Download construction of a model demonstrating neural pathways and reflex arcs

INNOVATIONS
A
CONSTRUCTION
N
OF A MODEL
NEURAL
PATHWAYS
D
I
D
E
A
S
DEMONSTRATING
AND
REFLEX
ARCS
Vivien Chan, Jeanna M. Pisegna, Rebecca L. Rosian, and Stephen E. DiCarlo
Department
of Physiology,
Northeastern
Ohio
Universities,
College
of Medicine,
Rootstown,
Ohio
44272
E
A&!.
PHYSIOL.
Key words:
271
(ADV
PHYSIOL.
EDUC.
16): SI4-S42,
patellar tendon reflex; education;
The standards of science education are undergoing
major reform. Currently, the new primary goal of
science educators is “scientific literacy” of all graduating high school students. Scientific literacy has become an imperative. Because of rapid technological
advances, a functional knowledge of mathematics and
science is a requirement for tomorrow’s workforce.
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laboratory
exercise
construction and manipulation of models. The use of
models allows broader scientific inquiry, enhances
understanding, and encourages future investigations
into the world of science. In response to these
concerns, our goal was to develop a physiologically
sound, inexpensive model that demonstrates neural
pathways and reflex arcs while also introducing basic
concepts of neurobiology.
To attain scientific literacy, the traditional methods of
lecture and rote memorization are inadequate. To
grasp scientific concepts, students must engage in
active learning. Passivity does not satisfy curiosity, nor
does it enhance understanding. Thus new and innovative teaching methods that encourage active learning
must be developed. One such method is through the
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We have found that high school students have few
appropriate physiological resources available to them.
Most physiology texts are written for the college level,
and laboratory experiments require expensive equipment. This is especially true in the subject area of
neurobiology. Many exercises in the investigation of
o 1996
THE AMERICAN
IN PHYSIOLOGY
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mployment opportunities
in the future will require higher skills and an understanding of mathematics and science. As a result of the growing number of careers that
require solid science and mathematics training, the methods of science education
are undergoing major reform. To adequately equip students for technologically
advanced
positions, new teaching methods must be developed that prepare tomorrow’s workforce
for the challenges of the 2 1st century. One such method is the use of models. By actively
building and manipulating
concrete models that represent scientific concepts, students
are involved in the most basic level of Piaget’s learning scheme: the sensorimotor
stage. Models are useful in reaching all students at the foundational levels of learning,
and further learning experiences
are rapidly moved through higher learning levels.
This success ensures greater comprehension
and understanding
compared with the
traditional methods of rote memorization.
We developed an exercise for the construction of an inexpensive, easy-to-build model demonstrating
neural pathways and reflex
arcs. Our exercise also includes many supplemental
teaching tools. The exercise is
designed to fulfill the need of sound physiological
teaching materials for high school
students.
INNOVATIONS
A
neurobiology are too detailed and too expensive for
the average high school science program. For example, although there are animated computer programs detailing the basics of neuroscience, these
programs are overly complex and too costly to be
useful at the high school level (3). In contrast, our
model was constructed with economical materials
readily available through local electronics or hardware
stores.l
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process. Questions are designed in a set, so that the
first few questions in the set review comprehension of
the previous paragraphs. The last question in a set
provokes thought on subsequent passages.At the end
of the laboratory exercise are questions for discussion
and integration of the entire learning experience.
BACKGROUND
TO NEUROBIOLOGY
Our rationale for using a model was because “evidence suggests that, with the use of activity-based
science programs, teachers can expect substantially
improved performances in science processes” (1).
Active participation with models alsoreaches all types
of learners in the visual, auditory, and kinesthetic and
tactile (VAK) scheme of learners. The V-type (visual)
learners are targeted by the actual presence of the
model, the supplied text, and instructions. A-type, or
auditory, learners are reached through discussion
during the laboratory exercise and teacher presentation. K-type learners are satisfied through the building
and manipulation of the model.
Questions are inserted within the text to help focus
thinking and test comprehension of the material.
Questions marked with arrows are comprehension
questions to review previous passages. Questions
marked with asterisks provoke thinking on subsequent passages.
Introduction
Models also satisfy pedagogical principles for “handson/minds-on” learning. This approach is supported by
the theory of constructivism. Advocates of constructivism point out that the importance of “hands-on”
science is that “students manipulate things physically
...for a purpose and engage in discussion about it” (4).
Structurally, the nervous system is divided into the
central nervous system (CNS) and the peripheral
nervous system (PNS). The CNS consists of the
brain and spinal cord. The PNS contains the spinal and
cranial nerves leading into and out of the CNS. There
are 12 cranial nerves. All other nerves in the body are
spinal nerves. Although the CNS and the PNS are
separated into two “systems,” it is important to realize
that they are connected to each other.
Our exercise not only provides an easy-to-build model
demonstrating neural pathways and reflex arcs, it also
comes with supplemental teaching tools. In addition
to detailed instructions concerning the construction
of the model, the supportive text contains discussion
questions, photographs of the model under construction, organizational concept maps, and instructive
background information on the physiology related to
the nervous system.
The nervous system is constantly bombarded by
stimuli, even during sleep. For example, as you read
this, your nervous system is receiving different types
of information gathered by your eyes, such as color,
light, texture of the paper, and the words on the
paper. This is known as sensory reception.
Within the text are questions for the students to
answer to help focus thinking and test comprehension of the material, thus facilitating the learning
1 Cost of the models
was based on purchasing
all the
supplies
needed. Supplies
were obtained
at Radio Shack. The
cost per one model came to an estimated
$25.00.
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A concept map that organizes the basic concepts of
BACKGROUND
TO NEUROBIOLOGY
text
material iS presented in Fig. 1. This map presents the nervous
system, with the components branching off into
smaller and smaller subunits. The text describing this
map is presented in detail below.
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- forebrain
- midbrain
- hindbrain
spinal
nerves
Peripheral
Nervous
System
Central Nervous
System
cranial
nerves
Electrical Action Potential Within
Chemical Neurotransmitters Between
- point of contact
between two
neurons
Components of a
- axon
- dendrites
- axon hillock
Interneurons or
Association
Neurons
Sensory (aff erent)
Neurons
Concept
map that
organizes
basic
concepts
of text
FIG. 1.
material
found
A specialized cell of the nervous system, the neuron,
conducts information that it receives. A neuron that
conducts sensory information is called an afferent
(sensory) neuron. Many billions of neurons are involved in processing sensory information.
Functionally, there are three types of neurons: sensory
neurons, motor neurons, and association neurons.
Sensory receptors receive information from outside
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T O NEUROBIOLOGY.
Motor neurons are output neurons. They conduct
information out to skeletal muscles, smooth muscles,
cardiac (heart) muscle, visceral (body) organs, and
glands. Motor neurons are also known as efferent
neurons. They make something happen. For example, efferent neurons can cause contraction in
muscles, changes in heart rate, changes in blood
pressure, sweating, and many other physiological
Neurons
16
BACKGROUND
the body and from internal organs. They pass their
information to sensory neurons that conduct this
information into the CNS. Thus sensory neurons
are input neurons. Sensory neurons can also be
called afferent neurons. An example of a sensory
neuron is shown in Fig. 2.
Different receptors senselight touch, deep pressure,
temperature, and many other tactile sensations. Finally, special olfactory cells are sensory receptors of
the nose.
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- ascending sensory tracts
- descending motor tracts
- horizontal direction of movem ent
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cell body of sensory
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neuron
finger
\
sensory
receptor
\
FIG. 2.
Example
of a sensory
neuron
with its structures
labeled.
Tack is providing
stimulus.
This sensory
neuron
is receiving
input from a sensory
receptor
in the
finger.
Sensory
neuron
is unique
in that it only
has an axon
by which
it
transmits
information.
Information
carried
by this neuron
continues
in the
body by way of a tract to reach the brain.
functions. Efferent neurons cause an appropriate response to the sensory information received. An example of a motor neuron is shown in Fig. 3.
The site of transmission between two neurons is
called a synapse. A synapse is an anatomic structure
that involves two neurons and the space between
them. The synaptic space is very small, and it can be
seen best with an electron microscope. A synapse is
different
from synaptic transmission. Synaptic transmission is an event that occurs at the synapse; the
synapse itself is a structure. A schematic of a synapse
is shown in Fig. 5.
Association neurons are also called interneurons.
Interneurons are found between afferent (incoming
sensory information) and efferent (outgoing motor
information) neurons. Interneurons serve many functions and can have many connections. Interneurons
are involved in information processing and are found
only in the CNS. An example of an interneuron is
shown in Fig. 4.
dend rites of
mot0 r neuron
1) -+Name the three types of neurons. Are they
afferent, efferent, or neither?
cell body of motor neuron
axon hillock of motor neuron
Nodes of Ranvier
axon of motor
\
target muscle
motor neuron
Motor
shown
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with
with its target
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structural
muscle.
1 - ADVANCES
FIG. 3.
components
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labeled.
Motor
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pain stimulus
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synapse
syqapse
neuron
neuron
FIG. 4.
Example
of an association
neuron
or interneuron.
ron is placed between
a sensory
(afferent)
neuron
neuron.
