Neurophysiology. 2.1 Equilibrium Potential

Transcription

1 2 Neurophysiology 2.1 Equilibrium Potential An understanding of the concepts of electrical and chemical forces that act on ions, electrochemical equilibrium, and equilibrium potential is a powerful tool for thinking about membrane potentials and the function of excitable cells, like neurons, muscle, and secretory cells. 1. Describe electrochemical equilibrium in one sentence or so. Be sure to include the conditions that must exist in order to reach electrochemical equilibrium for a single ion. The arrows on this diagram of a neuron illustrate the magnitude (shown by the length of the arrow) and the direction of the electrical (gray arrow) and chemical (black arrow) forces acting on K ions when the membrane potential is at the K equilibrium potential. 2. Given what you know about the distribution of K ions across the plasma membrane, will the membrane potential be negative or positive in this instance? 3. Use the Nernst equation to calculate the K equilibrium potential. where E ion equilibrium potential for a particular ion, in mv; E ion 61 log C o Z ( Ci ) C i intracellular concentration of the ion; C o extracellular concentration of the ion; Z the valence of the ion; and 61 a constant value that takes into account the universal gas constant (R), the temperature (37ºC), and the Faraday electrical constant (F).

2 4. Looking at the Nernst equation, which 3 variables can you say determine the equilibrium potential for a single ion? Under normal physiological conditions, these variables remain more or less constant, as do the equilibrium potentials for K and Na. (Later in this book, we will consider some extreme and/or pathological situations in which the normal distribution of K ions is changed and the consequences of that.) 5. Which cellular mechanism helps to keep these variables constant? 6. Calculate the Na equilibrium potential. 7. How does this value differ from the K equilibrium potential? Note 2 differences What accounts for these differences? 20 Cells to Systems

3 2.2 Resting Membrane Potential Most cells have a negative membrane potential at rest. The term resting membrane potential refers to the membrane potential of a cell that has mostly just leak channels open, without other channels, such as ligandgated or voltage-gated channels, open. Although neurons may not often be in this state, the concept of resting membrane potential provides a useful frame of reference when considering neuronal signaling (as well as the function of other cell types, such as muscle and secretory cells). At any given time, the membrane potential of a neuron (or any cell for that matter) will be determined by two major factors: 1) Which types of ion channels are open 2) How many of each type of ion channel are open This concept is described mathematically by the Goldman-Hodgkin-Katz (GHK) equation, which is simplified below as follows: V m 60 mv log P K[K ] out P Na [Na ] out P Cl [Cl ] in P K [K ] in P Na [Na ] in P Cl [Cl ] out where P ion is the relative permeability of the membrane to the ion (i.e., a reflection of how many channels are open). 1. Describe the similarities between the GHK equation and the Nernst equation. 2. Describe the differences between the GHK equation and the Nernst equation. 3. If you plug some numbers for a typical neuron at rest into the GHK equation, you get the following: V m 60 mv log 1(5) 0.04(145) 0.45(7) 1(150) 0.04(15) 0.45(100) Do the math: V m 4. The resting membrane potential of a neuron is closest to the equilibrium potential for which ion: Na or K? (circle one) Why? (Hint: Look closely at the numbers in the GHK equation.) 5. Increasing which one of the following parameters by 2 mm will cause the greatest change in resting membrane potential? Try predicting this first, then do the math. Explain your answer. [K ] out, [K ] in, [Na ] out, [Na ] in, [Cl ] out, or [Cl ] in Neurophysiology 21

4 2.3 Neuronal Structure and Function Neurons come in a variety of shapes and sizes, but it is helpful to generalize the structure/function relationships in order to think about neuronal signaling and interneuronal communication. 1. Draw a diagram of a typical neuron and label the main functional parts. Include dendrites, soma, initial segment of the axon (axon hillock), axon, myelin, and axon terminal. Draw arrows to indicate the direction of information flow. 2. List the main functional roles of each of the parts that you labeled in your drawing above. 1. dendrites 2. soma 3. initial segment of the axon (axon hillock) 4. axon, myelin 5. axon terminal 22 Cells to Systems

5 3. Describe an analogy to explain the structure/function relationships of neurons as signal processing and transmission units. For example, you can liken a neuron to an organization, organism, community, and so on anything that requires some information input, information processing or consultation, decision making, and output or communication with others. Note how the different components of your analogous system are similar to those of a neuron. Primary sensory neurons do not have the typical structure with the same components listed above. 4. Draw a picture of a primary sensory neuron and label the main functional parts. 5. Note how primary sensory neurons differ from the typical neuron and list the functional roles of each part. Note which parts play a similar role to those in the typical neuron and explain how they are designed to carry out their main purpose. Neurophysiology 23

