construction of a model demonstrating neural pathways and reflex arcs

INNOVATIONS A N D I D E A S 

CONSTRUCTION OF A MODEL DEMONSTRATING 

NEURAL PATHWAYS 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 

mployment opportunities in the future will require higher skills and an understand- 

ing 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 construc- 

tion 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. 

A&!. PHYSIOL. 271 (ADV PHYSIOL. EDUC. 16): SI4-S42, 1996 

Key words: patellar tendon reflex; education; laboratory exercise 

The standards of science education are undergoing 

major reform. Currently, the new primary goal of 

science educators is “scientific literacy” of all graduat- 

ing high school students. Scientific literacy has be- 

come an imperative. Because of rapid technological 

advances, a functional knowledge of mathematics and 

science is a requirement for tomorrow’s workforce. 

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 innova- 

tive teaching methods that encourage active learning 

must be developed. One such method is through the 

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. 

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 equip- 

ment. This is especially true in the subject area of 

neurobiology. Many exercises in the investigation of 

1043 - 4046 / 96 - $5.00 - COPYRIGHT o 1996 THE AMERICAN PHYSIOLOGICAL SOCIETY 

VOLUME 16 : NUMBER 1 -ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 

s14


neurobiology are too detailed and too expensive for 

the average high school science program. For ex- 

ample, although there are animated computer pro- 

grams 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 

Our rationale for using a model was because “evi- 

dence suggests that, with the use of activity-based 

science programs, teachers can expect substantially 

improved performances in science processes” (1). 

Active participation with models also reaches 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 presenta- 

tion. K-type learners are satisfied through the building 

and manipulation of the model. 

Models also satisfy pedagogical principles for “hands- 

on/minds-on” learning. This approach is supported by 

the theory of constructivism. Advocates of constructiv- 

ism 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). 

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 construc- 

tion, organizational concept maps, and instructive 

background information on the physiology related to 

the nervous system. 

Within the text are questions for the students to 

answer to help focus thinking and test comprehen- 

sion 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. 

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 

A concept map that organizes the basic concepts of 

BACKGROUND TO NEUROBIOLOGY text material iS pre- 

sented 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. 

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 subse- 

quent passages. 

Introduction 

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. 

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. 

VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 

s15

spinal 

nerves 

cranial 

nerves 

I N N 0 V A T I 0 N S A N D I D E A S 

Peripheral 

Nervous 

System 

Electrical Action Potential Within 

Chemical Neurotransmitters Between 

Components of a 

- axon 

- dendrites 

- axon hillock 

Sensory (aff erent) 

Neurons 

Central Nervous 

System 

FIG. 1. 

Concept map that organizes basic concepts of text material found in BACKGROUND TO NEUROBIOLOGY. 

Different receptors sense light touch, deep pressure, 

temperature, and many other tactile sensations. Fi- 

nally, special olfactory cells are sensory receptors of 

the nose. 

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 in- 

volved in processing sensory information. 

Neurons 

Functionally, there are three types of neurons: sensory 

neurons, motor neurons, and association neurons. 

Sensory receptors receive information from outside 

- forebrain 

- midbrain 

- hindbrain 

- ascending sensory tracts 

- descending motor tracts 

- horizontal direction of movem ent 

- point of contact 

between two 

neurons 

Interneurons or 

Association 

Neurons 

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. 

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 ex- 

ample, efferent neurons can cause contraction in 

muscles, changes in heart rate, changes in blood 

pressure, sweating, and many other physiological 


s16


finger 

\ 

\ sensory receptor 

pain stimulus 

, cell body of sensory neuron 

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 re- 

sponse to the sensory information received. An ex- 

ample of a motor neuron is shown in Fig. 3. 

Association neurons are also called interneurons. 

Interneurons are found between afferent (incoming 

sensory information) and efferent (outgoing motor 

information) neurons. Interneurons serve many func- 

tions 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 

cell body of motor neuron 

motor neuron 

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 trans- 

mission is an event that occurs at the synapse; the 

synapse itself is a structure. A schematic of a synapse 

is shown in Fig. 5. 

1) -+Name the three types of neurons. Are they 

afferent, efferent, or neither? 

axon hillock of motor neuron 

Nodes of Ranvier 

axon of motor 

\ 

target muscle 

FIG. 3. 

Motor neuron with structural components labeled. Motor neuron is 

shown with its target muscle. 


s17


syqapse 

I 

synapse 

:ov?E*> 

neuron neuron 

2) *What are the components of the CNS? 

