts here Action potential Action potential Saltatory conduction Na+ Na+ – – – + + + + + + + + + + + + + – – – – – – + + + + + + + + + + + + + – – – Na+ Na+ Na+ channel Axoplasm Myelin sheath Node of Ranvier Figure 5-17 Saltatory conduction along a myelinated axon. The flow of electrical current from node to node is illustrated by the arrows. 75 UNIT II Membrane Physiology, Nerve, and Muscle Threshold for Excitation and Acute Local Potentials. resting membrane potential level. Then, within another A weak negative electrical stimulus may not be able to small fraction of a second, the inactivation gates of the excite a fiber. However, when the voltage of the stimu- channels open, and a new action potential can be initiated. lus is increased, there comes a point at which excitation The period during which a second action potential can- does take place. Figure 5-18 shows the effects of suc- not be elicited, even with a strong stimulus, is called the cessively applied stimuli of progressing strength. A weak absolute refractory period. This period for large myelin- stimulus at point A causes the membrane potential to ated nerve fibers is about 1/2500 second. Therefore, one change from −70 to −65 millivolts, but this change is can readily calculate that such a fiber can transmit a maxi- not sufficient for the automatic regenerative processes mum of about 2500 impulses per second. of the action potential to develop. At point B, the stimu- lus is greater, but the intensity is still not enough. The Inhibition of Excitability—Stabilizers and Local stimulus does, however, disturb the membrane potential Anesthetics locally for as long as 1 millisecond or more after both In contrast to the factors that increase nerve excitability, of these weak stimuli. These local potential changes are membrane-stabilizing factors can decrease excitability. For called acute local potentials and, when they fail to elicit example, a high extracellular fluid calcium ion concentration an action potential, they are called acute subthreshold decreases membrane permeability to sodium ions and potentials. simultaneously reduces excitability. Therefore, calcium ions At point C in Figure 5-18, the stimulus is even stronger. are said to be what is called a stabilizer.␣ Now, the local potential has barely reached the threshold Local Anesthetics. Among the most important sta- level required to elicit an action potential, but this occurs bilizers are the many substances used clinically as local anesthetics, including procaine and tetracaine. Most of only after a short “latent period.” At point D, the stimulus is these agents act directly on the activation gates of the so- still stronger, the acute local potential is also stronger, and dium channels, making it much more difficult for these the action potential occurs after less of a latent period. gates to open and thereby reducing membrane excitabil- Thus, this figure shows that even a weak stimulus ity. When excitability has been reduced so low that the causes a local potential change at the membrane, but the ratio of action potential strength to excitability threshold intensity of the local potential must rise to a threshold (called the safety factor) is reduced below 1.0, nerve im- level before the action potential is set off.␣ pulses fail to pass along the anesthetized nerves. REFRACTORY PERIOD AFTER AN ACTION POTENTIAL, DURING WHICH A NEW Bibliography STIMULUS CANNOT BE ELICITED Alberts B, Johnson A, Lewis J