CHAPTER 44: THE NERVOUS SYSTEM
WHERE DOES IT ALL FIT IN?
Chapter 44 builds on the foundations of Chapter 32 and provides detailed information about animal
form and function Students should be encouraged to recall the principles of eukaryotic cell structure
and evolution associated with the particular features of animal cells. Multicellularity should also be
reviewed. The information in chapter 44 does not stand alone and fits in with the remaining chapters
on animals. Students should know that animals and other organisms are interrelated and originated
from a common ancestor of all living creatures on Earth.
SYNOPSIS
Communication by neurons is extremely fast acting and provides information to specific
locations. The nervous systems of vertebrates are composed of the brain, afferent nerves that
send information to the brain, and efferent nerves that transmit commands from the brain. The
two functional divisions are the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS is composed of the brain and the spinal cord. The PNS is composed of all of the
nerve pathways outside the CNS. Within the PNS, the afferent nerves are sensory pathways and
the efferent nerves are motor pathways. Motor pathways are divided into the voluntary (somatic)
nervous system that innervate the skeletal muscles and the involuntary (autonomic) nervous
system that innervate glands and non-skeletal muscles.
Electrical impulses are transmitted along individual neurons. The highly branched dendrites
carry incoming impulses from many different sources to the cell body. Its surface integrates the
information arriving from many dendrites and acts as the central processing unit. Generally a
neuron has only a single potentially lengthy axon. It carries electrical impulses away from the
cell body to a target cell. The end of the axon contains packets of neurotransmitters that
chemically transmit the nerve impulse to the next neuron or target cell. Specialized Schwann
cells wrap around the axon at specific intervals insulating the axon and forming the myelin
sheath. This insulation is interrupted at locations called nodes of Ranvier. Schwann cells
produce myelin in the PNS while oligodendrocytes produce myelin in the CNS. These cells are 2
of the most important kinds of neurolglia in vertebrates.
A neuron’s resting potential results from the charge differential between the inside and the
outside of the neuron. This gradient results from active outward transport of sodium and inward
active transport of potassium through voltage-gated membrane channels. This results in a cell
that is slightly more negative on the outside than on the inside. Graded potential, caused by the
opening of ligand-gated channels, can depolarize or hyperpolarize the membrane. Depolarizing
graded potentials summate, resulting in the rapid inward diffusion of sodium through sodium
channels [depolarization] wiping out the local electrical potential difference. This is followed by
the outward diffusion of potassium through potassium channels [repolarization]. This rapid
change in the membrane potential is called an action potential. The action potential of a neuron
is an all-or-nothing event, although the actual electrical value differs among various types of
neurons. A membrane is unable to respond to a new stimulus during the refractory period.
Saltatory conduction is an extremely rapid form of transmission that jumps from one node of
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Ranvier to the next.
An axon synapses with other neuron dendrites, with sites on muscles or with secretory cells. The
gap between the axon and the target cell is called the synaptic cleft. Nerve signals cross this gap
chemically. Chemicals released from the presynaptic side cause ion channels on the postsynaptic
side to open initiating depolarization in the target cell. Different chemicals in different junctions
allow a variety of responses not possible through direct electrical conduction. The neuromuscular
junction is a typical synapse in which acetylcholine is the neurotransmitter. This chemical is
rapidly degraded by acetylcholinesterase to allow for subsequent transmission. The cell body
integrates signals from inhibitory and excitatory synapses. The signals either cancel or reinforce
one another and affect the resulting signal sent out along the cell body’s axon. Many other
neurotransmitters occur throughout the nervous system. These include glutamate, glycine,
GABA, epinephrine, dopamine, norepinephrine, serotonin, neuropeptides, substance P, and even
a gas, nitric oxide. Addictive drugs alter a post-synaptic neuron’s response to neurotransmitters.
The vertebrate brain is divided into three regions: the hindbrain, the midbrain, and the forebrain.
The diencephalon of the forebrain integrates sensory information, while the telencephalon is
devoted to associative activity. Mammalian brains are particularly large with respect to overall
body mass as compared to those of fish and reptiles. In humans, there is great enlargement of the
cerebrum which functions in correlation, association, and learning. The human brain is divided
into left and right hemispheres connected by the corpus callosum. Each hemisphere is further
divided into frontal, parietal, temporal, and occipital lobes. The outer layer of the cerebrum is the
cerebral cortex, the site of most neural activity. Sensory integration is directed mostly by the
thalamus. The hypothalamus controls visceral responses, like temperature, respiration, and heartbeat
and secretions of the pituitary gland. It is connected to the cerebral cortex via the limbic system, the
center for emotional responses.
