Science Brain and Behavior contiuned 11

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In genetic engineering, genes can be introduced into an embryo or removed from
it. For example, the introduction of a new gene can allow goats to produce medicines
in their milk, and those medicines can be extracted from the milk to treat human diseases
(Niemann et al., 2003). It is also possible to produce chimeric animals, which have
genes from two different species. A cell from one species is introduced into the early
embryonic stage of a different species. The resulting animal has cells with genes from
both parent species and behaviors that are a product of those gene combinations.
The chimeric animal displays an interesting mix of the behaviors of the parent
species. For example, chickens that have received Japanese quail cells in early embryogenesis
display some aspects of quail crowing behavior rather than chicken crowing behavior,
thus providing evidence for the genetic basis of some bird vocalization (Balaban
et al., 1988). The chimeric preparation provides an investigative tool for studying the
neural basis of crowing because quail neurons can be distinguished from chicken neurons
when examined under a microscope.
One application of genetic engineering is in the study and treatment of human genetic
disorders. For instance, researchers have introduced into a line of mice the human
gene that causes Huntington’s chorea (Lione et al., 1999). The mice express the abnormal
huntingtin allele and display symptoms similar to the disorder in humans. This
mouse line is being used to study potential therapies for this disorder in humans.
As described at the beginning of this section (see “Knocking Out Genes” on page
100), knockout technology can be used to inactivate a gene so that a line of mice fails to
express it (Eells, 2003). That line of mice can then be used to study possible therapies
for human disorders caused by the loss of a single protein due to a mutant gene. Remarkably
interesting knockout animals can be bred. Examples are a knockout mouse
that grows up with a superior memory or with no memory and a mouse that is allowed
to grow up quite normally and the gene is then knocked out in adulthood.
It is potentially possible to knock out genes that are related to certain kinds of
memory, such as emotional memory, social memory, or spatial memory. Such technology
provides a useful way of investigating the neural basis of memory. So genetic
research is directed not only toward finding cures for genetic abnormalities in brain
and behavior, but also toward studying normal brain function.
In Review .
Researchers in genomics study how genes produce proteins, whereas those studying proteomics
seek to understand what individual proteins do. Each of our 46 chromosomes contains
thousands of genes, and each gene contains the code for one protein. The genes that
we receive from our mothers and fathers may include slightly different versions (alleles) of
particular genes, which will be expressed in slightly different proteins. Abnormalities in a
gene, caused by mutations, can result in an abnormally formed protein that, in turn, results
in the abnormal function of cells. Recessive or dominant alleles can result in neurological
disorders such as Tay-Sachs disease and Huntington’s chorea, respectively. Genetic engineering
is a new science in which the genome of an animal is altered. The genetic composition
of a cloned animal is identical with that of a parent or sibling; transgenic animals
contain new or altered genes; and knockouts have genomes from which a gene has been
deleted. The study of alterations in the nervous systems or in the behavior of animals produced
by these manipulations can be a source of insight into how genes produce proteins
and how proteins contribute to the structure and function of the nervous system.
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 107
Figure 3-28
A Clone and Her Offspring Dolly
(right) was cloned in 1996, when a team
of researchers in Scotland implanted a
nucleus from a mammary-gland cell of
an adult sheep into another ewe’s
unfertilized egg from which the nucleus
had been removed. The resulting embryo
was implanted into a third sheep’s
uterus. Dolly subsequently mated and
bore a lamb (left).
AP Photo/John Chadwick
To learn more about creating knockout
mice, visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Web links for Chapter 3.
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SUMMARY
What kinds of cells are found in the nervous system? The nervous system is composed
of two kinds of cells: neurons, which transmit information, and glia, cells that support
brain function. Sensory neurons send information from the body’s sensory receptors to
the brain,motor neurons send commands enabling muscles to move, and interneurons
link sensory and motor activities in the CNS. Like neurons, glial cells can be grouped by
structure and function. Ependymal cells produce cerebrospinal fluid; astrocytes structurally
support neurons, help to form the blood–brain barrier, and seal off damaged
brain tissue;microglia aid in the repair of brain cells; and oligodendroglia and Schwann
cells myelinate axons in the central and peripheral nervous systems, respectively.
What is the basic external structure of a neuron? A neuron is composed of three basic
parts: a cell body, or soma; branching extensions called dendrites designed to receive
information; and a single axon that passes information along to other cells.A dendrite’s
surface area is greatly increased by numerous dendritic spines; an axon may have
branches called axon collaterals, which are further divided into teleodendria, each ending
at a terminal button, or end foot. A synapse is the “almost connection” between a
terminal button and the membrane of another cell.
How is a cell internally structured? A surrounding cell membrane protects the cell and
regulates what enters and leaves it.Within the cell are a number of compartments, also
enclosed in membranes. These compartments include the nucleus (which contains the
cell’s chromosomes and genes), the endoplasmic reticulum (where proteins are manufactured),
the mitochondria (where energy is gathered and stored), the Golgi bodies
(where protein molecules are packaged for transport), and lysosomes (which break
down wastes). A cell also contains a system of tubules that aid its movements, provide
structural support, and act as highways for transporting substances.
Why are proteins important to cells? To a large extent, the work of cells is carried out
by proteins. The nucleus contains chromosomes, which are long chains of genes, each
encoding a specific protein needed by the cell. Proteins perform diverse tasks by virtue
of their diverse shapes. Some act as enzymes to facilitate chemical reactions; others
serve as membrane channels, gates, and pumps; and still others are exported for use in
other parts of the body.
