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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



the intermediate nucleus in the most medial part of 

 the base of the anterior horn, their axons being 

 directed to the chief flexor and extensor cell columns, 

 both ipsilateral and contralateral via the anterior 

 commissure, and to the propriospinal tracts. Certain 

 'funicular' cells along the outer margin of the anterior 

 horn probably serve a similar function (17, 79, 95) 

 and show marked chromatolysis following section of 

 dorsal roots (102). In the high cervical region these 

 poorly defined groups of cells appear to be devoted to 

 the coordination of neck reflexes and a lateral group 

 in the dorsal horn (n. cervicalis lateralis) projects to 

 the lateral part of the tegmentum of the midbrain 

 and to the cortex (72). It is possible that the severe 

 depression of ipsilateral reflexes in the cat and monkey 

 after high cervical hemisection results from asym- 

 metry of this coordination. In the medulla the corre- 

 sponding anatomical structure is the reticular forma- 

 tion the integrity of which allows even more general 

 integration of reflex behavior. Indeed, as a result of 

 his studies of the development of reflex beha\ior in 

 the amphibian embryo, C'oghill (15) concluded that 

 an integrated progression movement mediated by the 

 brain-stem reticulum was the primary response of the 

 developing nervous system. The spinal reflexes in 

 his view were fractions of this coordinated behavioral 

 unit and were more slowlv evolved. 



SUPRASEGMENT.^L INTEGR.'^TION 



Tonic Neck and Lahynntlune Re/lexa and 

 Decerebrate Rigidity 



If transection of the neuraxis is made above the 

 second cervical nerve roots, flexion, extension and 

 rotation of the neck and occipitoatlantal joints lead 

 to modifications of the posture of the limbs. Such 

 changes are minimal in degree with sections behind 

 the midpontine level. With a background of de- 

 cerebrate rigidity following transection above the 

 pons, the tonic neck reflexes become regularly demon- 

 strable (68, 77) as well as postural adjustments re- 

 lated to position of the labyrinths (tonic labyrinthine 

 reflexes). The facilitory effect that underlies decere- 

 brate rigidity must in part be related to innervation 

 of spinal segments reaching the spinal cord by the 

 vestibulospinal tracts, for section of these lessens its 

 intensity (60). The greater part of the facilitation of 

 all spinal reflexes that is associated with decerebrate 

 rigidity appears, however, to be related to the ac- 

 tivity of the pontine reticular formation. The general 



area concerned is the excitatory reticular formation 

 of Magoun and his associates (83, 86) with an efferent 

 pathway in the reticulospinal tracts. 



Decerebrate rigidity is lessened on the side of an 

 eighth nerve section and intensified on the opposite 

 side. It is little affected by bilateral section of the 

 eighth nerves. It is increased in one limlj b\ deafferen- 

 tation of the opposite limb, increased in both fore- 

 limbs by postbrachial section of the spinal cord 

 (Schiff-Sherrington phenomenon) (84) and in the 

 hind limbs by deafferenting the forelinibs (13). 

 These findings indicate an equilii^rium of effects, 

 the proprioceptors of each limb facilitating its own 

 stretch reflex and tending to inhibit those of other 

 parts, each labyrinth tending to facilitate ipsilateral 

 extensors and restrain contralateral. In addition each 

 fastigial nucleus of the cerebellum, through the 

 hook bundle of Russell to the reticular system, was 

 found by Moruzzi & Pompeiano (73) to influence 

 profoundly the distribution of rigidity. A lesion of the 

 caudal pole of one fastigial nucleus can abolish rigidity 

 on the opposite side and enhance that on the same 

 side, yet destruction of both fastigial nuclei leaves the 

 rigidity unaffected. The effect of a unilateral fastigial 

 lesion is ba.sed on inhibitory effects arising from the 

 proprioceptors of the ipsilateral muscles. Decerebrate 

 rigidity from intercollicular section is therefore a 

 disequilibrium of a complex postural integration of 

 proprioceptive facilitory and suppressor effects, 

 arising in the muscles of the limbs and reflected back 

 as a predominant extensor posture in which the tonic 

 neck and labyrinthine reflexes determine the major 

 pattern. The most fundamental single element in 

 this complex appears to us to be the enhancement 

 and integration of the proprioceptive positive sup- 

 porting reaction by the pontine reticular formation. 

 This is associated with increased discharge of the 

 small-fibered gamma motor system (33, 43). The 

 contactual type of positixe supporting reaction that 

 is grouped with the proprioceptive together as the 

 'Stiitzreaktion' by Rademaker (78) is notably absent. 



In decerebrate rigidity there is also an exaltation of 

 some flexion reflexes (20, 44) and of sympathetic and 

 parasympathetic (bladder) responses indicating that 

 an incongruous mixture of related and unrelated 

 reflex effects is released. We would doubt the exist- 

 ence of a pontine center (97) for urination, for ex- 

 ample. There is no good evidence that the pontine 

 reticulum is organized as a center in terms of a series 

 of specific categorical functions, although in ascend- 

 ing levels the first stage of simple grouping and 

 facilitation of excitatory (and to a less degree for 



