MINI LAB
Make a Model of a Brain
Some medical schools require students to build a model of
the brain. You can use jelly, other types of food, modelling
clay, or any other material that you think will be suitable to
build a 3-D model of the brain. Refer to the diagram on
page 145 and make each part of the brain (identified in
bold capital letters) a different colour. Include a card that
lists each part of the brain and its colour in your model.Analyze
1.List the functions of each part of the brain that is shown
in your model.
2.In what ways is your model similar to the human brain
(in terms of size, texture, and so on)?
3.In what ways is your model different from the human
brain?144 MHR • Unit 2 Homeostasis
(the conscious part of the brain that will be
discussed shortly). However, if the sensations are
strong, the thalamus triggers a more immediate
reaction while, at the same time, transferring the
sensations to the homeostatic control centre —
the hypothalamus.
The hypothalamus — to be studied in detail in
Chapter 6 — is an incredibly complex and
important bundle of tissues that acts as the main
control centre for the autonomic nervous system.
The hypothalamus enables the body to respond to
external threats by sending impulses to various
internal organs via the sympathetic nervous system.
It re-establishes homeostasis after the threat has
passed by stimulating the parasympathetic nerves.
The hypothalamus also controls the endocrine
hormone system, the focus of Chapter 6. Attached
to the hypothalamus is the pituitary gland, which
is sometimes called the master gland. The pituitary
gland produces hormones that control many of the
other endocrine glands. For example, the pituitary
gland produces the thyroid stimulating hormone
(TSH) that stimulates the thyroid gland to make
and release its hormone, thyroxine. (Thyroxine and
other hormones will be discussed in greater detail
in Chapter 6.)
The pituitary is actually two glands, both of which
are controlled by the hypothalamus. The back, or
posterior lobe of the pituitary is formed from the
cells of the hypothalamus during fetal development.
The hormones released from this lobe of the
pituitary — ADH and oxytocin — are produced in
the hypothalamus. The front, or anterior, lobe of
the pituitary is controlled by stimulating factors —
chemicals produced by the hypothalamus.
As you saw in Chapter 4, the hypothalamus
controls water levels in the body through the
release of ADH. It also monitors glucose levels to
determine satiety or a feeling of fullness when we
eat. The information on water and glucose levels is
relayed from the hypothalamus to the brain, whereit is combined with impulses from other organs
(such as the stomach) to create our awareness of
thirst and hunger.
As mentioned in Chapter 4, the hypothalamus
also monitors body temperature and can trigger
responses that increase or decrease that temperature.
The hypothalamus maintains a biological clock as
well. Generally, this clock is set to our 24-hour day
and establishes when we feel tired and when we
are alert. Have you ever experienced jet lag? If you
have, then you know what it feels like when your
biological clock is upset.
By monitoring and making us aware of all of
these biological functions, the hypothalamus
functions as the centre of homeostasis in the human
body. However, this biological marvel cannot
compare to the complexity of the human cerebrum.
Why is the cerebrum so complex? The cerebrum
is the part of the brain in which all the information
from our senses is sorted and interpreted.
Voluntary muscles that control movement and
speech are stimulated from this part of the brain.
Memories are stored and decisions are made in this
region as well. The cerebrum is what makes
humans different from any other animal on the
planet. It is the centre of human consciousness.
As shown in the photograph on page 136, the
cerebrum is divided into two halves, called the left
and right hemispheres. The surface of each half is
covered with convolutions that increase the
cerebrum’s surface area.
The cerebral cortex, the thin layer that covers
each hemisphere of the brain, contains over
one billion cells. It is this layer that enables us to
experience sensation, voluntary movement, and
all the thought processes we associate with
consciousness. The surface of the cerebral cortex
is made up of grey matter, composed primarily of
cell bodies and dendrites packed closely together
for maximum interaction. The two hemispheres are
joined by the corpus callosum, a layer of white