The cerebral cortex in the forebrain is the largest
part of the human brain. “Knowing” or a “conscious
awareness” of information is associated with the
cerebral cortex. Sensory, motor, and association areas
of the brain are found in the cerebral cortex. Association areas deal with higher brain functions and are
often called ‘ ‘silent’ ’ areas. They are involved in
memory, reasoning, concentrating, problem solving,
and many other complex functions.
2) *What are the components of the CNS?
Central Nervous System
Brain. The brain is made of neurons grouped together
according to their function. For example, neurons
dealing with vision are grouped together (sensory
areas), and neurons moving specific muscle groups
are placed together (motor areas). Although parts of
the brain are sectioned off by function, areas of the
brain are still interconnected so that the brain works
as a whole unit.
The cerebral cortex can be compared with the bossof
a company who must be informed about everything
going on. The boss makes most of the important
decisions in the company, just as the cerebral cortex
does in the body.
There are three main divisions of the brain: the
forebrain (front brain), the midbrain (middle brain),
and the hindbrain. These divisions are useful for
locating specific structures of the brain (Table 1). In
addition, Fig. 6, A and R, shows labeled structures of
the brain that correspond to Table 1.
neuron
Note that the interneuand a motor
(efferent)
The medulla in the hindbrain is anatomically the
lowest part of the brain. It controls the subconscious
activities of the body, which include heart rate,
respiration, sleeping and waking, digestive functions,
and electrolyte balance. Many of these functions are
also controlled by a region of the forebrain called the
hypothalamus.
The hypothalamus is involved in
body temperature control, water balance, and hormonal control, along with other functions.
1
TABLE
synapse
Structures
Hindbrain
neuron
between
5.
2 neurons
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H 1. Medulla
H2. Cerebellum
H3. Pons
1
of the brain
Forebrain
Midbrain
M 1. Cerebral
aqueduct
FIG.
Synapse
See Fig. 6 for schematic
is shown.
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F 1. Thalamus
F2. Hypothalamus
F3. Cerebral cortex
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F3. Cerebral Cortex
Thalamus
,H2.
Cerebellum
HI.
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f
Medulla
F3. Cerebral Cortex
/
1. Thalamus
F2. H
H3.
Ml.
erebellum
Cerebral
HI.
MGdulla
FIG. 6.
A: schematic
of labeled
brain structures
as if you were looking
from the
outside.
B: illustration
of a hemisected
brain (a brain that has been cut in
half) with labeled
structures.
Both illustrations
are labeled
in correspondence to the structures
listed in Table 1.
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Another important structure is the thalamus.
Although much research has been conducted on the
thalamus, most of its functions remain unknown.
However, many theories about thalamic function have
been proposed. The thalamus is a small, footballshaped structure that functions asthe “customs agent”
of all information going to the cerebral cortex. The
thalamus integrates and directs incoming information
along its way to the appropriate area of the cerebral
cortex. Also, all pathways with information exiting
the cerebral cortex must inform the thalamus about
what they are doing. The thalamus can therefore be
considered as a customs agent for information entering and leaving the cerebral cortex.
A
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A
7) *How does the spinal cord bring information to the
brain?
Spz’nal cord. The spinal cord is a long, cylindrical part
Information can travel through the spinal cord in two
different directions: horizontally and vertically. Nerves
from the PNS enter and exit at different levels of the
spinal cord. Information within the spinal cord (and
therefore also inside the CNS) travels vertically upward to the brain and vertically downward from the
brain to eventually reach different parts of the body.
Figure 7 is a representation of a section of the spinal
cord in a horizontal slice that illustrates the dorsal
(sensory) areasand ventral (motor) areas.
3) -The cerebellum and cerebral cortex are important structures of the brain. List a major function for
each.
4) -Name the three different types of areas in the
cerebral cortex.
5) +How are the cerebral cortex and the boss of a
company similar?
When information travels vertically, it is specially
organized into regions of the spinal cord known as
tracts.
Each tract carries its own specific type of
information. For example, one ascending tract carries
information about pain, (external) temperature, and
6) -What
is the most important function of the
thalamus?
dorsal (back) side of spinal cord section
dorsal half of spinal cord
that contains sensory information
ventral half of spinal cord
that contains motor information
ventral (front) side of spinal cord section
Section of a spinal
the dorsal (sensory)
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FIG. 7.
cord as it would
appear
in a horizontal
regions
and ventral
(motor)
regions
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slice. This action
of spinal cord.
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of the CNS extending downward from the hindbrain.
The spinal cord is protected by the vertebrae (backbone) asit passesdown the vertebral canal. The spinal
cord terminates between the first two lumbar vertebrae in most adults. Neurons in the spinal cord are also
functionally arranged so that areas dealing with the
same types of information are grouped together.
Incoming sensory information occupies one area, the
dorsal (back) portion of the cord, and neurons dealing
with motor output occupy another area, the ventral
(front) portion of the cord. Recall that neurons in the
brain are arranged in a similar way according to
function.
The cerebellum
is primarily involved in the coordination of motor activity. Coordination involves a complex mixture of balance, spatial orientation, and
motion. Recent research has shown that the cerebellum may also be involved with certain types of
learning and memory.
spinal ca al
\
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dorsal (back) side
sensory
neuron
tract containin
g
l\;;g:Ttion
incoming
f
information
neuron
ventral
(front) side
FIG. 8.
Schematic
representation
of different
directions
information
travels
in
the spinal cord. On the left half of the spinal cord, the horizontal
direction
of information
travel
in the spinal cord is shown.
Information
comes in
from
the sensory
neuron
to the dorsal
(back)
side of the spinal
cord.
Information
is passed by an interneuron
to the motor
neuron.
Motor
information
leaves the spinal
cord from the ventral
(front)
half of the
cord. On the right half of the spinal
cord,
sensory
information
in an
upgoing
tract is found
in the dorsal
half of the spinal
cord. This tract
continues
upward
through
the spinal cord to the thalamus
and then the
cerebral
cortex.
In the ventral
half of the spinal cord, motor
information
in a downgoing
tract is found. This tract originates
in the cerebral
cortex
and descends
to its target.
deep touch. Other tracts carry information about limb
position. Descending tracts carry motor information
destined for muscles, visceral organs, or glands in the
periphery. There are many different tracts in the
spinal cord.
spinal cord.2 A pictorial representation of the different
directions of information travel in the spinal cord is
found in Fig. 8.
8) -+Describe the two directions that information can
travel within the spinal cord.
The different directions of information travel within
the spinal cord are like people riding an escalator of a
busy skyscraper. People (information) can get on and
off at different floors (levels of the spinal cord). They
can also ascend and descend in an escalator. To speed
up efficiency, different professions ride their own set
of escalators. Likewise, different types of information
have their own tracts. Different types of sensory
information have their own upgoing tracts (up escalators) in the dorsal (back) half of the spinal cord, and
motor information has its own downgoing tracts
(down escalators) in the ventral (front) part of the
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9) *How does information travel in neurons?
2 Some students
may feel that an elevator
would be a more
practical
approach
to efficiency
and to this example.
However,
an elevator
can travel
both upward
and downward.
When
information
travels
in a tract, it travels
in only one direction:
upward
or downward
like an escalator,
not in both directions
like an elevator.
Information
cannot
“ride” the same tract to
ascend and descend.
Sensory
information
travels
upward
in
tracts located in the dorsal (back) half of the spinal cord, and
motor information
travels
downward
in tracts located in the
ventral
(front)
half of the spinal cord. Therefore,
the example
of
an upgoing
or downgoing
escalator
is preferred.
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~goingfl~~-LM~~~~or
interneuron
or associatior -I
motor
neuron
information
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is usually only one axon leading away from a neuronal
cell body. An axon branches when it reaches its target
(another neuron, muscle cell, organ, or gland). An
axon usually terminates on the next neuron’s dendrite
or cell body. The nerve impulse is then transmitted
across the tiny synaptic space. A schematic of synaptic
transmission is shown in Fig. 10.
Notice that in Fig. 2, there are no labeled dendrites.
This is because the type of sensory neuron involved is
unique. It only uses an axon to carry its information
toward the CNS, and it has no dendrites. Exceptions
like this to general classifications are commonplace in
the nervous system and make the nervous system one
of the most complex systems of the body.
Parts of an axon left uncovered by myelin are called
nodes of Ranvier. When information is carried by a
myelinated axon, the information
will jump from
node to node. This makes the transmission of information faster than if the information had to go straight
through the axon. Again, some axons are myelinated,
and some are not. However,
dendrites are never
myelinated (because they are extensions of the cell
body).
10) +Which
processes
neuronal cell body?
bring information
11) -Which
processes
the neuronal cell body?
take information
12) *In what
neuron?
Usually, there are multiple dendrites bringing information toward the neuron’s cell body. In contrast, there
forms
is information
axon
neuronal cell body
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with
axon
labeled
hillock,
1 - ADVANCES
the
away from
carried
by the
Information is carried along axons and dendrites in an
electrical form. The movement of differently charged
dendrites
neuron
cell body,
toward
FIG. 9.
structures,
and nodes
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such as the axon,
of Ranvier.
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The cell involved in carrying information
around
the body is the neuron. An illustration of a neuron is
found in Fig. 9. The neuron has two types of projections or processes from its cell body: axons and
dendrites.
Axon projections can be very long. Dendrites are shorter processes that bring information
toward the neuronal cell body. Dendrites are really
extensions of the cell body. Axons carry information
away from the cell body. Axons may be covered by
an insulating sheath of myelin that wraps around
them like a jelly roll. If an axon has myelin around it, it
is myelinated. Information moves faster along a myelinated axon than along an unmyelinated one.