How do genes work? A gene is a segment of a DNA molecule and is made up of a sequence
of nucleotide bases. Through a process called transcription, a copy of a gene is
produced in a strand of mRNA. The mRNA then travels to the endoplasmic reticulum,
where a ribosome moves along the mRNA molecule, and is translated into a sequence
of amino acids. The resulting chain of amino acids is a polypeptide. Polypeptides fold
and combine to form protein molecules with distinctive shapes that are used for specific
purposes in the body.
What do we inherit genetically from our parents? From each parent, we inherit one of
each of the chromosomes in our 23 chromosome pairs. Because chromosomes are
“matched” pairs, a cell contains two alleles of every gene. Sometimes the two alleles of
a pair are homozygous (the same), and sometimes they are heterozygous (different).
An allele may be dominant and expressed as a trait; recessive and not expressed; or
codominant, in which case both it and the other allele in the pair are expressed in the
individual organism’s phenotype. One allele of each gene is designated the wild type,
or most common one in a population, whereas the other alleles of that gene are called
mutations.A person might inherit any of these alleles from a parent, depending on that
parent’s genotype.
108 ! CHAPTER 3
neuroscience interact ive
There are many resources available for
expanding your learning online:
www.worthpublishers.com/kolb
Try the Chapter 3 quizzes and
flashcards to test your mastery of the
chapter material. You’ll also be able to
link to other sites that will reinforce
what you’ve learned.
http://www.bioteach.ubc.ca/
CellBiology/StudyingGeneFunction
Learn more about knockout technology
and gene function.
http://vector.cshl.org/dnaftb/
Review genetics at this Web site from
the Cold Spring Harbor Laboratory.
On your Foundations CD-ROM, you’ll
be able to begin learning about the cells
of the nervous system in the module on
Neural Communication. This module
includes animations and detailed
drawings of the neuron. In addition,
the Research Methods module includes
a video clip of neurons and glia, as well
as a detailed overview of various
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REVIEW QUESTIONS
1. Describe five kinds of neurons and five kinds of glia and their functions.
2. Describe the functions of the different parts of a cell.
3. Why can so many nervous system diseases be due to faulty genes?
FOR FURTHER THOUGHT
People often compare the “machine of the day” to the nervous system. Why can we
never understand our nervous system by comparing it to a computer and how it works?
RECOMMENDED READING
Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., & Watson, J. D. (2002).Molecular
biology of the cell (4th ed.). New York: Garland. This standard text provides a
comprehensive description of the cells and cell function in living organisms.
Levitan, I. B., & Kaczmarek, L. K. (2002). The neuron: Cell and molecular biology (3rd ed).
Oxford: Oxford University Press. An extremely readable text describing the function of
the neuron. The coverage is comprehensive, and the text is enjoyable to read,
accompanied by numerous illustrations that assist in explanation.
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 109
What is the relation among genes, cells, and behavior? Comprehending the links
among genes, cells, and behavior is the ultimate goal of research, but as yet these links
are only poorly understood. The structure and function of a cell are properties of all
its many genes and proteins, just as behavior is a property of the actions of billions of
nerve cells. It will take years to learn how such a complex system works. In the meantime,
the study of genetic abnormalities is a potential source of insight.
What causes genetic abnormalities? Genes can potentially undergo many mutations,
in which their codes are altered by one or more changes in the nucleotide sequence.
Most mutations are harmful and may produce abnormalities in nervous system structure
and behavioral function. Genetic research seeks to prevent the expression of genetic
and chromosomal abnormalities and to find cures for those that are expressed.
KEY TERMS
allele, p. 101
astrocyte, p. 84
axon, p. 76
axon collateral, p. 79
axon hillock, p. 79
bipolar neuron, p. 79
cell body (soma), p. 76
channel, p. 96
dendrite, p. 76
dendritic spine, p. 79
Down’s syndrome, p. 104
end foot (terminal button),
p. 79
ependymal cell, p. 82
gate, p. 96
glial cell, p. 82
heterozygous, p. 101
homozygous, p. 101
Huntington’s chorea, p. 104
hydrocephalus, p. 84
interneuron, p. 81
microglial cell, p. 84
multiple sclerosis (MS),
p. 85
mutation, p. 101
myelin, p. 85
oligodendroglial cell, p. 85
paralysis, p. 85
pump, p. 96
Purkinje cell, p. 81
pyramidal cell, p. 81
Schwann cell, p. 85
soma (cell body), p. 76
somatosensory neuron,
p. 79
synapse, p. 79
Tay-Sachs disease, p. 103
tumor, p. 82
wild type, p. 101
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110 !
C H A P T E R4
How Do Neurons Transmit
Information?
Focus on Disorders: Epilepsy
Electricity and Neurons
Early Clues to Electrical Activity in the Nervous
System
Modern Tools for Measuring a Neuron’s Electrical
Activity
How the Movement of Ions Creates Electrical Charges
Electrical Activity of a Membrane
Resting Potential
Graded Potentials
The Action Potential
Focus on New Research: Opening the
Voltage-Sensitive Gates
The Nerve Impulse
Saltatory Conduction and Myelin Sheaths
How Neurons Integrate Information
Excitatory and Inhibitory Postsynaptic Potentials
Focus on Disorders: Myasthenia Gravis
Summation of Inputs
The Axon Hillock
Into the Nervous System and Back Out
How Sensory Stimuli Produce Action Potentials
How Nerve Impulses Produce Movement
Focus on Disorders: Lou Gehrig’s Disease
Studying the Brain’s Electrical
Activity
Single-Cell Recordings
EEG Recordings
Event-Related Potentials
Left: Dr. David Scott/Phototake. Middle: Mason Morfit/
FPG International/PictureQuest. Right: CNRI/Phototake.