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 monosynaptic junction syna syna se sy 1 apse TsTv*E r receptor neuron neuron motor neuron target mkcle, organ,or gland FIG. 12. A: schematic representation of components of a reflex arc. This reflex arc schematic is for a monosynaptic reflex. BE components in a neurological schematic. Reflex arc presented in B includes an association neuron and is, therefore, a schematic for a polysynaptic reflex arc. 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. The monosynaptic component of the patellar tendon reflex is the essential component of the reflex and is diagrammed in Fig. 13. MONOSYNAPTIC 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 muscle fibers in the thigh (fibers of the quadriceps muscle) to stretch very slightly. Special sensory recep- tors in the quadriceps muscle sense this 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 afferent- efferent synapse is monosynaptic. The information carried by the efferent motor neuron causes the quadriceps muscle to contract. All of this happens automatically and very quickly, within 20 ms. 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 VOLUME 16 : NUMBER 1 - ADVANCES IN PMYSIQLOGY EDUCATION - DECEMBER 1996 527
1 INNOVATIONS A N D I D E A S reflex hammer provides stretch stimulus patellar tendon 2 tapping the tendon causes the leg to swing up 6 quadriceps muscle (extensor) containing sensory receptor for sense stretch I motor neuron 5 3 sensory / neuron section of spinal cord I monosynaptic junction 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. POLYSYNAPTIC 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 ac- tions. 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. For example, when the leg kicks out (extends), the movement is more efficient if the muscles that nor- 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. When the extensor muscles (quadriceps) actually produce the kick outward, the motor nerves to the leg flexor (hamstring) muscles are inhibited by an interneu- ron. 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 VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 S28
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