Myelinated
neuronal
S
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neuron
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1
/
synaptic vesicles
containing
chemical
neurotransmitters
synapse
neurotransmitters
released
v-
neuron
2
FIG. 10.
Schematic
showing
synaptic
transmission.
Axon from neuron
1 is shown
releasing
chemical
neurotransmitters
into the synaptic
space between
the
2 neurons. A dendrite
of neuron
2 is receiving
the chemical
neurotransmitters as they travel across the synaptic
space.
ions (positively charged substances and negatively
charged substances) causesan event called an action
potential. An action potential is the electrical current
form of information in the neuron.
There are many different neurotransmitters within the
nervous system. Some turn on the next neuron in line
and are called excitatory neurotransmitters. Excitatory neurotransmitters ensure that the action potential
is carried by the next neuron in line. Some neurotransmitters turn off the next neuron in line and are called
inhibitory neurotransmitters. These inhibitory neurotransmitters prevent the next neuron in line from
carrying the action potential.
The generation of an action potential occurs in a
special location close to the cell body of the neuron,
the axon hillock (Fig. 9). This is a probability event.
If enough charged ions reach the axon hillock to cause
an action potential, the action potential will occur. If
there are not enough ions to trigger an action potential, the action potential will not occur. This is
described as an all-or-none phenomenon. Information is either carried in its entirety through a neuron,
or it is not carried at all. If information is carried, it is
carried with its full strength and content. There is no
weakening or strengthening of a messagesent in an
action potential.
One neuron normally releasesonly one type of neurotransmitter, although it has recently been shown that
some neurons can release two or more types of
neurotransmitters. There are many combinations of
different neurotransmitter sequences in the body.
These different combinations make the body’s reactions to different stimuli unique.
13) +Describe the all-or-none phenomenon of an
action potential. Is it chemical or electrical?
The action potential within a neuron is an electrical
event. When a neuron passes its information
to
another neuron, a chemical event known as
synaptic transmission occurs. Synaptic transmission involves the release of proteins called neurotransmitters into the space between two neurons
(Fig. 10). Proteins are chemical substances;therefore,
the method of transmission becomes chemical, not
electrical.
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14) -+What are the two different classifications of
neurotransmitters?
15) -+Why is the axon hillock a special structure
involved in transmission of an action potential?
16) *How does the neuron handle both the chemical
and electrical forms of information?
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)
being
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Summary
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5) Just as the boss makes most of the important
decisions in a company and is kept informed about the
company’s activities, the cerebral cortex makes similar decisions and is aware of information concerning
the entire body.
6) The thalamus acts as a “customs agent” to the
“country” of the cerebral cortex by integrating and
directing all incoming information to the cerebral
cortex. The thalamus is also informed about all information exiting the cerebral cortex.
7) Information travels to the brain in special groups of
neurons that deal with the sametypes of information
called tracts. Information can reach the brain by way
of the spinal cord. The spinal cord is the site where
spinal nerves enter and exit to “deposit” their information into specialized tracts going to the brain.
1) The three types of neurons are sensory (afferent)
neurons, motor (efferent) neurons, and association
neurons or interneurons. Association neurons or interneurons are links between sensorv and motor neurons
and can, therefore, be classified as either afferent
(carrying information toward the CNS) or efferent
(carrying information away from the CNS), depending
on the situation. Therefore, in the strict sense,association neurons are neither afferent nor efferent.
Uniyuely, cranial nerves do not use spinal cord tracts
to take their information to the brain. Recall that the
spinal cord is an extension of the brain downward to
the coccyx (tailbone). The spinal cord no longer
exists at the level of the head. However, cranial nerves
carry their information into the hindbrain where the
information is segregated and distributed to appropriate areas of the brain. We will not be dealing with
cranial nerves and their pathways in this exercise.
2) The CNS is comprised of the brain and spinal cord.
It is important to realize that the separation of the
nervous system into two separate components is an
artificial one; all parts of the nervous system are
connected.
PHYSIOLOGY
S24
EDUCATION
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1996
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Association areas, or “silent areas,” of the cerebral
cortex are involved with higher brain processes:
memory, reasoning, problem solving, and concentrating, just to name a few.
Answers to Text Questions
IN
D
Motor areas of the cerebral cortex are involved with
actions. The actions can manifest in skeletal muscles,
smooth muscles, cardiac (heart) muscle, visceral organs, or glands.
18) *What is a reflex, and why is it important to the
nervous svstem?
1 - ADVANCES
I
deal with
incoming sensory information from the body and from
the body’s interpretation of external stimuli.
17) -+What is the difference between a synapse and
synaptic transmission?
: NUMBER
D
4) Sensory areas of the cerebral cortex
The electrical form of information is carried by the
dendrite toward the neuronal cell body. If enough
electrical charge reaches the axon hillock, a new
action potential is created. The whole process repeats
as the information is passedalong to the next neuron
and throughout the entire nervous system. Finally,
information reaches the motor neuron, which delivers
the highly processed messageto muscles, glands, or
body (visceral) organs.
16
N
3) The cerebellum is involved in coordinating motor
actions. The cerebral cortex is involved in almost all
processes of the nervous system. It is linked with the
conscious awareness of information and contains
sensory, motor, and association areas.
An action potential is generated at a neuron because
of a stimulus. This action potential travels along its
axon until it reaches the end of the axon. When the
action potential reaches the end of the axon, it causes
the release of chemical neurotransmitters. Because
neurotransmitters are proteins produced by the body,
they are forms of chemical, not electrical, transmission. Neurotransmitters are picked up by the dendrites of the next neuron. Synaptic transmission
has occurred. The type of neurotransmitter released,
whether excitatory or inhibitory, plays a part in how
the information will be passedalong this neuron. The
chemical form of information is converted to an
electrical form at the corresponding dendrite.
VOLUME
A
INNOVATIONS
A
8) Horizontally, information can travel within levels of
the spinal cord. At each level of the spinal cord, nerves
from the PNS enter and exit the spinal cord. Thus they
bring in and carry away information. This can be
compared with people getting on and off escalators at
different floors of a company building.
11) Axons are processes that carry information away
from the neuronal cell body.
12) Information can travel in electrical and chemical
modesin the nervous system. Electrical signaltransmission is found within a neuron, and chemical transmission is found between neurons or between neurons
and their target muscles, glands, or organs.
E
A
S
15) The axon hillock is the site where the information
is gathered to “decide” whether the information has
enough strength to be passedonward. Recall that this
is a probability event, and no actual conscious decision is involved.
17) A synapse is an anatomic structure. It is the site of
transmission between two neurons. Synaptic transmission is the event of chemical neurotransmitter release
from one neuron to another or from one neuron to its
target muscle, organ, or gland.
13) The all-or-none phenomenon is an electrical
process. It describes the process where information
passedbetween neurons is either passedin its entirety
or not at all. The generation of the action potential
(the electrical form of information within the neuron)
is a probability event. If enough electrical charge is
present at the axon hillock to generate an action
potential, the action potential will carry the informa-
1 - ADVANCES
D
18) A reflex is a predictable inotor outpt response to
a specific sensory stimulus. A reflex is n importa .nt
wav that the nervous system functions.
IN PHYSIOLOGY
ST5
EDUCATION
- DECEMBER
1996
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10) Dendrites are extensions of the neuronal cell body
that bring information toward the neuronal cell body.
: NUMBER
I
16) A stimulus generates an action potential. This is
carried by the axon of one neuron to another neuron.
When the electrical signal, or action potential, reaches
the end of the axon, it causeschemical neurotransmitters to be released. These chemical neurotransmitters
are released into the space between the two neurons
and are picked up by the dendrites of the next neuron
in line. The chemical neurotransmitters are converted
into electrical information at the dendrites. The type
of information that the dendrites carry is dependent
on the type of neurotransmitter received. The electrical information is carried bv the dendrite to the axon
hillock so that the probability decision can occur. As a
result of the probability event and the all-or-none
phenomenon, an action potential may or may not be
created. The entire process repeats itself throughout
the entire nervous svstem.
9) Inside the neuron, information is carried in an
electrical form called an action potential. Between
neurons and also between a neuron and its target
muscle, gland, or organ, information is carried chemically through a specific family of proteins called
neurotransmitters.
16
D
tion with its full strength and content. This is the “all”
part of the phenomenon. If there is not enough
electrical charge to generate an action potential, no
subsequent transmission of information will occur.
This is the “none” part of the phenomenon.
Vertically, information ascendsto and descends from
the brain in specialized regions called tracts. Tracts of
the spinal cord are organized by the information that
they carry. Specific information about different senses
each have their own tracts. These usuallv ascend to
the brain, much like an upgoing escalator. Information
going to specific muscle groups or glands also have
their own descending tracts, much like different
down escalators. The different types of information
can be compared with the different professions housed
in a large company. For efficiency, each profession
usesits own escalator.
VOLUME
N
I
N
N
0
V
A
T
I
0
N
S
A
N
Monosynaptic
Patellar Tendon
Reflex (an
extension reflex
D
I
D
E
A
S
Component
- causes excitation of quadriceps muscle
(an extensor muscle)
that results in muscle contraction
Polysynaptic
Component
- causes relaxation of flexor muscles
so that quadriceps muscle can do
its action unopposed
Withdrawal
Stimulus
1
- excitation of flexor muscle
causes mucle contraction that
results in pull away from painful
stimulus
- inhibition of extensor muscles allows
flexion to take place
upon Painful
- awareness of pain sensation
- creation of a memory
FIG.
