1. Multiple Choice, Fill-in-the blanks Questions

[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-Nervous System, Cumulative Assessment, M28″] [h]

Nervous System, Cumulative Assessment

[i]


[q] The cell that is the basic building block of the nervous system:

[hangman]

[c] neuron

[f] Good!

[q] The brain and spinal cord make up the ____________ nervous system

[hangman]

[c] central

[f] Excellent!

[q] The motor and sensory neurons in your arms, legs, etc., are part of the __________ nervous system

[hangman]

[c] peripheral

[f] Correct!

[q] The branches of the neuron responsible for input are the _________

[hangman]

[c] dendrites

[f] Correct!

[q] The part of the neuron responsible for output is the __________

[hangman]

[c] axon

[f] Correct!

[q] Neurons meet their targets at a

[hangman]

[c] synapse

[f] Great!

[q] The neurons that connect sensory neurons and motor neurons (and which are found in the brain and spinal cord) are

[hangman]

[c] interneurons

[f] Excellent!

[q multiple_choice=”true”] In the diagram below, ligand-gated ion channels would most likely be found at 

[c*] 3    [c]     [c] 6   [c] 8

[f] Excellent. Ligand-gated ion channels are most likely to be found at dendrites.

[f] No. 4 is the axon hillock. It’s where summation occurs. Here’s a hint: You’re looking for a part of a neuron that receives input.

[f] No. Number 6 is a Schwann cell, responsible for partially insulating the axon in a way that speeds up nerve impulses. Here’s a hint: which part of the neuron receives input, in the form of neurotransmitters, from other cells?

[f] No. Number 8 is the tip of an axon (called an axonal bulb). It’s where neurotransmitter will be released. Here’s a hint: You’re looking for a part of a neuron that receives input.

[q multiple_choice=”true”] In the diagram below, neurotransmitters will be released at 

[c] 3    [c] 4    [c] 6     [c*] 8

[f] No. Number 3 is a dendrite. It’s where neurotransmitters might be received, but not released. Here’s a hint: the neurotransmitter will communicate with the next cell. Which part of a neuron is responsible for output?

[f] No. Number 4 is the axon hillock. It’s where summation occurs. Here’s a hint: the neurotransmitter will communicate with the next cell. Which part of a neuron is responsible for output?

[f] No. Number 6 is a Schwann cell. Schwann cells partially insulate axons in a way that speeds nerve impulse transmission. Here’s a hint: the neurotransmitter will communicate with the next cell. Which part of a neuron is responsible for output?

[f] Excellent. Number 8 is the tip of an axon (called an axonal bulb). It’s where neurotransmitter will be released.

[q multiple_choice=”true”] In the diagram below, which number indicates a part where an action potential would occur?

[c] 2   [c] 3   [c] 6   [c*] 7

[f] No. Number 2 is the nucleus. Look for a part that’s connected to the neuron’s membrane.

[f] No. Number 3 is a dendrite. It’s where neurotransmitters are received and where graded depolarizations occur. An action potential would happen further on down the cell.

[f] No. Number six is a Schwann cell. Schwann cells partially insulate axons in a way that speeds nerve impulse transmission.

[f] Excellent. Number 7 is a Node of Ranvier. In a myelinated axon, action potentials jump from one Node of Ranvier to the next.

[q multiple_choice=”true”] In the diagram below, which number indicates a specialized cell that partially insulates the axon, resulting in saltatory conduction?

[c] 2      [c] 3    [c*] 6    [c] 8

[f] No. Number 2 is the nucleus. Look for a part that’s connected to the neuron’s membrane.

[f] No. Number 3 is a dendrite. Look for a part where an action potential might occur.

[f] Fabulous. Number 6 is a Schwann cell. Schwann cells partially insulate axons in a way that speeds nerve impulse transmission by creating the conditions for saltatory conduction (conduction which jumps from node to node).

[f] No. Number 8 represents the tip of an axon. Saltatory conduction occurs along the length of the axon.

