Electrical Properties of Nerves and Resting Membrane Potential Case File

Electrical Properties of Nerves and Resting Membrane Potential Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 3

A 39-year-old male on business in Japan was brought to the emergency department (ED) by paramedics with initial complaints of numbness and tingling around the face and mouth. His symptoms progressed to include lightheadedness and nausea/vomiting which prompted the visit. He had just completed a successful business venture and had celebrated the occasion with dinner at a fine restaurant by feasting on some puffer fish, a Japanese delicacy. Upon his arrival, he rapidly became unable to move and had severe respiratory distress. Telemetry recordings demonstrated an irregular heartbeat. He was quickly intubated and placed on mechanical ventilation. Unfortunately, he expired shortly after his presentation. Autopsy confirmed a diagnosis of tetrodotoxin (TTX) poisoning.

• What is the biochemical mechanism of this disease?
• How does TTX inhibit neural activity?
• What treatment options are available?

ANSWERS TO CASE 3: ELECTRICAL PROPERTIES OF NERVES AND RESTING MEMBRANE POTENTIAL

Summary: A 39-year-old businessman presents to the ED with complaints of numbness and tingling in his mouth which progresses to paralysis, respiratory failure, and death.

• Mechanism: TTX binds to the fast voltage-gated sodium channel at a site which is located in the extracellular pore opening of the ion channel, known as site 1. This binding temporarily disables the function of the ion channel, resulting in death by paralysis of muscles.
• Neural activity: TTX is a potent neurotoxin with no known antidote, which blocks action potentials in nerves by binding to the pores of the voltage-gated, fast sodium channels in nerve cell membranes. Specifically, the action potential is inhibited during the rapid depolarization phase.
• Treatment: There is no known antidote for TTX poisoning. Emptying the patient’s stomach, ingesting activated charcoal to bind the toxin, and supportive care with mechanical ventilation are the limits of current medical care.

CLINICAL CORRELATION

TTX is a potent neurotoxin commonly found in some species of fish in the order Tetradontiformes. TTX binds to the voltage-gated sodium channels in nerves and prevents Na+ entrance into the cell. While it does not bind irreversibly to the channel, TTX has a very strong affinity for the channel and is not easily removed. Symptoms may begin with numbness and paresthesias of the face, mouth, and extremities and progress to lightheadedness or dizziness, nausea/vomiting, paralysis, and respiratory distress. For up to 50% of individuals with tetrodotoxicity, death will generally occur within 24 hours of ingestion. Because of the potential dangers inherent in the ingestion of puffer fish or fugu, specially trained and licensed chefs are required to purchase and prepare this Japanese delicacy.

APPROACH TO ELECTRICAL PROPERTIES OF NERVES AND RESTING MEMBRANE POTENTIAL

Objectives

1. Describe the signaling mechanisms used in the nervous system.
2. Describe the necessary elements to create an electrochemical gradient.
3. Describe how the membrane and channels work to create the environment necessary for signaling.

Definitions

Electrochemical gradient: The gradient across a cell membrane created by the differential concentrations of charged ions on either side of the membrane, for example, the difference in concentrations of potassium and sodium ions inside and outside of a neuron that when regulated by their respective ion channels allow for signaling down an axon to occur.

Ion channels: Transmembrane proteins in the cell membrane which open and close to allow for the passage of ions.

Gating: The process by which channels undergo conformational changes to allow for the passage of ions.

Resting membrane potential: The potential created in a neuron at rest by resting channels and sodium-potassium pumps.

DISCUSSION

The nervous system depends on two types of signaling mechanisms, electrical and chemical, to propagate information throughout the nervous system. Rapid changes in the electrical potential across the neuronal cell membrane generate electrical signals that are transmitted down the length of the neuron. This system requires (1) an intact membrane to separate ions and maintain an electrochemical gradient and (2) ion channels to allow for the selective passage of ions of specific charges to generate the electrical signal.

The cell membrane of the neuron is formed by a lipid bilayer and is generally impermeable to charged particles. The double layer of phospholipids is hydrophobic. Charged ions are hydrophilic and as a result attract water molecules. This allows the neuronal cell membrane to separate charges across its surface to maintain the electrochemical gradient. However, to create and utilize the energy stored in the electrochemical gradient, structures must exist to allow for the passage of ions across this membrane. Ion channels, transmembrane spanning proteins, serve that specific function within the neuron. The basic structure consists of transmembrane proteins with carbohydrate groups attached to its surface and a central, pore-forming region to allow for the passage of ions. This pore-forming region spans the entirety of the membrane and is generally made up of two or more subunits.

Ion channels must also be selective for specific charged particles. One method by which channels select for specific ions is by size. Although the diameter of a K+ ion is larger than the diameter of a Na+ ion, the Na+ ions demonstrate a stronger electrostatic attraction for water molecules. Thus, in a solution the Na+ ion has a larger shell of water than K+ ions. Channels can therefore select for K+ ions based

upon the size differential in a solution. Other types of channels are selective for specific ions based upon the ion’s electrical affinity to charged portions of the channel. The attraction between an ion and the channel must be sufficiently strong enough to overcome the hydrostatic attraction of the ion. Once the shell of water surrounding the ion is shed, the ion can diffuse through the channel.