The reticular activating system monitors all signals to the brain and sorts out important signals. The
reticular system is also involved with sleep, which is studied via electroencephalograms. Higher
cerebral functions are associated with the motor, sensory, and associative areas of the cerebral cortex.
These regions direct sensory and motor input, higher mental activities, language, and memory.
Voluntary bodily functions are under direct conscious control of the associative cortex. Involuntary
homeostatic functions are not subject to conscious control. The spinal cord relays messages to and
from the brain and functions in relfexes. Most neuromuscular control is regulated by feedback loops,
the most simple being the muscle stretch receptors. Monosynaptic reflex arcs also provide feedback
without involving the CNS.
Neurovisceral control is directed by either the parasympathetic or the sympathetic divisions of the
autonomic nervous system. The neurotransmitter at the synapse between the CNS axon and the
autonomic dendrite is acetylcholine in both the sympathetic and the parasympathetic systems. The
second neurotransmitter in the parasympathetic system, between the autonomic axon and the target
organ, is again acetylcholine, while the sympathetic system uses epinephrine or norepinephrine. The
actions of the two neurotransmitters on a target organ are completely opposite. In general, the
parasympathetic system stimulates activity of normal body functions while the sympathetic system
prepares the body for greater, emergency activity.
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LEARNING OUTCOMES
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Differentiate among afferent, efferent, and interneurons; CNS and PNS; somatic and autonomic
motor neurons.
Describe the structure of a typical neuron and indicate the function(s) of each of its parts.
Explain what is meant by resting membrane potential and depolarization and understand how
they are associated with transmission of a nerve impulse.
Know the two kinds of gated membrane channels and how they are associated with ion
movement through a cell membrane.
Understand how an action potential is transmitted along a nerve and why it spreads only in one
direction.
Understand the importance of a chemical transmission of the nervous impulse across synapses.
Identify the chemical(s) and describe the events associated with transmission of a nerve impulse
across a neuromuscular junction.
Know the primary divisions of the human brain and the function of each.
Understand the anatomical and associative organization of the human cerebral cortex.
Understand how the brain controls higher functions like learning and memory.
Describe the structure of the spinal cord and the peripheral nervous system.
Understand how a monosynaptic reflex arc is organized and functions.
Describe the two systems associated with the antagonistic control of the autonomic nervous
system and identify the neurotransmitter(s) associated with each.
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 44 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students do not understand the evolution of endosymbionts in animal cells
Students are unsure that many of the lower animals are classified as animals
Students think that all animals evolved at about the same time
Students believe that most animals do not feel pain
Students believe that animals can sense emotions and danger
Students believe that only humans have a well-developed nervous system
Students believe that animals are purely instinctual
Students believe that most animals are vertebrates
Students do not equate humans with being animals
Students believe that all animals have identical organ system structures
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
Discuss the similarities and differences between a typical electrical system and the vertebrate
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nervous system. Which of the events typical with nervous conduction are lacking in electrical
conduction?
Short-term memory is analogous to files stored in a computer’s RAM (random access memory),
a volatile memory that requires a constant input of electricity. When the computer is shut off,
these files are lost. Long-term memory is analogous to ROM (read only memory) present in
special microchips or to files that have been written to or saved on a hard disk, floppy disk, zip
disk, or CD; they still exist when the power is off. The few absolutely necessary bits of
information (like time and date) are maintained in most computers by a tiny, internal battery on
the mother board.
Discuss the affects of the following on the function of synapses: cocaine, anti-depressants
known as SSRIs [selective seratonin re-uptake inhibitors], strychnine, and tetanus toxin. Explain
how death occurs due to strychnine and tetanus toxin and explain the addiction of cocaine.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 44.
Application
Analysis
Synthesis
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Have students explain the effects of diseases that cause a loss of
myelination.
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Have students explain the effects of blocking the movement of sodium
ions on nervous system function.
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Ask students to explain the peripheral nervous system components needed
to walk up the steps.
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Have students describe the effects of blocking acetylcholine on the
autonomic nervous system.
