932 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



These afferent fibers, henceforth called Group IB, 

 can be considered as falling exclusively in the Group 

 I band with the preponderance toward the lesser 

 diameter members of that band. 



Actually the question of lA and IB filler distribu- 

 tions in the Group I band is in a state of utter confu- 

 sion, and the interpretation of experiments, even in 

 some instances the results of experiments, is highly 

 controversial. A definitive statement cannot be 

 made at this time. To facilitate discussion of the 

 situation it should be noted that Group I afferent 

 fibers include those that form monosynaptic reflex 

 connection for mediation of the myotatic reflex 

 (54; see also below), and that it is generally conceded 

 that the muscle spindle is the myotatic reflex recep- 

 tor. 



The terms Group lA and Group IB first came 

 into use after Kuffler & Hunt (40, 44) proved that 

 the Group I band contained afferent fibers from the 

 two sorts of receptors, called A and B after the 

 original terminology of Matthews (72-74). Prior to 

 that it was known that the Group I band contained 

 the monosynaptic afferent fibers, and it was only at 

 that time that it became clear that the band con- 

 tained afferent fibers serving another reflex pattern 

 (50, 59). Subsequently Rail (80) studying the input- 

 output relation of the monosynaptic reflex of gastroc- 

 nemius found the monosynaptic reflex output to be 

 maximal at 60 to 70 per cent Group I input, an 

 observation that has repeatedly and consistently 

 failed of confirmation in the spinal preparation. For 

 instance, in spinal animals Hunt (38) and Lloyd & 

 Wilson (6g) have shown the monosynaptic reflex 

 output to increment progressively from reflex thresh- 

 old until measured Group I input is maximal, this 

 is in accord with the distribution of muscle spindle 

 afferent fibers of the gastrocnemius. Then Bradley & 

 Eccles (9) described the triphasically recorded con- 

 ducted afferent Group I spike of biceps-semitendi- 

 nosus and quadriceps, but not of other muscles, as a 

 'double negative spike.' These two peaks, in order 

 of latency, were designated lA and IB and it was 

 assumed that the A-type afferent fibers (muscle 

 spindle) occupied the first peak and that B-type 

 fibers (tendon organ) occupied the second. 



A curious aspect of this bifid allegedly Group I 

 triphasic spike is its inconstant occurrence. Bradley 

 & Eccles report it as being 'an almost invariable 

 finding,' whereas Laporte & Bessou (47) describe it 

 as having been encountered only after many experi- 

 ments. Lloyd & Mclntyre' (66) did not encounter 

 bifid spikes but, in the light of the foregoing, may 



2 4 6 8 10 12 14 16 18 // 

 12 24 36 48 60 72 84 96 10811/5 



FIG. 3. Distribution with respect to velocity (and diameter) 

 of afferent fibers IVoni B-typc muscle receptors from the soleus 

 compared with the histological aflfeient fiber spectrum. Manner 

 of construction as in fig. 2, [From Lloyd (6o); after Hunt (37).] 



not have persisted long enough. Another curious 

 aspect is that the threshold difference between lA 

 and IB according to Bradley & Eccles is greater 

 (i: i.5'2) than that between Group I and Group II 

 (1:1.48) according to Brock et al. (10), which cer- 

 tainly is a result inviting caution in the interpretation 

 and identification of the second peak of Bradley & 

 Eccles. 



It is unfortunate that Bradley & Eccles made no 

 observations on Group II fibers of quadriceps and 

 biceps-semitendinosus, for recordings illustrating re- 

 sponse of the two groups would ha\e facilitated 

 interpretation. 



Laporte & Bessou (47) report that most of the 

 afferent fibers characterized as arising in spindles 

 were located in their 'sous-groupe rapide' and those 

 from tendon organs were in their 'sous-groupe lent' 

 in the few instances in which they found bifid spikes. 

 L'nfortunately there is no indication that they fol- 

 lowed the rigid criteria established by Hunt (37) for 

 representative sampling, a most essential aspect for 

 quantitative study of units (61). Certainly the rigid 

 categorization by assumption presented by Bradley 

 & Eccles cannot hold and one must reserve judgment 



