Download Ch. 3 S. 1

Ch. 3 S. 1
The Nervous System
Obj: Explain how messages are
transmitted by neurons, and describe
the functions of the spinal cord and the
peripheral nervous system.
The Nervous System
The nervous system regulates our internal
functions. It is also involved in how we
react to the external world. Even learning
and memory are made possible by the
nervous system. When we learn a new
behavior or acquire new information, the
nervous system registers that experience
and changes to accommodate its storage.
Two main parts: the central nervous system
and the peripheral nervous system.
• Central nervous system – consists of the
brain and the spinal cord.
• Peripheral nervous system – made up of
nerve cells that send messages between
the central nervous system and all the
parts of the body.
Neurons
Nerve cells, called neurons, run through our
entire bodies and communicate with each
other. Neurons send and receive
messages from other structures in the
body, such as muscles and glands. These
messages can affect events ranging from
the sensation of a pinprick to the first steps
of a child, from the writing of a poem to the
memory of a past event. Each of us has
more than 100 billion neurons, most of
which are found in the brain.
Components of a Neuron
Neurons are somewhat like trees in
structure. Parts of neurons resemble the
branches, trunk, and roots of a tree. And,
as in forests, many nerve cells lie
alongside one another like a thicket of
trees. Unlike tress, however, neurons can
also lie end to end. Their “roots” are
intertwined with the “branches” of neurons
that lie below.
Every neuron consists of a cell body,
dendrites, and an axon. The cell body
produces energy that fuels the activity of
the cell. Branching out from the cell body
are think fibers called dendrites. The
dendrites receive information from other
neurons and pass the message through
the cell body.
While the dendrites carry information to the
cell body, the axon carries messages
away. A neuron has many dendrites but
usually only one axon.
Axons vary greatly in length. Some are just
a tiny fraction of an inch, while others
stretch to several feet. Because of the
length of their axons, some neurons in
your legs are several feet long.
Many axons are covered with myelin, a
white fatty substance that insulates and
protects the axon. This myelin sheath, or
casing, also helps to speed up the
transmission of the message. At the end of
the axon, smaller fibers branch out. These
fibers are called axon terminals.
The Communication Process
Messages are sent from the axon terminals of one
neuron to the dendrites of other neurons. In
order for a message to be sent from one neuron
to another neuron, it must cross the synapse.
The synapse is a junction between the axon
terminals of one neuron and the dendrites of
another neuron. Messages travel in only one
direction. Thus, messages are received by the
dendrites and travel through the cell body and
the axon to the axon terminals. From there, they
cross synapses to the dendrites of other
neurons. New synapses can develop between
neurons that were not previously connected, as
when we learn something new.
The information that is sent and the place to
which it goes depend on a number of
factors. These factors include the locations
of the neuron in the body and the events
that produced the message. Sensory
neurons are nerve cells that carry
information received by the senses to the
central nervous system. Motor neurons, on
the other hand, are nerve cells that carry
information from the central nervous
system to the muscles and the glands and
influence their functioning.
Occasionally, something happens to disrupt
the message-sending process. For
example, a hard blow to the head from a
car accident or a sports injury can cause a
concussion-an injury in which the soft
tissue of the brain hits against the skull.
Sometimes, the person is affected for only
a few seconds. Other times, the person
may experience effects for a much longer
time.
Neurotransmitters: Chemical
Messengers
Neurons send messages across synapses
through the release of neurotransmitters.
Neurotransmitters are chemicals that are
stored in sacs in the axon terminals. A
neuron fires, or sends its message, by
releasing neurotransmitters-much like
droplets of water shooting out of a spray
bottle. Neurons can fire hundreds of times
every second.
There are several types of neurotransmitters.
Each has its own structure and fits into a
receptor site on the next neuron, similar to
the way in which a key fits into a lock. The
message is converted into an electrical
impulse that travels the length of the
neuron. The message is then transmitted to
the next neuron by neurotransmitters. The
process continues until the message arrives
at its destination. This whole process takes
only a fraction of a second.
Neurotransmitters are involved in everything
people do. Whenever a person waves a
hand, yawns, or thinks about a friend,
neurotransmitters are involved. Some
diseases and psychological disorders may
also be caused by the presence of too much
or too little of various neurotransmitters.
Researchers have identified dozens of
neurotransmitters and their functions.
Examples: noradrenaline-prepares the body
for action; seratonin-emotional arousal and
sleep; dopamine-motor behavior, but a
deficiency plays a role in Parkinson’s and an
excess may contribute to schizophrenia.
The Central Nervous System
Consists of the neurons of the spinal cord
and the brain. The spinal cord extends
from the brain down the back. It is a
column of nerves about as thick as a
thumb, and it is protected by the bones of
the spine. It transmits messages between
the brain and the muscles and the glands
throughout the body.
The spinal cord is also involved in spinal
reflexes. A spinal reflex is a simple,
automatic response to something. For
example, if a person touches a hot stove,
a message goes immediately from his or
her hand to the spinal cord. A message to
remove the hand is then sent back to
motor neurons in the hand. The removal of
the hand is a spinal reflex. (The person
may also register pain in his or her brain.
But the pain is not what causes the reflex.
In fact, the pain may not even be felt until
after the hand has been removed.)
The Peripheral Nervous System
The peripheral nervous system lies outside
the central nervous system and is
responsible for transmitting messages
between the central nervous system and
all parts of the body. The two main
divisions of the peripheral nervous system
are the somatic nervous system and the
autonomic nervous system.
The Somatic Nervous System
The somatic nervous system transmits sensory
messages to the central nervous system. It is
activated by touch, pain, changes in
temperature, and changes in body position. The
somatic nervous system enables us to
experience the sensations of hot and cold and to
feel pain and pressure. For example, we can
feel the softness a cat’s fur. The somatic system
also alerts us that parts of the body have moved
or changed positions. It sends messages to the
muscles and the glands and helps us maintain
posture and balance.
The Autonomic Nervous System
The word autonomic means “occurring
involuntarily,” or automatically. The
autonomic nervous system regulates
the body’s vital functions, such as
heartbeat, breathing, digestion, and blood
pressure. We generally do not have to
think about these activities-they occur
automatically and are essential for keeping
us alive.
Psychologists are interested in the
autonomic system because of its
involvement in the experience of emotion.
The response of the autonomic nervous
system is particularly important when a
person experiences something stressful in
the environment.
This system has two divisions: the
sympathetic and the parasympathetic
nervous system. These systems generally
have opposing functions.
• The sympathetic system is activated when
a person is going into action. This is the
“fight or flight” response. It does this by
suppressing digestion, increasing the
heart and respiration rates, and elevating
the blood pressure.
• The parasympathetic system restores the
body’s reserves of energy after an action
has occurred. Heart rate and blood
pressure are normalized, breathing is
slowed, and digestion returns to normal.