[q multiple_choice=”true”] When an action potential reaches this area, voltage-gated calcium channels will open.

[c] 3  [c] 4  [c] 6   [c*] 8

[f] No. Number 3 is a dendrite. It’s where neurotransmitters and other input are received. Here’s a hint: the voltage gated calcium channels are associated with release of neurotransmitter. Where does that happen?

[f] No. 4 is the axon hillock. It’s where summation occurs. Here’s a hint: you’re looking for something that’s associated with neurotransmitter release. Where does that happen?

[f] No. Number six is a Schwann cell. Schwann cells are associated with saltatory conduction. Here’s a hint: you’re looking for something that’s associated with neurotransmitter release. Where does that happen?

[f] Excellent. Number 8 is the tip of an axon (called an axonal bulb). At this region of a neuron, an action potential will induce voltage-gated calcium channels to open. The influx of calcium ions will, in turn, induce the release of neurotransmitters.

[q] A nerve impulse is a wave of charged ______, diffusing across the membrane of a nerve cell, moving down the length of the cell.[hangman]

[c] ions

[f] Good!

[q multiple_choice=”true”] The best description of the diagram below is

[c] the height of an action potential.

[f] No. The action potential is a depolarization of the membrane. At its height, membrane potential is typically some positive value, about 35 mV.

[c] threshold potential.

[f] No. The threshold potential is typically about -50 mV.

[c*] resting potential.

[f] Nice job. Resting potential is typically about -70 mV

[c] the undershoot.

[f] No. The undershoot is a hyperpolarization of the membrane to a value that’s even more negative than the value during the resting potential. Undershoot values are about -80 mV.

[q labels = “top”]Drag the correct values into the table below.

Chlorine (Cl) Potassium (K+) Sodium (Na+) Anions (A-)
Value in the extracellular fluid _________ __________ __________ __________
Value in the cytoplasm _________ __________ __________  __________

[l]0 mM (A)

[f*] Good!

[fx] No, that’s not correct. Please try again.

[l]5 mM (K+)

[f*] Excellent!

[fx] No, that’s not correct. Please try again.

[l]10 mM (Cl)

[f*] Correct!

[fx] No. Please try again.

[l]15 mM (Na+)

[f*] Correct!

[fx] No. Please try again.

[l]100 mM (A)

[f*] Great!

[fx] No, that’s not correct. Please try again.

[l]120 mM (Cl)

[f*] Excellent!

[fx] No. Please try again.

[l]150 mM (K+)

[f*] Good!

[fx] No. Please try again.

[l]150 mM (Na+)

[f*] Excellent!

[fx] No, that’s not correct. Please try again.

[q multiple_choice=”true”] Which of the following is most directly responsible for creating a neuron’s resting potential?

[c] Voltage-gated ion channels

[f] No. Voltage-gated ion channels allow for facilitated diffusion, using a gradient created by the cell’s resting potential. The part that’s most directly responsible for creating the resting potential is shown in the diagram below.

[c] Ligand-gated ion channels

[f] No. Ligand-gated ion channels allow for facilitated diffusion, using a gradient created by the cell’s resting potential. The part that’s most directly responsible for creating the resting potential is shown in the diagram below.

[c*] The sodium-potassium pump

[f] Excellent. The sodium potassium pump (shown below at number 4) does the work that creates the resting potential.

[c] ATP synthase

[f] No. ATP synthase uses a proton gradient created by the electron transport train to synthesize ATP. But while ATP  (shown at 5 below) powers the creation of the cell’s resting potential, there’s a more direct answer in this list of choices (and it’s also shown in the diagram below).

[c] the mitochondrial electron transport chain.

[f] No. The mitochondrial electron transport chain creates a proton gradient, and that proton gradient is used by ATP synthase to create ATP. But while ATP  (shown at 5 below) powers the creation of the cell’s resting potential, there’s a more direct answer in this list of choices (and it’s also shown in the diagram below).

