The memories that concern us in everyday life, whether they are explicit memories or implicit memories, are far removed from nerve cells, just as our everyday world of food, cars, and people is far removed from the atoms that make them up.
Activity is electrical, nerve cells communicate with each other by releasing chemicals. This chemical release is a heritage of our past. When our ancestors were all just single cells, the only way to communicate was by releasing chemicals into primordial oceans. Later, as collections of multiple cells organized into primitive animals, the easiest way for cells to get messages across to one another was still to put out chemicals into the fluid that bathed them all.
When nerve cells developed, it appears that they adopted this existing transmission system for their own use. In some cases, these chemicals have retained some of the functions that they once had. In others, the functions have been modified beyond recognition. For example, the chemical people commonly know as adrenaline is actually a neurotransmitter as well. But it can get released into the blood when a special gland, the adrenal gland, gets stimulated. Adrenaline signals all the cells of the body to get ready for an emergency. It forces sugar into muscle cells, and slows down the digestive system. But adrenaline also operates deep within the brain, in the connections between some sets of nerve cells.
Memory at the nerve cell level is thought to involve changes in the strengths of connections between nerve cells. These changes can be both increases and decreases in the strength of connections. Since neurotransmitters are the major Way nerve cells communicate from one to another, changes in the way neurotransmitters are released, and changes in tie way neurotransmitters are received or interpreted by the nerve cell at the other end, must clearly be important in the formation of memory.
However, because we are concerned with the memories that come into our conscious experience, it is important to place our current knowledge of “memory” at the nerve cell level in the proper context. The memories that we are conscious of are not discrete files or pages inside our heads. Instead, they are a product of the electrical activity of an enormous number of nerve cells and nerve cell endings.
Some of these nerve cells and nerve cell endings are probably clustered together, and we identify them as specific regions of the brain. Other nerve cells involved in what we feel is a single memory probably are scattered widely all over the brain. The firing of the nerve cells is also probably spread out over time, as well. A single nerve cell takes about one-thousandth of a second to fire. However, the memories we see with techniques such as direct electrical recording seem to occupy a period of time at least two hundred times longer than this.