Download CHAPTER 3 THE STRUCTURE OF THE NERVOUS SYSTEM

CHAPTER 3
THE STRUCTURE OF THE NERVOUS SYSTEM
3.1. THE BASIC STRUCTURE OF THE NERVOUS SYSTEM.
The nervous system of all animals is made up of groups of neurons that receive
information from sensory systems, communicate with one another, and send information to
motor systems. In invertebrates such as slugs, insects, etc., there is no real brain, just collections
of neurons called ganglia. (singular = ganglion). The ganglia are distributed throughout various
parts of the body. In vertebrates (animals with backbones), the "head ganglion" or brain is
extremely highly developed. The nervous system of vertebrates can be divided into:
The Central Nervous System (CNS). The central nervous system includes the brain and
spinal cord).
The Peripheral Nervous System (PNS). The peripheral nervous system includes all of
the nerves located outside the CNS. These are classified as:
The cranial nerves, which originate from cell groups within the brain.
The spinal nerves, which originate from the spinal cord.
3.2. A SHORT HISTORY OF HOW WE THINK ABOUT THE NERVOUS SYSTEM.
Certain facts about sensory systems are obvious, and have presumably been known for
as long as humans have had self-awareness. For example, you know that if you close your eyes,
you stop seeing; you know, therefore, that the eyes are somehow involved in vision. Beyond this,
most of the knowledge we have about how information is processed within the brain has been
gained within the last century or two.
The ancient Greeks (e.g., Hippocrates) knew that the brain is somehow involved in
sensation, perception and intelligence, but had no idea how it functioned. The ancient Romans
and the Europeans through the time of the Renaissance believed that the brain moved the
muscles by pumping fluid or "humors" in and out through the nerves. The philosopher,
Descartes, around 1600, stated formally what was generally believed at that time - that the
"mind" is separate from the brain. According to this theory, the brain operates like a machine
and obeys the laws of nature, but the "mind" exists apart from the body (and brain) and is not
subject to the constraints of physical laws.
By the 1800's, the functioning of the brain was starting to be understood. The first hint
that the brain operates using electrical energy came from experiments by Galvani showing that
electrical stimulation of the nerves (of frogs) causes certain muscles to twitch. Medical scientists
(e.g., Flourens and Broca), were beginning to observe that injury to specific parts of the brain
causes specific deficits. Other scientists during the 19th century discovered that all body tissues
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are made up of cells, that nerves and muscles are capable of generating electrical activity, and
that electrical stimulation of the cortex can elicit movements. It was in the 19th century that
Darwin published his theory of evolution and that Mendel discovered the basic principles of
genetics.
During the 1900's as more sophisticated technology became available, many of the
details of how the nervous system processes information were revealed. We now know that brain
cells communicate with one another by releasing certain chemicals, that these chemicals affect
other nerve cells and cause them to generate electrical activity, and that this electrical activity
depends on the chemical makeup of the responding cells. It was not until the 1950s that the
molecular basis of genetics (DNA) was known. Much of what we now accept as basic
knowledge about the nervous system was unknown until the lifetime of your grandparents,
parents, or even yourselves. We are still far from understanding how the brain functions, and
new discoveries are being made every day.
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Thought question: The issue raised by Descartes is sometimes referred to as the "mind-body
problem", and remains a topic of discussion among some philosophers, theologians and
neuroscientists. Do you agree or disagree with Descartes? What evidence is there to support
your position?
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3.3. THE MAIN FEATURES OF THE CENTRAL NERVOUS SYSTEM.
In a mammal, the most obvious parts of the CNS from an external point of view are the
cerebrum, cerebellum, brainstem, and spinal cord.
Figure 3-1. The position of the brain and spinal cord relative to the head and neck. Left: A view of the left external
surface of the brain, showing the anatomical designations for direction. Rostral refers to the "front end", caudal the
"back end". Dorsal refers to the "top side" in a 4-legged animal, the top of the head or the back side of the body in a
human, ventral to the belly side. Right: The brain as it would appear if it were cut exactly in half and the right half
viewed from the center (inside). The main parts of the brain are labeled.
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If we were to cut a slice through the brain or spinal cord, we would see that it is made up
of:
grey matter (areas containing mostly neuron cell bodies)
white matter (areas containing mostly axons)
Figure 3-2. Three cross-sections through the human spinal cord at the levels indicated showing the grey matter
(hatched area) and the white matter (light areas). Cervical = neck region; thoracic = chest region; Lumbar = mid-to
lower back; sacral = lower back.
Looking at the ventral side of the brain, it is possible to identify a number of structures
including the olfactory bulbs and cranial nerves.
Just by cutting the brain in half, it is possible to see many more structures.