[q multiple_choice=”true”] The best description of A in the diagram below is a

[c*] mechanically-gated ion channel

[f] Excellent. The arrow at B represents a force that deforms the membrane. In response, the channel at A opens up, allowing ions to flow through.

[c] ligand-gated ion channel

[f] No. If this were a ligand-gated channel, it would look like D below.

[c] voltage-gated ion channel

[f] No. If this were a voltage-gated channel, it would look like E below (and open in response to a shift in voltage).

[c] active transport pump

[f] No. An active transport pump would be represented as something that moves particles or ions from lesser concentration to higher concentration (or up a concentration gradient), as shown in the diagram of the sodium-potassium pump below.

[q multiple_choice=”true”] What’s happening in diagram “F” below would be associated with which number in the graph on the left?

[c] 1 [c*] 3 [c] 4 [c] 5

[f] No. Number 1 represents the resting potential (about -70 mV). All voltage-gated ion channels would be closed. F is a voltage-gated sodium ion channel that opens when a graded depolarization raises the membrane potential to about -50 mV, and closes when it reaches about 30mV. Which part of the graph represents those conditions?

[f] Excellent! F is a voltage-gated sodium ion channel that opens when a graded depolarization raises the membrane potential to -50 mV, and closes when it reaches +30 mV. Those are exactly the conditions reflected by number 3 (which is also known as the rising phase of the action potential).

[f] No. Number 4 represents the falling phase of the action potential. F is a voltage-gated sodium ion channel that opens when a graded depolarization raises membrane potential to about -50 mV, and closes when it reaches about +30 mV. Which part of the graph represents those conditions?

[f] No. Number 5 represents the undershoot. F is a sodium ion channel that opens when a graded depolarization raises membrane potential to about -50 mV, and closes when it reaches about 30 mV.  Which part of the graph represents those conditions?

[q multiple_choice=”true”] What’s happening in diagram “G” below would be associated with which number on the graph on the left (note: the triangles represents potassium ions)?

[c] 1 [c] 3 [c*] 4 [c] 5

[f] No. Number 1 represents the resting potential (about -70 mV). All voltage-gated ion channels would be closed. G is a voltage-gated potassium ion channel that opens after the rising phase of the action potential, which causes a depolarization of about 30 mV.  Which part of the graph represents those conditions?

[f] No. Here’s a hint. Number 3 represents the rising phase of the action potential, which causes a depolarization of about 30 mV. G is a voltage-gated potassium ion channel that opens after the rising phase of the action potential.

[f] Fabulous! Number 4 represents the falling phase of the action potential. That’s when voltage-gated potassium ion channels (like the one represented by “G”) would open.

[f] No. Number 5 represents the undershoot. Here’s a hint. G is a voltage gated-potassium ion channel that opens after the rising phase of the action potential.

[q multiple_choice=”true”] The diagram in the inset below would be associated with which phase of the graph?

[c] 1 [c*] 2 [c] 3 [c] 4

[f] No. Number 1 represents the resting potential. In the inset, sodium ion channels are open and potassium ion channels are closed. When does that happen?

[f] Great job. Number 2 represents the rising phase of the action potential, during which voltage-gated sodium ion channels are open and voltage-gated potassium ion channels are closed.

[f] No. Number 3 represents the falling phase of the action potential. In the inset, sodium ion channels are open and potassium ion channels are closed. When does that happen?

[f] Number 4 represents the undershoot, or refractory period. In the inset, sodium ion channels are open and potassium ion channels are closed. When does that happen?

[q multiple_choice=”true”] How can you tell that the inset below does not represent the refractory period?

[c] Because the sodium-potassium pump is at work.

[f] No. The sodium-potassium pump is hard at work during the refractory period, as it restores the resting potential (moving sodium ions into the extracellular fluid, and potassium ions into the cytoplasm).

[c] Because both sodium ion and potassium ion voltage-gated channels are closed.

[f] No. During the refractory period, voltage gated ion channels for both sodium and potassium are closed.

