Cognitive Neuroscience Society

MODULE 2
The Neuron: At Rest and, Conducting

Though the millions of neurons in the nervous system interact in a vast number of ways, each of these nerve cells at any one moment is, like a light switch, either on or off -- conducting or at rest. When you want to turn a light on, you reverse the position of the light switch. When a neuron is "turned on," it, too, undergoes a kind of reversal. In the case of the neuron the reversal comes about by means of a rapid movement of electrically charged particles.

As you read the text, try to answer the following questions.

What is the nature of the resting neuron?
How does a neuron conduct an impulse?
How does a conducting neuron differ from a resting one?
What is the difference between excitatory and action potentials?

Whether the neuron is conducting or at rest is determined by the position of electrically charged particles called ions. Neural impulse transmission is, therefore, electrochemical in nature.

In the resting state -- the period during which a neuron is not conducting -- the fluids outside the cell membrane carry a relatively high positive charge. The fluids inside the cell membrane carry a less positive, or relatively negative, charge.

Potential difference

Figure 2. A nerve fiber at rest

Figure 2 is a diagram of a nerve fiber at rest. It shows the high concentration of positively charged ions outside the cell membrane and the relatively negative ionic concentration in the fluids inside the membrane. These charges do not cancel each other out. If we were to measure the difference in these charges, we would find that the difference in potential in a typical neuron is about 70 millivolts (0.070 volts). This potential is known as the resting potential.

Excitatory potential is stimulated in the dendrite

When a neuron is stimulated to conduct, the resting potential changes at the point of stimulation of a dendrite. The ions from outside the cell and the ions from inside the cell change places, reversing the resting state.

Excitatory potential of threshold value stimulates action potential in the axon

Figure 3 illustrates the changes that occur in an axon during neural conduction. The first diagram (a) shows the axon in the resting state. When stimulation begins (b), the cell membrane starts to admit positive ions and to expel the relatively negative ions. If a strong enough stimulus is given, the potential across the membrane is completely reversed (c); at this point, the inside of the membrane is positively charged and the outside negatively charged. In diagram (d), the action potential is propagated in an adjacent area; the preceding area has returned to its original resting state. In this way, the neural impulse is propagated along the axon at speeds from 10 feet per second to 390 feet per second (7 m.p.h. to 266 m.p.h.).

Electrochemical changes in axon

Figure 3. Electrochemical changes in a conducting axon

At first the process is slow and is called the excitatory potential. If the stimulus is strong enough, a critical threshold value is reached and an action potential results.

Another way to express this sequence is shown in Figure 4. The line on the graph represents the potential difference over time.

Potential difference

( Figure 4 Potential difference in neural transmission as a function of time

As can be seen in the diagram, the resting state shows no change in the potential difference. After stimulation the potential change accelerates rapidly to the peak of action potential. After the action potential is reached, the neuron returns to a normal resting state.

The excitatory potential is not self-propagating. It will die out unless the stimulus reaches the critical threshold value. The action potential, on the other hand, is self-propagating. That is, an action potential will cause a reversal of the potential in an adjacent area of the cell. This reversal causes another reversal and so on. This is neural conduction.

There are two important characteristics of the action potential.

1) It is all or none.
2) It is nondecremental.

Action potential is off or on in full force

The all-or-none characteristic of the action potential means that once a stimulus of threshold intensity is reached, the neuron conducts at the maximum intensity it is capable of at that time. The size of the action potential, then, is not at all related to the intensity of the stimulus.

The nondecremental characteristic of the action potential means that once a neuron begins conducting, the impulse is propagated to the very end of the axon without diminishing.

Excitatory potential depends on stimulus strength

The all-or-none and nondecremental characteristics apply only to the axon and those parts of the cell body involved in propagating the action potential. The dendrites of the neuron do not generate an action potential; their excitatory potentials are directly related to the intensity of the stimulus. A stronger stimulus will result in a large excitatory potential while a weaker stimulus will result in a weaker potential. Potentials in the dendrites are also decremental; they fade as they are propagated toward the cell body.

There are many different kinds of external stimuli. Simple touch is different from pain; an electric shock can be distinguished from a severe burn. But whatever the external stimulus the internal consequence is always the movement of chemicals resulting in the propagation of a neural impulse. All neural conduction is electrochemical.


MODULE 2
PROGRESS CHECK 1

Now test yourself without looking back.

1. Here is a diagram of a nerve fiber of a neuron showing the different charges on fluids inside and outside the cell membrane.

Neuron membrane

This neuron is in what state?______________________________

2. Check the velocity given below that might describe the speed of a neural impulse along an axon.
a. 2 m.p.h.
b. 1000 m.p.h.
c. 250 m.p.h.
d.25,000 m.p.h.

3. An action potential will be propagated only if a stimulus reaches a_______________________________ value.

4. The complete reversal of the neuron's resting state is called the:

a. excitatory potential.
b. action potential.
c. critical threshold.
d. potential difference.
e. all-or-none property.

5. The two most important characteristics of the action potential's transmission are:

a._______________________________

b.________________________________

6. Describe the all-or-none property.___________________________________________

7. Match.

1 ) Does not occur in accordance with the all-or-none property __________________
2) A Structure is nondecremental _____________________
3) An occurrence exhibited in the dendrites ________________
4) Highly concentrated inside the cell in the resting state ___________________________
5) A structure in which the generated potential is proportional to the intensity of the stimulus __________________________

a. Axon
b. Dendrite
c. Negative ions
d. Excitatory potential
e. Action potential
f. Critical threshold value
g. Nondecremental conductance

8. (Graph showing 1 and left, 2 going up slope, 3 at top of curve)

Name the potentials indicated by the numbers.

