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HANDBOOK OF PH'i'SIOLOGY 



NEUROPHYSIOLOGY II 



flow from vestibular organs (as distinguished from 

 the influences of vestibular nuclei) is not indispens- 

 able for standing of decerebrate preparations (240, 

 241) nor of intact animals, including man (68). 



RETICULAR iNFLLTENCEs. Yet anotlicr type of influence 

 upon postural reflexes derives from the reticular 

 formation, a region discussed in detail in Chapter 

 LII by French in this Handbook. This diffuse region, 

 fed continuously by impulses from a variety of 

 origins, including vestibular organs (85), muscles 

 and joints (42, 200), exerts a steady influence upon 

 segmental reflexes and tonic contractions (251). In 

 part, these influences may be expected to have spe- 

 cific and organized effects upon the body muscula- 

 ture since portions of the cephalic reticular formation 

 contain ill-defined centers for some of the righting 

 reflexes (188). In concept, at least, other activity 

 mav be more formless, a force for general motor 

 arousal. Destruction (of facilitatory portions) of the 

 reticular formation leads, indeed, to akinesia and 

 hypotonia along with heightened thresholds for gen- 

 eral arousal (189). In a positive sense the eff"ects of 

 the brain stem upon posture may be seen by con- 

 trasting the behavior of the decapitate cat with that 

 of the cat in which the brain stem is deprived of all 

 neck, labyrinthine and descending influences [ex- 

 cept, in part, the cerebellum (218)]. This preparation 

 exhibits a posture of waxy flexibility in which fore- 

 arms are partially flexed, while hind legs are held in 

 moderate extension. Attempts to flex any of the ex- 

 tremities meet moderate resistance which progres- 

 sively increases as the flexing is repeated, a behavior 

 which perhaps represents an 'arousal" of motor 

 performance by the reticular formation. 



A kindred example of a nonspecific central in- 

 fluence upon posture is the SchifT-Sherrington effect. 

 If in a decerebrate cat the cord is cooled or cut at 

 the thoracic level, the posture of the forelimbs is 

 tonically shifted toward extension; or if the lumbar 

 intumescence is the site of sectioning, enhanced ex- 

 tensor reflexes may be seen in muscles of the hind 

 leg innervated by the cephalic segment, such as the 

 quadriceps (43, 232). This release is not readily ex- 

 plainable in terms of simple pleurisegmental reflexes 

 for, although reflex effects between forelimbs and 

 hind limbs are elicitable in both ascending and de- 

 scending directions, a reverse Schiff-Sherrington ef- 

 fect is not generally described, although such an 

 observation has been made in the frog (281). Further- 

 more, the release of extensor activity in the forelimbs 

 is seen even when the posterior cord segment has 



been acutely or chronically deafiferented (231). Thus, 

 it appears that there is a separate type of activity 

 arising sui generis at lumbar cord levels or through 

 long return circuits from superior levels (231). 



Circulating endocrine secretions affect general 

 motor activity (12) which suggests that they may 

 enter into facilitation of postural behavior also. 

 Stimulation of the reticular formation through the 

 direct action of epinephrine (45), for example, could 

 provide such action. A central action of progesterone 

 has been postulated to explain the decreased tonus 

 and electromyographic activity in abdominal muscles 

 during pregnancy (140, 255), or following the ad- 

 ministration of this hormone (140, 142). This action 

 would favor development of the compensatory 

 lordosis of presjnancy. 



Significance to Postural Tone of Motor 

 Innervation of Spindles 



Conceivably muscle receptors of purely passive 

 nature 'in parallel' with contracting fibers could 

 provide a mechanism for reflex posture, as may be 

 the case in fish. Their afferent discharge, varying as 

 the length of the muscle, could at one optimal length 

 be exactly sufficient to resist reflexly the load imposed 

 on the muscle by gravity. Lengthening would arouse 

 increased discharge, a greater contraction and a 

 return of the muscle toward the optimal length. 

 Posture under such a mechanism would be singularly 

 inflexible, for muscles would always seek the fixed 

 position of repose. 



The problem of resetting sensitivity of spindle 

 receptors, to permit establishment of an inflow- 

 outflow equilibriimi at any length of the muscle, may 

 be solved by making the discharge of the sensory 

 wrappings dependent also on the pull of a special 

 muscle fiber which through central innervation 

 may be held in partial contraction or 'bias.' In frogs 

 this innervation consists of thin branches of motor 

 axons leading to extrafusal muscles (136), so that 

 compensation occurs for the unloading and de- 

 creased firing of spindles in active shortening. Each 

 combination of length and contractile force of the 

 gross muscle is presumably linked with a specific 

 rate of afferent discharge, and upon the equational 

 constants of this relationship depends the fitness of 

 the external circuit through the spindle for sustaining 

 the contraction without additional aid. 



Mammals have added flexibility to spindle func- 

 tion by dissociating totally the peripheral innerva- 

 tion of extra- and intrafusal muscle. In tiiis new 