Central Nervous System 

FIG. 4. 

Example of an association neuron or interneuron. Note that the interneu- 

ron is placed between a sensory (afferent) neuron and a motor (efferent) 

neuron. 

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. 

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 1 

synapse 

neuron 2 

FIG. 5. 

Synapse between 2 neurons is shown. 

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. Associa- 

tion 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. 

The cerebral cortex can be compared with the boss of 

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. 

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 hor- 

monal control, along with other functions. 

TABLE 1 

Structures of the brain 

Hindbrain Midbrain Forebrain 

H 1. Medulla M 1. Cerebral aqueduct F 1. Thalamus 

H2. Cerebellum F2. Hypothalamus 

H3. Pons F3. Cerebral cortex 

See Fig. 6 for schematic representation. 


Sl8


F2. H 

H3. 

F3. Cerebral Cortex 

/ 

Thalamus 

f ,H2. Cerebellum 

HI. Medulla 

F3. Cerebral Cortex 

1. Thalamus 

Ml. Cerebral erebellum 

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 correspon- 

dence to the structures listed in Table 1. 


s19


Another important structure is the thalamus. Al- 

though 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, football- 

shaped structure that functions as the “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 enter- 

ing and leaving the cerebral cortex. 

The cerebellum is primarily involved in the coordina- 

tion of motor activity. Coordination involves a com- 

plex mixture of balance, spatial orientation, and 

motion. Recent research has shown that the cerebel- 

lum may also be involved with certain types of 

learning and memory. 

3) -The cerebellum and cerebral cortex are impor- 

tant 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? 

6) -What is the most important function of the 

thalamus? 

spinal ca al 

\ 

dorsal (back) side of spinal cord section 

ventral (front) side of spinal cord section 

7) *How does the spinal cord bring information to the 

brain? 

Spz’nal cord. The spinal cord is a long, cylindrical part 

of the CNS extending downward from the hindbrain. 

The spinal cord is protected by the vertebrae (back- 

bone) as it passes down the vertebral canal. The spinal 

cord terminates between the first two lumbar verte- 

brae 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. 

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 up- 

ward 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) areas and ventral (motor) areas. 

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 

dorsal half of spinal cord 

that contains sensory information 

ventral half of spinal cord 

that contains motor information 

FIG. 7. 

Section of a spinal cord as it would appear in a horizontal slice. This action illustrates 

the dorsal (sensory) regions and ventral (motor) regions of spinal cord. 


s20


interneuron 

or associatior 

neuron 

sensory neuron 

incoming f 

l\;;g:Ttion 

~goingfl~~-LM~~~~or 

-I 

dorsal (back) side 

tract containin 

motor neuron information 

information 

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. 

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 escala- 

tors) 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 

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. 

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. 


s21 

g


Basic Concepts of Neurobiology 

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 projec- 

tions or processes from its cell body: axons and 

dendrites. Axon projections can be very long. Den- 

drites 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 myelin- 

ated axon than along an unmyelinated one. 

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 informa- 

tion 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). 

Usually, there are multiple dendrites bringing informa- 

tion toward the neuron’s cell body. In contrast, there 

dendrites 

neuronal cell body 

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. 

10) +Which processes bring information toward the 

neuronal cell body? 

11) -Which processes take information away from 

the neuronal cell body? 

12) *In what forms is information carried by the 

neuron? 

Information is carried along axons and dendrites in an 

electrical form. The movement of differently charged 

axon 

FIG. 9. 

Myelinated neuron with labeled structures, such as the axon, dendrites, 

neuronal cell body, axon hillock, and nodes of Ranvier. 


s22


synapse 

/ neuron 1 

synaptic vesicles 

containing 

chemical 

neurotransmitters 

neurotransmitters being 

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 neurotransmit- 

ters as they travel across the synaptic space. 

ions (positively charged substances and negatively 

charged substances) causes an event called an action 

potential. An action potential is the electrical current 

form of information in the neuron. 

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 poten- 

tial, the action potential will not occur. This is 

described as an all-or-none phenomenon. Informa- 

tion 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 message sent in an 

action potential. 

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 transmis- 

sion involves the release of proteins called neuro- 

transmitters into the space between two neurons 

(Fig. 10). Proteins are chemical substances; therefore, 

the method of transmission becomes chemical, not 

electrical. 

