MODULE 3
Neural Influences

A complex set of interacting forces underlies the movement of our bodies. Some of these forces cause our muscles to contract, others cause them to relax. In most cases these forces are all present in the same muscle at the same time. They work together to keep our movements smooth and coordinated.

As you read the text, try to answer the following questions.
How do neurons influence other neurons?

How do neurons influence muscles?

What is inhibition?

What is facilitation?

What factors determine the nature of neural influences over muscles,
glands, and other neurons?

In an earlier module we learned that a neuron will not conduct an action potential until a stimulus intensity of critical threshold value is reached. This critical value is not absolute. It varies with the internal influences on the neuron. The effect of these influences on the action potential may be inhibitory; they may inhibit the action of the neuron. The influences may also be facilitative; they may make neural action easier.

Inhibition means that the critical threshold value of a neuron has been raised, thus decreasing the chances that an action potential will- occur. An inhibited neuron needs a stronger-than-usual stimulus to begin conducting.

Facilitation means that the chances that an action potential will occur are increased, since the critical threshold value is lowered. A facilitated neuron can begin conducting with a weaker-than-usual stimulus.

The nervous system is a complex interconnection of millions of neurons, each capable of influencing one or more nerves, muscles, and glands. The point at which one neuron exerts influence over a second neuron is called the synaptic junction, or synapse. Here is a diagram showing the synapse between two interconnected neurons.

Chemicals released by an axon either inhibit or facilitate action potential in another neuron

The synapse is simply the point where the hair-like structure, or terminal arborization, of the axon of one neuron lies close to the dendrites of another. The influencing neuron is called the presynaptic neuron. The neuron that is influenced is called the postsynaptic neuron.

The action potential is propagated along the axon of the presynaptic neuron. When it reaches the end of the axon, a chemical substance is released. This substance will either inhibit or facilitate an action potential in the postsynaptic neuron. If the chemical is acetylcholine (ACH), it will facilitate the neuron. There are a number of possible inhibitor chemicals that have been suggested, but the actual substance is not known.

The axons of many neurons often converge on the dendrites of a single neuron. In these cases, the excitation of an action potential in the postsynaptic neuron depends on the net effect of the many presynaptic neurons. If the amount of inhibitor substance released by presynaptic neurons is greater than the amount of the facilitative substance (ACH) released, the response will be inhibited. If the amount of the facilitative substance is greater than the inhibitory substance, the response will occur at a lower-than-usual threshold value.

Whether action potential is reached depends on the chemicals released by many neurons

In Figure 5, four presynaptic neurons are converging on a single postsynaptic neuron that controls the muscle fiber. Whether or not the muscle fiber contracts depends on whether or not an action potential is generated in the postsynaptic neuron. This, in turn, depends on the net effect of all of the presynaptic neurons.

Figure 5. Presynaptic neurons converging on a postsynaptic neuron

If the net effect is inhibitory, the muscle fiber will not contract. If it is facilitative, the muscle fiber will contract.

Examples of how inhibition and facilitation work together can be found in many skeletal muscles. The muscles of your upper arm, for instance, are coupled in antagonistic pairs. When one muscle extends, the other contracts. When you "make a muscle," causing the biceps muscle to bulge, the neural influence on the upper muscle is facilitative, and the muscle contracts. The influence on the lower muscle is inhibitory, so this muscle extends. The neuron influence on your leg muscles while you are walking is another good example of this mechanism called reciprocal innervation. The neurons that influence, or innervate, the leg muscles act reciprocally. Opposed sets of muscles are stimulated to contract at different times, never together.


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MODULE 3
PROGRESS CHECK 1

Now test yourself without looking back.

1. The influence of one neuron upon another may be either

2. The facilitative transmitter is or 3. The point at which one neuron influences another is called the_____________________________________

4. In a certain synaptic junction only acetylcholine was emitted. In this case, the response was_____________________________.

5.

Label the diagram above.
a.
b.
c.

6. Facilitative influences do what to the critical threshold value?

7. The neural mechanism that coordinates activity of paired muscles is called

ANSWER KEY


MODULE 3
EXERCISES

The point at which one neuron influences another is called the ________________________________________________________ 2

When one neuron influences another in such a way as to keep it from conducting, what has taken place? _______________________________________________5

Write the correct identifying number for each of the following.
a. Synaptic junction where the neural impulse is transmitted from one neuron to another ____
b. Presynaptic neuron which develops the impulse____
c. Postsynaptic neuron which receives the impulse____
_________________________________________________4

The nature of the postsynaptic influence (inhibitory or facilitative is determined best by:
a. the width of the synaptic junction.
b. the frequency of presynaptic conduction.
c. the net effect of excitatory and inhibitory chemicals acting on the postsynaptic cell.
d. the intensity of the presynaptic stimulus.

_______________________________________________ 8

Inhibitory influences do what to the critical threshold value?

____________________________________________________1

The extension of the forearm shows how inhibitory and facilitative influences work together. This working together is called____________________________________________ innervation.

_________________________________________________________________________________6

ANSWERS:

1) Raise it
2) synapse or synaptic junction postsynaptic cell
4) a. 3
b. 1
c 2

5) inhibition
6) reciprocal
8) c

NOW TAKE PROGRESS CHECK 2


MODULE 3
PROGRESS CHECK 2

1. This diagram shows two neurons and a muscle fiber. The muscle fiber will contract when it receives a neural impulse. Identify:

a.

b.

c.

d.

2. If only acetylcholine (ACH) is emitted at the synapse, the muscle fiber will________________________

3.. If the muscle fiber does not contract, we may suppose that the threshold of the postsynaptic neuron has been__________________________________

4. If the muscle does contract, we may assume that the chemical_ was emitted at the synapse.

5. Many of the complicated muscular movements of our bodies are influenced by a mechanism that cord- notes the activity of antagonistic pairs of muscles. This mechanism is called ____________________________.

ANSWER KEY

September 30, 2002