Cell Bio - Ch. 22

Neurons, or nerve cells, are essential components of the nervous system. Neurons provide inputs from sensory systems and outputs to motor or other effector systems, and they make up the information processing network that connects these inputs and outputs. This tutorial focuses on one important property of neurons---resting potential. Before beginning the tutorial, watch the How Neurons Work animation. You can review relevant parts of the animation at any point in the tutorial. Refer to this key as you watch the animation: Na+ = small yellow spheres K+ = red diamonds ATP = starbursts ADP = large yellow ovals areas of net positive (+) charge = blue regions above or below the axon membrane areas of net negative (-) charge = yellow regions above or below the axon membrane *Part A - Neuron structure* Drag the labels to their appropriate locations on the diagram of the neurons below. Use only the pink labels for the pink targets (which indicate the locations of gated ion channels).

(A) Cell body (B) Myelin sheath (C) Synaptic terminal (D) Axon hillock (E) Nucleus (F) Location of voltage-gated channels (G) Location of ligand-gated channels (H) Axon (I) Dendrite

Glutamate causes the membrane of postsynaptic neurons to become hyperpolarized. How would THC be expected to affect the rate of nerve impulse transmission in postsynaptic neurons that make synapses with the glutamatergic neuron? (A) The rate would increase. (B) The rate would decrease. (C) The rate would not change.

(A) The rate would increase.

Which of the following would induce synaptic release of neurotransmitter in the absence of an action potential in a neuron? (A) application of a calcium ionophore that equilibrated calcium between the cytoplasm of the synaptic terminal and the extracellular space (B) demyelination of the axon (C) inhibition of neurotransmitter receptors in the dendrites of the axons (D) increasing the rate of neurotransmitter removal from the synaptic cleft

(A) application of a calcium ionophore that equilibrated calcium between the cytoplasm of the synaptic terminal and the extracellular space

*Part B - Ion movements at resting potential* The diagram below shows the five main transport proteins that control the distribution of Na+ and K+ ions across the plasma membrane of an axon. Assume that the membrane is at resting potential---the membrane potential of the axon remains constant at about -70 mV. Drag the arrows onto the diagram to show the direction of Na+ (gray arrows) and K+ (red arrows) movement through each transport protein at resting potential. If no ions move through a transport protein at resting potential, leave that target blank.

(A) blank (RMP - no movement) (B) blank (RMP - no movement) (C) Na+ out, K+ in (D) K+ out (E) Na+ in

*Part B - Voltage-gated channels and the action potential* The fixed pattern of changes in membrane potential during an action potential is coordinated by the sequential opening and closing of voltage-gated ion channels. Can you identify the status (open/closed) of the voltage-gated Na+ and K+ channels during each phase of an action potential? Drag the appropriate labels onto the graph to indicate the status (open or closed) of the voltage-gated Na+ and K+ channels during each phase of an action potential. Labels may be used once, more than once, or not at all.

(A) resting potential Na+ channels closed K+ channels closed (B) rising phase Na+ channels open K+ channels closed (C) falling phase Na+ channels closed k+ channels open (D) undershoot Na+ channels closed K+ channels open (E) resting potential Na+ channels closed K+ channels closed

CB1 agonists such as Tetrahydrocannabinol (THC) reduce calcium influx by blocking the activity of voltage-dependent calcium channels. What effect would this have on the release of glutamate? (A) Exocytosis of vesicles containing glutamate from the nerve terminal would increase. (B) Exocytosis of vesicles containing glutamate from the nerve terminal would decrease. (C) There will be no effect.

(B) Exocytosis of vesicles containing glutamate from the nerve terminal would decrease.

A mutant sodium channel has been discovered that opens faster and allows more sodium to enter the cell compared to the wild-type sodium channel. What experimental evidence would verify this? (A) The width of the peak of the action potential would increase. (B) The slope of the rising phase of the action potential would be greater and the peak would be higher. (C) The slope of the falling phase of the action potential would be greater. (D) The hyperpolarizing phase would move to more negative values.

(B) The slope of the rising phase of the action potential would be greater and the peak would be higher.

Which of these structures transmit signals from the central nervous system to muscles? (A) sensory neurons (B) motor neurons (C) Schwann cells (D) glial cells

(B) motor neurons

What is the role of calcium at the synaptic terminal? (A) Depolarization of the synaptic terminals by an invading action potential opens voltage-gated calcium channels at the terminal, thus permitting diffusion of calcium out of the synaptic terminal. (B) Calcium enters the synaptic terminal, permitting water to enter, increasing hydrostatic pressure, and forcing neurotransmitter out of the terminal into the synapse. (C) Depolarization of the synaptic terminals by an invading action potential releases intracellular calcium stores or allows calcium channels to open to allow calcium to enter the synaptic terminal from the extracellular environment, thus permitting neurotransmitter release. (D) Calcium, like sodium, is out of both ionic and electrical equilibrium. Therefore, it counteracts the loss of free energy following depolarization of the synaptic terminal.

(C) Depolarization of the synaptic terminals by an invading action potential releases intracellular calcium stores or allows calcium channels to open to allow calcium to enter the synaptic terminal from the extracellular environment, thus permitting neurotransmitter release.

