construction of a model demonstrating neural pathways and reflex arcs

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INNOVATIONS A N D I D E A S synapse / neuron 1 synaptic vesicles containing chemical neurotransmitters neurotransmitters being released ) v- neuron 2 FIG. 10. Schematic showing synaptic transmission. Axon from neuron 1 is shown releasing chemical neurotransmitters into the synaptic space between the 2 neurons. A dendrite of neuron 2 is receiving the chemical neurotransmitters as they travel across the synaptic space. ions (positively charged substances and negatively charged substances) causes an event called an action potential. An action potential is the electrical current form of information in the neuron. The generation of an action potential occurs in a special location close to the cell body of the neuron, the axon hillock (Fig. 9). This is a probability event. If enough charged ions reach the axon hillock to cause an action potential, the action potential will occur. If there are not enough ions to trigger an action potential, the action potential will not occur. This is described as an all-or-none phenomenon. Informa- tion is either carried in its entirety through a neuron, or it is not carried at all. If information is carried, it is carried with its full strength and content. There is no weakening or strengthening of a message sent in an action potential. The action potential within a neuron is an electrical event. When a neuron passes its information to another neuron, a chemical event known as synaptic transmission occurs. Synaptic transmission involves the release of proteins called neurotransmitters into the space between two neurons (Fig. 10). Proteins are chemical substances; therefore, the method of transmission becomes chemical, not electrical. There are many different neurotransmitters within the nervous system. Some turn on the next neuron in line and are called excitatory neurotransmitters. Excita- tory neurotransmitters ensure that the action potential is carried by the next neuron in line. Some neurotransmitters turn off the next neuron in line and are called inhibitory neurotransmitters. These inhibitory neurotransmitters prevent the next neuron in line from carrying the action potential. One neuron normally releases only one type of neurotransmitter, although it has recently been shown that some neurons can release two or more types of neurotransmitters. There are many combinations of different neurotransmitter sequences in the body. These different combinations make the body’s reac- tions to different stimuli unique. 13) +Describe the all-or-none phenomenon of an action potential. Is it chemical or electrical? 14) -+What are the two different classifications of neurotransmitters? 15) -+Why is the axon hillock a special structure involved in transmission of an action potential? 16) *How does the neuron handle both the chemical and electrical forms of information? VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 S23
Summary I N N 0 V A T I 0 N S A N D I D E A S An action potential is generated at a neuron because of a stimulus. This action potential travels along its axon until it reaches the end of the axon. When the action potential reaches the end of the axon, it causes the release of chemical neurotransmitters. Because neurotransmitters are proteins produced by the body, they are forms of chemical, not electrical, transmission. Neurotransmitters are picked up by the den- drites of the next neuron. Synaptic transmission has occurred. The type of neurotransmitter released, whether excitatory or inhibitory, plays a part in how the information will be passed along this neuron. The chemical form of information is converted to an electrical form at the corresponding dendrite. The electrical form of information is carried by the dendrite toward the neuronal cell body. If enough electrical charge reaches the axon hillock, a new action potential is created. The whole process repeats as the information is passed along to the next neuron and throughout the entire nervous system. Finally, information reaches the motor neuron, which delivers the highly processed message to muscles, glands, or body (visceral) organs. 17) -+What is the difference between synaptic transmission? a synapse and 18) *What is a reflex, and why is it important to the nervous svstem? Answers to Text Questions 1) The three types of neurons are sensory (afferent) neurons, motor (efferent) neurons, and association neurons or interneurons. Association neurons or interneurons are links between sensorv and motor neurons and can, therefore, be classified as either afferent (carrying information toward the CNS) or efferent (carrying information away from the CNS), depending on the situation. Therefore, in the strict sense, association neurons are neither afferent nor efferent. 2) The CNS is comprised of the brain and spinal cord. It is important to realize that the separation of the nervous system into two separate components is an artificial one; all parts of the nervous system are connected. 3) The cerebellum is involved in coordinating motor actions. The cerebral cortex is involved in almost all processes of the nervous system. It is linked with the conscious awareness of information and contains sensory, motor, and association areas. 4) Sensory areas of the cerebral cortex deal with incoming sensory information from the body and from the body’s interpretation of external stimuli. Motor areas of the cerebral cortex are involved with actions. The actions can manifest in skeletal muscles, smooth muscles, cardiac (heart) muscle, visceral organs, or glands. Association areas, or “silent areas,” of the cerebral cortex are involved with higher brain processes: memory, reasoning, problem solving, and concentrat- ing, just to name a few. 5) Just as the boss makes most of the important decisions in a company and is kept informed about the company’s activities, the cerebral cortex makes simi- lar decisions and is aware of information concerning the entire body. 6) The thalamus acts as a “customs agent” to the “country” of the cerebral cortex by integrating and directing all incoming information to the cerebral cortex. The thalamus is also informed about all information exiting the cerebral cortex. 7) Information travels to the brain in special groups of neurons that deal with the same types of information called tracts. Information can reach the brain by way of the spinal cord. The spinal cord is the site where spinal nerves enter and exit to “deposit” their information into specialized tracts going to the brain. Uniyuely, cranial nerves do not use spinal cord tracts to take their information to the brain. Recall that the spinal cord is an extension of the brain downward to the coccyx (tailbone). The spinal cord no longer exists at the level of the head. However, cranial nerves carry their information into the hindbrain where the information is segregated and distributed to appropri- ate areas of the brain. We will not be dealing with cranial nerves and their pathways in this exercise. VOLUME 16 : NUMBER 1 - ADVANCES IN PHYSIOLOGY EDUCATION - DECEMBER 1996 S24
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