Can Sympathetic Outflow Influence Afferent Activity in Man?
The total afferent barrage of signals traveling centripetally through the CNS input systems far exceeds the number of impulses that can be analysed by the brain. Various inhibitory mechanisms exist which filter out irrelevant signals so that the organism can concentrate on important infomation vital for its survival. The mechanisms of presynaptic inhibition at the afferent terminals and postsynaptic inhibition at the earliest synaptic levels in the CNS have been extensively studied (see Eccles, 1964) and it is generally accepted that these forms of inhibition are predominant in the vertebrate nervous system. Direct influence on afferent activity at the receptor level is common in the somesthetic and kinesthetic system of invertebrates, but not in vertebrates (Schmidt, 1973). However, some physiological studies in animals have yielded data suggesting that even in vertebrates a peripheral modulation of the afferent input may exist in the somesthetic system (see Table 1). Futhermore, clinical observations initially made over a hundred years ago (Mitchell, 1872) on some patients with peripheral nerve lesions are most readily explained by modulation of certain afferent signals at the peripheral level. A number of findings suggests that the postganglionic sympathetic fibers which travel to the periphery could be involved in such a modulatory function.
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