[c*] Because of the concentration gradients of the sodium and potassium ions.

[f] Nice job. During the refractory period, the membrane has returned to a situation where the outside of the membrane has a positive charge relative to the inside. But, relative to the resting potential, the ions are in the wrong place (with lots of potassium ions outside the cell, and lots of sodium ions inside the cell). What you see above is the resting potential (with sodium ions mostly outside, and potassium ions mostly inside).

[q multiple_choice=”true”] Which letter below represents a depolarization?

[c] A      [c] B     [c] C   [c*] D

[f] No. Letter A, at just about -50 mV, represents the threshold potential.

[f] No. B is the resting potential, about -70 mV.

[f] No. C is a hyperpolarization: a response to a stimulus that makes the membrane potential more negative, moving it away from the threshold and making the neuron less likely to fire an action potential.

[f] That’s right. D is a depolarization: a response to a stimulus that makes the membrane potential less negative, moving it toward the threshold and making the neuron more likely to fire an action potential.

[q multiple_choice=”true”] In the diagram below, an action potential is represented by

[c*] 1 [c] 2 [c] 3 [c] 4

[f] Excellent. Number 1 represents an action potential.

[f] No. Number 2 represents a portion of the membrane that’s still at its resting potential.

[f] No. Number 3 represents a portion of the membrane that’s in its refractory period.

[f] No. Number 4 represents a portion of the membrane that has returned to its resting potential.

[q multiple_choice=”true”] In the diagram below, which region contains a portion of membrane that would be experiencing a partial depolarization?

[c] 1 [c*] 2 [c] 3 [c] 4

[f] No. Number 1 represents an action potential (a complete depolarization)

[f] Way to go. The part of number 2 that’s immediately adjacent to the action potential (at 1) would experience a partial depolarization caused by the influx of sodium ions from region 1.

[f] No. Number 3 represents a portion of the membrane that’s in its refractory period. It’s repolarizing.

[f] No. Number 4 represents a portion of the membrane that has returned to its resting potential.

[q] When axons are myelinated (as shown below), nerve impulses move by _________ conduction.

[hangman]

[c] saltatory

[f] Great!

[q multiple_choice=”true”] When “a” binds with “e,”

[c] an action potential might occur

[f] No. An action potential might occur at the axon hillock, but only if the membrane depolarizes all the way to the threshold potential. What causes the membrane potential to reach the threshold potential?

[c*] a graded depolarization might occur

[f] Exactly. The receptor at “d” is a ligand-gated ion channel. When it binds with neurotransmitter, the channel will open. If that channel is a sodium ion channel, a graded depolarization will occur.

[c] voltage-sensitive calcium ion channels might open

[f] No. Voltage sensitive calcium channels open when action potentials arrive at the axonal bulb, which happens at the presynaptic axon. Find something that happens at the postsynaptic dendrite.

[c] axonal vesicles might fuse with the membrane

[f] No. That’s something that happens at the presynaptic axon. Find something that might happen at the postsynaptic dendrite.

[q multiple_choice=”true”] The relationship between a neurotransmitter and a receptor is closest to

[c] a vesicle and its contents

[f] No. Here’s a hint. The neurotransmitter is a ligand. How do ligands and receptors interact?

[c] a ribosome and an amino acid

[f] No. Ribosomes combine amino acids into proteins. There’s no similar connection between neurotransmitters and receptors. Here’s a hint. The neurotransmitter is a ligand. How do ligands and receptors interact?

[c] a nucleic acid and a nucleotide

[f] No. Nucleotides are the monomers of nucleic acids. There’s no similar connection between receptors and neurotransmitters. Here’s a hint. The neurotransmitter is a ligand. How do ligands and receptors interact?

[c*] a substrate and an enzyme

[f] Excellent. In the same way that an enzyme has a shape that fits its substrate, an receptor has a shape that fits a specific neurotransmitter.

