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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. - 4046 VOLUME / 96 - $5.00 16 : NUMBER - COPYRIGHT 1 -ADVANCES 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 1043 1996 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 s14 PHYSIOLOGICAL EDUCATION - DECEMBER SOCIETY 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 N D I D E A S 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. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY s15 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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. I N N 0 V A T I 0 N A S N D I D E A S - 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 : NUMBER 1 - ADVANCES 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. VOLUME in IN PHYSIOLOGY s16 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 - ascending sensory tracts - descending motor tracts - horizontal direction of movem ent I N N 0 V A T I 0 N S D I D A N , cell body of sensory E A S 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 VOLUME 16 neuron with with its target : NUMBER structural muscle. 1 - ADVANCES FIG. 3. components IN PHYSIOLOGY s17 labeled. Motor EDUCATION neuron - DECEMBER is 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 pain stimulus I N N 0 V A T I 0 N A S N D I D E A S 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 VOLUME 16 : NUMBER 2 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. 1 - ADVANCES IN PHYSIOLOGY Sl8 EDUCATION representation. - DECEMBER 1996 F 1. Thalamus F2. Hypothalamus F3. Cerebral cortex Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 I :ov?E*> I N N 0 V A T I 0 N S A N D I D E A S F3. Cerebral Cortex Thalamus ,H2. Cerebellum HI. Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY s19 EDUCATION - DECEMBER 1996 I N N 0 V A T I 0 N S 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 N D I D E 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) VOLUME 16 : NUMBER FIG. 7. cord as it would appear in a horizontal regions and ventral (motor) regions 1 - ADVANCES IN PHYSIOLOGY s20 slice. This action of spinal cord. EDUCATION illustrates - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 \ S I N N 0 V A T I 0 N S A N D I D E A S 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 VOLUME 16 : NUMBER 1 - ADVANCES 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. IN PHYSIOLOGY s21 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 ~goingfl~~-LM~~~~or interneuron or associatior -I motor neuron information I Basic Concepts N N 0 V A T I 0 N S of Neurobiology A N D I D E A 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 VOLUME 16 : NUMBER 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 IN PHYSIOLOGY s22 such as the axon, of Ranvier. EDUCATION dendrites, - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 INNOVATIONS A N neuron D I D E A S 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. VOLUME 16 : NUMBER 1 - ADVANCES 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? IN PHYSIOLOGY S23 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 ) being I N N 0 V A T I 0 N S Summary E A S 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 - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 S26 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 ,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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 S33 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 s34 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 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 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 s35 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 #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 s36 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 s37 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 SW EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 ous model.) I N 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 s39 EDUCATION - DECEMBER 199G Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 s40 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 s41 EDUCATION - DECEMBER 1996 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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 N N 0 V A T 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 Downloaded from http://advan.physiology.org/ by 10.220.32.246 on June 16, 2017 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. 16 : NUMBER 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