Discharge patterns of the slowly adapting mechanoreceptor afferent units innervating the non-warty skin of the frog.

Discharge patterns of slowly adapting (SA) mechanoreceptor afferent units innervating the non-warty skin of frogs, were analyzed statistically. Because SA units in the frog did not always produce sustained discharges for a long period, analysis was carried out on ensembles of interspike intervals (ISIs) obtained in two ways: (1) a series of successive ISIs obtained in the quasi-stationary state of maintained discharges and (2) an ensemble of ISIs (X(1),X(2),..., X(50)), obtained on repeated trials of stimulation at about 1,000 msec following the onset of stimulation. In some units these two types of ensembles of ISIs were collected. Statistical analysis of a series of successive ISIs in the quasi-stationary state disclosed two kinds of discharge patterns, irregular and regular, corresponding to those of the SA cutaneous units of mammals and reptiles. We named these two types of units Frog type I and Frog type II (Ft I and Ft II) units, respectively. In addition, the quantitative study of an ensemble of ISIs in the non-stationary state revealed that Ft II units discharged regularly in response to an optimum range of indentation but irregularly to indentation outside this range, although Ft I units produced irregular discharges irrespective of the magnitude of indentation applied.