6 2.4 Synapses and Synaptic Potentials When neurons communicate with one another at a synapse, the presynaptic axon terminal releases a chemical signal (neurotransmitter) into the synaptic cleft. The neurotransmitter molecule diffuses the short distance across the cleft to the plasma membrane of the postsynaptic cell, which contains receptors. If the receptors are ligandgated ion channels, binding of the neurotransmitter will cause these channels to open. When considering what will happen to the membrane potential of a postsynaptic cell when ligand-gated ion channels are opened, it is helpful to think first about the forces acting on ions when the membrane potential is at rest (i.e., 70 mv). 1. Draw arrows on the diagram to indicate the magnitude and direction of electrical and chemical forces (as we did in Exercise 2.1) on Na ions when the membrane potential is at 70 mv. Na + 2. If the membrane potential were at 70 mv and Na channels were opened, in which direction would the Na ions move? +60 mv 3. In which direction would the membrane potential change from 70 mv? Draw an arrow on the scale to the right to illustrate this. 0 mv 70 mv 90 mv 4. Draw a diagram of a neuron, with arrows indicating the magnitude and direction of electrical and chemical forces on K ions when the membrane potential is 70 mv. 5. If the membrane potential were 70 mv and K channels were opened, in which direction would K ions move? 6. Draw an arrow on the scale to show in which direction the membrane potential would change from 70 mv. 24 Cells to Systems

7 7. Write a simple generalization (or rule of thumb ) about the direction that the membrane potential will go when a specific type (i.e., K or Na ) of channels is opened. (Hint: Think about the equilibrium potential.) This rule of thumb will be a useful tool for thinking about how neurons use changes in membrane potential for signaling and interneuronal communication. A postsynaptic potential (psp) is a change in membrane potential over time that occurs in a postsynaptic cell. Excitatory postsynaptic potentials (epsp; see A below) are depolarizing, and inhibitory postsynaptic potentials (ipsp; see B below) are hyperpolarizing. 8. Which type of channels might be opening to cause the depolarizing and hyperpolarizing potentials shown in A and B below? A. B. A B. 9. Describe what would be happening at a synapse at each of the times labeled by the numbers on the epsp shown in A on the previous page. Note in the table below what is happening at the presynaptic axon terminal as well as the postsynaptic membrane (use the drawing of some of the key components of a synapse on the previous page to help you think through this). Presynaptic axon terminal Time Point Neurotransmitter release or reuptake No neurotransmitter released Postsynaptic neuron Receptors Ion movement Membrane potential (with respect to rest) No receptors bound by neurotransmitter, hence no channels open No net movement of ions into or out of postsynaptic neuron At rest ( 70mV) How would any of the above differ for the ipsp shown in B? Neurophysiology 25

8 2.5 Action Potentials Neurons receive information from other neurons in the form of neurotransmitters binding to plasma membrane receptors at synapses. Often the result of neurotransmitters binding to plasma membrane receptors on postsynaptic neurons is a change in membrane potential, as we saw in the previous exercise about postsynaptic potentials. Neurons have many synapses on their dendrites and soma, and the postsynaptic potentials generated at all of the synapses sum (as shown in the diagram below, where epsp and ipsp from synapses A, B, C, and D sum in various combinations). Whether or not a neuron will send a signal to another neuron will depend on whether or not the initial segment of the axon is depolarized sufficiently by the summed postsynaptic potentials to open voltage-gated channels that mediate the action potential (AP). If this threshold depolarization (see dotted line in diagram below) is reached, an AP will be initiated and then propagated along the axon to its terminal (not shown on this diagram), where it will trigger neurotransmitter release. Membrane potential at the initial segment of the axon AP B A C A A C A B D A+C D Summation of Postsynaptic Potentials 1. In the drawing above, what type of summation is shown by the two closely spaced arrows labelled A in the membrane potential trace on the right? Explain. 2. What type of summation results in the triggering of an action potential in the membrane potential trace on the right? Explain. 26 Cells to Systems

9 Voltage-Gated Ion Channels Just as synaptic potentials are often mediated by ligand-gated ion channels, action potentials are mediated by voltage-gated ion channels. Understanding the properties of these channels makes it easier to understand important characteristics of action potentials, including threshold, the all-or-none nature of the action potential, and the afterhyperpolarization and refractory periods. All these characteristics of action potentials shape the way neurons and other excitable cells (i.e., cells that generate action potentials) function. 1. Compare and contrast the following properties of voltage-gated Na and K channels that mediate action potentials. Property Voltage-Gated Na Channel Voltage-Gated K Channel Gating (What causes it to open? Be specific and give an example) Depolarization from rest (around 70mV) to around 50 mv (threshold) Depolarization from rest Speed of opening Change in MP as a result of channel opening Inactivation? (if yes, on what two factors does this depend?) What causes it to close? Positive or negative feedback mechanism? Neurophysiology 27

