Biology & Life Sciences Chapter 48 Homework Ions Must Either Actively Transported Diffuse

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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 Kconcentration 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 Kvoltage-gated ion channels

axon NaNagates or channels

cell body negative organic ions Kgates 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 Naions 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 Naand Kcannot

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 Nachannels 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 Cachannels.

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 Ca2is 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

Na50 440 50 440

K400 20 400 40

a. You change the solution bathing the neuron by increasing the Kconcentration

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 Kto the outside, you added

more Nato the fluid bathing the neuron? Explain.

As in part a, Naions cannot be added without adding a counterbalancing

326 Activity 48.3