Concept
map that organizes
and flexor
withdrawal
reflex
LABORATORY
ACTION
IN THE
EXERCISE:
A MODEL
NERVOUS
SYSTEM
basic concepts
are presented.
11.
of laboratory
Reflexes are predictable motor output responses to
specific sensory stimuli. Reflexes are involuntary or
“automatic” becausethey occur without people thinking about them. Most reflexes are polysynaptic
(contain more than one synapse). Polysynaptic reflexes involve interneurons. Some reflexes are known
as monosynaptic reflexes. They only involve two
1 - ADVANCES
tendon
reflex
A reflex arc is the pathway for a reflex. Reflex arcs
must have the following parts. A sensory receptor
must be present to receive stimuli. The afferent
(sensory) neuron carries the stimulus information
from the sensory receptor. The sensory information
goes through the sensory neuron and into the CNS.
There, at least one synapse is made with the efferent
(motor) neuron. The efferent (motor) neuron carries
information out to the target muscle, organ, or gland.
The muscle, organ, or gland must be present to
execute the action. A schematic of the components of
a monosynaptic reflex arc is presented in Fig. 1U, and
a schematic of the components of a polysynaptic
reflex arc is presented in Fig. 12B.
Introduction
16 : NUMBER
patellar
neurons and one synapse. Monosynaptic
reflexes
are the simplest reflexes of the nervous system.
OF REFLEX
A concept map (Fig. 11) is presented that organizes
the basic concepts of the text material found in the
laboratory exercise. The concept map begins with REFLEXESand branches off into the components of the
patellar tendon reflex and flexor withdrawal reflex.
VOLUME
exercise:
IN PHYSIOLOGY
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EDUCATION
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1996
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,Polysynaptic Component:
1 flexion away from stimulus
I
N
N
0
V
A
T
I
0
N
S
syna
N
D
I
D
E
A
S
junction
se
syna
sy apse
1
r
TsTv*E
receptor
neuron
neuron
motor neuron
target mkcle,
organ,or gland
FIG.
A: schematic
representation
of components
of a reflex
reflex.
BE components
in a neurological
schematic.
neuron
and is, therefore,
a schematic
for a polysynaptic
12.
arc. This reflex
arc schematic
is for a monosynaptic
arc presented
in B includes
an association
reflex
arc.
Reflex
1) Patellav
tendon
reflex (knee jerk reflex).
The
stretch reflex is the classic example used to
demonstrate monosynaptic reflexes. The stretch
reflex is a component of the patellar tendon reflex,
but the complete patellar tendon reflex is a polysynaptic one.
muscle fibers in the thigh (fibers of the quadriceps
muscle) to stretch very slightly. Special sensory receptors in the quadriceps muscle sensethis stretch.
The afferent neuron carries the stretch information
into the spinal cord. In the spinal cord, there is a
synapse between the afferent (sensory) neuron and
the efferent (motor) neuron. This direct afferentefferent synapse is monosynaptic.
The information carried by the efferent motor neuron causesthe
quadriceps muscle to contract. All of this happens
automatically and very quickly, within 20 ms.
The monosynaptic
component of the patellar tendon reflex is the essential component of the reflex
and is diagrammed in Fig. 13.
STRETCH.
The setup for testing this reflex
is very simple. Someone sits elevated with dangling or
crossed legs. The patellar tendon below the kneecap
(patella) is tapped with a reflex hammer. Tapping the
tendon is the stimulus. Tapping the tendon causes
MONOSYNAPTIC
VOLUME
16 : NUMBER
1 - ADVANCES
Contraction of the quadriceps causes the leg to kick
out. This is aided by the polysynaptic component of
the reflex. Note that in an anatomic sense, the leg is
IN PMYSIQLOGY
527
EDUCATION
- DECEMBER
1996
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monosynaptic
A
INNOVATIONS
A
quadriceps muscle
(extensor) containing
sensory receptor
for sense stretch
2
patellar
tendon
N
D
I
D
E
A
S
section of spinal cord
3
sensory
/ neuron
1
reflex
hammer
provides
stretch
stimulus
I
monosynaptic
junction
I
motor neuron
5
6
FIG. 13.
Illustration
of patellar
tendon
reflex.
Components
of the reflex
are numbered
in
accordance
with
order
in which
information
travels.
Stretch
stimulus
for reflex
is
provided
by reflex
hammer
when it taps the patellar
tendon.
Sensory
neuron
that carries
afferent
information
is shown
going from the muscle into the dorsal (back)
portion
of the
spinal
cord. Dendrite
of this cell, which
is bringing
information
from the muscle
to the
neuronal
cell body, extends
all the way from the quadriceps
muscle to the neuronal
cell
body. Sensory
cell body lies just outside
the spinal
cord. Because
there is a direct link
between
afferent
(sensory)
and efferent
(motor)
neurons,
there is only one synapse,
i.e.,
monosynaptic.
Motor
neuron
leaves the ventral
(front)
part of the spinal
cord and
innervates
the quadriceps
muscle.
It is important
to note that the cell body of the motor
neuron
is actually
found inside the ventral
(front)
part of the spinal cord. Its axon carries
information
away from the cell body and stretches
from its origin
in the spinal
cord all
the way to the quadriceps
muscle.
For illustrative
purposes,
it is shown
outside
the spinal
cord in this figure.
For completion,
the opposing
flexor
muscle (the hamstring)
is shown.
Hamstring
group
of muscles
in the thigh
has antagonistic,
or opposite,
action to the
quadriceps
group
of muscles.
Hamstring
muscles
insert (attaches)
to the lower leg bone
(tibia)
and flex the knee.
only the part of the lower limb from the knee
downward.
mally bend (flex) the leg are relaxed. Muscles have a
constant level of muscle tone, and they must be
“turned off” to be relaxed. To “turn off” a muscle, or
to prevent it from contracting, the motor nerve going
to the muscle must be inhibited.
COMPONENT.
Many muscles of the body are
functionally paired. There are muscle groups that flex
limbs or pull them toward the body. There are also
muscle groups that extend limbs or straighten them
out again. These muscle groups have opposing actions. Both types of muscle groups are attached to any
one bone. So, to produce smooth, coordinated movement, one group of muscles has to relax for the other
group to work efficiently.
POLYSYNAPTIC
When the extensor muscles (quadriceps) actually
produce the kick outward, the motor nerves to the leg
flexor (hamstring) musclesare inhibited by an interneuron. In this way, more synapses than just one are
involved in producing the patellar tendon reflex.
Gamma-aminobutyric
acid (GABA) is a major
inhibitory
transmitter in the brain and spinal
cord. Glycine, a less common transmitter, is used
For example, when the leg kicks out (extends), the
movement is more efficient if the muscles that nor-
VOLUME
16 : NUMBER
1 - ADVANCES
IN PHYSIOLOGY
S28
EDUCATION
- DECEMBER
1996
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tapping the tendon
causes the leg to
swing up
I
N
in the spinal
cord
antagonist
muscles.
2)
Withdrawal
N
0
V
by interneurons
reflex
upon
painful
A
T
I
that
0
N
S
N
D
I
D
E
A
S
cord. In the spinal cord, the information is passedby
an interneuron to the efferent (motor) neuron. The
efferent (motor) neuron carries its information out to
the muscle to cause flexion of the limb away from the
stimulus.
inhibit
stimulus.
A
The
Again, because muscles work in functional pairs, the
group of musclesthat works to extend your arm or leg
is inhibited. Muscles
are inhibited when the nerves to
them are inhibited. Motor neurons receive their information from nerves in the spinal cord. This is the same
mechanism asthe patellar tendon reflex except that it
is for a flexor muscle and not an extensor one. Also, it
is polysynaptic and involves an interneuron to link the
sensory (afferent) and motor (efferent) neurons.
The withdrawal reflex is a polysynaptic one, but it can
be broken down into basic components. One part of
the withdrawal reflex causesyour arm or leg to flex
away from the offensive stimulus. This part is similar
to the patellar tendon reflex; a schematic representation of the components of the withdrawal reflex is
found in Fig. 14. It is important to note that, while the
muscular component of the withdrawal reflex is
similar
to the patellar tendon reflex, it differs because
it is a polysynaptic
reflex involving an interneuron.
Information causing the reflex
portion of the withdrawal reflex enters and exits at
the same level of the spinal cord. Additionally, the
information reaches the brain through an ascending
tract.
INVOLVING
THE
BRAIN.
The information coming from the afferent (sensory)
neuron reaches the spinal cord. When it enters the
spinal cord, the information about pain hops through
one synapse, its destination: the neurons in the tract
that carry pain, temperature, and deep touch sensations. The tract ascends to the thalamus where it
synapsesagain. Then, the information is relayed to the
correct region of the cerebral cortex.
The other part of the reflex involves a sensory
awareness of a painful sensation. Further processing
of this information leads to learning and memory.
In the withdrawal reflex, sensory receptors
receive the “painful” stimulus. This information is
carried by afferent (sensory) neurons into the spinal
THE REFLEX.
