Problem 15 - Biology

Question

During a relative refractory period, (a) IPSPs are generated (b) opioid neurotransmitters are released (c) voltage-activated sodium channels are inactivated (d) an axon can transmit impulses but the threshold is higher (less negative) (e) an axon cannot transmit an action potential

Step-by-Step Solution

Answer

Option (d) is correct: an axon can transmit impulses, but the threshold is higher.

Step 1: Understanding the Refractory Period

The relative refractory period is the phase following an action potential during which a neuron can generate another action potential, but a larger-than-normal stimulus is required to reach the threshold.

Step 2: Evaluating the Options

Analyze each option to see which fits the description of the relative refractory period: - (a) IPSPs (Inhibitory Postsynaptic Potentials) refer to the input from another neuron, not related to the relative refractory period. - (b) Opioid neurotransmitters relate to specific neurotransmitter release, not the state of refractory. - (c) Voltage-activated sodium channels are typically inactivated during the absolute refractory period, not the relative. - (d) During the relative refractory period, an axon can indeed transmit impulses, but it requires a stronger stimulus, which means a higher (less negative) threshold level. - (e) An axon cannot transmit an action potential during the absolute refractory period, not the relative refractory period.

Step 3: Selecting the Correct Option

From the analysis, option (d) correctly describes the conditions during the relative refractory period. The axon can transmit impulses, but the threshold for activation is higher, requiring a stronger stimulus.

Key Concepts

Axon Potential

When neurons communicate, they send electrical signals along axons. The axon potential, also known as action potential, is a rapid rise and fall in voltage across the axon's membrane. This process initiates when a neuron decides to send a signal.
The axon potential begins when the neuron receives enough stimulus to reach a threshold. Different ions play a crucial role:

• Na⁺ (sodium ions) rapidly enter the cell, making the inside more positive.
• K⁺ (potassium ions) exit the cell to restore the resting state.

This exchange change in voltage is what enables the transmission of the impulse down the axon. Understanding axon potential is crucial to decipher how signals travel in the nervous system.

Higher Threshold

A higher threshold occurs during the relative refractory period. In this phase, the neuron requires a stronger-than-normal stimulus to fire another action potential.
The threshold is essentially the level of stimulus needed to trigger an action potential. Under normal conditions, neurons have a specific threshold level. However, after an initial action potential, this threshold temporarily becomes less negative.

• It ensures neurons don't fire too easily and provides a breather.
• Prevents neurons from becoming overly excited and fatigued.

The phenomenon of a higher threshold safeguards the nervous system from excessive firing, maintaining balance.

Neuron Action Potential

A neuron action potential is the core electrical signal that neurons use to communicate. It involves a sudden change in electrical charge across the cell membrane, moving from one end of the neuron to the other.
This signal is crucial for:

• Transmitting information rapidly over long distances in the body.
• Enabling complex processes, such as movement, sensation, and thought.
• Coordinating responses in different parts of the body.

Action potentials are like messages sent through the neuronal network, orchestrating the body's response to internal and external stimuli.

Sodium Channel Inactivation

Sodium channel inactivation is a vital process that occurs immediately after an action potential. During an action potential, sodium channels open briefly to allow Na⁺ ions to flood into the neuron, depolarizing the cell.
However, these channels soon inactivate during the absolute refractory period, halting any further entry of sodium ions temporarily.
This inactivation is critical because it:

• Prevents the neuron from being depolarized again too quickly.
• Ensures unidirectional flow of the action potential down the axon.
• Keeps neurons from becoming overstimulated, allowing recovery before the next potential fires.

In summary, sodium channel inactivation is protective, ensuring that neurons can only fire action potentials at appropriate intervals.