POSTURE AND LOCOMOTION 



1073 



contract more readily tlian the I'ed semitendinosus 

 (112). 



In voluntary contraction of a muscle, activity is 

 consistently aroused in some single units earlier than 

 in others (47, 50, 127, 157, 234, 249) Also, units 

 activated in willed "primary' movements are not the 

 same ones that first appear when the muscle is acting 

 in accessory postural fixation (47, 234). Although in- 

 terpretation of disparity in the size of potentials as 

 picked up by unifocal electrodes is difficult (47, 131), 

 it is suggestive that the units activated in fixation {47) 

 and those appearing only secondarily on voluntary 

 contraction {157, 208) have relati\ely large action 

 potentials. This is somewhat paradoxical, for units 

 with high thresholds in voluntary contractions are 

 late to respond to galvanic stimulation of motor 

 nerves which indicates that their axons are small 

 (157). In accord with the latter finding, fragmentary 

 histological evidence indicates that motoneurons to 

 red, or tonic, muscles are smaller than those to their 

 immediate companions of paler composition, e.g. 

 the soleus versus the gastrocnemius (46, 54, 245, 256); 

 this is, perhaps, also true for the medial versus the 

 lateral head of triceps brachii (61 ), and the semitendi- 

 nosus as compared with the .semimembranosus (61). 

 It can only be said at present that relationships be- 

 tween the axon size of a motor unit and its evoked 

 EMG potential are not well known (208). 



Tokizane and other Japanese workers (70, 138, 139, 

 141, 260), in studies on the motor units of human 

 muscles, have come to recognize a 'kinetic' type of un- 

 even discharge and a more uniformly firing 'tonic' 

 type upon the basis of irregularity of ,spike-to-spike 

 intervals in the discharge of single units. It is inferred 

 that these units mediate phasic and tonic contractions, 

 respectively, and in accord with this the relative 

 number of kinetic and tonic units varies with the 

 functional nature of the muscle. The gastrocnemius, 

 for example, has more kinetic units than the soleus 

 (70). Similarly, the abductor pollicis muscle is 

 largely kinetic (138), while the .sphincter ani is tonic 

 (139). Perhaps related to this characterization is the 

 observation that units which are preferentially acti- 

 vated in 'primary' movements tend to fire in asyn- 

 chronous showers, whereas those activated in con- 

 tractions serving purposes of fixation have a periodic 

 discharge (47). A claim has been made that two 

 types of electromyographic units in man and other 

 mammals may be separated by curarization or by 

 hypno.sis (230). 



Recently, attention has been directed to the moto- 

 neurons themselves for evidence of two kinds of units. 



Some gastrocnemius motoneurons, for example, 

 may consistently be induced to fire at les.ser degrees 

 of stretch of the homonymous muscle, or inten.sity 

 of electrical stimulation of the muscle nerve, than 

 other units (3). Granit and his co-workers have 

 separated the units in the motor pool of the triceps 

 surae muscle (i.e. both red and white muscle) into 

 phasic and tonic units, according to the duration of 

 firing in response to single shock stimulation of the 

 muscle nerve after a preceding period of tetanic 

 stimulation (96). Also, Eccles and his co-workers, 

 through the use of intracellular electrodes, have 

 distinguished tonic and phasic motoneurons in the 

 motor pools of a variety of muscle complexes (53) 

 They characterize the tonic units as exhibiting long- 

 lasting after-hyperpolarization, more general accessi- 

 bility to monosynaptic acti\ation from afferents of 

 synergistic muscles and .slower conduction rates in 

 their axons. Separation of units into tonic and phasic 

 classes is not sharp, however (53), and wide \ariation 

 in responsiveness of units is found if rigorous steps are 

 taken to ensure revelation of all units capable of 

 firing monosynaptically upon stimulation of com- 

 ponents of the triceps muscle nerve (179). Further- 

 more, since firing indices of individual units may be 

 shifted drastically by alteration in the background 

 level of facilitation, it is difficult to decide whether 

 variation in firing capacity is truly bimodal or not 

 (179). Certainly curves relating the height of mono- 

 synaptic response to stimulus strength do not show 

 the bimodality (175) which would be required if 

 there were two classes of neurons of greatly differing 

 thresholds to monosynaptic testing. 



Conceivably, even though motoneurons of a given 

 motor pool are all of the same type, muscle fibers of 

 dissimilar nature might be diflferentially stimulated 

 through partial blocking of the passage of impulses 

 onto some branches of the a.xon, such as the thin 

 'accessory fibers', or across some myoneural junctions. 

 Such exception to the principle of all-or-none con- 

 traction of the motor unit has been adduced to ac- 

 count for the graded augmentations of muscle action 

 potentials produced by stretch or a preceding period 

 of indirect stimulation of the muscle [see review by 

 Hodes (120)]. In chickens, for example, it is claimed 

 that one quarter of the gastrocnemius fibers, which 

 respond after a conditioning shock or tetanic stimu- 

 lation of the motor nerve, do not contract following 

 an isolated shock to the nerve (29). Normal mam- 

 malian muscle may also exhibit augmentation of 

 potential during a series of supramaximal indirect 

 stimuli (208), but generally the phenomenon is 



