Fluctuations in Neural Thresholds 



159 



As in the study by Pi chlr, the degree of correlation of thresJiold variation 

 for members of the same pool of motoneurons was investigated. The extent 

 of correlated and uncorrelated fluctuations is a measure of the relative impor- 

 tance in producing fluctuations of events extrinsic and intrinsic to the fiber. 

 In the spinal cord there is reason to believe that threshold fluctuation is, at 

 least in part, the eff'ect of background activity in other fibers. Such activity 

 would presumably aff'ect many fibers in a neighborhood; the threshold fluctua- 

 tions of these fibers would therefore show definite correlations. 



To determine the extent of correlated variation Rall and Hunt (6) recorded 

 the response of a ventral root together with the response of a single moto- 

 neuron belonging to an adjacent root; an example of such a recording is 

 shown in Fig. 7. Fig. 8 shows the results of an experiment based on a thousand 



-n 



nHHi 



« 1 



Fig. 7. Simultaneous recording of the responses of a single motoneuron (hori- 

 zontal deflection) and of an adjacent ventral root (vertical deflection) 

 upon repeated stimulation of the gastrocnemius nerve with identical shock stimuli. 



From Rall and Hunt (6). 



such responses. The population response amplitudes were divided into class 

 intervals, and the number of responses within each class interval was plotted. 

 For each population response within a class interval, the occurrence or failure 

 of a unit response was noted and the number of unit responses plotted 

 (shaded area). The unit responded a total of 697 times out of 1000. If 

 the population response and the unit response were not correlated, the firing 

 index of the unit would be about the same in each class interval. This is clearly 

 not the case. Instead, firing occurs infrequently when the population response 

 is small, and more often as the population response grows. The probability 

 of unit firing when the population response amplitude is in a given class interval 

 — that is, the ratio of shaded to unshaded amplitude — is plotted in the lower 

 part of the figure. If unit response and population amplitudes were uncorrelated 

 this function would be a horizontal line at about 0.7. However, it is also clear 

 that correlation of unit and population response is not complete. In other 

 words, the thresholds of the units within the population vary with respect 

 to one another, in addition to their collective (that is, correlated) fluctuation. 

 If this were not so, a particular unit would respond only after all units of 

 lower threshold had responded; therefore its probability of response would 

 be zero if the population response were smaller than a certain value, and would 

 be one if the population response were larger than that value. The lower curve 

 would therefore be a step function. 



III. POSSIBLE SOURCES OF THRESHOLD VARIATIONS 



Fatt and Katz (7) have found that at motor endplates miniature end-plate 

 potentials occur more or less randomly even though no stimulus is present. 



