Q48.3-3CC - Campbell Biology

Question

How do both negative and positive feedback contribute to the changes in membrane potential during an action potential?

Step-by-Step Solution

Answer

Positive feedback causes the opening of numerous voltage-gated sodium channels, resulting in an outflow of sodium ions that drives the action potential to rise. The voltage-gated channels activate in the form of negative feedback as the membrane potential becomes positive. 

 

Since Na⁺ has a greater concentration gradient, it will enter the cell even after the membrane potential has decreased to zero, causing the voltage around the pore to rise.

Step 1: Changes in the membrane potential during action potential

A change in membrane potential in the form of an action potential happens when the membrane potential of the axon hillock of a neuron exceeds the threshold. There are three phases of this changing membrane potential.

 

Depolarization comes first, then repolarization, and finally, a brief period of hyperpolarization. These three actions occur in a matter of milliseconds.

Step 2: Positive feedback of the action potential

The process for generating action potentials is a positive feedback loop; the activation of the voltage-dependent Na⁺ conductance increases Na⁺ entry into the neuron, which causes the membrane potential to depolarize.

 

Positive feedback remains intact until the membrane potential is restored to its resting level by inactivating the Na⁺ conductance and activating the K⁺ conductance.

 

The voltage-dependent activation of Na⁺ conductance is driven by membrane depolarization, which quickly begins a positive feedback cycle.

Step 3: Negative feedback of the action potential

In the cell membrane, negative feedback is obtained when the membrane becomes more permeable to the transportation of K ions instead of Na ions.

 

The cell membrane cannot achieve the optimum concentration due to continuous K⁺ permeability because the increasing permeability of K⁺ ions makes the cell membrane negative.

 

Thus, due to the positive and negative feedback, the membrane potential will cause a change in the action potential. The action potential activity on the membrane is always positive and will be negative if the flow of K⁺ ions increases in the membrane.

 

The positive feedback will cause the sodium channels to increase the action potential.