Direct Contact with the World
Critical survey of all available data suggests that there is lasting merit in the nineteenth-century scheme dividing primary afferent neurons of skin nerves into strictly specialized, quality-specific submodalities (or labelled lines), of mechanoreceptors, thermoreceptors, and nociceptors. Among the mechanoreceptive fibers further subdivisions have been discerned. Whether or not thermoreceptors fall into two categories, cold-sensitive and warm-sensitive, is in dispute, but the weight of evidence seems to fall in favor of the notion that there indeed are two categories of temperature sensors.
It is suggested that theories of quality-distinctions of cutaneous nerve signals based on ‘patterning’ of nerve discharges failed, to date at least, to demonstrate convincingly that information of the nature of skin stimuli is contained in the temporal sequencing of neural discharges or in their spatial distribution (the latter seems reserved for transmitting information of the spatial distribution of the stimuli, not of their quality). While specificity of the adequate stimulus appears to be the code for the quality in cutaneous peripheral nerves, the coding of the same information in the central nervous system is not understood as yet.
A discussion of the chemosensitivity of nociceptive C-fibers is deferred until the end of Chapter 7.
The concept of the receptive field is introduced. This will later emerge as a cornerstone in the coding of information not only in the tophographic senses (cutaneous and optical) but also for the receptor sheet of the basilar membrane of the cochlea.
KeywordsReceptive Field Nerve Ending Mechanical Stimulation Sense Organ Myelinated Fiber
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