1) _____________________________

2) _____________________________

3) _____________________________

9. Write the numbers 1 through 5 to show the proper order of the following events.

______ Expulsion of positive ions, influx of negative ions

_____ Action potential (complete reversal of the resting state)

_____ Stimulus of threshold strength given

_____ Excitatory potential (influx of positive ions, expulsion of negative ions)

_____ Resting state (after propagation of impulse to adjacent area of axon)

ANSWER KEY


MODULE 2
EXERCISES

The sequence of events from stimulus to impulse propagation is:
1) Stimulus strength given
2) Propagation of excitatory potential (influx of positive ions, expulsion of negative ions)
3) Action potential (complete reversal of resting state)
4) Expulsion of positive ions, influx of negative ions
5) Return to resting state (propagation of impulse in adjacent area of axon

Refer to this sequence as you work through the rest of these exercises.

A neuron generating a neural impulse is said to be in a _____________________________ (resting/conducting) state. _______________________________________________ 3

Just after stimulation (before the action potential), a(n)_____________________________________ potential is generated. __________________________________2

At the instant of action potential:

a. there is a high concentration of positive ions outside the cell membrane.
b. the neuron is at rest.
c. the difference in resting potential is completely reversed.
d. positive ions are expelled from the cell's interior.
e. an excitatory potential is generated.
_________________________________________________________4

Two properties of the action potential are the all-or-none and nondecremental properties. Which property states that once a stimulus of threshold intensity is reached, the neuron conducts at the maximum strength of which it is capable? _________________________________________________ 5

The propagation of an impulse to the very end of the axon without fading is which characteristic of the action potential? _______________________________________________ 6

Fill in the following chart to describe neurons in the resting and conducting state.
Charge inside membrane Charge outside membrane
Resting
Conducting

ANSWERS:
5 all-or-none property
2 excitatory
6 nondecremental
3 conducting
4 c

__________________________________________________

Decremental transmission is a characteristic of what part of the neuron?

_______________________________________________ 1

In the fluids outside the cell membrane of the resting neuron, there is a high concentration of ions that carry a relatively________________________________ charge

____________________________________________________________ 8

When the difference in resting potential across a cell membrane is completely reversed, a(n)_____________________________ potential is generated.

__________________________________________________ 7

When an area of the axon has ended its conduction it returns to a(n) _________________________________________________________ state. ___________________________________________________ 6

In a graph showing potential changes, the resting potential would not change at all. The excitatory potential would be shown as a gradual change. The action potential would be shown as a peak in the curve.

Here is a graph showing potential changes.

Name the potentials indicated by the arrows.

1)____________________________ potential

2) __________________________________potential

3)____________________________potential

_____________________________________________2

The part of the nerve cell that performs In accordance with the all-or-none property is the ______________________________ 5

Neural transmission in some axons is fairly rap d, Which of the follow speeds would be a good description of a rapid neural transmission?

a. A brisk walk (7 m.p.h.)
b. A cross- country runner (12 m.p.h.)
c. A racing boat (250 m.p.h.)
d. The speed of light (186,000 m.p.sec.)

_____________________________________________________ 4

Match.
1) Beginning of return to resting state __________
2) Complete reversal of resting state __________
3) Gradual inflow of positive ions, outflow of negative ions ________
a. Resting potential
b. Action potential
c. Expulsion of positive ions
d. Excitatory potential ___________________________________________________ 3

NOW TAKE PROGRESS CHECK 2

ANSWERS

1 the dendrites
2 1) resting
2) excitatory
3) action

3
1, c
2 b
3 d

4 c

5 axon

6 resting

7 action

8 positive


MODULE 2
PROGRESS CHECK 2

1. This diagram represents a portion of a neuron when it is:
a. at rest.
b. conducting.

2. Match.

) Highly concentrated outside the resting neuron's membrane _________
2) A neuron not conducting _______
3) Exists across the cell membrane in a neuron at rest _______
4) Concentrated inside the resting neuron's cell membrane ________

a. Negative ions
b. Positive
c. Difference in potential
d. Resting state
e. Neural impulse

3. The speed of the fastest neural conduction is about ____________ miles per hour.

4. When a stimulus reaches a critical threshold value, a(n) _____________ potential is generated.

5. A complete reversal of the neuron's resting state is called a(n) ___________ potential.

6. Two characteristics of the action potential are:
______________
______________

7. When a stimulus reaches a critical threshold value, an action potential is generated at the greatest strength of which the neuron is capable. This is a statement of what property of neural transmission? _____________________________

8. On the graph below, label the resting potential, the action potential, and the excitatory potential.

Potentials

9. Write the numbers 1 through 5 to show the proper order of the following events.
_______ Expulsion of positive ions, influx of negative ions

_______ Action potential (complete reversal of the resting state)

_______ Stimulus of threshold strength given

_______ Excitatory potential of positive ions, expulsion of negative ions)

_______ Resting state (propagation of impulse in adjacent area of axon)

10. Match.

1) The all-or-none property does not apply ___________

2) A structure in which transmission is nondecremental ___________

3) Occurs in the dendrites ___________

4) Highly concentrated inside the cell in the resting state _____________

5) A structure in which the generated potential is proportional to the intensity of the stimulus __________

a. Axon
b. Dendrite
c. Negative charge
d. Excitatory potential
e. Action potential
f. Critical threshold value
g. . Nondecremental conductance

ANSWER KEY