Which answer shows the correct order of occurrence during an action potential? (A) depolarizing phase, hyperpolarizing phase, repolarizing phase, resting state (B) resting state, hyperpolarizing phase, depolarizing phase, repolarizing phase (C) resting state, depolarizing phase, repolarizing phase, hyperpolarizing phase (D) hyperpolarizing phase, depolarizing phase, repolarizing phase, resting state

(C) resting state, depolarizing phase, repolarizing phase, hyperpolarizing phase

*Part B - Conduction of an action potential along an axon* Diagram showing an action potential moving from left to right along an axon membrane. The axon membrane is labeled from left to right: a, b, c, d, e, f, g. The action potential starts at the leading edge, labeled (f), and ends at the trailing edge, labeled (a). Label g is at the right of the leading edge. Labels b, c, d, and e are within the action potential. At resting, the charge outside the cell is positive and the charge inside the cell is negative. As the action potential moves left to right, it temporarily reverses the charges inside and outside the cell. Match the letter of each location along the axon with the correct description of what is occurring at that position.

1. At location (C), the membrane potential changes sign (from a positive value to a negative value) and the voltage-gated K+ channels are open. 2. At location (F), the axon membrane reaches threshold and the voltage-gated Na+ channels open. 3. At location (A), the voltage-gated Na+ channels reactivate. 4. At location (D), the voltage-gated Na+ channels are inactivating and the voltage-gated K+ channels are opening. 5. At location (G), the axon membrane is at resting potential. 6. At location (B), the voltage-gated K+ channels are closing. 7. At location (E), the membrane potential changes sign (from a negative value to a positive value) and the voltage-gated Na+ channels are open.

*Part C - Predicting movement through an artificial non-gated K+ channel* Suppose that an artificial non-gated K+ channel could be inserted into the plasma membrane of an axon at resting potential (membrane potential = -70 mV). Assume that the axon has not recently produced an action potential. What would happen when an artificial K+ channel is inserted into an axon membrane at resting potential? 1. In what direction will the K+ ions move through the artificial channel? 2. Dies the K+ concentration gradient promote or impede the movement of K+ ions through the artificial channel? 3. Does the membrane potential promote or impede the movement of K+ ions through the artificial channel? 4. How does the movement of K+ ions through the artificial channel affect the membrane potential?

1. Out of the cell 2. Promote 3. Impede 4. Causes a hyperpolarization

Which statement correctly describes what causes the second voltage-gated Na+ channel to open?

After the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open.

Which of the following characteristics determines when the refractory period ends?

How long it takes for the voltage-gated Na+ channels to reactivate at the end of an action potential

*Part A - Initiating an action potential* Under most circumstances, once an axon's membrane potential reaches threshold (about -55 mV in mammals), an action potential is automatically triggered. The graph below shows the changes in membrane potential that occur in an axon membrane that is initially at resting potential. In response to a stimulus, the membrane slowly depolarizes until the membrane potential reaches a particular value, called threshold. At threshold, a rapid depolarization of the membrane occurs and an action potential is initiated. Drag the labels onto the flowchart to show the sequence of events that occurs once the membrane potential reaches threshold. You may use a label once or not at all.

Membrane potential reaches threshold --> (A) many voltage-gated Na+ channels open --> (B) Na+ ions rush into the cell --> (C) Membrane potential rises (depolarized rapidly)

*Part D - Conduction speed of an action potential* There are two properties that affect the conduction speed of an action potential along an axon: the axon's diameter and whether or not the axon is myelinated. Rank the axons from slowest to fastest conduction speed. If two axons have the same conduction speed, place one on top of the other.

Slowest non myelinated invertebrate axon -20μm --> non myelinated invertebrate axon - 30μm --> non myelinated invertebrate axon - 40μm --> myelinated vertebrate axon 30μm Fastest

Why does an action potential move in only one direction down the axon?

The refractory period - once a region of the membrane has experienced an action potential, it becomes *temporarily* unresponsive and *incapable* of undergoing another action potential. It can be divided into two periods known as the *absolute* and *relative* refractory periods. *Absolute* refractory periods are due to the inactivation of sodium channels. *Relative* refractory periods are those where a new action potential is possible but difficult to initiate.

*Part C - Direction of action potential conduction* In the diagram, (a), (b), and (c) represent three points along a vertebrate axon where electrodes were implanted to detect action potentials. Under normal conditions, when this neuron produces an action potential, the action potential passes through point (a) first, followed by point (b), and then point (c). Diagram of a neuron, showing the cell body (left), axon (middle), and synaptic terminals (right). Points on the axon from left to right are labeled a, b, and c. An action potential is indicated by a red arrow between points (a) and (b), but closer to point (b). Suppose, however, that an action potential is artificially triggered at the point indicated by the red arrow. In what sequence would the action potential pass through points (a), (b), and (c)? Enter the sequence in which the action potential would pass through the points. Enter the letters in the correct order separated by commas. For example if the order is point (c), then (b), then (a), enter c, b, a. If the action potential would not pass though a point, do not include that point in your answer.

b,a,c. (Under the artificial conditions, the axon membrane is initially at resting potential everywhere except at the location indicated by the red arrow (where the membrane is artificially brought to threshold). Voltage-gated Na+ channels at that location open and Na+ ions rush into the cell. The depolarization caused by this influx of Na+ ions is "felt" both to the right and left of the open channels. Voltage-gated Na+ channels on both sides open, resulting in an action potential that moves in both directions. It reaches nearby locations (on both sides) first, then locations farther away.)