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Have students assess the effects on the nervous system of too much
calcium in the diet.
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Ask students to explain why complete damage to the brain still leaves the
body with many intact functions.
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Ask students explain why certain people are able to control autonomic
nervous system responses.
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Have students find a medical application for a chemical that blocks
sodium transport across the cell membrane.
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Evaluation
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Ask the students design an experiment to investigate the role of
oligodendrites on central nervous system function.
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Ask students evaluate the benefits and risks of using mood altering drugs
that inhibit the uptake of certain brain neurotransmitters.
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Ask students to evaluate the accuracy science fiction books that envision a
future in which human thoughts are controlled by drugs.
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Ask students to evaluate the effectiveness and safety of wasp control
insecticides that block the action of acetylcholine.
VISUAL RESOURCES
Pass a message around the room using three different methods. Have students verbally pass
instructions from one to another across a row to represent communication via gap junctions.
Have another set of students walk a written message from one end of a row to another imitating
hormonal communication. The third method requires buying or borrowing a pair of walkie
talkies to simulate nervous communication. The speed at which the message passes should be
obvious to your students. They may additionally find that the slowest is also the least accurate as
the message may get garbled especially if it is complicated.
A copper wire wrapped with electrical tape resembles a myelinated nerve both in structure and
function. A very simple mechanical associative activity is illustrated by a light that is activated
only when it is dark outside (photosensitive) or when there is someone in the vicinity (heat,
sound, or motion sensitive).
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Virtual Neurophysiology
Introduction
This demonstration uses a virtual electrophysiology to teach nervous system function in
animal models.
Materials
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Biological Clocks Biointeractive website at
http://www.hhmi.org/biointeractive/vlabs/neurophysiology/index2.html
Procedure & Inquiry
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1. Explain to the class that they are going to observe a virtual neurophysiology
demonstration using a model experimental system.
2. Load up the website and click on “Overview of equipment used in the lab”.
3. Then follow the sequence until completed with the virtual demonstration.
4. Then ask the class to the class to review what they saw and explain why the leech is used
as an animal neurophysiology model.
USEFUL INTERNET RESOURCES
1. Images of animals are available from the University of California at Berkeley CalPhotos:
Animal website. These images are valuable teaching resources for lecture and laboratory
sessions related to animal anatomy and physiology. The site is available at
http://calphotos.berkeley.edu/fauna/.
2. Faculty and students will find value in websites that simplify animal anatomy and
physiology concepts. The information can be used for projects that educate children and
civic groups about animals. Biology-4-kids is a model website for animal education. The
website can be found at http://www.biology4kids.com/files/systems_nervous.html.
3. Case studies are an effective tool for stimulating interest in a lesson on fungi. The
University of Buffalo has a case study called “It Takes A Lot of Nerve” which has
students investigating the complexity of the nervous system in an entertaining way. The
case study can be found at
http://www.sciencecases.org/nervous_system/nervous_system.pdf.
4. Case studies are an effective tool for stimulating interest in a lesson on animals. The
University of Buffalo has a case study called “Bad Fish” which has students investigating
the physiology of the action potential. The case study can be found at
http://www.sciencecases.org/badfish/badfish_genbio.pdf.
LABORATORY IDEAS
A. Brine Shrimp as a Nervous System Model
This activity has students design an experiment in which brine shrimp are used as a
model of nervous system function.
a. Explain to students how animals and animal cells are used in medicine and research as
models for human studies.
b. Provide students with the following materials
a. Large brine shrimp at room temperature
b. Microscope
c. Microscope slides
d. Plastic pipettes
e. Test reagents :
i. 3% W/V sodium chloride solution
ii. 3% W/V potassium chloride solution
iii. 3% W/V calcium chloride solution
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iv. 1-2 dilution organophosphate insecticide
v. Black coffee
c. Ask students to design an experiment to test the effects of the different solutions on the
nervous system activity of the brine shrimp. Review the experiment before they progress
with the activity.
d. Have them record their findings and look up the potential effects of the treatments on the
nervous system.
e. Students should explain if their findings are consistent with the expected results.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students do a lesson do a hands-on program on the animal behavior.
2. Have students tutor high school students studying animal anatomy and physiology.
3. Have students volunteer on environmental restoration projects with a local conservation
group.
4. Have students volunteer at the educational center of a zoo or marine park.
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