VISUAL ANALOGY

A CHAIN REACTION

FIGURE 31–4 With a strong enough push, the fall of one domino leads to the fall of the next. An action potential moves along a neuron in a similar manner. Use Analogies Compare and contrast how an action potential traveling along an axon is like the fall of a row of dominoes.

Once the impulse passes, sodium gates close and gated potassium channels open, allowing K⁺ ions to flow out. This restores the resting potential so that the neuron is once again negatively charged on the inside. All the while, the sodium-potassium pump keeps working, ensuring that the axon will be ready for more action potentials.

A nerve impulse is self-propagating; that is, the flow of ions at the point of the impulse causes sodium channels just ahead of it to open. This allows the impulse to move rapidly along the axon. You could compare the flow of an impulse to the fall of a row of dominoes. As each domino falls, it causes the next domino to fall.

In Your Notebook In your own words, summarize what happens across a neuron's membrane when it is at rest and during an action potential.

FIGURE 31–5 The Moving Impulse Once an impulse begins, it will continue down an axon until it reaches the end. In an axon with a myelin sheath, the impulse jumps from node to node.

d

Threshold Not all stimuli are capable of starting an impulse. The minimum level of a stimulus that is required to cause an impulse in a neuron is called its threshold. Any stimulus that is weaker than the threshold will not produce an impulse. A nerve impulse is an all-or-none response. Either the stimulus produces an impulse, or it does not produce an impulse.

The threshold principle can also be illustrated by using a row of dominoes. If you were to gently press the first domino in a row, it might not move at all. A slightly harder push might make the domino teeter back and forth but not fall. A push strong enough to cause the first domino to fall into the second, and start the whole row falling, is like a threshold stimulus.

If all action potentials have the same strength, how do we sense if a stimulus, like touch or pain, is strong or weak? The brain determines this from the frequency of action potentials. A weak stimulus might produce three or four action potentials per second, while a strong one might result in as many as 100 per second. If you accidentally hit your finger with a hammer, those action potentials fire like mad!

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