Biology  5/e   Raven/Johnson
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Sodium/potassium pump: Cells expend a lot of energy actively pumping Na⁺ ions out of cells and actively pumping K⁺ ions into cells, both against their concentration gradients. This active transport occurs through protein channels in the cell membrane. These special channels are called sodium/potassium pumps. Nerve function depends on the ion imbalance established by the pump.

Ion channels: Many transmembrane protein channels allow the passage of ions into or out of the cell by diffusion, without any expenditure of energy. The ions follow their concentration gradients, moving from areas of high concentration to areas of low concentration. Many of the ion channels are highly selective and allow the passage of only specific ions. Nerve cells have ion channels for both Na⁺ and K⁺ ions which are essential for the proper functioning of the cells.

Primitive invertebrates: The primitive invertebrates include the phyla Porifera (sponges), Cnidaria (coelenterates), Ctenophora (comb jellies), Platyhelminthes (flatworms), Rhynchocoela (ribbon worms), Nematoda (nematodes), and Rotifera (rotifers). These are the simplest members of the animal kingdom, but they illustrate the evolutionary history of the major characteristics associated with the more advanced animal phyla. Although these invertebrates are primitive, they are the foundation upon which the more advanced forms were built. In them we can trace the evolutionary history of various organ systems and body architecture. With the nervous system, for example, we can trace the evolutionary trend from the sponge with no nerve cells, to the cnidarian with its nerve net, to the planaria with its central versus peripheral nervous system.

Neuron: Neurons are cells that are specialized for transmitting nerve impulses (i.e., conducting electric current). Together with their supporting cells they make up nerve tissue, one of the four main tissue types in the vertebrate body. Their ability to function depends on maintaining an electrical charge difference between the interior and exterior of the cell.

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