BE 312 Lab #3 LAB 3: Stretch Receptors and Reflexes I. Background Studying the vertebrate stretch reflex is a good way to introduce students to the topics of stretch receptors, nerve conduction velocity, electromyograms (EMG), and motor control. Specialized receptors in the muscle respond to the stretching of the tendon attached to the muscle, and then send signals to motorneurons through a single synapse. The muscle fibers depolarize and twitch (contract) in response to the incoming impulse from the motorneuron. The Stretch Receptor Skeletal muscles have specialized receptors which convey information about muscle length, tension, and pressure to the central nervous system. The sensory receptors responsible for providing information about the length, or the rate of change of the length, of a muscle are called muscle spindles. Arranged in parallel with muscle fibers, the spindles are stretched when the muscle is stretched by an external force. Therefore, these receptors play a significant role in developing antigravity reflexes and maintaining muscle tone. Muscle spindles contain a small bundle of intrafusal fibers which do not contribute to the overall tension of the muscle, but regulate the excitability of the sensory afferent spindle nerves by mechanically deforming the receptors. These fibers are innervated by gamma motor neurons. Much of a muscle consists of extrafusal fibers, which are innervated by alpha motor neurons and are responsible for developing muscle tension. Motor Fibers Sensory Fiber Intrafusal Fibers Fluid Cavity Extrafusal Fibers Muscle Muscle Spindle Sensory Neuron Motor Endplates Motor Neuron Spinal Cord Figure HN-2-B1: A monosynaptic stretch reflex arc. The Stretch Reflex When a muscle is stretched, excitation of its muscle spindles causes a reflex contraction of the muscle. This reflex response is known as a stretch (myotatic) reflex. The minimal delay between the muscle stretching and the reflex contraction is due to its monosynaptic pathway. The sensory afferent nerves from the spindles synapse directly with motor neurons; there are no interneurons. This pathway constitutes the shortest possible reflex arc. 1 BE 312 Lab #3 Rectus Femoris Vastus Lateralis Patellar Ligament Tibialis anterior Tibia -Biceps Femoris Femur -Soleus -Gastrocnemius (Lateral Head) - Fibula Achilles Tendon HN-2-1 Figure HN-2-B2: The major extensors and flexors of the human knee and ankle joints. The stretch reflexes used in this exercise are elicited by striking the patellar tendon or the Achilles tendon. As an example of the stretch reflex, consider the reflex response that occurs when a person jumps from a low stool to the floor. The extensor muscles of the legs are stretched on landing, lengthening all their muscle spindles. The discharge of the muscle spindles is conveyed to the central nervous system through the fast-conducting afferent axons. These sensory axons enter the spinal cord through the dorsal root and synapse with the motor neurons of the same extensor muscle. In turn, the motor neurons trigger the contraction of the extensor muscle to oppose the stretch produced by landing, completing the reflex arc. This reflex is one of the main reasons you keep your balance and do not fall when changing certain body positions. In this lab, students will record electromyograms (EMGs), the summation of asynchronous electrical activity (muscle action potentials) in the multiple fibers in the muscle, and use them to determine the time between the stretch of the tendon and the arrival of the motor impulse at the muscle. Two reflexes in a human subject will be studied: the Achilles tendon reflex, and the patellar tendon (knee-jerk) reflex. Conduction times and nerve velocities for each reflex arc will be determined and compared. The effect of pre-existing tension in the effector muscle, or motor activity in other muscle groups, upon reflex responses will be measured. The coordination of motor activity in antagonistic muscles will also be studied. 2 BE 312 Lab #3 II. Setup Equipment Required ● ● ● PC or Mac Computer IXTA, USB cable, IXTA power supply ROAM EMG Alcohol swabs Disposable EMG electrodes PRH-100/PRH-200 Patellar reflex hammer Settings -> Human Nerve-> StretchReceptorReflex-ROAM EMG Cable and Reflex Hammer Setup 1. Locate Patellar Reflex Hammer and plug the BNC connector into Channel A3. 