3
target
muscle
that flexes
away from
offending
pain
I
I
synapse
synapse
synapse
FIG. 14.
Schematic
of reflex arc components
involved
in withdrawal
reflex.
A sensory
receptor
in the skin receives
the pain stimulus
and transmits
it to the afferent
(sensory)
neuron.
There is a synapse
between
the
afferent
neuron
and the interneuron
or association
neuron.
There
is another
synapse
between
the
interneuron
and the efferent
(motor)
neuron.
This makes
the neural
circuit
a polysynaptic
one.
Information
from the efferent
neuron
is transmitted
to the target muscle
also by a synapse.
VOLUME
16 : NUMBER
1 - ADVANCES
IN
PHYSIOLOGY
S29
EDUCATION
- DECEMBER
1996
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withdrawal reflex is an important protective reflex.
This reflex prevents excessive injury to the body. The
withdrawal reflex is used when you step on something sharp or when you touch something hot. Your
first reaction to painful stimuli like these is to withdraw your hand or leg or flex it away from the
stimulus. This happens very rapidly, even before your
brain can sensethe pain.
INNOVATIONS
In the cortex,
This becomes
Although
this
ately compared
at a relatively
occurred.
A
the information
is interpreted
as pain.
the first conscious
awareness
of pain.
response seems to occur almost immediwith the reflex component,
it comes
long period of time after the reflex has
l
N
Construct
reflex
upon
I
D
D
E
A
S
a working
mod .el of the
painful stimul us
Use the model to explain
stage of the withdrawal
reflex
l
withdrawal
what happens
at each
upon painful sti .mulus
Compare a monosynaptic
reflex as found in the
patellar tendon reflex to the polysynaptic
withdrawal
reflex upon painful stimulus.
l
cortex
in the
knowlin an
injured
Finally, the cortex interprets
more information
concerning the pain and its results over time. The cerebral
cortex
compares
this pain to other
experiences.
Dealing with the information
over a period of time
leads to the creation of a memory. Therefore,
the next
time a painful stimulus is encountered,
it tends to be
avoided.
BUILDING THE REFLEX MODELS
(TEACHER’S COPY)
Purpose
Through
the construction
and manipulation
of the
model,
students will develop
an appreciation
and
understanding
of neural pathways and the monosynaptic reflex.
Introduction
This model is designed to illustrate reflex mechanisms
of the nervous system. It is important
to realize that
this model is only an electrical representation
of what
happens
in the nervous system. In the body, both
electrical
current
and chemical
transmission
are involved
in information
transfer.
Chemical
synaptic
transmission
cannot be shown by this solely electrical
model.
Prelab Preparation
The prelab
preparation
consists
of preparing
the
student packets. In addition,
the prelab preparation
also consists of construction
of the synaptic junctions
and motor units. The student
packets contain
the
materials required
to assemble the reflex models. All
materials used in the construction
of the models can
be purchased
from Radio Shack or through
any
Electrical
Objectives
On completion
be able to
of this laboratory
unit, students
should
supplies
Electrical
4-E-10 Miniature
l
List the components
monosynaptic
l
and describe the function
of a
reflex
Construct
reflex
a working
model of a monosynaptic
arc (knee jerk/patellar
tendon reflex)
Components
threaded
the neural
Neural
base lamps
Component
Synaptic
junctions
Cerebral
cortex
(#272-357)
Diagram and describe the neural pathways
involved in the withdrawal
reflex upon painful stimulus
and the monosynaptic
reflex/patellar
tendon reflex
l
TABLE 2
required
to build
reflex
models
Knife switch (#275-l 537)
Low-voltage
(m 2.3 V) threaded light
bulbs
1.5- to 3.0-V DC miniature
buzzer
(#273-053)
AA batteries and 1 battery holder
1.5-3.0 VDC motor
Monosynaptic
and polysynaptic
wire packets
4 Mini alligator clips (#270-380A)
27 Solderless insulated spade tongues
Stimulus/receptor
Muscle effector
Neurons
(#64-3033)
Use the model to explain what happens during the
monosynaptic
portion of the patellar tendon reflex
l
VOLUME
16 : NUMBER
1 - ADVANCES
Alligator clips and spade tongues are optional, but would be helpful
to students when assembling
the reflex models. DC, direct current.
IN PHYSIOLOGY
S30
EDUCATION
- DECEMBER
1996
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In addition
to giving awareness
of pain, the
simultaneously
pinpoints
the location
of pain
body. A common
secondary
reaction
to the
edge of where the pain has occurred
results
outward
behavioral
action, such as holding the
hand or foot.
INNOVATIONS
A
electronics company or catalogue. Table 2 presents a
list of supplies necessary to build the model. In Table
2, Radio Shack catalogue numbers are in parentheses
beside each item.
D
E
A
S
is represented
by a knife
switch
to assemble
the synaptic
junction.
placement
of the wires between
the
Prepare four synaptic junctions for each lab group.
Label two of the synaptic junctions as SENSORY
NEURON with tape or colored wire. In the same
manner, label one synaptic junction as MOTOR NEURON and the last synaptic
junction
as INTERNEURON.
E.
A. Use wire strippers to remove approximately 1 cm of the plastic insulation cover from both
ends of each wire piece.
INSTRUCTIONS.
Loosen the two screws
I
Wrap the free end of one of the wires attached to the
mini-lamp around the shaft of the screw and tighten
the screw. Repeat this same procedure for the other
wire. The junction apparatus should resemble that
shown in Fig. 15.
Part 1: construction
of synaptic junctions.
MATERIALS
NEEDED. The materials needed to construct
the model
are two 6-cm pieces of 22-gauge stranded wire, one
mini-lamp base with bulb, and one knife switch.
B.
D
Part
2: assembling
the w.mscle effector. MATERIALS
The materials needed are 1.5- to 3-V direct
current motor, two S-cm pieces of wire same color as
motor neuron, and two mini alligator clips (optional).
on the mini-lamp base.
NEEDED.
c. Place one end of the bare wire under the metal strip
on each side of the bulb holder and tighten the screw.
Repeat this same procedure for the other side.
INSTRUCTIONS.
D. There are six screws on the knife switch.
the two screws adjacent to the ‘U-shaped”
VOLUME
16
: NUMBER
1 - ADVANCES
A. USe
the plastic insulation
piece of wire.
Loosen
lever.
IN PHYSIOLOGY
s31
EDUCATION
StripperS
t0 remove 1 cm Of
cover from both ends of each
Wire
- DECFMBER
1996
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FIG. 15.
Synaptic
junction
located
between
2 neurons
connected
to a lamp. Left: materials
necessary
Right:
completed
synaptic
junction
with correct
knife switch
and the lamp.
N
INNOVATIONS
A
N
D
I
D
E A
S
B. There are two metal terminals
at the base of the
motor. Attach one piece of wire to one terminal.
Attach the second piece of wire to the remaining
terminal. If the motor does not have extensions to
connect the wires, solder the wire to the terminal.
wire labeled SENSORYNEURON, four 17-cm pieces of
wire labeled MOTOR NEURON, two 7-cm pieces of
wire labeled INTERNEURON, and 26-27 insulated
spadetongues (optional wire connectors).
IiWrRucTIo~s.A. Use wire strippers to remove 1 cm of
the plastic insulation covering from both ends of each
wire.
c. OPTIONAL:
The assembly of the pathways to the
synaptic junctions will be easier if alligator clips are
attached to the ends of each wire extending from the
motor (Fig. 16). Thread the bare wire end from the
motor through the opening on the alligator clip. Then
either crimp the alligator clip to the wire or solder it to
the wire. This will secure the alligator clips to the wire
extending from the motor.
OPTIONAL:
Connect a solderless insulated spade
tongue or wire terminal to both ends of each stripped
wire to make the assembly of the model pathways
easier. To do this, thread the bare end of the wire
through the spade tongue (terminal) and crimp the
two together. If only solder terminals are available,
they can be used in place of the solderlessterminals.
B.
Mini alligator clip can also be attached to
the wire ends of the battery holder (Fig. 16).
D. OPTIONAL:
Part 3:preparation of student wirepackets. MATERIALS
NEEDED.
The materials needed are two 40-cm pieces of
wire labeled SENSORYNEURON, two 55-cm pieces of
wire labeled SENSORYNEURON, two 30-cm pieces of
wire labeled SENSORYNEURON, two 35-cm pieces of
VOLUME
16 : NUMBER
1 - ADVANCES
c. Place the 40-cm pieces of wire (sensory) and 24-cm
pieces of wire (motor) in a bag. Label the bag
MONOSYNAPTIC MODEL.
D. Connect the 30- and 35-cm pieces of wire as shown
in Fig. 17 to form a double-wire connector. The
IN PHYSIOLOGY
S32
EDUCATION
- DECEMBER
1996
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017
FIG. 16.
Motor-wire-alligator
clip complex
shown
represents
target or muscle
effector portion
of the patellar
tendon
reflex.
Le@ use of mini
alligator
clips
where
1 end of the wire is threaded
through
the hole in the alligator
clip. This
will help students
assemble
the models more quickly.
INNOVATIONS
A
N
D
I
D
E
A
S
FIG. 17.
Double-wire
connector
illustrated
refers
to the sensory
neuron
units
described
in the instructions
(Part
2, c and D). The wires are crimped
together
at A. Shorter
wire (30 cm) will connect
to left side of S-2 unit (B)
and longer
wire (35 cm) will connect
to left side of S-l unit (C). There is
another
sensory
neuron
unit that needs to be attached
in the same
manner.
