Neuron signals handout, Oct. 2001
STUDY QUESTIONS FOR BASIC NEUROPHYSIOLOGY
1. What types of ion channels and pumps are present in the neuron membrane? What is the ion specificity of each type of channel and pump? What type of membrane transport occurs through each of these membrane proteins (i.e., simple diffusion, osmosis, facilitated diffusion, active transport, or bulk transport)? In which direction do the ions cross the membrane in each case? Which of these channels and pumps are distributed throughout the membrane, and which are mainly restricted to either dendrite and soma membrane, or to axon membrane? Which are always open or operating, and which are sometimes closed or not operating? For those that are sometimes closed or not operating, what are the conditions that would activate them (open them or cause them to start operating)?
2. Which ions cross the neuron membrane, resulting in flow of charge which becomes a signal (a stereotyped potential change, usually just called a potential)? What are the driving forces that cause the ions to cross the membrane? Do these ions cross the membrane at all times? In which direction (into or out of the cell) should each ion move in response to each force, when
a) Vm = -70 mV (or any value more negative than 0 mV)?
b) Vm is greater (positive) than 0 mV?
3. What are the two classes of signals (stereotyped potential changes) generated by a neuron? For each class, which types of ion channels are involved in generating the signal? How do each of these types of signals spread away from the area of origin? How does the difference in mechanism of spread relate to the usefulness of each type of signal for long-distance communication? Draw the types of signals, in the form of labeled graphs.
4. Explain why each voltage-regulated ion channel is accurately described as a battery: Which ion channel function is analogous to turning on a switch in a battery-powered device? Which ion channel function is analogous to turning off this switch? What happens when the ion channel is "ON?" What happens when the ion channel is "OFF?" What prevents these membrane batteries from "dying" like a typical battery? What is voltage (potential)? Why is it more accurate to refer to a voltage as a voltage difference? What is a current? What is resistance? What is a conductor? What is an insulator?
5. Where does an action potential originate, in a multipolar neuron? In a bipolar neuron? In a unipolar neuron? How does this origination point differ from other parts of the neuron membrane? What causes or triggers ion channels in this region to generate an action potential? What causes or triggers ion channels in other regions to generate an action potential? Does a single action potential spread to the end of the axon, or is it more accurate to say that each patch of membrane generates a new, identical, action potential?
6. Where does the graded potential originate, in all neuron types? How does this origination point differ from other parts of the neuron membrane? What causes or triggers ion channels in this region to generate a graded potential? As this signal spreads, why isn’t it regenerated (boosted), like an action potential is? Does a single graded potential spread to the end of the axon, or is it more accurate to say that each patch of membrane generates a new, identical, graded potential?
7. What is the role of the sodium/potassium pump? Does this pump contribute to the production of either class of neuron signal? Is this pump regulated (is its operation controlled)? Does the pump require ATP to operate? What happens to the membrane potential if the pumps in a neuron quit working? What type of membrane transport do pumps perform?
8. Do leak channels contribute to the production of either class of neuron signal? Do leak channels promote the maintenance of the resting potential? Do leak channels promote the differential distribution of ions inside and outside the cell?
9. Why is the neuron described as electrically polarized? What would be different if the cell were not polarized? What is a depolarization, and what causes it? What is a hyperpolarization, and what causes it?
10. Describe the resting (non-signaling) state of a neuron – what is the resting membrane potential? Is the resting membrane potential steady or changing? Are any ion channels or pumps open or operating when the neuron is at rest? Are ions crossing the membrane (is there a resting current)? Does the resting membrane potential function as a signal? In terms of potential, what is different in about a neuron when it is generating a signal compared to when it is in a non-signaling state?
11. At any given moment, what determines whether or not an action potential is produced at a trigger zone? If an action potential occurs at a trigger zone, will it spread to the axon terminal or can it "die out?" What is neural integration? What is summation? What is an EPSP? An IPSP? Where in a cell do PSPs occur? Which ions and ion channels are involved in PSPs? What are the two types of specificity that these ion channels have?
12. What is the role of the myelin sheath in propagation (spread) of neural signals? How does propagation differ in a myelinated axon versus an unmyelinated axon? How does the distribution of voltage-regulated ion channels differ in a myelinated axon compared to an unmyelinated axon?