Problem 3 - Biology Concepts and Applications

Question

Action potentials occur when ________. a. potassium gates close b. a stimulus pushes membrane potential to threshold c. sodium-potassium pumps become active d. neurotransmitter is reabsorbed

Step-by-Step Solution

Answer

The correct answer is b: a stimulus pushes membrane potential to threshold.

Step 1: Understanding Action Potentials

An action potential is a rapid rise and fall in electrical potential across a cell membrane as a nerve impulse passes. It plays a critical role in cellular signaling, especially in muscle cells and neurons.

Step 2: Identify the Key Event to Trigger Action Potential

To initiate an action potential, a specific event must occur. This event involves reaching the 'threshold' membrane potential, which is often triggered by an external stimulus. The threshold is typically around -55 mV for many neurons.

Step 3: Analyze the Options

Review each option: - Option a: 'potassium gates close' is not the primary trigger; it usually follows the action potential's peak. - Option b: 'a stimulus pushes membrane potential to threshold' accurately describes the trigger. - Option c: 'sodium-potassium pumps become active' maintains resting potential, not directly causing action potential. - Option d: 'neurotransmitter is reabsorbed' occurs after an action potential.

Step 4: Selecting the Correct Answer

Based on the analysis, option b, which involves a stimulus pushing the membrane potential to the threshold, is the correct trigger for an action potential.

Key Concepts

Membrane Potential

Membrane potential is essentially the voltage difference across a cell's plasma membrane. It is crucial for the function of nerve and muscle cells.
Cells maintain a negative membrane potential during rest, typically around -70 mV in neurons. This potential is established primarily by ion channels and pumps, especially the sodium-potassium pump, which actively exchanges ions across the membrane.
Some important aspects of membrane potential include:

• Maintaining balance: Potassium ions (K⁺) are more concentrated inside cells, while sodium ions (Na⁺) are more outside.
• Negative resting potential: This is due to the movement of more potassium ions out than sodium ions in through leak channels.
• Ion channels: Specialized proteins create pathways for ion flow, altering the membrane potential.

Understanding membrane potential is essential for grasping how cells communicate, especially during the generation of action potentials.

Threshold Potential

Threshold potential is a critical value that a neuron's membrane potential must reach to initiate an action potential. It's like a trigger point, often around -55 mV for many neurons.
This point is reached when an external stimulus causes a depolarization strong enough to open voltage-gated sodium channels.
Key points about threshold potential:

• Rapid depolarization: When reached, rapid influx of Na⁺ leads to depolarization, pushing the potential closer to zero.
• All-or-none principle: Once threshold is reached, an action potential occurs automatically and cannot be stopped.
• Importance in signaling: Ensures that only significant stimuli result in nerve impulses, filtering out minor fluctuations.

Determining how stimuli affect threshold potential is crucial for understanding how signals are transmitted in the nervous system.

Nerve Impulse

A nerve impulse, or action potential, is the way neurons communicate with each other and with muscles.
It's a wave of electrical activity that travels along the axon of a neuron, triggered when the membrane potential reaches the threshold.
Why are nerve impulses vital?

• Transmission of information: Enabling responses to external stimuli; they underpin the functioning of the entire nervous system.
• Speed and precision: Nerve impulses travel at high speed, ensuring timely responses.
• Propagation: Moves along neurons as graded potentials build, reaching subsequent parts of the neuron and triggering action potentials.

This electrical phenomenon is fundamental in processes like muscle contraction, pain perception, and brain activities, forming the basis of our sensory and motor experiences.