1072 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



myograpliic monitoring of activity in opposing pairs 

 of leg muscles in man indicates that the contractions 

 are reciprocal (167). 



Patterns of Motoneuron Activation 



The participation of neuronal and muscular ele- 

 ments in postural contraction is imperfectly known 

 despite innumeraijle obser\ations beginning with 

 those of Ran\'ier in 1874 (224). Much older work 

 attempted to attribute tonic contraction to supple- 

 mentary innervation of striated muscle by sympa- 

 thetic or parasympathetic systems [for reviews see 

 (118, 259)] or to gamma-sized motoneurons (iio). 

 These efforts have failed of conxincing demonstration, 

 and current thought holds that all skeletal muscular 

 contraction in mammals is mediated by alpha moto- 

 neurons (146, 154, 259). Assuming for the present 

 that all muscle fibers in a motor unit contract in 

 response to a single axonal impulse, three possibilities 

 exist by which a motor pool mav maintain subniaxi- 

 mal contraction of a muscle: a) all neurons may be in 

 continual but submaximal action; b) all neurons of 

 the pool may participate but only a few discharge 

 at any one time; or c) selected motor units may be in 

 continual action, the others remaining idle until 

 recruited for phasic or stronger tonic contraction. 



The fact that motor units are activated at higher 

 rates as the strength of contraction increases is well 

 known. It appears, though, that this mechanism 

 may have limited application for, in moderate con- 

 traction of the muscle, this increase in rate is not 

 large as compared with the minimum rate at which 

 the unit fires (17). At any rate, most accounts of 

 single unit electromyographic recording note the 

 entry of new units as contractions are intensified. 



The persistent idea that units engaged in tonic 

 contraction are in rotational activity (7) first received 

 wide attention following a review by Forbes (67) 

 in which it was mentioned as a mere speculation. 

 Evidence from electromyographic sampling of single 

 units of muscles in sustained reflexes, decerebrate 

 rigiditN' or \oluntary contractions (i, 17, 121, 221, 

 225, 249, 272, 278), however, has not revealed such 

 rotation. Active units, on the contrary, ma\' fire 

 continuously for over one-half hour in sustained (171) 

 or repeated contractions (88). Presumably, such units 

 do not fatigue becau.se the low rates of motoneuron 

 discharge (i, 17, 22 ij are below the frequencies re- 

 quired for tetanic fusion of single units (90). Even 

 extraocular muscles, to which motoneurons may dis- 

 charge at rates up to 170 per sec, are not maximally 



taxed, for fusion thresholds are correspondingly in- 

 creased to 350 per .sec. (225). Although rotation in 

 the above sense is unsubstantiated, fluctuations in 

 activity between motor pools or segments of the pool 

 of a single muscle might be expected to result from 

 the kaleidoscopic adjustments of balance made by 

 the animal's body. These waverings are seen impres- 

 sively on cephalograms taken during Romberg test- 

 ing in man (113). They could, through localized 

 stresses within a muscle, cause myotatic contractions 

 of single heads ( i 70) or even minute slips of the mus- 

 cle (37, 170). The alternative to equipotentiality 

 among units in a motor pool, and the implied rotation 

 during submaximal contraction, is differential sensi- 

 tivity among the motoneurons. The amphibian pro- 

 vides a complete illustration of this, for tonic contrac- 

 tion, attaining perhaps 15 per cent of the available 

 ntuscle strength, is mediated by a distinct class of 

 ventral root fibers of small caliber which lead to 

 special small muscle fibers (153, 155, 156, 256). 

 The latter ha\e slow, sustained and nonpropagated 

 contractions, whereas the larger mu.scle fibers, which 

 arc inner\ated by neurons of greater size, contract 

 only phasically (153). The small nerve system has a 

 lower threshold to cutaneous stimulation than the 

 large one and is thought to be the instrument of 

 postural tone (146, 155). Crustacea also have a dual 

 contractile mechanism (152), sometimes with the 

 additional complication of inhibiting neurons (279). 

 In mammals little is known concerning the rela- 

 tions of the motoneuron to red and white, bright and 

 dark, field and fibrillar or other descriptive types of 

 muscle fibers (21, 150, 282). Even the functional in- 

 ference that red and white muscle types correlate 

 with speeds of contraction is questioned (46), although 

 the generalization tiiat red muscle is concerned pri- 

 marily with sustained contraction (112, 207) continues 

 to receive support. Se\eral investigators, for example, 

 have noted that the soleus, a relatively red muscle 

 e\en in man (268), is more active in standing than 

 other heads of the triceps surae (i, 47, 70, 133, 144, 

 206). Units in deeper portions of muscles, w-here lie 

 muscular heads (46) or strata (48, 91) of redder fibers, 

 often ha\e low thresholds in tonic contraction (138), 

 tendon reflexes (47) or in the spastic activity in para- 

 plegia (47). Even in the anterior tibial muscle, a 

 physiological flexor, red portions have a lower 

 threshold and longer after-discharge to elicitation of 

 flexor reflexes or stimulation of the motor cortex than 

 paler and faster-contracting superficial layers (91). 

 Similar elicitation of flexor reflexes, however, seems 

 in the rabbit to cause the pale semimembranosus to 