The flow of ions through a channel is passive and governed by the electrochemical gradient. Some ion channels are highly selective for a specific anion or cation, while others are more indiscriminate. Ion channels also open and close based upon the needs of the neuron. This change in state requires a conformational change of the proteins that form the channel, a process called gating.

To understand the electrical properties of the neuron, we must have an understanding of the electrochemical gradient. Particular ions are distributed unequally across the cell membrane. The [Na+] and [Cl-] are greater on the outside of the cell, while [K+] and organic anions, such as charged amino acids and proteins, are greater on the inside of the cell (Table 3-1). Only the organic anions are incapable of passing across the cell membrane.

Because the cell membrane is essentially impermeable to charged particles, ions must rely on specific channels and transporters to gain entry or exit from the neuron. There are two general types of ion channels in the neuron, resting and gated channels. Resting channels are normally open, as their name implies, in the resting state of the neuron and is important for the establishment of the resting membrane potential. Gated channels are typically closed in the resting state and open only in response to an external signal to allow for the rapid electrical potential changes necessary for cell signaling.

Resting channels help to create the equilibrium potential, the point at which there is no net flow of ions across the cell membrane. The equilibrium potential is created by the concentration gradient of a single ion across the cell membrane as calculated by the Nernst equation:

Vm = RT/FZ × ln [Ion]out/[Ion]in

where Vm is the membrane potential in volts, R is the gas constant (8.3143 joules/ mole degree), T is the absolute temperature, F is Faraday’s constant (96,487 coulombs/mole), Z is the ionic valence, and [Ion]out/[Ion]in is the concentration gradient for an ion across the cell membrane. Using potassium as an example, the equilibrium potential represents the voltage at which the chemical gradient driving K+ ions out of the cell is exactly balanced by the electrical gradient keeping K+ ions inside the cell, resulting in no net movement of K+ ions.

Table 3-1 APPROXIMATE ION CONCENTRATION ACROSS NEURONAL CELL MEMBRANE∗
ION	[Out] IN mmol	[In] IN mmol	EQUILIBRIUM POTENTIAL (mV)
Na+	150	15	+55
K+	5.5	150	−90
Cl-	125	9	−70
Organic anions	−	385	−

∗[Out] and [In] represent concentrations of ions outside and inside of the cell membrane respectively.

While the Nernst equation accounts for a single ion, the Goldman- Hodgkin-Katz equation accounts for the major ions which contribute to the resting membrane potential and each ion’s permeability coefficient, the ease with which the ions pass across the cell membrane:

Vm = RT/F × ln (pK[K+]out + pNa[Na+]out + pCl[Cl-]out)/(pK[K+]in 

         

         + pNa[Na+]in + pCl[Cl-]in)

where the permeability coefficient for a specific ion is represented by pK, pNa, and pCl. If we are to take a hypothetical cell membrane with only resting potassium channels open, we would find that K+ ions leave the cell because of its concentration gradient. However, the resulting net negative charge inside the cell (from the remaining organic anions) would limit the K+ ions efflux by balancing the concentration gradient with the increasingly negative electrical gradient inside the cell. Adding Na+ channels into the equation, Na+ ions enter the cell down its concentration gradient, but also down its electrical gradient. This continues until the system reaches equilibrium with a balance of concentration and electrical gradients for Na+ and K+ ions until there is no net movement of ions across the membrane. It is this steady state that creates the resting membrane potential for neurons, −65 mV. Because the permeability of K+ ions is greater than Na+ ions, the resting membrane potential is closer to the equilibrium potential of K+ than it is to Na+. Since the resting membrane potential for Cl- ions is similar to the resting membrane potential for the neuron as a whole, Cl- ions do not contribute much to the overall membrane potential.

If resting channels were unopposed, the resting membrane potential would continue to decrease. Thus, there must be some mechanism to offset the continuous, slow leakage of Na+ and K+ ions across the membrane. The −65-mV resting membrane potential is maintained by the sodium-potassium pump (Na+/K+ pump), a large membrane-spanning protein with binding sites for Na+, K+, and ATP (see Figure 3-1). The intracellular portion of the pump has binding sites for three Na+ ions. The extracellular portion contains two binding sites for K+ ions along with a portion for ATPase activity. As one molecule of ATP is hydrolyzed to ADP and inorganic phosphate, the resulting energy is used to move Na+ and K+ against their net electrochemical gradients. It is this unequal movement of ions which maintains the negative resting membrane potential of the neuron. But at rest, the concentration of Na+, K+, and Cl- ions inside and outside of the cell are balanced and constant owing to the previously mentioned forces.

Signaling through the nervous system requires large changes in electrical potential to propagate signals through and between neurons. These electrical potentials are created by substantial changes in permeability to Na+, K+, and Cl- ions. The large changes in electrical potential, however, are created by only a very small net movement of ions. During an action potential, there is very little change in the concentration gradients of the ions.

