Excitable Properties of Neurons PDF

MUSC Medical Curriculum Excitable Properties of Neurons EXCITABLE PROPERTIES OF NEURONS John J Woodward, Ph.D Professor, Department of Neuroscience Office: Gazes 627B 843-792-5225 [email protected] Readings: Purves, et al, 5th edition, pp. 25-76. Additional resources: Hille, Ion Channels of Excit...

MUSC Medical Curriculum Excitable Properties of Neurons EXCITABLE PROPERTIES OF NEURONS John J Woodward, Ph.D Professor, Department of Neuroscience Office: Gazes 627B 843-792-5225 [email protected] Readings: Purves, et al, 5th edition, pp. 25-76. Additional resources: Hille, Ion Channels of Excitable Membranes, 3rd Edition, Sinauer. Nicholls, From Neuron to Brain, 4th Edition, Sinauer. I. Ionic Basis of Resting Membrane Potential • • • • • II. Introduction Passive and Active Membrane Properties Ion Gradients and Channels Reversal Potential and Nernst Equation Goldman Equation Generation of Action Potentials • • • • • Ion Permeability Changes Anatomy of an Action Potential Mechanisms Underlying Action Potential Voltage-gated Channels Drugs, Toxins and Mutations III. Propagation of Action Potentials • • • • • • Membranes as Resistors Length Constant Membranes as Capacitors Time Constant Propagation in Un-Myelinated Axon Propagation in Myelinated Axon Illustrations: The figures used in the handout are taken primarily from Neuroscience (Purves, 5th edition; Sinauer). Other images are used under the Fair Use Statute. 1 MUSC Medical Curriculum Excitable Properties of Neurons Ionic Basis of Resting Membrane Potential John J Woodward, PhD Professor, Department of Neuroscience Office: Gazes 627B 843-792-5225 [email protected] Learning Objectives: After completing your study of this material, you should be able to: 1. Describe the difference between passive and active properties of membranes. 2. Understand the meaning of the following terms: membrane potential, depolarization, hyperpolarization, leak channels. 3. List the two major factors that underlie the resting membrane potential. 4. Give the approximate values for concentrations of sodium and potassium inside and outside of mammalian neurons. 5. Describe the concept of equilibrium (reversal) potential and how electrical and chemical gradients determine this value. 6. Discuss how the Nernst and Goldman equations can be used to predict the resting membrane potential. 7. Describe how changes in the concentrations of ions inside and outside the neuron affect the resting membrane potential 2 MUSC Medical Curriculum Excitable Properties of Neurons Introduction Neurons are the specialized cells of the nervous system that are responsible for signaling information over long distances. They do this by integrating incoming chemical signals and transducing this information into electrical signals that can be propagated over long distances. The figure below shows two examples of neurons, one from the cortex and one from the retina in the eye. Both are characterized by a distinct anatomy consisting of dendrites (green)-branches off the cell body that receive incoming information from other neurons at specialized structures called synapses, a large cell body or soma (purple), and an axon (brown) that can form extensive branches and that extend for long distances. For example, the axons of motor neurons that control skeletal muscle can be over 1 meter long. Passive and Active Membrane Properties Examples of different types of neuronal signaling are shown in the figure to the left. Each panel on the left shows a particular type of neuron including sensory (top), brain (middle), and motor (bottom). Stimulation of each neuron induces measurable changes in the membrane potential of the neuron as shown in the panels to the right. Prior to the stimulus, note that the membrane potential is negative ranging between -60 and -70 mV. Following the stimulus, the membrane potential becomes more positive (depolarized) and then returns to its baseline value. In the case of the motor neuron, a large and rapid depolarization called an action potential is elicited followed by a brief period of hyperpolarization. We will discuss the underlying mechanisms of these changes in a later section. 3 MUSC Medical Curriculum Excitable Properties of Neurons The ability to measure changes in the membrane potential of cells is called electrophysiology and this tool is essential for the study of the electrical signaling properties of neurons. As shown in the figure below, microelectrodes can be inserted into neurons and used to both record and stimulate changes in membrane potential. Neurons display two major types of electrical responses, those that are passive and simply follow the stimulus and those that are active and usually exceed the amplitude or duration of the initial stimulus. For example, if negative current is injected into the neuron via the stimulation microelectrode, the membrane potential of the neuron becomes more negative (hyperpolarization) while the opposite is true if positive current is injected (depolarization). These are passive responses since they simply follow the direction of the injected current and decay back to baseline once the stimulus is stopped. However, if the injected current induces a depolarization that reaches a certain threshold, a dynamic and active process takes over that results in an action potential; a rapid and reversible active depolarization of the nerve cell membrane. If the stimulus is large enough or is sustained, a series of action potentials is generated, each with the same amplitude and time course. This type of active response is termed all or none as they either occur completely