[q multiple_choice=”true”] The antidepressant Prozac is a selective serotonin reuptake inhibitor. By blocking reuptake of the neurotransmitter serotonin, it keeps serotonin active in the synaptic cleft, which enhances stimulation of the postsynaptic dendrite. Based on this description, Prozac most directly interacts with

[c] b [c] d [c] e [c*] f

[f] No. Letter b is a voltage-sensitive calcium ion channel. If you inhibited “b” there’d be less neurotranmitter released, and less neurotransmitter in the synaptic cleft. Here’s a hint: find something which, if inhibited, would increase the amount of neurotransmitter in the synaptic cleft.

[f] No. Letter d is a receptor on a ligand gated ion channel. The neurotransmitter will bind with d, but d doesn’t impact the amount of neurotransmitter available. Here’s a hint: find something which, if inhibited, would increase the amount of neurotransmitter in the synaptic cleft.

[f] No. Letter e shows a vesicle that has just released its contents into the synaptic cleft. If you inhibited “e,” there’s be less neurotransmitter in the synaptic cleft. So here’s a hint: find something which, if inhibited, would increase the amount of neurotransmitter in the synaptic cleft.

[f] Fantastic. Letter f is a neurotransmitter reuptake channel. It removes neurotransmitter from the synaptic cleft. If this were inhibited, the neurotransmitter would stay in the synaptic cleft, and continue to bind with the receptor (which is how drugs like Prozac work).

[q] Summation occurs at the axon __________.

[hangman]

[c] hillock

[f] Excellent!

[q]When ____________ ions diffuse into the cytoplasm, the result is a depolarization,

[hangman]

[c]sodium

[q]When ____________ ions diffuse into the extracellular fluid, the result is a hyperpolarization,

[hangman]

[c]potassium

[q]The adding together of postsynaptic potentials is called

[hangman]

[c]summation

[q]A single word description for the process shown below:

[hangman]

[c]summation

[q]The four letter acronym for the impulses represented by the red line segments below.

[hangman]

[c]IPSP

[q]The four letter acronym for the impulses represented by the green line segments below.

[hangman]

[c]EPSP

[q]The type of summation shown below.

[hangman]

[c]spatial

[q]The type of summation shown below.

[hangman]

[c]temporal

[q]Which of the following is something that neurons DON’T do?

[c] Conducting signals

[c*] Forming the myelin sheath

[c] Receiving information

[c] Integrating information

[c] Coordinating responses to the environment

[f]No. To “conduct” means the same as “transmit,” and neurons do that all the time. Look at the axon in the diagram below for the answer.

[f]Correct. Cells called Schwann cells (in the peripheral nervous system) and oligodendrocytes (in the central nervous system) form the myelin sheath.

[f]No. Neurons receive information at their dendrites. Look at the axon in the diagram below for the answer.

[f]No. Neurons integrate multiple impulses from their dendrites and then use a process called summation to compute a response. Look at the axon in the diagram below for the answer.

[f]No. A key role of the nervous system is coordinating our response to the environment. Look at the axon in the diagram below for the answer.

[q]Within a single neuron, nerve impulses travel from

[c*]Dendrite to cell body to axon to synapse

[c]Synapse to cell body to axon to dendrite

[c]Cell body to dendrite to axon to synapse

[c]Synapse to axon to cell body to dendrite

[c]Synapse to dendrite to axon cell to cell body

[f]Way to go! The correct sequence is dendrite to cell body to axon to synapse, as shown below.

[f]No. Here’s a hint: the synapse (4 below) comes last. Study the diagram below to get the correct sequence.

[f]No. Here’s a hint: the cell body (1 below) comes second. Study the diagram below to get the correct sequence.

[f]No. Here’s a hint: the axon (3 below) comes third. Study the diagram below to get the correct sequence.

[f]No. Here’s a hint: the synapse (4 below) comes last. Study the diagram below to get the correct sequence.

