construction of a model demonstrating neural pathways and reflex arcs

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INNOVATIONS A N D I D E A S FIG. 20. This figure should be used in conjunction with directions for assembling the monosynaptic reflex/patellar tendon reflex. Battery unit represents the stimulus/receptor. Wires labeled by color or tape follow the path of the sensory neurons to the spinal cord. Knife switch and lamp correspond to the synaptic junction between the sensory and motor neurons. Wires labeled by color or tape follow the path of the motor neuron to the muscle effector. In this model the muscle effector is represented by a motor. neuron. The chemical neurotransmitters have also been converted into an electrical form. By pushing down the lever, you have decided that enough electrical signal has accumulated at the axon hillock for an action potential to be created. D. Now the action potential, as represented by the electrical current in the model, travels through the wires labeled MOTOR NEURON. At the end of the wire (the end of the axon), the information turns on the motor. The motor represents the resulting action caused by the stimulus (tapping of the patellar tendon). The quadriceps muscle contracts. This, along with help from the polysynaptic component of the patellar tendon reflex, causes the leg to kick out. QUESTIONS. I) Discuss why the lamp and switch were together as a unit when you received them. Think about the difference between a synapse and synaptic transmission when developing your answer. 2) Is the cerebral cortex involved in a monosynaptic reflex arc? Explain your answer thoroughly. 3) Describe the polysynaptic component of this monosynaptic reflex. ANSWERS (PATEUAR TENDON REFLEX). 1) Recall that a synapse is an anatomic structure that includes the junction of two neurons and the space between them. Synaptic transmission is an event that involves the VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 s37
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 message to continue along its pathway. Keep in mind that synaptic transmission is a chemical event, and our model is a mechanical/electrical model. The electrical action of a light bulb lighting up represents the synaptic transmission of 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. 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 messages are interpreted, and an action is initiated immediately. 3) The polysynaptic component of the patellar tendon reflex involves two groups of muscles with opposing actions. Most muscles in the body are functionally paired. The muscles involved with the patellar tendon reflex are examples of such pairing. These muscles include the quadriceps muscle, which extends the leg (kicks out), and the hamstring muscles, which flex (bend) the leg. Obviously, kicking out the leg is the exact opposite of flexing the leg. You might guess that extension would occur much more easily if the flexors were inhibited. Interestingly, the body thinks so, too. When the quadriceps receive their monosynaptic signal to 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. packet, four synaptic junctions (lamp/switch connec- tions) appropriately labeled SENSORY, MOTOR, and INTERNEURON, four AA batteries with holder, one buzzer, one low-voltage motor, tape (masking or clear), and a Phillips screwdriver. INSTRUCTIONS. (You will need to disconnect your previ- ous model.) A. The polysynaptic reflex diagram sheets are num- bered l-4 in the upper left-hand corner. Arrange the four sheets on the lab table in the order shown below and in Fig. 19. Tape the four pages together and tape the entire diagram to the table. B. Place the four synaptic junction units (switch/lamp) on the diagram at the sites labeled S-l, S-2, S-3, and S-4. The knife switch on all units should be in the perpendicular position. c. Sort the wires in the polysynaptic wire packet into MOTOR, SENSORY, and INTERNEURON wires. There should be two MOTOR, two SENSORY, and two INTERNEURON wires. Additionally, there should be two SENSORY neuron units that resemble the wires shown in Fig. 17. n, The skin receptor is the starting point. Position both SENSORY neuron units so that the wires that are crimped together (see A in Fig. 17) lie near the battery unit. The remaining wires should extend toward the synaptic units S-l and S-2. Do not connect the SENSORY neuron units to the battery unit at this time. E. Loosen the screws on either side of the bulbs on the synaptic junction units S&l and S&2, Connect one wire of a SENSORY neuron unit to the screw located to the left of the bulb on S-l. Connect the remaining wire of that SENSORY neuron unit to the screw to the left of the bulb on S-2. Repeat this procedure for the other sensory neuron unit. However, connect the wire ends VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 SW
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