354
UNIT THREE
nerve f
ber, carries the inFormation away From the cell in the
Form oF bioelectric signals called
nerve impulses
(f g. 10.1)
.
Nerves
are bundles oF axons. Neuroglia were once thought
only to f
ll spaces and surround or support neurons. Today,
we know that they have many other Functions, including
nourishing neurons and perhaps even sending and receiv-
ing messages.
An important part oF the nervous system at the cellu-
lar level is not a cell at all, but the small space between a
neuron and the cell(s) with which it communicates called a
synapse
(sin
aps). Much oF the work oF the nervous system
is to send and receive electrochemical messages between
neurons and other cells at synapses. Biological messenger
molecules called
neurotransmitters
(nu
ro-trans-mit
erz)
are the actual conveyors oF this neural inFormation.
The organs oF the nervous system can be divided into
two groups. One group, consisting oF the brain and spinal
cord, Forms the
central nervous system (CNS)
, and the
other, composed oF the nerves (cranial and spinal nerves)
that connect the central nervous system to other body parts,
is called the
peripheral nervous system (PNS)
(f g. 10.2)
.
10.1
INTRODUCTION
The nervous system oversees all that we do and largely deter-
mines who we are. Through a vast communicating network oF
cells and the biochemicals that they send and receive, the ner-
vous system can detect changes in the body, make decisions
on the basis oF the inFormation received, and stimulate muscles
or glands to respond. Typically, these responses counteract
the eFFects oF the changes, and in this way, the nervous system
helps maintain homeostasis. Clinical Application 10.1 discusses
how environmental changes may trigger migraine headaches,
a common medical problem attributed to the nervous system
that may involve its blood supply as well as neurons.
The nervous system is composed predominantly oF
neural tissue, but also includes blood vessels and connec-
tive tissue. Neural tissue consists oF two cell types: nerve
cells, or
neurons
(nu
ronz), and
neuroglia
(nu-ro
gle-ah)
(or neuroglial cells). Neurons are specialized to react to
physical and chemical changes in their surroundings. Small
cellular processes called
dendrites
(den
drı¯tz) receive the
input, and a longer process called an
axon
(ak
son), or
metabolism, as well as the development of the incredibly complex human brain
from initial stem and progenitor cells. The material in brain and stem cell banks
is also being used in drug discovery and in developing new treatments based on
cell implants. The chapter opening image shows neural progenitor cells and the
photo accompanying this essay shows neural stem cells.
I
n a large room at the Croatian Institute for Brain Research, rows of
shelves hold a variety of F
uid-±
lled jars, a human brain suspended
in each. Their sizes di²
er, reF
ecting their origins from embryos up to
the elderly. Researchers can use the more than 1,000 brains and more
than 130,000 histological slides at the bank to investigate brain-
based diseases and injuries that a²
ect many millions of people worldwide
and also to better understand the functioning of the normal human brain.
In the United States, several brain banks o²
er tissue sections from thou-
sands of people who willed their brains to science. Unlike donated hearts,
lungs, or corneas, which directly help other people, donated brains go to
research labs.
Many brain banks are specialized. The bank at Harvard University is
devoted to neurodegenerative diseases, such as Alzheimer and Parkinson
diseases, while the resource at the University of Maryland in Baltimore
focuses on developmental disorders, including Down syndrome and autism.
The brain bank at the University of Miami has brains from people who had
schizophrenia, depression, amyotrophic lateral sclerosis, and several other
disorders, as well as undiseased brains for comparison.
Brains must be removed from the skull within twelve hours of death.
Then they are halved and cut into one-centimeter thick sections and frozen
in plastic bags. The specimens are provided free to researchers.
Study of brain function and malfunction is also possible at the cellular
level. The National Human Neural Stem Cell Resource provides neural stem
cells, which function after death longer than neurons because their ener-
getic and oxygen requirements are not as high as those of the more special-
ized cells. Hospitals collect brain material upon autopsy and send it to the
facility, where a special protocol is used to obtain and preserve the cells from
several brain areas. These techniques were perfected on the brains of pigs,
cats, and sheep. Investigators use the human neural stem cells to study neu-
rodegenerative disorders, stroke, traumatic brain injury, rare inborn errors of
BRAIN BANKS
Neurospheres cultured in the laboratory consist of neural stem cells. These
cells can divide and di²
erentiate to give rise to neural progenitor cells, which
in turn divide and di²
erentiate, yielding neurons and neuroglia. In the brain,
neural stem cells occupy certain areas but are exceedingly rare. Researchers are
attempting to harness the natural ability of neural stem and progenitor cells to
divide and replace damaged or diseased neural tissue.
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