[q]Neurons communicate with their targets (other neurons, muscles, glands) at a

[hangman]

[c]synapse

[q]For a stimulus to generate an action potential, it has to be above a

[hangman]

[c]threshold

[q] When an action potential arrives at a patch of  membrane,

[c] K+ channels open

[c*] Na+ channels open

[c] the Na+/K+ pump starts pumping

[c] All ion channels open

[f]No. When an action potential arrives at a patch of membrane, it causes a depolarization that generates another action potential. Here’s a hint: What ions are flowing in during time “A” below?

[f]Great job! When an action potential arrives at a patch of membrane, it causes a depolarization that generates another action potential. The depolarization (shown at “A” below) occurs as voltage-sensitive sodium ion channels open, leading sodium ions to enter the cell.

[f]No. When an action potential arrives at a patch of membrane, it causes a depolarization that generates another action potential. Here’s a hint: What ions are flowing in during time “A” below?

[f]No. You’re right about ion channels opening, but it’s not all channels. When an action potential arrives at a patch of membrane, it causes a depolarization that generates another action potential. Here’s a hint: What ions are flowing in during time “A” below?

[q]The function of the sodium-potassium pump is to pump

[c] both sodium and potassium ions into the cell.

[c] both sodium and potassium ions out of the cell.

[c] ATP, using sodium and potassium gradients to power the pumping.

[c] sodium ions into the cell and potassium ions out of the cell.

[c*] potassium ions into the cell and sodium ions out of the cell.

[f]No. In the model of the sodium-potassium pump shown below, 4 represents sodium and 5 represents potassium. Figure out what’s going on, and remember your answer the next time you see this question.

[f]No. In the model of the sodium-potassium pump shown below, 4 represents sodium and 5 represents potassium. Figure out what’s going on, and remember your answer the next time you see this question.

[f]No. In the model of the sodium-potassium pump shown below, 4 represents sodium, 5 represents potassium, and 6 represents ATP. Figure out what’s going on, and remember your answer the next time you see this question.

[f]No. In the model of the sodium-potassium pump shown below, 4 represents sodium and 5 represents potassium. Figure out what’s going on, and remember your answer the next time you see this question.

[f]Excellent. As you can see in the model of the sodium-potassium pump shown below, the pump pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell.

[q]A key difference between a non-myelinated axon and a myelinated one is that

[c] a nonmyelinated axon does saltatory conduction; a myelinated one does not.

[c] a nonmyelinated axon transmits sensory impulses; a myelinated axon transmits motor impulses.

[c*] a nonmyelinated axon transmits impulses much more slowly than a myelinated axon.

[c] a nonmyelinated axon transmits impulses much more quickly than a myelinated axons.

[c] a nonmyelinated axon is associated with Schwann cells; a myelinated axon is associated with oligodendrocytes.

[f]No. Notice that the myelinated axon shown below is doing saltatory conduction. Here’s a hint: how does saltatory conduction change the nerve impulse?

[f]No. Myelination occurs in both sensory and motor neurons. Here’s a hint: notice that the myelinated axon shown below is doing saltatory conduction. How does saltatory conduction change the nerve impulse?

[f]Great job. Myelination leads to saltatory conduction, and saltatory conduction is much faster than non-saltatory conduction.

[f]No: you’ve got it reversed.

[f]No. Myelinated axons in the central nervous system are associated with Schwann cells; in the central nervous system, myelination is associated with oligodendrocytes.Here’s a hint: notice that the myelinated axon shown below is doing saltatory conduction. How does saltatory conduction change the nerve impulse?

[x]

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[/qwiz]

2. Free Response Questions

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Nervous System, Final FRQ, M28″]

[h]Nervous system FRQs

[i]Accidentally, you’ve touched something that’s burning hot. Almost instantly, a reflex moves your arm away from the source of the heat. Awareness of the pain and injury follows a moment later.

Use what you’rve learned about the nervous system to answer the FRQs that follow.

[q]Explain how a stimulus (such as a burning object) can generate an action potential. In your answer, 1) sketch an action potential, 2) fully describe what happens during an action potential (ending at the falling phase) and 3) explain how the membrane’s resting potential is restored.