However,
the wire endings
will connect
to the right side of S-l
and S-2 units.
double-wire connector represents the sensory neuron
from the skin.
Each student or group of
students will need a packet containing the following
items to assemblethe models: four synaptic junctions
(light/lamp and switch connected together), one lowvoltage buzzer, one low-voltage motor, four AA batteries with holder, one monosynaptic wire packet in
large plastic Ziploc bag, one polysynaptic wire packet
in large plastic Ziploc bag.
Part 5: student
Place the 55-cm pieces of wire (sensory), the
21-cm pieces of wire (motor), the lo-cm pieces of
wire (interneuron), and the double-wire connector
(sensory) in a bag. Label the bag POLYSYNAPTIC
MODEL.
E.
4: neuralpathway
diagrams.
Each neural pathway diagram consists of four 8 X 1l-in. pages that
serve as maps for the students to follow when
constructing their own model.3 Make enough copies
of each pathway so that each lab group has a complete
set for the patellar tendon reflex and the polysynaptic
withdrawal reflex upon painful stimulus models (Figs.
18 and 19).
Part
TIPS
lab packets.
FOR TEACHERS
Options
We have presented two options for classroom presentation of the laboratory exercise. These two options
are only suggestions, and individual teachers may have
other ideas for the presentation of this exercise.
demonstration
(one class period).
The
laboratory would consist of demonstrating the model
that has been constructed before the students start the
lab. With this option, four students can work with one
model.
I. Interactive
3 To request
a master
copy of the reflex
pathways
for
duplication
purposes,
write
to authors
at NEOUCOM,
4209
State Route 44, PO Box 95, Rootstown,
OH 44272-0095
or fax
(216) 325-2524.
VOLUME
16 : NUMBER
1 - ADVANCES
IN PHYSIOLOGY
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end to the left side of
the bulb of the S -1 unit.
remaining wire to the
left side of the bulb
of the S - 2 unit .
INNOVATIONS
A
N
D
I
D
E
A
S
I. Student construction
Investing in solderless
terminal) will add a little
of the student packets.
will ultimately save time
the models.
of the model (2~0 classperio&~.
l
The laboratory experience would include constructing
the model and demonstrating how it resembles neural
pathways. With this option, we suggest that only two
students work on one model. To allow time for students
to build the model, this approach will take approximately two class periods of 50 mm.
Helpful
BUILDING
(STL~DENT~
Hints
The wires needed for the model do not need to be
different colors for sensory, motor, and interneurons.
If one color is used throughout the model, another
form of designating each wire would be appropriate,
i.e., labeling the wires with tape.
THE REFLEX
COPY)
insulated spade tongues (wire
more time in the preparation
However, the time invested
when the students assemble
MODELS
l
VOLUME
16 : NUMBER
1 - ADVANCES
purpose
Through the construction and manipulation
of the
model, students will develop an appreciation and
IN PHYSIOLOGY
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EDUCATION
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1996
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FIG. 18.
Monosynaptic
reflex/patellar
tendon
reflex is shown.
Arrange
the 4 sheets of paper in order shown here. Trace path of
the reflex
by following
the numbered
sheets clockwise.
Sheet 1 contains
muscle receptor
that senses stretch
placed on
the quadriceps
muscle when the patellar
tendon
is tapped.
Sensory
neuron
extends
from muscle
receptor
to the spinal
cord as illustrated
on sheet 3. Sensory
neuron
synapses
with the motor
neuron
in the spinal
cord. Motor
neuron
extends
from the spinal
cord to the muscle
effector
as shown
in sheet 4. Muscle
effector
is the quadriceps
muscle,
which
contracts
and causes the leg to kick out when the reflex
ls initiated.
Sheet 2 represents
Input from the cerebral
cortex.
Patellar
tendon
reflex
does not require
input to or from the cerebral
cortex.
I
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S
A
N
D
IDEAS
#2
FIG. 19.
Polysynaptic
withdrawal
reflex
upon painful
stimulus
is illustrated.
Arrange
the 4 sheets of paper ln the order shown
here. Trace the path of the reflex
by following
the numbered
sheets clockwise.
Sheet 1 contains the skin receptor
that
senses the painful
stimulus.
Sensory
neuron
extends
from the skin receptor
to the spinal cord illustrated
on sheet
3.
Sensory
neuron
synapses
with an interneuron
in the spinal cord at the S-l unit and with a second interneuron
in the
spinal cord at the S-2 unit. Interneuron
from the S-2 unit passes its information
to the ascending
tract for pain and
temperature.
S-4 unit is a synapse
in the thalamus
that continues
to the cerebral
cortex
illustrated
on sheet
2. The
interneuron
from the S-l unit synapses
with the motor
neuron
at the S-3 unit on sheet
3. Motor
neuron
travels
to the
muscle effector
shown
in sheet 4, which
allows an individual
to pull away from the painful
stimulus.
understanding
tic reflex.
of neural pathways and the monosynap-
Construct
a working model of a monosynaptic
reflex arc (knee jerk/patellar tendon reflex)
l
Objectives
On completion
be able to
l
of this laboratory unit, students should
Diagram
and describe
the neural pathways involved in the withdrawal reflex upon painful stimulus
and the monosynaptic reflex/patellar tendon reflex
a working model
reflex upon painful stimulus
* Construct
l
List the components
monosynaptic reflex
l
VOLUME
and describe
16 : NUMBER
of the withdrawal
Use the model to explain what happens at each
stage of the withdrawal reflex upon painful stimulus
the function of a
1 - ADVANCES
Use the model to explain what happens during the
monosynaptic portion of the patellar tendon reflex
l
IN PHYSIOLOGY
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#4
INNOVATIONS
AN
Compare
a monosynaptic
reflex as illustrated by
the patellar tendon reflex to the polysynaptic
withdrawal reflex upon painful stimulus.
Introduction
This model is designed to illustrate reflex mechanisms
of the nervous system. It is important to realize that
this model is only an electrical representation
of what
happens in the nervous system. In the body, both
electrical current and chemical transmission
are involved in information
transfer. Chemical synaptic
transmission cannot be shown by this solely electrical
model.
Loosen the two screws on the synaptic junction unit
farthest from the lamp/bulb. Attach one end of the
wire labeled MOTOR NEURON to one of the screws
and tighten. Repeat this procedure for the remaining
wire labeled MOTOR NEURON. Tape the wires on the
diagram over the area labeled MOTOR NEURON.
E.
Place the motor on the diagram at the site labeled
MUSCLE EFFECTOR. Connect the muscle effector to
the motor neuron by attaching an alligator clip on the
muscle effector to one wire representing the motor
neuron. Do the samewith the remaining alligator clip
and wire. Your model is complete and should resemble Fig. 20. Check Fig. 20 before asking your
teacher to check your model.
F.
the Models
I: monosynaptic
reJex/patellar
tendon
reflex.
The materials needed are the monosynaptic reflex diagram sheets, monosynaptic wire
packet, one synaptic junction (lamp/switch connections), four AA batteries with holder, one motor, tape
(masking or clear), and a Phillips screwdriver.
MATERIALS
NEEDED.
DEMONSTRATION
A. The monosynaptic reflex diagram
sheets are numbered l-4 in the upper left-hand
corner. Arrange the four sheets on the lab table in the
order shown below and in Fig. 18. Tape the four pages
together and tape the entire diagram to the table.
&I
REFLEX.
A. COIl-
Notice that the first light bulb (in the S-l unit) lights
up. This signifies that synaptic transmission has occurred. Chemical neurotransmitters from the axon of
the sensory neuron have been released to the next
neuron in line, the motor neuron.
c. Loosen the screws on either side of the bulb on the
synaptic junction unit. Attach the wires designated as
SENSORYNEURON to each of the screws. To do this,
wrap the bare end of the wire around the screw and
tighten the screw to secure the wire, or, if connectors
have been attached to the ends of the wire, slide the
connector under the head of the screw and tighten
the screw to secure the wire.
1 - ADVANCES
TENDoN
B.
Place the synaptic junction unit (switch/lamp) on
the diagram at the site labeled SYNAPSE. The knife
switch should be in the perpendicular position.
B.
16 : NUMBER
OF THE PATELLAR
nect the wires labeled SENSORY NEURON to the
power source. The power source is the battery pack.
The battery pack represents the stimulus in this reflex:
tapping the patellar tendon with a reflex hammer. The
stimulus is picked up by stretch receptors located in
the quadriceps muscle. The stimulus is transmitted by
the sensory receptor to the sensory neuron. The
sensory neuron is represented by the wires labeled
SENSORY NEURON. All of this happens when you
connect the wires to the battery pack.
INSTRUCTIONS.
VOLUME
EAS
c. Push down the switch/lever. The flow of energy in
the model mimics the flow of nerve signals in the
body. When you flip the switch, you are making the
decision in the all-or-none phenomenon that occurs at
the axon hillock. Recall that the chemical neurotransmitters released by the axon of the sensory neuron
have been picked up by the dendrites of the motor
IN PHYSIOLOGY
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Part
ID
D. Place the wires
labeled SENSORY NEURON over
the area labeled SENSORY NEURON on the diagram.