There are many different neurotransmitters within the 

nervous system. Some turn on the next neuron in line 

and are called excitatory neurotransmitters. Excita- 

tory neurotransmitters ensure that the action potential 

is carried by the next neuron in line. Some neurotrans- 

mitters turn off the next neuron in line and are called 

inhibitory neurotransmitters. These inhibitory neuro- 

transmitters prevent the next neuron in line from 

carrying the action potential. 

One neuron normally releases only one type of neuro- 

transmitter, 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 reac- 

tions to different stimuli unique. 

13) +Describe the all-or-none phenomenon of an 

action potential. Is it chemical or electrical? 

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? 


S23

Summary 


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, transmis- 

sion. Neurotransmitters are picked up by the den- 

drites 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 passed along this neuron. The 

chemical form of information is converted to an 

electrical form at the corresponding dendrite. 

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 passed along to the next neuron 

and throughout the entire nervous system. Finally, 

information reaches the motor neuron, which delivers 

the highly processed message to muscles, glands, or 

body (visceral) organs. 

17) -+What is the difference between 

synaptic transmission? 

a synapse and 

18) *What is a reflex, and why is it important to the 

nervous svstem? 

Answers to Text Questions 

1) The three types of neurons are sensory (afferent) 

neurons, motor (efferent) neurons, and association 

neurons or interneurons. Association neurons or inter- 

neurons 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, associa- 

tion neurons are neither afferent nor efferent. 

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. 

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. 

4) Sensory areas of the cerebral cortex deal with 

incoming sensory information from the body and from 

the body’s interpretation of external stimuli. 

Motor areas of the cerebral cortex are involved with 

actions. The actions can manifest in skeletal muscles, 

smooth muscles, cardiac (heart) muscle, visceral or- 

gans, or glands. 

Association areas, or “silent areas,” of the cerebral 

cortex are involved with higher brain processes: 

memory, reasoning, problem solving, and concentrat- 

ing, just to name a few. 

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 simi- 

lar 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 infor- 

mation exiting the cerebral cortex. 

7) Information travels to the brain in special groups of 

neurons that deal with the same types 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 informa- 

tion into specialized tracts going to the brain. 

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 appropri- 

ate areas of the brain. We will not be dealing with 

cranial nerves and their pathways in this exercise. 


S24


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. 

Vertically, information ascends to 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 

uses its own escalator. 

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 chemi- 

cally through a specific family of proteins called 

neurotransmitters. 

10) Dendrites are extensions of the neuronal cell body 

that bring information toward the neuronal cell body. 

11) Axons are processes that carry information away 

from the neuronal cell body. 

12) Information can travel in electrical and chemical 

modes in the nervous system. Electrical signal transmis- 

sion is found within a neuron, and chemical transmis- 

sion is found between neurons or between neurons 

and their target muscles, glands, or organs. 

13) The all-or-none phenomenon is an electrical 

process. It describes the process where information 

passed between neurons is either passed in 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- 

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. 

15) The axon hillock is the site where the information 

is gathered to “decide” whether the information has 

enough strength to be passed onward. Recall that this 

is a probability event, and no actual conscious deci- 

sion is involved. 

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 causes chemical neurotransmit- 

ters 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 electri- 

cal 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. 

17) A synapse is an anatomic structure. It is the site of 

transmission between two neurons. Synaptic transmis- 

sion is the event of chemical neurotransmitter release 

from one neuron to another or from one neuron to its 

target muscle, organ, or gland. 

18) A reflex is a predictable i notor outpt 

a specific sensory stimulus. A reflex is 

wav that the nervous system functions. 


ST5 

response to 

n importa .nt


Patellar Tendon 

Reflex (an 

extension reflex 

Withdrawal upon Painful 

Stimulus 

Monosynaptic 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 

,Polysynaptic Component: 

1 flexion away from 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 

- awareness of pain sensation 

- creation of a memory 

FIG. 11. 

Concept map that organizes basic concepts of laboratory exercise: patellar tendon reflex 

and flexor withdrawal reflex are presented. 

LABORATORY EXERCISE: A MODEL OF REFLEX 

ACTION IN THE NERVOUS SYSTEM 

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 RE- 

FLEXES and branches off into the components of the 

patellar tendon reflex and flexor withdrawal reflex. 

