Central Coding in the Somatic Senses
If at the end of this chapter the reader feels confused, he should be consoled by the thought that he is not alone. Reports are quoted from the literature supporting each of the following entirely factual statements: the dorsal columns of the spinal cord transmit information concerned with fine discrimination of mechanical stimuli; damage to these columns does not degrade performance based on such discriminations; all the input from spinal cord to the nuclei of the dorsal columns comes by way of the dorsal columns; the dorsal columns do not convey information on the position of joints; some of the neurons in the nuclei of the dorsal columns do fire as a function of joint position; the dorsolateral pathways are an alternative route for information on fine mechanical stimuli, including kinesthesis; the principal target of the dorsolateral pathway, the lateral cervical nucleus, contains no neurons which respond to either the movement or the position of joints; the neurons of the spinothalamic pathways are entirely concerned with crude (‘protopathic’) information, have large receptive fields, and are not modality-specific; some of the cells of origin of the spinothalamic pathways in the dorsal horns of the spinal cord encode finely discriminated tactual mechanical information, and some fibers of the spinothalamic tract end in the ventrobasal nuclei of the thalamus, where all neurons have small receptive fields and are strictly modahty specific. For homework the reader is invited to complete this inventory of paradoxical findings.
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