Mechanisms of Graded Potentials in Excitable Cells

Local and Decremental Properties

Unlike action potentials, which maintain their strength as they travel, graded potentials possess two defining characteristics: they are local and decremental.

1. Local Effect: The change in the membrane potential is most intense at the exact site where the chemical binds to the receptor.
2. Decremental Effect: As the ions move away from the initial site of entry, their influence on the membrane potential diminishes.

This happens because the ions move via diffusion through the cytosol (the liquid inside the cell). As they spread out, they become more equally distributed, which dilutes their ability to change the electrical charge of the membrane the further they travel from the source.

Exam Tip: If a question asks why a signal gets weaker as it moves away from a synapse, the answer is the decremental nature of graded potentials caused by ion diffusion.

Depolarization vs. Hyperpolarization

The direction in which the membrane potential shifts depends entirely on which ions are allowed to pass through the open chemically gated channel.

Depolarization

When a channel allows positive ions to enter the cell, the inside of the membrane becomes less negative (more positive). This shift is called depolarization. Common ions involved in depolarization include:

• Sodium ($Na^+$)
• Calcium ($Ca^{2+}$)

Hyperpolarization

Conversely, if the opening of a channel allows positive ions to leave the cell, or negative ions to enter, the inside of the membrane becomes more negative. This is called hyperpolarization. Common ions involved in hyperpolarization include:

• Potassium ($K^+$)
• Chloride ($Cl^-$)

Reaching the Axon Hillock

The ultimate "decision-maker" in a neuron is the axon hillock. This is the region where the cell body (soma) meets the axon. The axon hillock does not have many chemically gated channels itself; instead, it acts as a measuring station that sums up all the various graded potentials occurring in the dendrites and soma.

For a signal to be successful, the net effect of all incoming graded potentials must be strong enough to reach a specific electrical level called the threshold. If the sum of the depolarizations is sufficient to reach this threshold at the axon hillock, it will trigger the voltage-gated channels located at the initial segment of the axon, thereby initiating an action potential.

To ensure a signal is strong enough to reach the threshold despite the decremental effect, the cell can use two main strategies:

• Duration: Leaving a single gate open for an extended period to allow a massive influx of ions.
• Quantity: Opening a large number of gates simultaneously to increase the total volume of ions moving across the membrane.

By summing these "additions" and "subtractions" of charge, the axon hillock determines the net membrane potential and decides whether the message continues down the line.