23.4–23.7: How do neurons work?

Neurons within the brain of a mouse reveal rich connectivity among the cells.

“Fire!” “Don’t fire!” “Fire!” “Don’t fire!” Dendrites are on the receiving end of an almost constant barrage of (sometimes conflicting) signals. A neuron may have hundreds or even thousands of dendrites, which give it a huge amount of surface area over which to make connections with other neurons and receive signals.

Dendrites receive stimuli in one of two ways. Those on motor neurons and interneurons generally connect with and receive signals from other neurons. Sensory neuron dendrites, on the other hand, are modified to respond to a specific external stimulus such as a touch or sound, light, or a chemical (FIGURE 23-8).

Figure 23.8: “Fire!” (or “Don’t fire!”).

When a neuron is not transmitting a signal, the inside of the cell has a negative charge relative to the outside of the cell. This difference in charge, referred to as a membrane potential, is the neuron’s resting potential. The resting potential is produced as proteins within the plasma membrane of the neuron pump sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell. Three sodium ions are pumped out of the cell for each two potassium ions moved in, contributing to establishment of a greater positive charge outside the cell than inside the cell. The cell is described as “polarized” and can be thought of as “ready to fire.”

On stimulation, receptors within the cell membrane of the dendrite respond by briefly opening up ion channels (usually the sodium channels). These channels, made from proteins, allow the passage of charged ions (usually sodium ions) down their concentration gradient. This ion flow momentarily alters the negative electrical charge inside the cell—increasing it or decreasing it, depending on the type of ion flow. When opening the channels makes the neuron more negatively charged (that is, the electrical charge inside the cell relative to that in the fluid surrounding the cell becomes even more negative), this says, “Don’t fire!” When it makes the neuron more positively charged, such as when sodium channels open and sodium ions rush into the cell, this says, “Fire!”

As stimuli cause channels to open in the dendrites of a neuron—some making the neuron more negatively charged, others making the neuron more positively charged—the changes in the cell’s electrical charge, occurring in all these dendrites, converge at the cell body. The cell body then integrates them, much like tallying the votes in an election. If the sum total of signals coming in is sufficiently positive—exceeding a threshold favoring “Fire!”—then the neuron initiates an action potential, an electrical signal that travels down its axon. If the sum total is negative, no action potential is generated.