Groups of neuron cell bodies are often segregated into populations that perform a specific
function. A group of neuron cell bodies is designated by a different name depending on whether
it is in the central nervous system or outside it.
A nucleus is a spatially segregated group of neurons with related function, locatedwithin
the central nervous system.
A ganglion is a spatially segregated group of neurons with related function, located
outside the central nervous system.
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Thought question: The brain of each species of animal is adapted to make the best possible use
of information from the environment in which it lives. What specializations might be seen in the
sensory systems of animals that are active during the day versus those that are active at night?
What about animals that use their front limbs strictly for walking versus those that use them to
manipulate objects?
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Figure 3-3. Ventral view of a human brain (left) and a cat brain (right). The brain sizes are not to scale; obviously,
the human brain is really much larger than the cat brain. The cerebral cortex of both cats and humans is convoluted,
or folded. The folds are called sulci (singular = sulcus) or fissures, and the protruding parts are called gyri (singular
= gyrus). On the views of both brains, the cranial nerves have been cut, but the stumps are visible where they exit
the brain (indicated by Roman numerals on the cat brain).
3.3.1. Species differences.
Obviously, brains of different vertebrates (and even different mammals) look very
different. Nevertheless, they have many features in common. For example, all of the same basic
subdivisions can be identified.
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Thought question: The brain of each species of animal is adapted to make the best possible use
of information from the environment in which it lives. What specializations might be seen in the
sensory systems of animals that are active during the day versus those that are active at night?
What about animals that use their front limbs strictly for walking versus those that use them to
manipulate objects?
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3.3.2. The blood supply to the brain.
The brain is a highly vascular structure. Blood vessels are evident over the entire surface
of the brain. The brain is shielded from potentially harmful substances (especially large
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molecules) by a special interface between the capillary walls and the neural tissue. This interface
is referred to as the blood-brain barrier.
3.3.3. The covering of the brain.
The CNS is protected by the skull and vertebral column, and by several fibrous
membranes, collectively called the meninges.
Figure 3-4. Comparison of a rat brain with a human brain. A few differences: The cortex of the rat is smooth,
whereas the cortex of the human is highly convoluted. In the rat, the forebrain makes up a much smaller proportion
of the total brain volume than in the human. The rat's olfactory bulb is huge compared to the human's. Despite these
differences, all of the same structures are present in both brains.
3.3.4. The ventricles.
If we were to cut open the brain, we would see that there are hollow spaces inside, called
ventricles. The cerebrospinal fluid is produced in the ventricles and escapes into the surrounding
spaces, where it is absorbed into the bloodstream.
3.3.5. The spinal cord.
The spinal cord is located within the spinal column, or vertebral column. At every level,
the spinal nerves enter and exit the cord. Each spinal nerve has a dorsal root and a ventral root.
A cross section through the spinal cord reveals that it contains white matter and grey
matter. The grey matter contains the cell bodies of motor neurons (neurons that send input to
muscles) and interneurons (neurons that receive input from and connect to other neurons in the
spinal cord). Motor neurons' cell bodies are located in the ventral horn of the grey matter and
send their axons out via the ventral root of each spinal nerve.
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The cell bodies of sensory neurons are located in the dorsal root ganglion, a group of cell
bodies located just outside the spinal cord. They send their axons in via the dorsal root of each
spinal nerve, where they contact projection neurons (neurons that send information to the next
level, in the brain) or interneurons in the dorsal horn of the grey matter.
3.4. THE FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM.
In the broadest sense, the nervous system consists of sensory systems, motor systems,
and integrative systems.
3.4.1. Sensory systems.
In sensory systems, information about the environment is transduced by a receptor array,
then transmitted via the sensory ganglia and nerves to primary sensory areas of the brain. As we
will see later, considerable processing takes place even at the earliest stages of the sensory
pathway.
3.4.2. Motor systems.
In motor systems, information is transmitted from primary motor areas of the brain via
the motor ganglia and nerves to effector organs, mainly muscles. The body's motor systems can
be divided into two broad classifications:
The somatic motor system is responsible for moving the skeletal muscles.
The autonomic system regulates activity in internal organs and glands.
3.4.3. Integrative systems.
Throughout the spinal cord and brain there are numerous specialized circuits that analyze
and integrate the information that comes in through sensory systems and that determine how the
organism will react to that information.
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Thought Question: It is probably obvious to you that there need to be inputs from sensory
systems to motor systems. Think of reasons why connections in the opposite direction are also
needed, i.e., connections from motor systems to sensory systems.
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Figure 3-5. General schematic of integrative processing in the brain.
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Thought Question: What are some ways in which physiological state could influence sensory
processes or perception?
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Thought Question: Is your perception of the world the same from day to day, or from year to
year? What aspects of perception remain the same? What aspects of perception change? If
perception does change, why do you think this occurs?
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