Biomedical Engineering Reference
In-Depth Information
3.4.3 The Nervous System
The most exciting and mysterious part of the human body is the magical 3½ pounds of
tissue we carry around inside our skulls: the brain. For centuries, the brain has frustrated
those daring enough to explore its secrets. Encased not only in its bony protective covering
but also in a shroud of mysticism, it has been an extremely difficult structure to study.
Even with the invention of the microscope and the discovery of electricity, generations of
Western scientists refrained from investigating the activity of the human brain out of
respect for it as the seat of a human's immortal soul.
In recent years, this convoluted mass, the source of all thought and emotion, has been the
focal point of intense scientific investigation. There has been a great flurry of activity in assem-
bling interdisciplinary teams consisting of physiologists, psychologists, biochemists, and engi-
neers in order to gain a better understanding of brain function. To many of these individuals,
the brain represents a symbolic Mt. Everest, an obstacle to be scaled and conquered before it
will be better understood. And yet, in spite of all the efforts to date, we are still only in the foot-
hills of such a climb. The mechanisms and processes that enable the brain to convert the variety
of electrical and chemical activity occurring within it into thoughts, feeling, dreams, and mem-
ories—the fundamental awareness of self—are still beyond our understanding.
However, in spite of the difficulties encountered and the frustrations experienced by
explorers in this world of the mind, significant progress has been made in deciphering
the cryptic flow of electrical energy coming from the brain. In reviewing these electrical
signals, it has been possible to detect the presence of certain patterns or rhythms that occur
in the brain that represent a “language” that can be recognized and understood by neural
circuits in the brain itself. The fundamental building block of this neuronal communication
network is the individual nerve cell: the neuron. Figure 3.16d is a schematic drawing of just
such a cell. It consists of three major components: the cell body itself, or
soma
; the receptor
zone, or
, which carries electrical signals from the
main body of the cell to the muscles, glands, or other neurons. Numbering approximately
20 billion in each human being, these tiny cells come in a variety of sizes and shapes. How-
ever, nowhere is more variety displayed than in the length of the axonal terminating fiber.
In the human body, it ranges from a few thousandths of an inch up to three feet or more,
depending on the type of neuron involved. Consider, for example, the long pathways from
the extremities to the brain. In these communication channels between the periphery and
the “central data processor” that we call the brain, only a few neurons may be connected
to one another. As a result, the axon of these nerve cells may be as long as 2 or 3 feet, even
though the cell body is quite small. Some axons are surrounded by sheaths of myelin that
are formed by specialized nonneural cells called Schwann cells. Gaps, called “Nodes of
Ranvier,” in the myelin sheath allow the action potential generated by the neuron to travel
more rapidly by essentially jumping from one node to the next.
Neurons are anatomically distinct units with no physical continuity between them. The
transmitting portion of a neuron, its axon, ends in a series of synapses, thereby making
contact with other neurons (see Figure 3.28). Under the microscope this often stands out
as a spherical enlargement at the end of the axon to which various names have been given,
for example, boutons, end-feet, or synaptic terminals. This ending does not actually make
contact with the soma or the dendrite but is separated by a narrow cleft (gap) that is, on
dendrites
; and a long fiber called the
axon
