Introduction

Membrane potential is an electrochemical gradient across a cell membrane that is created by ion pumps and channels. It is an important component of cell physiology, as it influences many physiological processes such as cellular excitability, signal transduction, and secretion. Understanding the changes in membrane potential is essential for understanding the functioning of cells and tissues. This article reviews the changes in membrane potential in different physiological states.

Resting Potential

The resting potential of a cell is the membrane potential when the cell is at rest and not responding to any stimuli. The resting potential of a cell is determined by the ionic balance of the cell. The resting potential of most cells is around -70 mV, meaning that the inside of the cell is negative relative to the outside. This resting potential is maintained by the selective permeability of the cell membrane, as well as ion pumps and channels.

The resting potential is maintained by the activity of Na+/K+ ATPase, an enzyme that pumps three sodium ions out of the cell for every two potassium ions that it pumps in. This creates a net negative charge inside the cell, resulting in the resting potential.

Action Potential

The action potential is an all-or-nothing response of a cell to a stimulus. It is an electrical impulse that is propagated along the cell, allowing for signal transduction and cellular communication. It is caused by the opening and closing of voltage-gated ion channels.

When a stimulus is applied, the voltage-gated sodium channels open, allowing for a large influx of sodium ions into the cell. This causes the membrane potential to become positive, and the cell depolarizes. This depolarization causes more voltage-gated sodium channels to open, allowing for a greater influx of sodium ions, further depolarizing the cell. This process continues until the membrane potential reaches the threshold potential, which is usually around -55 mV.

At this point, the voltage-gated sodium channels close, and the voltage-gated potassium channels open, allowing for a large efflux of potassium ions out of the cell. This causes the cell to repolarize, and the membrane potential returns to the resting potential.

Hyperpolarization

Hyperpolarization is an increase in the resting membrane potential from the normal -70 mV to a more negative potential. It is caused by an increase in the number of ions inside the cell, or by the opening of ion channels that allow for the influx of ions into the cell.

Hyperpolarization is important for several physiological processes. It can act as a modulator for synaptic transmission, as the hyperpolarization of a neuron can make it less likely to fire an action potential. It can also act as a signal for cell proliferation, as hyperpolarization can activate specific signaling pathways that lead to cell growth and division.

Hypopolarization

Hypopolarization is a decrease in the resting membrane potential from the normal -70 mV to a more positive potential. It is caused by a decrease in the number of ions inside the cell, or by the opening of ion channels that allow for the efflux of ions out of the cell.

Hypopolarization is important for several physiological processes. It can act as a modulator for synaptic transmission, as the hypopolarization of a neuron can make it more likely to fire an action potential. It can also act as a signal for cell death, as hypopolarization can activate specific signaling pathways that lead to cell death and apoptosis.

Summary

Membrane potential is an important component of cell physiology, as it influences many physiological processes such as cellular excitability, signal transduction, and secretion. This article reviewed the changes in membrane potential in different physiological states, including the resting potential, action potential, hyperpolarization, and hypopolarization. Understanding the changes in membrane potential is essential for understanding the functioning of cells and tissues.