10 Action Potential 1. Draw a picture of a neuronal action potential and label the time and voltage (i.e., membrane potential) axes. Label the different phases of the action potential. Phases of the Action Potential Depolarization/Overshoot Phase 2. Which ion channels are responsible for the depolarization phase of the action potential? 3. Why does the membrane depolarization and overshoot happen so quickly? 4. Why doesn t the membrane potential get all the way to 60 mv (i.e. E Na ) during the overshoot? Repolarization Phase 5. Why does the membrane potential begin to repolarize after the overshoot? Note all of the ion channels that play a role here. Afterhyperpolarization 6. Which channels are responsible for the afterhyperpolarization phase of the action potential? 28 Cells to Systems

11 7. How can the membrane potential be more negative than resting membrane potential at this time? 8. Why does the membrane potential go back to resting membrane potential after the afterhyperpolarization phase? Refractory Periods Absolute Refractory Period 9. During the absolute refractory period, what is the state of voltage-gated Na and K channels? 10. Why is it not possible to generate another action potential at this time? Relative Refractory Period 11. Explain the state of voltage-gated Na and K channels and the membrane potential during the relative refractory period. 12. During the relative refractory period, why is it possible to generate another action potential? 13. What is necessary to generate another action potential during the relative refractory period? 14. Why? Neurophysiology 29

12 2.6 Action Potential Propagation and Conduction Velocity Action potentials represent a means of transmitting an electrical signal from one part of a neuron to its axon terminal, where it triggers neurotransmitter release. Axons are designed such that the propagation of action potentials occurs without a decrease in signal strength (i.e., amplitude of the action potential) from the initial segment to the axon terminal. 1. Draw a neuron with a myelinated axon and a neuron with an unnmyelinated axon. Use symbols to indicate the location of voltage-gated ion channels that mediate action potentials. 2. Explain how action potentials propagate along an unmyelinated axon, and why the action potential that reaches the axon terminal is as large as the action potential that was initiated at the initial segment. 3. What is the critical design parameter that will ensure that action potentials will be propagated along an unmyelinated axon? 4. Explain how propagation of an action potential along a myelinated axon differs from propagation along an unmyelinated axon. 5. Put a number in the box beside each of the axons below to rank them based on conduction velocity, with the fastest conduction velocity being 1 and the slowest being 3. Rank 6. What are the two most important factors that determine conduction velocity? 7. Explain how each of the two factors affects conduction velocity. 8. What would happen to action potential propagation in axons subjected to a demyelinating disease like multiple sclerosis? 30 Cells to Systems

13 2.7 Organization of the Nervous System 1. Mine your brain. Based on your own experience and knowledge, make the most complete list that you can of all of the functions that the brain and spinal cord perform. These functions can be very general (like think ), but also make an effort to drill down to as much detail as you can. Note which functions are specific to only one or the other or to both the brain and the spinal cord. 2. For each of the following brain regions, write a single phrase or sentence to summarize its functional role. Include after each summary some of the specific functions that each region is involved in. This need not be complete, but try to fill in as much as you can. For example: Brainstem: The brainstem is involved in many automatic or unconscious functions that serve to keep the body functioning without the need for conscious control. For example, the brainstem is involved in the control of respiration, heart rate, vomiting, and so on. All axons that go between the brain and the spinal cord must pass through the brain stem. It is essential for life since it is required to drive respiration. Cerebellum: Limbic system: Hypothalamus: Thalamus: Cerebral hemispheres: Neurophysiology 31

14 Peripheral Nervous System 3. Define the afferent and efferent division of the peripheral nervous system. 4. Draw a picture of the spinal cord with an afferent and efferent neuron as well as an interneuron. Use arrows to show the direction of information flow. 5. Name the two divisions of the efferent division of the peripheral nervous system and describe in general terms how their functions and target organs differ. 6. Give an example of the parasympathetic and sympathetic innervation of cells in a specific target organ of your choice and discuss circumstances when one is more likely to be active than the other. Include in your discussion how neurons from the two different branches of the autonomic nervous system can have different effects on the same target cell and how the specific effects on the target cell fit into the general concepts of flight or fight and rest and digest. 32 Cells to Systems