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Notes to Instructors
Chapter 48 Nervous Systems
What is the focus of these activities?
In vertebrates, the endocrine and nervous systems maintain primary control over
What are the particular activities designed to do?
Activity 48.1 How do ion concentrations affect nerve function?
This activity is designed to give students an understanding of both the experimental
Activity 48.2 How do neurons function to transmit information?
This activity is designed to help students understand how an action potential can be
Activity 48.3 What would happen if you modified a particular aspect of neuron
function?
The questions in this activity are designed to help students test their understanding of
What misconceptions or difficulties can these activities reveal?
Activity 48.1
Most students have no idea why some axons in squid should be much larger than others.
Notes to Instructors 315
Activity 48.2
Activity 48.3
Many students have difficulty understanding that an action potential is recorded at a
single point along an axon and results from highly localized changes in ion
concentrations. These changes require the movement of only a very few ions through
gated channels in the membrane.
Answers
Activity 48.1 How do ion concentrations affect neuron function?
Much of our understanding of neuron function was based on studies of the squid giant
axon. Squid move through the water by contracting the muscles of the mantle. This
compresses water inside the mantle that is forced out the siphon. Squid can change the
direction of movement by directing the flow from the siphon either forward or backward
(relative to the head or anterior end).
1. In which direction would the squid move if water flow from the siphon was directed
toward the head end of the organism?
The squid would move backward (or toward its tail end). Conversely, if the siphon
2. When squid are startled or in danger, they can simultaneously contract all muscles
of the mantle to jet water forcefully out of the siphon and escape rapidly. Assume the
mantle of the squid is 30 cm in length (about 12 inches). The brain sends a signal to
major nerve ganglia in the mantle, which relay the signals to axons innervating the
mantle muscles. For all muscles of the mantle to contract simultaneously, all nerve
signals sent along these axons must reach all parts of the mantle at the same time.
316 Activity 48.1
a. On the diagram of the squid above, draw and number three neurons. Assume all
are simultaneously stimulated by a single motor neuron from the brain. Neuron 1
innervates the mantle muscles nearest the brain. Neuron 2 innervates the muscles
in the midregion of the mantle. Neuron 3 innervates the muscles at the tail end of
the mantle.
b. In invertebrates, like the squid, how must the nervous system be structured to
allow both the muscles nearest the brain and those farthest from it to contract
simultaneously? Modify your drawing to indicate any differences in the size or
structure of the three neurons that would be required. Explain your reasoning.
Invertebrates do not contain myelinated nerves. As a result, speed of nerve
3. Researchers discovered that they could remove these “giant axons” of the squid.
With some skill, the axons could be maintained outside the body if held in ion
solutions of the same concentrations as the squid’s extracellular fluids. If stimulated
with a microelectrode, these isolated axons would generate action potentials. By
recording the potential difference between an electrode in the axon versus one in the
fluid bathing the axon, the scientists could also record the potential difference at rest
and any change in potential difference that occurred as a nerve impulse, or action
potential, was generated.
Activity 48.1 317
ganglion
siphon mantle
The ion concentrations the researchers recorded for the squid giant axon are listed in
the table below.
318 Activity 48.1
Some of these ion concentrations for the human are listed in the table below.
Ion concentrations (mM) for intra- and extracellular fluids of the squid axon
Intracellular (mM) Extracellular (mM)
Ion concentrations (mM) for intra and extracellular fluids of the human axon
Intracellular (mM) Extracellular (mM)
Potassium (K)140 5
Sodium (Na)15 150
Using the Nernst equation, your text book calculates that in humans the equilibrium
potential for potassium is 90 mV and that for sodium is 62 mV.
What are the equilibrium potentials for potassium and sodium for the squid
4. In a typical neuron, which ion has the greatest influence on the membrane potential
at rest? In other words, flux of which ion contributes the most to the resting
membrane potential? Explain why this occurs.
At rest almost all sodium voltage-gated channels are closed. However, while most of
5. In calculating the resting and action potentials of the axon, we generally don’t worry
about the concentration differences of calcium or chloride ions. Explain.
The concentration differences are set up by active transport of the ions across the
6. What effects would the following changes in extracellular Kconcentration be
likely to have on the resting membrane potential of neurons in a human?
a. Change extracellular K from 5 mM to 2 mM
b. Change extracellular K from 5 mM to 10 mM
EK=62/1 log(20/140) = 52 mV The normal potassium equilibrium
Activity 48.2 How do neurons function to transmit information?
Working in groups of three or four, construct a dynamic (claymation-type) model of the
transmission of an action potential along a neuron and then across a synapse to generate
Activity 48.2 319
Neurons or Parts of
Neurons Ions Gates
dendrite Kvoltage-gated ion channels
axon NaNagates or channels
cell body negative organic ions Kgates or channels
Building the Model
• Using chalk on a tabletop or a marker on a large sheet of paper, draw the
membranes of two neurons and the synaptic region between them. Each neuron
should each be at least 4 inches across and a foot or more in length.
Use your understanding of how action potentials are generated and propagated
to answer the questions.
1. All cells maintain an ionic (and therefore electrical) potential difference across their
membranes. In most cells, this potential difference is between 50 and 100 mV.