3. 2. Disconnect the ROAM EMG from the dock and place the electrodes as shown in Figure 1. Use an alcohol swab to clean and abrade two regions on the calf of the left leg for electrode attachment. One area is in the middle of the calf muscle, and the second area is about 3 inches below the back of the knee (see Figure 2). Let the areas dry. 4. Remove the plastic disk from a disposable electrode and apply it to one of the abraded areas. Repeat for the other area. Worx TA-ROAM Figure 2. Electrode placement on leg. A4 AS AB REKAM (a) (b) Figure 1. (a) The PRH-200 Reflex Hammer to be plugged into Channel A3. (b) The ROAM device for wireless measurements. 3 BE 312 Lab #3 5. Attach the two color-coded electrode leads as shown in Figure 2 for the Achilles Reflex. III. Experiments Exercise 1: Achilles Tendon Reflex Aim: To determine conduction time from tendon tap to response of the gastrocnemius muscle in the Achilles tendon reflex arc. Approximate Time: 20 minutes Procedure 1. Instruct the subject to sit on a lab bench so that the subject's thighs are supported by the top of the bench and his or her calves hang freely. 2. The Achilles tendon is located above the heel and connects the gastrocnemius muscle to the tarsal bone of the foot. Tap the tendon with the wide end of the reflex hammer a few times to locate a point on the tendon which produces a consistent contraction of the gastrocnemius muscle and a downward movement of the foot (plantar flexion). The opposite, upward movement is known as dorsiflexion. 3. Click Record and then instruct the subject to move his or her foot up and down to demonstrate the type of EMG that occurs during plantar flexion and dorsiflexion. Click AutoScale All. 4. Type Achilles in the Mark box. Click the mark button to mark the recording. Continue recording. 5. Tap the subject's Achilles tendon to elicit the stretch reflex. Record a total of five trials using the same tapping force. 6. After the fifth trial, click Stop to halt recording. 7. Select Save As in the File menu, type a name for the file. 8. Repeat this exercise on the same subject using three different amounts of force. Exercise 2: Patellar Tendon (Knee Jerk) Reflex Aim: To determine conduction time from tendon tap to response of the quadriceps muscle in the patellar tendon reflex arc. Approximate Time: 40 minutes Software Changes Click on the Settings menu and select the PatellarStretchReflex settings file. Procedure 1. Instruct the subject to sit on a lab bench so that the subject's thighs are supported by the top of the bench and his or her calves hang freely. 2. Remove the lead wires of the EMG recording cable from the electrodes over the subject's calf muscle. Keep these electrodes on the subject's calf muscle. 3. Place a new set of recording electrodes on the quadriceps muscle of the subject on the medial side of the thigh, so that: the black (-1) lead wire is attached to an electrode which is about 12 cm from the knee. 4 BE 312 Lab #3 the red (+1) lead wire is attached to an electrode which is about 10 cm above the negative electrode (see Figure 3). RAM Figure 3. Circuit diagram for recording EMGs from the thigh muscles. Using the iWireB3G on the left and the wireless ROAM on the right. Note: Ignore the green wire 4. Feel the position of the patellar tendon just below the kneecap. Place one hand on the patella (kneecap), and use the other hand to tap the patellar tendon with the reflex hammer. Find the point on the patellar tendon that causes the greatest response from the quadriceps muscle. 5. Click Record and then instruct the subject to raise and lower his or her lower leg to demonstrate the type of EMG that occurs during quadriceps contraction and relaxation. Click AutoScale All. Click Stop to halt the recording. 6. Type Patellar in the Mark box. 7. Click Record and then click the mark button to mark the recording. 8. Instruct the subject to relax his or her quadriceps muscle and that the exercise has begun. 9. Tap the subject's patellar tendon to elicit the stretch reflex. Record a total of five trials using the same tapping force. 10. After the fifth trial, click Stop to halt recording. 11. Select Save in the File menu. 12. Repeat this exercise on the same subject while the subject is voluntarily contracting his or her quadriceps. 5