or not at all. This type of of signaling is often compared to the binary system of information transfer used in microprocessors where information is manipulated in bits of 0 and 1. Because the amplitude of the action potential is always the same, information is coded by the frequency of action potential firing, not the amplitude. Frequency =/= Amplitude 4 MUSC Medical Curriculum Excitable Properties of Neurons Passive Transmission of Electrical Signals is Limited In the figure below, a segment of an axon is shown and a stimulating electrode is used to supply a brief injection of positive current. Recording the change in membrane potential at different distances reveals that the response is greatest at the site of stimulation and then slowly decays as the recording site moves further away. The bottom panel plots the relationship between the amplitude of the membrane potential and the distance that the recording electrode is from the stimulus. The decrease in the amplitude of the signal arises from leak of charges across the membrane and illustrates that passive conduction of electrical signals is local and is not a particularly useful system for conveying information over the long distances that axons often travel. Active Transmission of Electrical Signals In the figure below, a similar axon is shown except now with an active process that allows the generation of action potentials. In this case, the electrical signal is regenerated down the length of the axon and each recording electrode reports the same waveform regardless of how far away it is from the 5 MUSC Medical Curriculum Excitable Properties of Neurons stimulus. In this way, electrical signals can be transmitted both quickly and over relatively long distances with no decrease in their signal strength. We will discuss how these action potentials are generated in a later section. Ion Gradients and Channels Generate the Resting Membrane Potential Electrical potentials are generated across neuronal membranes because of two important factors: a. Differences in concentrations of specific ions inside and outside of the cells b. Differences in the permeability of the membrane to those ions The table below lists the concentrations of the major ions inside and outside of two different types of neurons. The squid neuron was a key experimental preparation and was used to study how electrical signals are generated in axons. Also shown are values for a typical mammalian neuron. The key point of this table in terms of the resting membrane potential is the large differences in concentrations of ions inside and outside the cell. For example, in mammalian neurons, the potassium concentration inside the cell is approximately 140 mM while the concentration outside is only 5 mM. Differences in concentrations of ions across the membrane arise from the actions of active transporters that actively move ions against their concentration gradient and specialized proteins called ion channels that only allow certain ions to diffuse through their pore. 6 MUSC Medical Curriculum Excitable Properties of Neurons At rest, neuronal membranes have a high permeability to K+ ions while permeability to ions such as sodium, calcium and chloride is almost zero. This occurs because ions do not easily cross phospholipid membranes without an aqueous channel and only channels permeable to potassium (called leak channels; e.g., 2-pore K+ channels; see page 15) are open at rest. The combination of a high intracellular potassium concentration and a selective permeability to potassium is what generates the negative resting membrane potential in neurons. This is illustrated in the figure below that shows a compartment divided by a membrane that is selectively permeable to potassium. A voltmeter measures the difference in electrical charge across the membrane. If an equal concentration of K+ ions is placed on each side (panel A; 1 mM in this case), then the system is at chemical and electrical equilibrium and the voltmeter reads 0 mV indicating no potential difference between the two sides. However, if the left side (intracellular) of the compartment has a 10-fold excess of potassium (10 mM K+), a chemical gradient has been established and K+ ions will begin to flow from left to right (middle panel). The removal of positively charged particles from the inside generates a negative resting electrical potential that stabilizes at approximately -58 mV (right panel). This equilibrium occurs not because the concentrations of K+ are now equal from inside to outside (they aren’t) but because the flow of K+ ions from inside to outside is balanced by the electrical repulsion of the positive K+ ions that accumulate on the extracellular side. This type of equilibrium is called electrochemical equilibrium because it involves both the chemical gradient and the electrical gradient. Interestingly, the number of ions that actually need to flow from inside to outside to generate the resting membrane potential is extremely small meaning that overall concentration of ions on the inside and outside of the cell is minimally affected. In neurons, the loss of K+ ions through the K+ selective ion channels is balanced by the action of active, ATP-dependent transporters that transport K+ ions back into the cell thus maintaining the concentration gradient. When cells become damaged or are incapable of generating ATP, this gradient is lost and membrane potential goes to 0 mV. 7 MUSC Medical Curriculum Excitable Properties of Neurons The Nernst Equation Predicts Membrane Potential The membrane potential that is generated at the electrochemical equilibrium of an ion is called the equilibrium