Problem 7 - Biology: The Dynamic Science

Question

Which of the following does not contribute to propagation of action potentials? a. As the area outside the membrane becomes negative, it attracts ions from adjacent regions; as the inside of the membrane becomes positive, it attracts negative ions from nearby in the cytoplasm. These events depolarize nearby regions of the axon membrane. b. The refractory period allows the impulse to travel in only one direction. c. Each segment of the axon prevents the adjacent segments from firing. d. The magnitude of the action potential stays the same as it travels down the axon. e. Up to a limit, increasing the intensity of the stimulus increases the number of action potentials.

Step-by-Step Solution

Answer

Option (c) does not contribute to the propagation of action potentials because it describes an inhibitory function not present in typical axon activity.

Step 1: Eliminate correct contributors

Options that accurately describe mechanisms that contribute to the propagation of action potentials are eliminated from consideration. (a) describes the process of depolarization spreading action potentials, (b) states the refractory period ensures unidirectional travel, (d) recognizes equal magnitude of action potentials along the axon, and (e) notes that stimulus intensity affects the frequency of action potentials.

Step 2: Identify incorrect statements

Option (c) claims that each segment of the axon prevents adjacent segments from firing. This is not a description of a mechanism that aids in the propagation of action potentials, but instead suggests an inhibitory function not typically associated with the axon during action potential propagation.

Step 3: Choose the correct answer

The option that does not contribute to the propagation of action potentials is the one that inaccurately describes the process or suggests a contrary function. In this case, (c) Each segment of the axon prevents the adjacent segments from firing is the correct answer because it describes a scenario that would inhibit the spread of action potentials rather than contribute to it.

Key Concepts

Depolarization

Depolarization is a critical phase in the generation and propagation of action potentials along the axon of a neuron. It occurs when the normally negative interior of the axon becomes less negative or even positive due to the influx of sodium ions (Na⁺). This sudden change in electrical charge happens when a neuron is stimulated by a physical or chemical signal strong enough to open voltage-gated Na⁺ channels in the axon membrane.

The action potential begins at the axon hillock when the membrane potential reaches a certain threshold. At this point, the selective permeability of the axon membrane changes momentarily, allowing Na⁺ ions to rush into the cell. This rapid inflow of positive charges causes the membrane potential to rise, resulting in depolarization.

• The threshold potential typically ranges from -55 to -50 millivolts (mV).
• Upon reaching the threshold, the action potential is triggered, and depolarization ensues.
• The peak of depolarization occurs when the membrane potential reverses and becomes positive compared to the outside of the cell.

Understanding depolarization is crucial because it is the first step that leads to the generation of a nerve impulse, which allows neurons to communicate with each other rapidly and effectively.

Refractory Period

Following depolarization, the axon experiences a refractory period, which is a brief period of time during which the neuron is unable to fire another action potential. The refractory period ensures that each action potential is a separate and distinct event and enforces one-way transmission of nerve impulses along the axon.

There are two types of refractory periods:

Absolute Refractory Period

The absolute refractory period occurs immediately after an action potential has been initiated. During this time, no new action potentials can be generated, regardless of the strength of the stimulus. This is due to the fact that the voltage-gated Na⁺ channels, which opened to initiate the action potential, are temporarily inactivated.

Relative Refractory Period

Following the absolute refractory period, there is a relative refractory period during which a larger-than-normal stimulus can initiate a new action potential. This occurs as the voltage-gated K⁺ channels open, allowing potassium ions (K⁺) to exit the cell, initiating the repolarization process that eventually restores the neuron to its resting state.
The refractory periods play a pivotal role in the directional flow of action potentials and in setting the maximum frequency at which action potentials can be generated by a neuron.

Axon Membrane

The axon membrane, or axolemma, is the phospholipid bilayer that encases the axon, a long extension of a neuron. It is fundamental to neuronal function, as it maintains the electrical potential of the neuron and houses various proteins, including ion channels and pumps that are essential for action potential propagation.

Key aspects of the axon membrane include:

• The lipid bilayer provides a barrier that separates the internal environment of the axon from the extracellular space.
• Voltage-gated ion channels, which open or close in response to changes in membrane potential, are critical for the initiation and propagation of action potentials.
• Ion pumps like the sodium-potassium pump (Na⁺/K⁺ ATPase) actively maintain the resting membrane potential by moving Na⁺ out of the cell and K⁺ into it against their concentration gradients.
• Both the structural integrity and the protein composition of the axon membrane are crucial for nerve impulse transmission.

The combined actions of ion channels and pumps not only generate the resting membrane potential but also restore it after each action potential, making successive signaling possible.