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 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 s39
I N N 0 V A T I 0 N S A N D I D E A S 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 junctions. 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 neurotransmitters 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 interneuron 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 information 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. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 s40
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