or reversal potential (Erev) and can be calculated for each ion by using the Nernst equation. Eion= (RT/zF) ln ([ion]o/[ion]i) Where: R=universal gas constant=8.31 joule/(oK mole) T=absolute temperature=degrees Kelvin z=valence of the ion F=Faraday’s constant; 96,500 coulombs/mole ln= natural logarithm [ion]o=concentration of the ion in the extracellular fluid [ion]i=concentration of the ion in the cytoplasm This equation can be simplified by calculating the RT/zF constant and by converting to base 10 logarithms. For room temperature, this results in the following equilibrium potential for potassium given a 10-fold difference in concentration between inside and out. Exceptions(2) EK+= 58 log ([1]o/[10]i)= -58 mV If the concentration of potassium inside the cell were to be increased to 100 mM, then the resulting EK+ would be -116 mV. Note that for negative ions such as chloride, we invert the ratio ([ioni]/[iono]) to account for the difference in charge. For multivalent ions (e.g. Ca++), we use 2Z in the formula to account for the presence of two charges (e.g. for calcium, the equation is 29 log ([ion]o/[ion]i)). Understanding the concept of the reversal potential is key to understanding what happens when an ion channel with selective permeability to a given ion opens. As long as the channel remains open, ions will flow until the membrane potential reaches that ion’s reversal potential. In our example, that value is -58 mV for potassium ions as the hypothetical membrane is only permeable to potassium and the ratio of the concentrations from inside to outside is 10:1. In reality, the K+ gradient is higher than this for most mammalian neurons (see Table on page 6) and using values of 140 mM inside and 5 mM outside yields a reversal potential for potassium of -84 mV. Substituting in values for the other ions listed in the Table yields calculated reversal potentials of: ENa +84 mV (assuming 5 mM intracellular) ECa +125 mV (assuming 2 mM extracellular) ECl -60 mV (assuming 10 mM intracellular) This means that if a sodium permeable channel opens, sodium ions would flow down their concentration gradient until the membrane potential reached +84 mV. If a calcium channel opened, calcium would flow until a value of +125 mV was reached. If a chloride channel opened, the membrane potential would stabilize at -60 mV. In reality, most membranes don’t quite reach these maximums since several different channels with different permeability are open at the same time. As we will see in the next section; it is the coordinated opening and closing of ion selective channels that allow neurons to precisely control their electrical signaling properties and to conduct impulses down long lengths of axons. 8 MUSC Medical Curriculum Excitable Properties of Neurons The Goldman Equation Accounts for the Presence of Additional Permeant Ions In the example given above, we considered situations in which only the movement of a single permeant ion was involved. As neurons are surrounded by a mixture of cations and anions, the Nernst equation is not able to account for membranes that are permeable to more than one ion at a time. The Goldman equation was developed in response to this need and introduces a permeability factor for each ion. For example, in neurons where K+, Na+ and Cl- are the primary permeant ions, the equation is: Em=58 log PK[K]o + PNa[Na]o + PCl[Cl]i PK[K]i + PNa[Na]i + PCl[Cl]o 1 0.04 0.45 At rest, the estimated permeability ratios of K+, Na+ and Cl- for a typical neuron are 1 : 0.04 : 0.45. Using this information and the values shown in table on page 6, calculate the resting membrane potential of a neuron using the Goldman equation. Because K+ has the highest permeability of any ion at rest, it has the greatest effect on the resting membrane potential of neurons. Another way to think of this is that the membrane potential at rest is always closer to EK than to ENa. This also means that small changes in the extracellular concentration of potassium can have dramatic effects on neuronal excitability by changing the resting membrane potential. This is shown in the figure above that plots the resting membrane potential of the squid giant axon as a function of the extracellular potassium concentration (note that seawater is much higher in potassium than mammalian extracellular fluid). This graph shows how relatively small increases in extracellular potassium markedly depolarize the neuronal membrane. In brain neurons, the levels of extracellular potassium are maintained in large part by the action of glial cells that surround neurons and actively take up and sequester potassium. Changes in this function can lead to hyper-excitability since neurons are closer to the threshold needed to generate action potentials. 9 MUSC Medical Curriculum Excitable Properties of Neurons Ionic Basis of Action Potentials John J Woodward, Ph.D Professor, Department of Neuroscience Office: Gazes 627B 843-792-5225 [email protected] Learning Objectives: After completing your study of this material, you should be able to: 1. Discuss the concept of selective membrane permeability as it applied to action potentials. 2. Be familiar with the following terms regarding action potentials: duration, over-shoot, amplitude, threshold, refractory period, all or none behavior. 3. Understand how voltage-gated sodium and potassium channels interact to generate the action potential. 4. Describe the basic structure of the sodium and potassium channels and how they are arranged in the membrane. 5. Describe examples of drugs, toxins and genetic mutations that affect the function of voltagegated sodium and potassium channels. 10