Tape the wires to the paper. Do not connect the wire
ends to the power source (stimulus/sensory
receptor)
until the model has been completed and checked by
your teacher.
l
Constructing
D
INNOVATIONS
A
N
D
I
D
E
A
S
neuron. The chemical neurotransmitters
have also
been converted into an electrical form. By pushing
down the lever, you have decided that enough electrical signal has accumulated at the axon hillock for an
action potential to be created.
I) Discuss why the lamp and switch were
together as a unit when you received them. Think
about the difference between a synapse and synaptic
transmission when developing your answer.
QUESTIONS.
2) Is the cerebral cortex involved in a monosynaptic
reflex arc? Explain your answer thoroughly.
Now the action potential, as represented
by the
electrical current in the model, travels through the
wires labeled MOTOR NEURON. At the end of the
wire (the end of the axon), the information turns on
the motor. The motor represents the resulting action
caused by the stimulus (tapping of the patellar tendon). The quadriceps muscle contracts. This, along
with help from the polysynaptic
component of the
patellar tendon reflex, causes the leg to kick out.
D.
VOLUME
16 : NUMBER
1 - ADVANCES
3) Describe
monosynaptic
the polysynaptic
reflex.
component
of this
(PATEUAR
TENDON
REFLEX).
1) Recall that a
synapse is an anatomic structure that includes the
junction of two neurons and the space between them.
Synaptic transmission
is an event that involves the
ANSWERS
IN PHYSIOLOGY
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EDUCATION
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FIG. 20.
This figure should be used in conjunction
with directions
for assembling
the monosynaptic
reflex/patellar
tendon
reflex.
Battery
unit represents
the stimulus/receptor.
Wires labeled by color or tape follow
the path of the sensory
neurons
to the spinal cord. Knife switch
and lamp correspond
to the synaptic
junction
between
the sensory
and
motor
neurons.
Wires labeled by color or tape follow
the path of the motor
neuron
to the muscle effector.
In this
model the muscle effector
is represented
by a motor.
INNOVATIONS
A
N
D
I
D
E
A
S
release of chemical neurotransmitters from an axon of
one neuron to the dendrite of the next neuron in line.
This light bulb-and-switch unit represents all of the
components of the synaptic junction: the neurotransmitter release and the signal transmission across the
synapse (light bulb lighting up) and the creation of an
action potential as an electrical, probability event
(pushing down the knife switch). By keeping these
components together, we are reinforcing the idea
that, although there are many components of the
synaptic junction, they work together for one purpose: to bridge the gap between neurons and allow
the messageto continue along its pathway.
packet, four synaptic junctions (lamp/switch connections) appropriately labeled SENSORY, MOTOR, and
INTERNEURON, four AA batteries with holder, one
buzzer, one low-voltage motor, tape (masking or
clear), and a Phillips screwdriver.
INSTRUCTIONS.
(You will need to disconnect your previ-
The polysynaptic reflex diagram sheets are numbered l-4 in the upper left-hand corner. Arrange the
four sheetson the lab table in the order shown below
and in Fig. 19. Tape the four pages together and tape
the entire diagram to the table.
A.
Keep in mind that synaptic transmission is a chemical
event, and our model is a mechanical/electrical model.
The electrical action of a light bulb lighting up
represents the synaptic transmission of neurotransmitters. The mechanical action of pushing the knife
switch represents a chemical-to-electrical conversion
of information and also represents the all-or-none
phenomenon at the axon hillock.
Place the four synaptic junction units (switch/lamp)
on the diagram at the sites labeled S-l, S-2, S-3, and
S-4. The knife switch on all units should be in the
perpendicular position.
B.
2) The cerebral cortex is not involved in a monosynaptic reflex arc. This type of reflex allows a very quick,
automatic response and is usually protective in nature.
Monosynaptic reflex arcs do include the spinal cord.
Incoming messagesare interpreted, and an action is
initiated immediately.
c. Sort the wires in the polysynaptic wire packet into
MOTOR, SENSORY,and INTERNEURON wires. There
should be two MOTOR, two SENSORY, and two
INTERNEURON wires. Additionally, there should be
two SENSORY neuron units that resemble the wires
shown in Fig. 17.
3) The polysynaptic component of the patellar tendon
reflex involves two groups of muscles with opposing
actions. Most muscles in the body are functionally
paired. The muscles involved with the patellar tendon
reflex are examples of such pairing. These muscles
include the quadriceps muscle, which extends the leg
(kicks out), and the hamstring muscles, which flex
(bend) the leg.
n, The skin receptor is the starting point. Position both
SENSORY neuron units so that the wires that are
crimped together (seeA in Fig. 17) lie near the battery
unit. The remaining wires should extend toward the
synaptic units S-l and S-2. Do not connect the
SENSORYneuron units to the battery unit at this time.
Obviously, kicking out the leg is the exact opposite of
flexing the leg. You might guessthat extension would
occur much more easily if the flexors were inhibited.
Interestingly, the body thinks so, too. When the
quadriceps receive their monosynaptic signal to extend the leg, the hamstrings simultaneously receive
their polysynaptic signal for inhibition. This allows the
quadriceps to extend the leg without the opposing
influence of the hamstrings.
VOLUME
16
: NUMBER
1 - ADVANCES
Loosen the screws on either side of the bulbs on the
synaptic junction units S&l and S&2,Connect one wire
of a SENSORYneuron unit to the screw located to the
left of the bulb on S-l. Connect the remaining wire of
that SENSORYneuron unit to the screw to the left of
the bulb on S-2. Repeat this procedure for the other
sensory neuron unit. However, connect the wire ends
E.
IN PHYSIOLOGY
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EDUCATION
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1996
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ous model.)
I
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N
0
V
A
T
I
0
N
S
A
N
D
I
D
E
A
S
to the screws on the right side of the bulbs in the S-l
and S-2 units.
interneuron wire between the screw on the left side
of the S-l unit and the left side of the S-3 unit.
F.
Tape these wires together so that they form a single
unit with two terminals at one end (toward the battery
unit) and four terminals attached to the synaptic units
S-l and S-2 (Fig. 21).
H.
Loosen the two screws on the knife switch that are
located farthest from the lamp on the S-l synaptic
unit. Also loosen the two screws on the lamp of the
S-3 unit. Attach one interneuron wire between the
screw on the right side of the S-l unit and the screw
on the right side of the S-3 unit. Attach the other
I. The second part of this pathway projects to the
cerebral cortex. Therefore, there needs to be a circuit
from the S-2 junction to the cerebral cortex. To do
this, go back to the S-2 unit and loosen the two screws
To complete the reflex part of the pathway, loosen
the screws on the knife switch farthest from the lamp
on the S-3 unit. Connect the terminal ends of the
motor neuron wires, one on each side, and tighten the
screws to secure. Attach the alligator clips on the
motor to the opposite ends of the wires.
G.
VOLUME
16 : NUMBER
1 - ADVANCES
IN PHYSIOLOGY
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FIG. 21.
This figure
should
be used in conjunction
with directions
for assembling
the polysynaptic
withdrawal
reflex
upon painful
stimulus.
Battery
unit represents
the stimulus/receptor.
Wires labeled by color or tape
follow
the path of the sensory
neurons
to the spinal cord. Knife switch and lamp complexes
correspond
to
the synaptic
junctions
located between
the neurons.
Wires labeled by color or tape follow
the path of the
motor neuron
to the muscle effector.
An additional
set of wires labeled by color or tape follow the sensory
neurons
traveling
to the cerebral
cortex
along the ascending
tracts. Buzzer represents
the cerebral
cortex.
In this model the muscle effector
is represented
by a motor.
I
N
N
0
V
A
T
I
0
N
on the knife switch farthest from the lamp. On the S-4
synaptic unit, loosen the screws on either side of the
bulb.
Attach one SENSORYwire between the right side of
the S-2 unit and the right side of the S-4 unit. Attach
the second SENSORYwire between the left side of S-2
and left side of the S-4 unit.
OF THE
WITHDRAWAL
REFLEX
UPON
PAINFUL
Notice that there are four synaptic junctions. This model is polysynaptic
because the reflex
consists of more than one synaptic junction.
c. Connect the wires labeled SENSORY NEURON to
the power source. Recall that, asin the monosynaptic
reflex/patellar tendon reflex, the battery pack acts as
the stimulus and sensory receptor. In this withdrawal
reflex upon painful stimulus model, the battery pack
represents a painful or extremely hot stimulus. These
could be produced by touching a hot pan or stepping
on a sharp nail.
A
S
At the end of the wires labeled as MOTOR NEURON, the information will be transmitted from the
neuron to the target muscle. The motor will be turned
on. This is the resulting action caused by the stimulus.
The action is manifest asflexing the injured limb away
from the painful stimulus. Note that the flow of
electrical current in the model mimics the way information travels in the body. Discuss with your lab
partner the direction of signal transmission in the
polysynaptic reflex pathway.
G.
The stimulus of connecting the wires labeled SENSORY NEURON to the power source is received by
the sensory receptor (also represented by the battery
pack). The stimulus is then transmitted as electrical
current, just asit is in an electrical form in the body’s
nervous system, through the wires labeled SENSORY
NEURON.
Follow the same procedure for the pathway that
goes to the brain (buzzer). Push the knife switch on
the S-2 unit. An action potential is created in the
neuron found in the tract carrying information about
pain, (external) temperature, and deep touch. The
light bulb will light at the S-4 unit.
