Activity in single cutaneous afferents: spinal pathways and cortical evoked potentials.

The production of cortical evoked potentials by large afferent volleys is a well documented phenomenon6,7,12,22, 24. Considerable evidence has been amassed that the maximal evoked response results from a small afferent volley in a single nerve (see e.g. ref. 18). In fact, Amassian 2 has calculated that activity in as few as 12 dorsal column fibers can produce the maximum cortical evoked response. Furthermore synaptic transmission in the afferent channels to the brain requires very little summation 14,16,18. Effects elicited in the central nervous system by very small peripheral inputs have only recently begun to be studiedg,15,20,al, 32. This is surprising in the light of the work cited above and behavioral experiments which show that extremely small stimuli can be most effective in eliciting simple 20 as well as complex behaviors 2s. Weak stimuli applied to small areas of the skin can produce evoked cortical responses1,19; however, even these small stimuli probably evoke a number of action potentials in a number of fibers. In contrast, McIntyre et al. 15 have shown that a single action potential elicited in a single afferent fiber associated with a Pacinian corpuscle can produce an evoked potential in SII cortex of the cat. Similar results might be expected for other kinds of somatic afferents; however, the inconvenience of obtaining such strict control of the afferent activity has perhaps discouraged investigators from tackling the problem. Such control is now possible with new techniques developed in our laboratory (see e.g. ref. 4). These techniques and information on the functional organization of the type I cutaneous afferent system have encouraged us to undertake a similar study for these particular somatic afferents10,1a,29,3k In addition, a study of the location of pathways which carry activity of type I afferents to the cerebral cortex was prompted by the apparent absence of activity from type I afferent fibers in the major projection pathways to the cerebral cortex the spino-