Introduction 

Reflexes are predictable motor output responses to 

specific sensory stimuli. Reflexes are involuntary or 

“automatic” because they occur without people think- 

ing about them. Most reflexes are polysynaptic 

(contain more than one synapse). Polysynaptic re- 

flexes involve interneurons. Some reflexes are known 

as monosynaptic reflexes. They only involve two 

neurons and one synapse. Monosynaptic reflexes 

are the simplest reflexes of the nervous system. 

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. 1 U, and 

a schematic of the components of a polysynaptic 

reflex arc is presented in Fig. 12B. 


S26


monosynaptic junction 

syna 

syna se 

sy 

1 

apse 

TsTv*E 

r 

receptor neuron neuron 

motor neuron 

target mkcle, 

organ,or gland 

FIG. 12. 

A: schematic representation of components of a reflex arc. This reflex arc schematic is for a monosynaptic 

reflex. BE components in a neurological schematic. Reflex arc presented in B includes an association 

neuron and is, therefore, a schematic for a polysynaptic reflex arc. 

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 polysyn- 

aptic one. 

The monosynaptic component of the patellar ten- 

don reflex is the essential component of the reflex 

and is diagrammed in Fig. 13. 

MONOSYNAPTIC 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 

muscle fibers in the thigh (fibers of the quadriceps 

muscle) to stretch very slightly. Special sensory recep- 

tors in the quadriceps muscle sense this 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 afferent- 

efferent synapse is monosynaptic. The informa- 

tion carried by the efferent motor neuron causes the 

quadriceps muscle to contract. All of this happens 

automatically and very quickly, within 20 ms. 

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 

VOLUME 16 : NUMBER 1 - ADVANCES IN PMYSIQLOGY EDUCATION - DECEMBER 1996 

527

1 


reflex 

hammer 

provides 

stretch 

stimulus 

patellar tendon 

2 

tapping the tendon 

causes the leg to 

swing up 

6 

quadriceps muscle 

(extensor) containing 

sensory receptor 

for sense stretch 

I 

motor neuron 

5 

3 

sensory 

/ neuron 

section of spinal cord 

I 

monosynaptic 

junction 

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. 

POLYSYNAPTIC 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 ac- 

tions. Both types of muscle groups are attached to any 

one bone. So, to produce smooth, coordinated move- 

ment, one group of muscles has to relax for the other 

group to work efficiently. 

For example, when the leg kicks out (extends), the 

movement is more efficient if the muscles that nor- 

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. 

When the extensor muscles (quadriceps) actually 

produce the kick outward, the motor nerves to the leg 

flexor (hamstring) muscles are inhibited by an interneu- 

ron. 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 


S28


in the spinal cord by interneurons that inhibit 

antagonist muscles. 

2) Withdrawal reflex upon painful stimulus. The 

withdrawal reflex is an important protective reflex. 

This reflex prevents excessive injury to the body. The 

withdrawal reflex is used when you step on some- 

thing sharp or when you touch something hot. Your 

first reaction to painful stimuli like these is to with- 

draw your hand or leg or flex it away from the 

stimulus. This happens very rapidly, even before your 

brain can sense the pain. 

The withdrawal reflex is a polysynaptic one, but it can 

be broken down into basic components. One part of 

the withdrawal reflex causes your arm or leg to flex 

away from the offensive stimulus. This part is similar 

to the patellar tendon reflex; a schematic representa- 

tion 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. 

The other part of the reflex involves a sensory 

awareness of a painful sensation. Further processing 

of this information leads to learning and memory. 

THE REFLEX. In the withdrawal reflex, sensory receptors 

receive the “painful” stimulus. This information is 

carried by afferent (sensory) neurons into the spinal 

I 

synapse 

cord. In the spinal cord, the information is passed by 

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. 

Again, because muscles work in functional pairs, the 

group of muscles that works to extend your arm or leg 

is inhibited. Muscles are inhibited when the nerves to 

them are inhibited. Motor neurons receive their infor- 

mation from nerves in the spinal cord. This is the same 

mechanism as the 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. 

INVOLVING THE BRAIN. 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. 

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 sensa- 

tions. The tract ascends to the thalamus where it 

synapses again. Then, the information is relayed to the 

correct region of the cerebral cortex. 

I 

synapse 

synapse 

3 

target 

muscle 

that flexes 

away from 

offending 

pain 

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. 


S29


In the cortex, the information is interpreted as pain. 

This becomes the first conscious awareness of pain. 

Although this response seems to occur almost immedi- 

ately compared with the reflex component, it comes 

at a relatively long period of time after the reflex has 

occurred. 

