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.
