Development of the resting membrane potential. (a) Active transport creates a concentration gradient across the cell membrane for sodium ions (Na+) and potassium ions (K+). K+ diffuses out of the cell rather slowly, but nonetheless faster than Na+ can diffuse in. (b) This unequal diffusion causes a net loss of positive charge. Combined with the presence of impermeable anions within the cell (PO4-2, SO4-2, and proteins), this results in a relative excess of negative charge inside the membrane. The negative membrane potential tends to draw Na+ into the cell and restricts K+ leaving the cell. Despite the differences in permeability of these ions, their movements across the membrane are now more nearly equal. The Na+/K+ pump balances these movements and maintains the concentration gradients for these two ions.

The difference in electrical charge between two points is measured in units called volts. It is called a potential difference because it represents stored electrical energy that can be used to do work at some future time. The potential difference across the cell membrane is called the membrane potential (transmembrane potential) and is measured in millivolts.

In the case of a resting neuron, one that is not sending impulses or being affected by other neurons, the membrane potential is termed the resting potential (resting membrane potential) and has a value of -70 millivolts. The negative sign is relative to the inside of the cell and is due to the excess negative charges on the inside of the cell membrane. To understand how the resting potential provides the energy for sending a nerve impulse down the axon, we must first understand how neurons respond to signals called stimuli.

With the resting membrane potential established, a few sodium ions and potassium ions continue to diffuse across the cell membrane. The negative membrane potential helps sodium ions enter the cell despite sodium's low permeability, but it hinders potassium ions from leaving the cell despite potassium's higher permeability. The net effect is that three sodium ions "leak" into the cell for every two potassium ions that "leak" out. The Na+/K+ pump exactly balances these leaks by pumping three sodium ions out for every two potassium ions it pumps in.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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