In addition to giving awareness of pain, the cortex 

simultaneously pinpoints the location of pain in the 

body. A common secondary reaction to the knowl- 

edge of where the pain has occurred results in an 

outward behavioral action, such as holding the injured 

hand or foot. 

Finally, the cortex interprets more information con- 

cerning 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 monosynap- 

tic reflex. 

Objectives 

On completion of this laboratory unit, students should 

be able to 

l Diagram and describe the neural pathways in- 

volved in the withdrawal reflex upon painful stimulus 

and the monosynaptic reflex/patellar tendon reflex 

l List the components and describe the function of a 

monosynaptic reflex 

l Construct a working model of a monosynaptic 

reflex arc (knee jerk/patellar tendon reflex) 

l Construct a working mod .el of the withdrawal 

reflex upon painful stimul us 

l Use the model to explain 

stage of the withdrawal reflex 

what happens at each 

upon painful sti .mulus 

l Compare a monosynaptic reflex as found in 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 in- 

volved 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 

TABLE 2 

Electrical supplies required to build the neural 

reflex models 

Electrical Components Neural Component 

4-E-10 Miniature threaded base lamps Synaptic junctions 

(#272-357) 

Knife switch (#275-l 537) 

Low-voltage 

bulbs 

(m 2.3 V) threaded light 

1.5- to 3.0-V DC miniature buzzer 

Cerebral cortex 

(#273-053) 

AA batteries and 1 battery holder 

Stimulus/receptor 

1.5-3.0 VDC motor 

Muscle effector 

Monosynaptic and polysynaptic wire packets Neurons 

4 Mini alligator clips (#270-380A) 

27 Solderless insulated spade tongues 

l Use the model to explain what happens during the Alligator clips and spade tongues are optional, but would be helpful 

monosynaptic portion of the patellar tendon reflex to students when assembling the reflex models. DC, direct current. 

(#64-3033) 


S30


FIG. 15. 

Synaptic junction located between 2 neurons is represented by a knife switch 

connected to a lamp. Left: materials necessary to assemble the synaptic junction. 

Right: completed synaptic junction with correct placement of the wires between the 

knife switch and the lamp. 

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. 

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. 

INSTRUCTIONS. A. Use wire strippers to remove approxi- 

mately 1 cm of the plastic insulation cover from both 

ends of each wire piece. 

B. Loosen the two screws on the mini-lamp base. 

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. 

D. There are six screws on the knife switch. Loosen 

the two screws adjacent to the ‘U-shaped” lever. 

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 


E. 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 NEU- 

RON and the last synaptic junction as INTER- 

NEURON. 

Part 2: assembling the w.mscle effector. MATERIALS 

NEEDED. 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). 

INSTRUCTIONS. A. USe Wire StripperS t0 remove 1 cm Of 

the plastic insulation cover from both ends of each 

piece of wire. 

VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECFMBER 1996 

s31


FIG. 16. 

Motor-wire-alligator clip complex shown represents target or muscle effec- 

tor 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. 

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. 

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. 

D. OPTIONAL: Mini alligator clip can also be attached to 

the wire ends of the battery holder (Fig. 16). 

Part 3:preparation of student wirepackets. MATERIALS 

NEEDED. The materials needed are two 40-cm pieces of 

wire labeled SENSORY NEURON, two 55-cm pieces of 



wire labeled SENSORY NEURON, four 17-cm pieces of 

wire labeled MOTOR NEURON, two 7-cm pieces of 

wire labeled INTERNEURON, and 26-27 insulated 

spade tongues (optional wire connectors). 

IiWrRucTIo~s. A. Use wire strippers to remove 1 cm of 

the plastic insulation covering from both ends of each 

wire. 

B. 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 solderless terminals. 

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 


S32

double-wire connector 

from the skin. 


remaining wire to the 

left side of the bulb 

of the S - 2 unit . 

end to the left side of 

the bulb of the S -1 unit. 

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. 

represents the sensory neuron 

E. 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. 

Part 4: neuralpathway diagrams. Each neural path- 

way diagram consists of four 8 X 1 l-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). 

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. 

Part 5: student lab packets. Each student or group of 

students will need a packet containing the following 

items to assemble the models: four synaptic junctions 

(light/lamp and switch connected together), one low- 

voltage buzzer, one low-voltage motor, four AA batter- 

ies with holder, one monosynaptic wire packet in 

large plastic Ziploc bag, one polysynaptic wire packet 

in large plastic Ziploc bag. 