[c*] Show the answer

[f] PART 1: Sketch of an action potential

1 Resting potential

A. graded depolarization.

2. Threshold

3. Rising phase of action potential

4. Falling Phase.

5. Refractory Period.

PART 2: Explanation of how an action potential is generated.

At the tip of a sensory neuron are dendrites. These contain heat sensitive sodium ion channels, pain sensors, etc. In response to the injury, these channels open up, which allows sodium ions to flow from outside the cell, where they’ve been pumped by the sodium-potassium pump. As these sodium ions enter, they cause a graded depolarization.

This depolarization is probably happening at multiple dendrites, each of which has multiple ion channels. Through a process called summation, the axon hillock of the cell body adds all of these depolarizations together.  When the depolarization reaches the threshold potential (about -50 mV), voltage-sensitive sodium ion channels in the axon-hillock open. This is an all or none-response, and generates an action potential.

An action potential begins with a rising phase, during which voltage gated sodium ion channels open. As sodium ions rush in, they completely depolarize the membrane so that it becomes positive on the inside relative to the outside (a value of about 35 mV). At that point, these voltage gated sodium ion channels close, and voltage gated potassium ion channels open. This allows potassium ions to rush out of the cell. As they do, they carry out their positive charge, making the membrane potential switch back to negative. This outflow of potassium ions is called the falling phase of the action potential.

PART 3: How the resting potential is restored.

At the end of the falling phase, so much potassium has rushed out of the cell that the the membrane becomes hyperpolarized (to a value even more negative than the resting potential of -70 mV). This is called the undershoot. At that point, potassium ion channels close. Note that while the membrane potential is negative (as it is during the resting phase), the membrane now has many potassium ions on the outside, and many sodium ions on the inside. This ionic situation causes a refractory period, when another action potential becomes temporarily impossible. To restore the resting potential, the membrane’s sodium-potassium pump goes to work, pumping 3 sodium ions out of the cell for every two potassium ions that are pumped into the cell. In a few milliseconds, the resting potential is restored.

[q]Describe how an action potential can move down the length of an axon.

[c*] Show the answer

[f]During an action potential,  voltage-sensitive sodium ion channels allow sodium ions to flow from the extracellular fluid into the cytoplasm. While this causes a complete depolarization in the area where the action potential is occurring, it causes a partial depolarization in the patch of membrane that is immediately adjacent to the action potential. At a certain point, as more and more sodium ions enter the cytoplasm, the membrane potential will reach the threshold. At the threshold, voltage gated sodium ion channels will open. That will cause sodium ions to rush in, and a new action potential occurs.  Continuing this process, the action potential will continue to propagate itself, moving all the way to the end of the axon.

[q]Explain how the message about the injury can be communicated from a sensory neuron to a motor neuron.

[c*] Show the answer

[f]At the end of the axon, within the spinal cord in the central nervous system, the sensory neuron synapses with a motor neuron. As the action potential arrives at the axon’s tip, it causes voltage sensitive calcium channels to open. As these channels open, they allow calcium ions to diffuse into the tip of the axon. These calcium ions induce vesicles filled with neurotransmitter to fuse with the membrane at the axon’s tip, releasing the neurotransmitter into the synaptic cleft.

The neurotransmitter diffuses across the synaptic cleft, and then binds with receptors on the dendrites of the postsynaptic motor neuron. These receptors are part of ligand-gated ion channels. In response to the binding, the receptors cause the ion channels to open. If the receptors are ligand-gated sodium ion channels, they’ll cause a depolarization of the membrane. If enough neurotransmitter binds with enough receptors at enough channels, the influx of sodium could be enough to cause a depolarization that reaches the threshold, inducing an action potential in the motor neuron (through the same mechanism described above).

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[/qwiz]

Congratulations! That’s the end of this module about the nervous system. Click to return to the Nervous System Main Menu, or choose another option from the menu above!