How Do Neurons Work?

Neurons are often referred to as excitable cells. Excitable cells are able to respond to a stimulus by changing the electrical properties of their plasma membrane. Only muscle and nerve cells are excitable and the basis for excitability lies in the electrolytes (ions) that are dissolved into our extracellular and intracellular fluids. As mentioned before, the concentrations of the different electrolytes are not the same across the plasma membrane (Figure 2.4). In general the concentrations of sodium (Na+), chloride (Cl−), and calcium (Ca2+) are much greater in the extracellular fluid, while the concentration of potassium (K+) is greater in the intracellular fluid. This means that these electrolytes have the potential to move across the plasma membrane, down their concentration gradient, when their respective ion channels open up.
 

When an excitable cell is stimulated, ion channels open in a specific and

timely fashion. This allows electrolytes to move either into or out of the cell depending on the direction of their concentration gradient. The movement of the charged electrolytes changes the electrical nature of the plasma membrane at the site of the stimulus. Furthermore, when the cell is stimulated at one point on its plasma membrane, the excitability or impulse then moves along the plasma membrane like a ripple on a pond. Thus the excitability spreads and is often called a nerve impulse, as shown in Figure 2.5.