construction of a model demonstrating neural pathways and reflex arcs

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I N N 0 V A T I 0 N S A N D I D E A S 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 something 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 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. 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 information 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. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 S29
INNOVATIONS A N D I D E A S 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 monosynaptic reflex. Objectives On completion of this laboratory unit, students should be able to l Diagram and describe the neural pathways involved 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 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 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) VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 S30
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