That is, the inside of the cells is more negative than the outside by 50 to 100 mV.
Although all cells in the body maintain this potential difference across their
membranes, only certain cells (for example, neurons) are capable of generating
action potentials.
a. How is this potential difference across the cell membrane generated?
Sodium-potassium pumps in cell membranes actively pump Naions out of cells
320 Activity 48.2
b. What characteristics of membranes allow cells to concentrate or exclude ions?
Cell membranes are selectively permeable. Ions such as Naand Kcannot
c. What is it about neurons (nerve cells) that make their properties different from
those of other cells? In other words, what enables nerve cells to produce action
potentials?
All cells generate a potential difference across their membranes. However, only
neurotransmitters released from presynaptic neuron terminals.
d. How is an action potential started and propagated?
Refer to Figure 48.11 of Campbell Biology, 9th edition, and keep in mind that all
of the following responses are recorded at a single site on the axon.
Activity 48.2 321
e. Is any direct or indirect energy input required to generate an action potential?
If so, when and where is the energy used?
Other than whatever energy is required to generate a depolarizing stimulus, no
f. What happens in time and space (along the axon) once an action potential
begins?
In part d, we looked at what happened at a single site on the axon as it was
g. What factors ultimately limit the ability of the nervous system to respond (that is,
to continue to generate impulses)?
Ultimately, the operation of the sodium-potassium pump limits the ability of the
system to respond. The sodium-potassium pump establishes the potential
difference (70 mV, for example) across the neuron membrane. In experimental
2. If an axon is stimulated in the middle of its length, nervous signals (action
potentials) will move out from the point of stimulus in both directions. Normally,
however, nerve signals move in only one direction along neurons. Explain.
322 Activity 48.2
Normally, the refractory period prevents the action potential from moving in more
3. Whether or not an action potential is generated in a postsynaptic neuron depends on
a number of factors. What are they? What ultimately determines whether or not an
action potential is generated in the postsynaptic neuron?
Whether or not an action potential is generated in a postsynaptic neuron depends on
4. Diffusion is efficient over only very short distances. In fact, as you can see in this
table, diffusion is efficient only for distances of about 1 to 100 m.
Diffusion Distance (m) Time Required for Diffusion
1 0.5 msec
10 50 msec
100 5 sec
1,000 (1 mm) 8.3 min
a. How wide is a synapse?
b. If a synapse were two times as wide, what effect would it have on the
transmission of nerve signals from one neuron to the next? How would this
change affect the response time of an organism?
If the synapse were two times as wide, the time for diffusion would increase by
Activity 48.2 323
5. If you examine neuron transmission within an organism, you discover that every
action potential generated is stereotyped; for example, every action potential reaches
the same maximum height and the same minimum height. In addition, the generation
of action potentials is an all-or-none phenomenon. That is, once the potential
difference across the membrane reaches threshold, an action potential will be
generated. Given this, how does the nervous system signal differences in intensity of
signal?
Intensity of signal is generally determined by the number of action potentials fired
Activity 48.3 What would happen if you modified a particular
aspect of neuron function?
In the following questions, test your understanding of the various parts of the nervous
system by asking yourself what would happen if a certain part was damaged. What would
the system still be able to do? What would it be unable to do?
1. Some nerve gases and insect poisons work by destroying acetylcholine esterase.
Acetylcholine esterase is normally present in acetylcholine synapses and acts to
degrade acetylcholine. What is likely to happen to nervous transmission in insects
exposed to this type of insect poison?
In the central nervous system (of vertebrates), acetylcholine can cause inhibition or
excitation of the postsynaptic neuron. Which it does depends on the type of
2. The pufferfish (fugu) contains the poison tetrodotoxin. Some shellfish produce a
paralytic poison called saxotoxin. Both of these poisons block the Nachannels in
neurons. What specific effects could these toxins have on neuron function?
324 Activity 48.3
3. A type of spider (the funnel-web spider) produces a toxin that blocks the Cachannels.
a. Can a neuron exposed to this toxin fire an action potential? Explain.
b. Can a neuron transmit a signal across the synapse using neurotransmitters?
Explain.
In chemical synapses, an influx of Ca2is required to cause the synaptic vesicles
4. You isolate a section of a squid giant axon and arrange an experiment so that
you can change the solution bathing the axon. You insert an electrode into the axon
and place another electrode outside the cell so that you can measure the potential
across the cell membrane. With the axon bathed in normal extracellular fluid, you
observe a resting potential of 70mV and action potentials, when stimulated, that
reach 55 mV.
Activity 48.3 325
mM concentration of each ion
Normal concentrations Experimental concentration in (a)
Ion Inside neuron Outside neuron Inside neuron Outside neuron
Na50 440 50 440
K400 20 400 40
a. You change the solution bathing the neuron by increasing the Kconcentration
to 40 mM. What effect will this have on the neuron? For example, will it
depolarize the membrane and make it easier to start an action potential? Will it
hyperpolarize the membrane and make it more resistant to starting an action
potential? Or will it have no effect? Explain your answer.
To answer this question, keep these facts in mind:
b. What would happen if, instead of adding more Kto the outside, you added
more Nato the fluid bathing the neuron? Explain.
As in part a, Naions cannot be added without adding a counterbalancing
326 Activity 48.3