TIPS FOR TEACHERS 

Options 

We have presented two options for classroom presen- 

tation of the laboratory exercise. These two options 

are only suggestions, and individual teachers may have 

other ideas for the presentation of this exercise. 

I. Interactive 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 



S33


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. Student construction of the model (2~0 classperio&~. 

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 approxi- 

mately two class periods of 50 mm. 

l Investing in solderless insulated spade tongues (wire 

terminal) will add a little more time in the preparation 

of the student packets. However, the time invested 

will ultimately save time when the students assemble 

the models. 

Helpful Hints BUILDING THE REFLEX MODELS 

(STL~DENT~ COPY) 

l 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. 

purpose 

Through the construction and manipulation of the 

model, students will develop an appreciation and 


s34

#4 

I N N 0 V A T I 0 N 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 of neural pathways and the monosynap- 

tic reflex. 

Objectives 

On completion of this laboratory unit, students should 

be able to 

l Diagram and describe the neural pathways in- 

volved in the withdrawal reflex upon painful stimulus 

and the monosynaptic reflex/patellar tendon reflex 

l Construct a working model of a monosynaptic 

reflex arc (knee jerk/patellar tendon reflex) 

l Use the model to explain what happens during the 

monosynaptic portion of the patellar tendon reflex 

* Construct a working model of the withdrawal 

reflex upon painful stimulus 

l List the components and describe the function of a l Use the model to explain what happens at each 

monosynaptic reflex stage of the withdrawal reflex upon painful stimulus 


s35

INNOVATIONS AN D ID EAS 

l Compare a monosynaptic reflex as illustrated by 

the patellar tendon reflex to the polysynaptic with- 

drawal 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 in- 

volved in information transfer. Chemical synaptic 

transmission cannot be shown by this solely electrical 


Constructing the Models 

Part I: monosynaptic reJex/patellar tendon reflex. 

MATERIALS NEEDED. The materials needed are the mono- 

synaptic reflex diagram sheets, monosynaptic wire 

packet, one synaptic junction (lamp/switch connec- 

tions), four AA batteries with holder, one motor, tape 

(masking or clear), and a Phillips screwdriver. 

INSTRUCTIONS. 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 

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. 

c. Loosen the screws on either side of the bulb on the 

synaptic junction unit. Attach the wires designated as 

SENSORY NEURON 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 

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. 

E. 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. 

F. 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 same with the remaining alligator clip 

and wire. Your model is complete and should re- 

semble Fig. 20. Check Fig. 20 before asking your 

teacher to check your model. 

DEMONSTRATION OF THE PATELLAR TENDoN REFLEX. A. COIl- 

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. 

B. Notice that the first light bulb (in the S-l unit) lights 

up. This signifies that synaptic transmission has oc- 

curred. Chemical neurotransmitters from the axon of 

the sensory neuron have been released to the next 

neuron in line, the motor neuron. 

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 neurotrans- 

connector under the head of the screw and tighten mitters released by the axon of the sensory neuron 

the screw to secure the wire. have been picked up by the dendrites of the motor 


s36


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. 

neuron. The chemical neurotransmitters have also 

been converted into an electrical form. By pushing 

down the lever, you have decided that enough electri- 

cal signal has accumulated at the axon hillock for an 

action potential to be created. 

D. 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 ten- 

don). The quadriceps muscle contracts. This, along 

with help from the polysynaptic component of the 

patellar tendon reflex, causes the leg to kick out. 

QUESTIONS. 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. 

2) Is the cerebral cortex involved in a monosynaptic 

reflex arc? Explain your answer thoroughly. 

3) Describe the polysynaptic component of this 

monosynaptic reflex. 

ANSWERS (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 


s37


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 message to continue along its pathway. 

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 neurotransmit- 

ters. 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. 

2) The cerebral cortex is not involved in a monosynap- 

tic 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 messages are interpreted, and an action is 

initiated immediately. 

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. 

Obviously, kicking out the leg is the exact opposite of 

flexing the leg. You might guess that 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 ex- 

tend 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. 

packet, four synaptic junctions (lamp/switch connec- 

tions) 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- 

ous model.) 

A. The polysynaptic reflex diagram sheets are num- 

bered l-4 in the upper left-hand corner. Arrange the 

four sheets on 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. 

