ANATOMICAL TERMS AND ORGANIZATION
Throughout the study of the brain, there are many ways one can describe, divide, and
organize the brain. The following is a summary of the various ways this can be done.
Below is a diagram showing the major divisions of the human nervous system. In this
lecture we will concentrate on the brain and some important structures within it.
A COMMON BRAIN DIVISION
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Before examining the brain in more detail you will need to be familiar with a few
anatomical terms these are simply the words used to describe the relationship in space
between parts of the brain (and body):
Directions and Planes of Section
There are a number of special words
that are used to describe the position
and direction of brain structures.
These words help describe the
location of structures relative to other
structures. For example, we can say
that the frontal lobe is "rostral" to the
occipital lobe.
The brain, like all biological
structures, is three dimensional. So,
any point on or inside the brain can be
localized on three "axes" or "planes" the x, y and z axes or planes. The
brain is often cut ("sectioned") into
pieces for further study. These slices
are usually made in one of three
planes: the coronal plane, the
horizontal plane or the sagittal plane.
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The coronal plane, horizontal plane and
sagittal plane are shown in the figure on
the right. The coronal plane is also called
the frontal plane. Slices of the brain taken
in the coronal plane are similar to the slices from a loaf
of bread. Horizontal cuts are made as if you were
slicing a hamburger bun or bagel.
The sagittal plane divides the right and left side of the
brain into parts. The midsagittal plane would divide
the right and left sides of the brain into two equal
parts, like cutting down the middle of a baked potato
before you put on the toppings.
The figures below show the human brain in the three planes of section on "synthetic MR"
images produced by BrainWeb:
Coronal Section Sagittal Section Horizontal Section
You can find some photographs of coronal sections from the human brain at the
Comparative Mammalian Brain Collection.
The LONI Resource is also available for viewing in coronal, horizontal and sagittal
planes.
While visiting a new city or country, people often bring along a map.
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Neuroscientists]who study the brain also use maps to identify exactly what part of the
brain they are examining. These maps of the brain are called stereotaxic atlases. Just like
maps, stereotaxic atlases use words to describe direction. However, instead of "north",
"south", "east" and "west", the following words are used to describe direction in the brain
(and other parts of the body too):
Directional Terms of the Body
Direction
Description
Ventral
Toward the
belly (front)
Dorsal
Toward the
back
Rostral
Toward the
nose
Caudal
Toward the
tail
Superior
Toward the
top of the
head/body
Lateral
Away from
the middle
Medial
Toward the
middle
Bilateral
On both sides
Ipsilateral
On the same
side
Contralateral
On the
opposite side
Side View
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Front View
EVOLUTION AND BRAIN ANATOMY
During the evolution of the vertebrate brain the following scheme prevailed, with the
relative sizes of these areas reflecting the animal’s life style. As animals evolved from
fish to birds the relative size of the forebrain increased to accommodate neural networks
which underlie increasingly sophisticated behavior.
One convenient way of thinking about the brain is to consider it as three structures (the
triune brain model) the outer ones being added to the inner ones during evolution.
Paul MacLean, the former director of the Laboratory of the Brain and Behavior at the
United StatesNational Institute of Mental Health, developed a model of the brain based
on its evolutionary development. It is referred to as the "triune brain theory" because
MacLean suggests that the human brain is actually three brains in one. Each of the layers
or "brains" were established successively in response to evolutionary need. The three
layers are the reptilian system, or R-complex, the limbic system, and the neocortex. Each
layer is geared toward separate functions of the brain, but all three layers interact
substantially.
The Reptilian Complex
The R-complex consists of the brain stem and the cerebellum. Its
purpose is closely related to actual physical survival and maintenance of
the body. The cerebellum orchestrates movement. Digestion, reproduction,
circulation, breathing, and the execution of the "fight or flight" response in
stress are all housed in the brain stem. Because the reptilian brain is
primarily concerned with physical survival, the behaviors it governs have
much in common with the survival behaviors of animals. It plays a crucial
role in establishing home territory, reproduction and social dominance.
The overriding characteristics of R-complex behaviors are that they are
automatic, have a ritualistic quality, and are highly resistant to change.
The Limbic System
The limbic system, the second brain to evolve, houses the primary
centers of emotion. It includes the amygdala, which is important in the
association of events with emotion, and the hippocampus, which is active
in converting information into long term memory and in memory recall.
Repeated use of specialized nerve networks in the hippocampus enhances
memory storage, so this structure is involved in learning from both
commonplace experiences and deliberate study. However, it is not
necessary to retain every bit of information one learns. Some
neuroscientists believe that the hippocampus helps select which memories
are stored, perhaps by attaching an "emotion marker" to some events so
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that they are likely to be recalled. The amygdala comes into play in
situations that arouse feelings such as fear, pity, anger, or outrage.
Damage to the amygdala can abolish an emotion-charged memory.
Because the limbic system links emotions with behavior, it serves to
inhibit the R-complex and its preference for ritualistic, habitual ways of
responding.
The limbic system is also involved in primal activities related to food
and sex, particularly having to do with our sense of smell and bonding
needs, and activities related to expression and mediation of emotions and
feelings, including emotions linked to attachment. These protective, loving
feelings become increasingly complex as the limbic system and the
neocortex link up.
The Neocortex
Also called the cerebral cortex, the neocortex constitutes five-sixths of
the human brain. It is the outer portion of our brain, and is approximately
the size of a newspaper page crumpled together. The neocortex makes
language, including speech and writing possible. It renders logical and
formal operational thinking possible and allows us to see ahead and plan
for the future. The neocortex also contains two specialized regions, one
dedicated to voluntary movement and one to processing sensory
information.
We have mentioned that all three layers of the brain interact. The
layers are connected by an extensive two-way network of nerves. Ongoing communication between the neocortex and the limbic system links
thinking and emotions; each influences the other and both direct all
voluntary action. This interplay of memory and emotion, thought and
action is the foundation of a person’s individuality.
The full extent of this interconnectedness is unclear. However, it is
entirely incorrect to assume that in any situation one of our three "brains"
is working and the others are not. What we can do, tentatively, is assume
that at times one particular focus may be dominant while the rest of the
brain acts in support and that education can influence which focus
dominates.
Caine, Renate Nummela and Geoffrey Caine. Making Connections:
Teaching and the Human Brain. Nashville, TN: Incentive Publications,
1990.
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