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Remaining questions from Problem Set #2 4. Information into and out of the brain is often carried in more than one tract at a time. Because these channels (tracts) run parallel to one another, they can handle the same information in different ways. For example, touch information is carried in both the dorsal columns and in the anterolateral tracts. These two separate tracts conduct signals at different rates; the dorsal columns consist of large, myelinated fibers with no synapses intervening between the periphery and the brain, so these neurons conduct information much more rapidly than do the anterolateral pathway, which includes at least one synapse in the spinal cord. The two tracts also end in different places, spreading the information to more brain centers than might occur if the information were carried along a single channel. Synapses within each of the parallel pathways also transform the information. For example, the pathway from the periphery to the cortex by way of the dorsal columns includes synapses at least in the dorsal column nuclei, in the thalamus, and onto cortical neurons. At each synapse, the sensory information is integrated with other information, and the sum of all the inputs determines whether a signal will be in the post-synaptic neuron . The parallel nature of signal processing increases the potential for complexity in information processing and allows different aspects of information processing to proceed simultaneously, speeding up the process. It also provides redundancy, so if one of the tracts is damaged, others remain to convey the same information, even though they may handle it in a somewhat different fashion. Signals from cortex to motor neurons are handled in about the same way. That is, parallel pathways are common, and each pathway consists of chains of neurons connected by synapses. 5. a. Notice an action potential in the primary sensory neuron does not necessarily produce an action potential in the second neuron along the pathway. b. Notice there is not a 1:1 correspondence between action potentials in these two neurons. c. Page 1 of 4 prob_set3.htm 5/1/2009
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d. e. A sensory neuron encodes information about stimulus intensity in the number and frequency of action potentials carried along its axon. The frequency of action potentials can lead to more or less temporal summation at its synapses onto the next neuron in line. As a result, a high frequency barrage of action potentials may produce larger P.S.P.s in the next cell in the pathway and be more likely to bring it above
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