POSTURE AND LOCOMOTION 



1077 



freedom, coactivity or reciprocal activity between 

 alpha and gamina motoneurons may and does 

 occur. Usually in spontaneous cortically-induced or 

 reflexly-elicited motor responses, the acti\ity of both 

 neurons increases together so that the afferent dis- 

 charge from spindles rises rather than drops during 

 contractions. Parallel inhibition, too, of both types 

 of motoneurons is found (94, 123). 



Alterations in activity of alpha and gamma moto- 

 neurons are not simultaneous, or rather, the thresh- 

 old for gamma effects is lower than that for alpha 

 effects (63, 146). Distention of the bladder of the 

 spinal cat, for example, may readily lead to spindle 

 activation preceding, or in the absence of, alpha 

 activation (Abdullah, A. & E. Eldred, unpublished 

 oijservations). Furthermore, firing of lumbosacral 

 gamma fibers may persist despite bilateral multiseg- 

 mental section of dorsal roots, under which condi- 

 tions alpha activity, of course, is absent in the 

 decerebrate cat (57). Amphii^ian and mammalian 

 propriocepti\e systems, thus, differ not only in the 

 greater independence of action possible in mammals 

 but in that this independence is exercised to afford 

 gamma efferents a greater sensitivity to reflex and 

 descending influences. 



The aijove observations, together \sith the knowl- 

 edge that gamma efferents are readily influenced 

 froin a variety of brain structures (55, 94), indicate 

 that this efferent system does not merely serve to 

 adjust locally afferent inflow to changes in muscle 

 length but has wide functional significance. The 

 prominence of gamma activation in decerebrate 

 animals (57, 123) and the known dependence of the 

 stretch reflex upon annulospiral afferents suggest 

 that postural maintenance is a part of this function. 

 The observation that the net effect of afferent dis- 

 charge from passively stretched and de-efferented 

 extensor muscle is largely inhibitory to homonymous 

 alpha (115, 124) and gamma neurons (123; r/., 

 however, 146) makes it appear that some degree of 

 intrafusal tone may be requisite for the very existence 

 of a stretch reflex. Simultaneous stimulation of a num- 

 ber of isolated gamma efferents leading to a muscle 

 does, in fact, result in facilitation of the mono- 

 synaptic reflex (124). And of course, in the decere- 

 brate animal, which has vigorous gamma efferent 

 activity, stretch of the muscle leads to sustained 

 facilitation of homonymous motoneurons (3). 



The gamma efferent system, although a major 

 mechanism behind postural tone, is not necessarily 

 the sole one, for alpha neurons are also directly ac- 

 cessible to postural influences (252, 265). The cat. 



for example, which has been chronically deafferented 

 bilaterally in the cervical and thoracic segments, 

 then terminally decerebrated and spinalized at a 

 low thoracic level, still demonstrates labyrinthine 

 effects upon the forelimbs (219). Also, tonic contrac- 

 tions in deafferented limbs may be produced and 

 coactivation of alpha and gamma neurons disturbed 

 by alterations of cereijellar function (98). 



Nor is it implied that tonic contraction is the sole 

 concern of the mammalian gamma efferent system. 

 Many considerations point to a close tie with phasic 

 motility; the presence of spindles in nearly all types 

 of muscle, the strong control exerted over them by 

 motor areas of the cortex (60, 95), the rapidity of 

 pathways descending to gamma-efferents (97) and, 

 if an analogy may be used, the fact that intrafusal 

 fibers in frog spindles are innervated by branches of 

 the large phasic motoneurons as well as by those 

 mediating tonic contraction (151). Even a role in 

 adjustment of sensory perception (52) cannot be ex- 

 cluded since some muscle afferents project to the 

 sensory cortex (80, 82, igi, 204). 



Postural Adjustment 



B.oiSic MECH.^NISMS. Postural maintenance is but 

 another facet of the organism's efforts at homeostasis 

 and as such is not a reflex state of wholly unvarying 

 afferent-efferent exchange, but one in which adjust- 

 ments for nuances of imbalance are continually being 

 made. Basically, these corrections are either resistive 

 or compensatory. The stretch reflex itself serves to 

 resist changes in position and to encourage the return 

 of disturbed parts (although the latter is obstructed 

 by the lengthening and shortening reactions). The 

 standing dog, for example, in swaying forward 

 stretches tendons of digital flexors (physiological ex- 

 tensors), thus facilitating restitutive contractions of 

 these muscles (19, 233). If the dog leans too far 

 forward, compensatory mechanisms enter for new 

 receptors which favor flexor action and inhibit ex- 

 tensor tone are excited until finally the paw is raised 

 and replaced in a position of better support, the 

 hopping reaction (iii, 220). Resistive mechanisms 

 are sometimes at play in operation of the 'righting 

 reflexes'; thus the body-on-head reflex would seem 

 to be ever ready to oppo.se tilting of the head. Full 

 elicitation of righting reflexes, however, involves 

 compensatory efforts to re-establish the postural 

 status quo. The actions in which visual stimuli cause 

 the head to turn, in which vestibular organs cause the 



