Problem 9 - Biological Psychology

Question

Suppose researchers find that axon \(A\) can produce up to 1,000 action potentials per second (at least briefly, with maximum stimulation), but axon \(\mathrm{B}\) can never produce more than 200 per second (regardless of the strength of the stimulus). What could we conclude about the refractory periods of the two axons?

Step-by-Step Solution

Answer

Axon A has a shorter refractory period than axon B.

Step 1: Understanding the Problem

We are given two axons, A and B, with different maximum rates of action potentials. Axon A can produce up to 1,000 action potentials per second, while axon B can only produce a maximum of 200 action potentials per second. We need to compare their refractory periods.

Step 2: Recall the Concept of Refractory Periods

The refractory period is the time following an action potential during which a neuron is unable to fire another one. The absolute refractory period refers to the time when a neuron cannot respond to any stimulus, while the relative refractory period is the time when a neuron can fire a new action potential but requires a stronger stimulus.

Step 3: Calculate Maximum Firing Rate

The maximum firing rate is inversely related to the length of the refractory period. If a neuron fires 1,000 action potentials per second, its refractory period is 1/1,000 seconds, or 1 ms (millisecond). A neuron firing 200 action potentials per second has a refractory period of 1/200 seconds, or 5 ms.

Step 4: Conclusion Based on Calculations

Since axon A has a shorter refractory period (1 ms) compared to axon B (5 ms), it can fire more frequently. Thus, the axonal refractory period is a key factor determining the frequency of action potentials.

Key Concepts

Action Potentials

Action potentials are the electrical signals that carry information along neurons. They are essential for everything the nervous system does, from simple reflexes to complex thoughts. To put it simply, when a neuron is activated, a rapid change in electrical charge occurs across its membrane. This brief electrical impulse travels along the neuron, enabling communication between nerve cells. When a neuron reaches a certain threshold, voltage-gated sodium channels open, causing sodium ions to rush inside, making the inside of the neuron more positive. After peaking, this electrical impulse quickly returns to its resting state.

• A neuron at rest has a negative charge inside compared to outside.
• Action potentials are initiated once the neuron is sufficiently stimulated.
• The all-or-nothing nature means it either fully happens or not at all, like a light switch.

Absolute Refractory Period

During the absolute refractory period, a neuron absolutely cannot fire another action potential, regardless of stimulus strength. This period occurs immediately after an action potential has been initiated and generally lasts a millisecond or less.

• The neuron’s ion channels are either open or in a certain state where they cannot reopen yet.
• The absolute refractory period ensures each action potential is a separate, distinct signal, ensuring signals don't overlap.
• This limits how often a neuron can fire to avoid constant, uncontrollable firing.

Relative Refractory Period

Following the absolute refractory period is the relative refractory period, during which a neuron can fire again but only if the stimulus is stronger than usual. The neuron has started to recover from the action potential, but is still slightly negatively charged compared to its resting state.

• Ion channels gradually reset, allowing a new action potential with the right conditions.
• The relative refractory period helps fine-tune neuron signaling, as it requires more effort to fire the neuron.
• It allows for variable firing rates based on stimuli strength.

Neuron Firing Rate

The neuron firing rate refers to how often a neuron can generate action potentials over a given timeframe. It is closely tied to the length of its refractory periods, which act as natural boundaries limiting the frequency of neuron firing. For example, if a neuron can fire 1,000 action potentials per second, each action potential's refractory period is only 1 ms long. A longer refractory period, like 5 ms, means fewer action potentials per second, such as 200.

• The firing rate is an indicator of how quickly a neuron can respond to new stimulation.
• The shorter the refractory period, the higher the potential firing rate.
• Differences in firing rates indicate variations in neuronal refractory period lengths.