H.
Notice that two light bulbs are on (they should be
found on the S-l and S-2 units). These signify that
synaptic transmission has occurred. In the S-l unit,
chemical neurotransmitters have been released and
taken up by the interneuron. The chemical neurotransmitters have been released and converted to an
electrical signal in the dendrites of the interneuron. In
D.
1 - ADVANCES
E
Now flip the knife switch of the S-3 unit. Again, you
have made the probability decision of the all-or-none
phenomenon to create an action potential in the
motor neuron.
Make sure that all switches are in the upright
position.
: NUMBER
D
F.
B.
16
I
E. Follow the pathway that produces a motor response
to the pain stimulus. First, flip the knife switch found
in the S-l unit. You have made the probability
decision of the all-or-none phenomenon to create an
action potential in the interneuron. The light bulb of
the S-3 unit will light. Again, this signifies that
synaptic transmission has occurred between the interneuron and the next neuron in line: the motor neuron.
A.
VOLUME
D
The S-4 unit represents a synaptic junction in the
thalamus. Flipping the knife switch of the S-4 unit
represents thalamic function. The thalamus directs
the information that it receives to the appropriate area
of cerebral cortex.
I.
IN PHYSIOLOGY
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K. To connect the buzzer (brain) to the pathway,
loosen the screws on the knife switch farthest from
the bulb. Connect the wires on the buzzer, one to
each screw. Your model is now complete and should
resemble the model shown in Fig. 21. Have your
teacher check the model before you connect the
power source.
STIMULUS.
N
the S-2 unit, chemical neurotransmitters have been
released and taken up by a neuron in the tract that
carries information about pain, external temperature,
and deep touch. The chemical-to-electrical conversion
has also occurred in the dendrites of the neuron in the
tract. Here, the pathway for the withdrawal reflex
upon painful stimulus diverges into two components.
The component involving muscular movement will be
described in parts E-G. The component that involves
the brain will be described in parts H-J.
J.
DEMONSTRATION
A
S
INNOVATIONS
A
When the action potential is relayed to the cerebral
cortex, the buzzer will sound. This signifies that the
cerebral cortex has received information concerning
the painful stimulus. You are now aware of pain in
your body, and you alsoknow exactly where that pain
is located. Again, discuss with your partner what is
happening along this pathway.
N
D
I
D
E
A
S
2) Two synaptic junctions are simultaneously stimulated by the axon of the initial sensory neuron. This
allows two action potentials to be initiated at the same
time. One will cause an immediate withdrawal of the
limb at which the stimulus is received, and the other
informs the brain that pain has occurred somewhere
in the body. It also causes subsequent actions as
needed.
J.
2) Describe the difference between the
monosynaptic reflex/patellar tendon reflex and the
withdrawal reflex upon painful stimulus.
QUESTIONS.
2) Explain why both lamps light up in the withdrawal reflex upon painful stimulus when the system
is initially turned on.
3) In the pain reflex, discuss the advantages of a
pathway that reaches higher brain centers over a
monosynaptic pathway.
4) The involvement of the cerebral cortex provides an
awareness of pain and simultaneously pinpoints the
location of pain in the body. This can lead to a
secondary reaction such as grabbing the injured body
part or placing a burned finger in your mouth or
holding it under cold water. The cerebral cortex can
also interpret more information over time that leads to
the creation of a memory. This leads to the avoidance
of a situation that involves a painful stimulus.
4) How is the cerebral cortex involved in a pain/
temperature reflex? Explain your answer thoroughly.
the receptor were nonfunctional? ...the afferent (sensory) neuron were cut? ...the
efferent (motor) neuron were cut? ...the neuron
between the thalamus and cerebral cortex were cut?
5)WHATWOULDHAPPENIF:...
receptor were nonfunctional? If the receptor were nonfunctional, there
would be no sensory input received. Therefore, there
would be no withdrawal from the harmful stimulus.
Bodily harm could occur. For example, someone
without a functional receptor might severely burn or
cut their hand without realizing it.
When answering this question, use the approach that
cutting a neuron would be equivalent to driving down
a road that suddenly dead-ended.
ANSWERS
(WITHDRAWAL
REFLEX UPON
PAINFIJL
STIMULUS).
5) WHAT WOIJI,D
1)
There are two major differences between the two
reflexes. Obviously, the polysynaptic withdrawal reflex upon painful stimulus is more complex. It contains an interneuron between the sensory neuron and
motor neuron. In contrast, the corresponding part of
the patellar tendon reflex is monosynaptic with a
direct link between the alfimxt and e@erentneurons.
Note that the muscle would still be functional, and
other neural impulses that caused the muscle to move,
such asduring exercise, would function normally.
...the afferent (sensory) neuron were cut? If the
afferent (sensory) neuron were cut, the receptor
would be able to receive the stimulus. The receptor
could also pass on the information it received to the
sensory neuron, but the sensory neuron would not be
able to transmit its information. So, no sensory information would be passed on to the interneuron or to the
The second difference involves the conscious awareness of pain. This leads to further processing for
learning and memory. This second component of the
pathway involves the thalamus and the cerebral cortex. Note that the monosynaptic reflex does not
involve the brain at all.
VOLUME
16
: NUMBER
1 - ADVANCES
HAPPEN TF.. the
IN PHYSIOLOGY
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3) Because the cortex has the ability to store memories and interpret information over time, this will lead
to the avoidance of situations that cause pain. If the
pain withdrawal reflex did not involve the cerebral
cortex, there would be no previous knowledge about
pain or learning involved. We would therefore not
know to avoid painful sensations, and we would be
destined to repeat them.
I
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0
V
A
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cerebral cortex. All of the above information
also hold true he re.
I
0
N
S
would
I
D
E A
S
V. Chan was supported
by the Summer
Fellowship
Program
at
Northeastern
Ohio Universities
College of Medicine. J. Pisegna was
supported
by the American
Physiological
Society’s Frontiers
in
Physiology
Science Research Program for Teachers.
. ..the neuron between
the thalamus and cerebral
cortex were cut? If the neuron between the thalamus
and the cerebral cortex were cut, the withdrawal
part
of the pathway would not be affected. Therefore, the
automatic, reflexive component would exist.
Address for reprint
requests:
S. E. DiCarlo,
Dept. of Physiology,
Northeastern
Ohio Universities,
College of Medicine,
PO Box 95,
Rootstown,
OH 44272 (E-mail:[email protected]).
Destroying this specific connection between the cerebral cortex and thalamus would result in not knowing
where the pain had occurred. Recall that the thalamus
has a relay function, and it directs information to the
appropriate area of cerebral cortex. Because of its
relay function, the thalamus is still able to relay the
information it received about the painful stimulus to
the cerebral cortex. Whereas the specific connection
going to the region of the cerebral cortex that deals
with pinpointing pain does not exist, the thalamus can
still send the information to other areas of the brain.
Received
25 August
1995; accepted
in final form
14 August
1996.
References
1. Bredderman,
T. What research
says-Activity
science-The
evidence shows it matters. Science Children
20: 41, 1982.
2. Kandel,
E. A., J. H Schwartz,
and T. M. Jessel. Principles
of
NWWUZ Science (3rd en.). New York: Elsevier, 1991.
3. Teyler,
T. J., and T. J. Voneida.
Use of computer-assisted
courseware
in teaching neuroscience:
the Graphic Brain. Am, J
Physiol. 263 (Adv. Pbysiol. Educ. 8): S37-S44, 1992.
4. Tobin, K. The Practice of Constructivism
in Science Education.
Washington.
DC: AAAS Press, 1993.
IN PHYSIOLOGY
S42
EDUCATION
- DECEMBER
1996
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One of the authors (J. M. Pisegna) used this model in
her senior-level biology class. The following will be a
brief discussion of how the material was received by
her students. In general, the students had no difficulty
in following the directions as written. They found the
actual experience of constructing
the model enjoyable and seemed genuinely surprised
when they
realized they understood the model. It was apparent
that constructing and manipulating the model made it
much easier for the students to grasp the concepts.
This was due, we believe, in part to the hands-on,
concrete nature of model building as well as the ability
to keep the students attention and interest. The
students truly appeared to be immersed in the entire
process. Similar observations
were noted by a colleague who used this same model construction experience with her anatomy and physiology class. On the
basis of these experiences,
we believe that more
teachers should use model construction
in teaching
advanced concepts.
People can avoid excessive bodily damage with this
injury by using their other functional limbs to jerk the
damaged one away from the painful stimulus. They
would be able to do this because they would be aware
of the pain sensation and where it occurred. They
could then do something about it.
1 - ADVANCES
D
DISCUSSION
The perception of pain in the cerebral cortex would
exist because that component of the pathway would
remain intact. Information
can still pass from the
sensory receptor to the sensory neuron and on to the
neuron in the tract carrying information about pain,
(external) temperature, and deep touch sensations.
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N
Thus a general awareness
of a painful sensation
results, but you would be unable to pinpoint its
location.
. ..the efferent (motor) neuron were cut? If the efferent
(motor) neuron were cut, the withdrawal
reaction
would not occur because the muscle would not get
the message to contract. The information would be
received normally through the sensory receptor, sensory neuron, and interneuron. However, at the interneuron, the information would not be able to pass
through the injured motor neuron. So, the muscle
would not be moved away from the painful stimulus.
VOLUME
A