Problem 30 - Anatomy and Physiology of Animals

Question

A channel opens on a postsynaptic membrane that causes a negative ion to enter the cell. What type of graded potential is this? a. depolarizing b. repolarizing C. hyperpolarizing d. non-polarizing

Step-by-Step Solution

Answer

The graded potential is hyperpolarizing.

Step 1: Understanding Graded Potentials

Graded potentials refer to changes in membrane potential in a neuron that vary in size, as opposed to the all-or-none response of action potentials. They can be depolarizing or hyperpolarizing.

Step 2: Analyzing Ion Movement

When negative ions enter the cell, the interior of the cell becomes more negative compared to the outside. This movement of ions generally causes hyperpolarization.

Step 3: Defining Hyperpolarization

Hyperpolarization is a change in a cell's membrane potential that makes it more negative. This occurs when negative ions enter or positive ions leave the neuron, moving the membrane potential further from the threshold for firing an action potential.

Step 4: Identifying Correct Option

Given that negative ions entering the cell leads to hyperpolarization, the correct answer is that this situation describes a hyperpolarizing graded potential. Thus, the correct option is (c) hyperpolarizing.

Key Concepts

Neuron Membrane Potential

Every neuron has a resting membrane potential, which is typically around -70 millivolts (mV). This potential is the difference in electric charge inside and outside of the neuron. The resting membrane potential is primarily maintained by the distribution of ions across the neuron's membrane.
Neurons have ion channels that allow ions to pass through their membranes. At rest, more negative ions are present inside the neuron compared to the outside, maintaining its negative charge.
The resting membrane potential is crucial because it sets the stage for the neuron to respond to signals. This potential can change in response to different stimuli, leading to either graded potentials or action potentials.

Hyperpolarizing Potential

Hyperpolarization is a specific type of graded potential where the interior of the neuron becomes more negative relative to the outside. This occurs when negative ions, like chloride ions ( ext{Cl}^-), enter the neuron or positive ions ( ext{e.g., Na}^+) exit the cell.

• When hyperpolarization happens, the membrane potential moves further away from zero and farther from the threshold needed to initiate an action potential.
• This makes it less likely for the neuron to "fire" or send a signal.

Hyperpolarizing potentials serve to inhibit neural activity, making them essential for calming neural circuits and preventing over-excitation.

Ionic Movement in Neurons

Ionic movement is critical for neuron function, as it determines the changes in membrane potential. Neurons rely on ions, specifically ext{Na}^+, ext{K}^+, ext{Cl}^-, among others, moving through the membrane to generate various potentials.

• Ion channels in the membrane can open or close to allow specific ions to pass through. Their movement is influenced by gradients of concentration and charge, which drive ions to move between the inside and outside of the neuron.
• When a negative ion enters the neuron, it adds to the negative charge inside, causing hyperpolarization.

This movement of ions not only supports resting and active potentials but also contributes to the graded potentials that can inhibit or facilitate neural firing.