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 interneuron or associatior neuron sensory neuron incoming f l\;;g:Ttion ~goingfl~~-LM~~~~or -I dorsal (back) side tract containin motor neuron information information ventral (front) side FIG. 8. Schematic representation of different directions information travels in the spinal cord. On the left half of the spinal cord, the horizontal direction of information travel in the spinal cord is shown. Information comes in from the sensory neuron to the dorsal (back) side of the spinal cord. Information is passed by an interneuron to the motor neuron. Motor information leaves the spinal cord from the ventral (front) half of the cord. On the right half of the spinal cord, sensory information in an upgoing tract is found in the dorsal half of the spinal cord. This tract continues upward through the spinal cord to the thalamus and then the cerebral cortex. In the ventral half of the spinal cord, motor information in a downgoing tract is found. This tract originates in the cerebral cortex and descends to its target. deep touch. Other tracts carry information about limb position. Descending tracts carry motor information destined for muscles, visceral organs, or glands in the periphery. There are many different tracts in the spinal cord. The different directions of information travel within the spinal cord are like people riding an escalator of a busy skyscraper. People (information) can get on and off at different floors (levels of the spinal cord). They can also ascend and descend in an escalator. To speed up efficiency, different professions ride their own set of escalators. Likewise, different types of information have their own tracts. Different types of sensory information have their own upgoing tracts (up escalators) in the dorsal (back) half of the spinal cord, and motor information has its own downgoing tracts (down escalators) in the ventral (front) part of the spinal cord.2 A pictorial representation of the different directions of information travel in the spinal cord is found in Fig. 8. 8) -+Describe the two directions that information can travel within the spinal cord. 9) *How does information travel in neurons? 2 Some students may feel that an elevator would be a more practical approach to efficiency and to this example. However, an elevator can travel both upward and downward. When information travels in a tract, it travels in only one direction: upward or downward like an escalator, not in both directions like an elevator. Information cannot “ride” the same tract to ascend and descend. Sensory information travels upward in tracts located in the dorsal (back) half of the spinal cord, and motor information travels downward in tracts located in the ventral (front) half of the spinal cord. Therefore, the example of an upgoing or downgoing escalator is preferred. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 s21 g
I N N 0 V A T I 0 N S A N D I D E A S Basic Concepts of Neurobiology The cell involved in carrying information around the body is the neuron. An illustration of a neuron is found in Fig. 9. The neuron has two types of projections or processes from its cell body: axons and dendrites. Axon projections can be very long. Den- drites are shorter processes that bring information toward the neuronal cell body. Dendrites are really extensions of the cell body. Axons carry information away from the cell body. Axons may be covered by an insulating sheath of myelin that wraps around them like a jelly roll. If an axon has myelin around it, it is myelinated. Information moves faster along a myelinated axon than along an unmyelinated one. Parts of an axon left uncovered by myelin are called nodes of Ranvier. When information is carried by a myelinated axon, the information will jump from node to node. This makes the transmission of information faster than if the information had to go straight through the axon. Again, some axons are myelinated, and some are not. However, dendrites are never myelinated (because they are extensions of the cell body). Usually, there are multiple dendrites bringing information toward the neuron’s cell body. In contrast, there dendrites neuronal cell body is usually only one axon leading away from a neuronal cell body. An axon branches when it reaches its target (another neuron, muscle cell, organ, or gland). An axon usually terminates on the next neuron’s dendrite or cell body. The nerve impulse is then transmitted across the tiny synaptic space. A schematic of synaptic transmission is shown in Fig. 10. Notice that in Fig. 2, there are no labeled dendrites. This is because the type of sensory neuron involved is unique. It only uses an axon to carry its information toward the CNS, and it has no dendrites. Exceptions like this to general classifications are commonplace in the nervous system and make the nervous system one of the most complex systems of the body. 10) +Which processes bring information toward the neuronal cell body? 11) -Which processes take information away from the neuronal cell body? 12) *In what forms is information carried by the neuron? Information is carried along axons and dendrites in an electrical form. The movement of differently charged axon FIG. 9. Myelinated neuron with labeled structures, such as the axon, dendrites, neuronal cell body, axon hillock, and nodes of Ranvier. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 s22
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