Figure 3-1. Sodium-potassium pump. Na+ and K+ flux through the resting nerve cell membrane.

COMPREHENSION QUESTIONS

[3.1] A 25-year-old female presents to the ED because of protracted vomiting and diarrhea from what she believed to be food poisoning. Although she is afebrile, she is pale and obviously volume depleted. She also complains of generalized weakness and fatigue. Initial laboratory tests are remarkable for a K+ of 2.8 mEq/L (normal value 4.0 mEq/L). How would the physician expect the marked hypokalemia to affect the resting membrane potential of the nerves?

A. Hypopolarization of the nerve

B. No effect on the resting membrane potential

C. Hyperpolarization of the nerve

[3.2] A 47-year-old woman presents to your office complaining of paresthesias. After rather extensive workup, you are unable to discover the source of her problem and you decide to check the resting membrane potential of her sensory nerves. The microelectrode is inserted, and the intracellular potential

is measured as −65 mV (which is normal). What relative ionic concentrations are responsible for maintaining this membrane potential?

A. [Na+]out > [Na+]in, [K+]out > [K+]in

B. [Na+]out > [Na+]in, [K+]out < [K+]in

C. [Na+]out < [Na+]in, [K+]out > [K+]in

D. [Na+]out < [Na+]in, [K+]out < [K+]in

[3.3] A researcher in a neuroscience laboratory is investigating the behavior of neuronal membrane potentials in the immediate postmortem period in rats. She notes that immediately following death, the resting membrane potential remains the same as when the animal was alive, but that it slowly decreases toward zero over the following hours. What cellular mechanism is responsible for maintaining the resting potential?

A. Na+/K+ ATPase

B. NKCC (Na+, K+, Cl- cotransporter)

C. Ca2+ ATPase

D. Na+/glucose symporter

Answers

[3.1] C. Decreasing extracellular potassium ion concentration as in this case, will result in hyperpolarization of the nerve. Because potassium is the major intracellular cation in nerves, major alterations in the body’s store of potassium can have a significant effect on the resting membrane potential of the nerve and thus its ability to propagate electrical signals. From the Nernst equation, we can easily see that decreasing the extracellular potassium ion concentration will result in a larger negative value for the resting membrane potential for potassium. The nerve is said to be hyperpolarized or more negative, thus making it more difficult for the nerve to depolarize to propagate an electrical signal. Likewise, elevated potassium levels or hyperkalemia affect the resting membrane potential of the neuron in the opposite manner, resulting in a depolarization of the membrane potential.

[3.2] B. The negative resting membrane potential of sensory neurons is maintained by the relative concentrations of ions across the membrane, as well as the permeability of the membrane to these ions. The high relative intracellular concentration of K+ coupled with the membrane’s relatively high resting permeability to K+ results in a negative Vm. The high relative extracellular concentration of Na+ and the membrane permeability to Na+ actually result in a positive Vm. However, since the permeability of the membrane to Na+ is considerably less than that of K+ (roughly 100 times less), the primary driving force of the membrane potential is K+.

[3.3] A. The Na+/K+ATPase pump maintains the resting potential. Because the neuronal membrane is permeable to both sodium and potassium, the ions slowly diffuse down their electrochemical gradients at rest, and without compensatory mechanism the membrane potential would eventually reach zero. However, there is a mechanism that counteracts this diffusion: the Na+/K+ ATPase, which transport Na+ out of cells and K+ into cells, against their concentration gradients. This pump is driven by ATP, and therefore no longer functions after the cellular metabolism has ceased, as occurs following death. The NKCC (Na+, K+, Cl- cotransporter) is an iontransporting ATPase involved in renal function, the Ca2+ ATPase is a membrane ATPase important in muscle cells, and the Na+/glucose symporter allows absorption of glucose in the intestines. These transporters are not important in neuronal membrane electrochemistry.

NEUROSCIENCE PEARLS ❖ The Na+ and Cl- ion concentrations are greater on the outside of the cell, while the K+ ion concentration and organic anions (ie, charged amino acids and proteins) are greater on the inside of the cell. ❖ The neuron contains two types of channels: resting channels and gated channels. ❖ Resting channels are normally open in the resting state of the neuron and are important for the establishment of the resting membrane potential. ❖ Gated channels open in response to an external signal and allow for the rapid electrical potential changes necessary for cell signaling. ❖ The equilibrium potential is created by the concentration gradient of a single ion across the cell membrane and is calculated by the Nernst equation: Vm = RT/FZ × ln [Ion]out/[Ion]in  ❖ The resting membrane potential occurs when there is a balance of concentration and electrical gradients for Na+ and K+ ions. ❖ Resting membrane potential for neurons is at −65 mV and is maintained by the sodium-potassium pump.

REFERENCES

Bear MF, Connors B, Paradiso M, eds. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006. 

Kandel E, Schwartz J, Jessell T. Principles of Neural Science. 5th ed. New York: McGraw-Hill; 2000. 

Zigmond MJ, Squire LR, Bloom FE, Landis SC, Roberts JL, eds. Fundamental Neuroscience. 2nd ed. San Diego, CA: Academic Press; 1999.

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