B. 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. 

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 


n, The skin receptor is the starting point. Position both 

SENSORY neuron units so that the wires that are 

crimped together (see A 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 

SENSORY neuron units to the battery unit at this time. 

E. Loosen the screws on either side of the bulbs on the 

synaptic junction units S&l and S&2, Connect one wire 

of a SENSORY neuron unit to the screw located to the 

left of the bulb on S-l. Connect the remaining wire of 

that SENSORY neuron 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 


SW


to the screws on the right side of the bulbs in the S-l interneuron wire between the screw on the left side 

and S-2 units. 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). 

G. 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 

H. 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. 

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 

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. 

VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 199G 

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on the knife switch farthest from the lamp. On the S-4 

synaptic unit, loosen the screws on either side of the 

bulb. 

J. Attach one SENSORY wire between the right side of 

the S-2 unit and the right side of the S-4 unit. Attach 

the second SENSORY wire between the left side of S-2 

and left side of the S-4 unit. 

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. 

DEMONSTRATION OF THE WITHDRAWAL REFLEX UPON PAINFUL 

STIMULUS. A. Notice that there are four synaptic junc- 

tions. This model is polysynaptic because the reflex 

consists of more than one synaptic junction. 

B. Make sure that all switches are in the upright 

position. 

c. Connect the wires labeled SENSORY NEURON to 

the power source. Recall that, as in 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. 

The stimulus of connecting the wires labeled SEN- 

SORY 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 as it is in an electrical form in the body’s 

nervous system, through the wires labeled SENSORY 

NEURON. 

D. 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 neurotrans- 

mitters have been released and converted to an 

electrical signal in the dendrites of the interneuron. In 

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. 

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 inter- 

neuron and the next neuron in line: the motor neuron. 

F. 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. 

G. At the end of the wires labeled as MOTOR NEU- 

RON, 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 as flexing the injured limb away 

from the painful stimulus. Note that the flow of 

electrical current in the model mimics the way infor- 

mation travels in the body. Discuss with your lab 

partner the direction of signal transmission in the 

polysynaptic reflex pathway. 

H. 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. 

I. 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. 


s40


J. 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 also know exactly where that pain 

is located. Again, discuss with your partner what is 

happening along this pathway. 

QUESTIONS. 2) Describe the difference between the 

monosynaptic reflex/patellar tendon reflex and the 

withdrawal reflex upon painful stimulus. 

2) Explain why both lamps light up in the with- 

drawal 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) How is the cerebral cortex involved in a pain/ 

temperature reflex? Explain your answer thoroughly. 

5)WHATWOULDHAPPENIF:... the receptor were nonfunc- 

tional? . . . the afferent (sensory) neuron were cut? . . .the 

efferent (motor) neuron were cut? . ..the neuron 

between the thalamus and cerebral cortex were cut? 

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). 1) 

There are two major differences between the two 

reflexes. Obviously, the polysynaptic withdrawal re- 

flex upon painful stimulus is more complex. It con- 

tains 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@erent neurons. 

The second difference involves the conscious aware- 

ness of pain. This leads to further processing for 

learning and memory. This second component of the 

pathway involves the thalamus and the cerebral cor- 

tex. Note that the monosynaptic reflex does not 

involve the brain at all. 

2) Two synaptic junctions are simultaneously stimu- 

lated 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. 

3) Because the cortex has the ability to store memo- 

ries 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. 

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. 

5) WHAT WOIJI,D HAPPEN TF.. the receptor were nonfunc- 

tional? 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. 

Note that the muscle would still be functional, and 

other neural impulses that caused the muscle to move, 

such as during 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 informa- 

tion would be passed on to the interneuron or to the 


s41

cerebral cortex. All of the 

also hold true he re. 


above information would 

. . .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, sen- 

sory neuron, and interneuron. However, at the inter- 

neuron, 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. 

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. 

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. 

. . .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. 

Destroying this specific connection between the cere- 

bral 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. 

Thus a general awareness of a painful sensation 

results, but you would be unable to pinpoint its 

location. 

DISCUSSION 

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 enjoy- 

able 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 col- 

league who used this same model construction experi- 

ence 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. 

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. 

Address for reprint requests: S. E. DiCarlo, Dept. of Physiology, 

Northeastern Ohio Universities, College of Medicine, PO Box 95, 

Rootstown, OH 44272 (E-mail:sdicarlo@riker.neoucom.edu). 

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. 


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construction of a model demonstrating neural pathways and reflex arcs

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