1296 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



tern. Additionally, however, striopallidal influences 

 must be strongly energized by cortical inputs and are 

 probably expressed through cortical re-entrant cir- 

 cuits. It is difficult, if not impossible, to label these 

 pallidal contributions exclusively facilitatory (57, 

 170) or inhibitory (47, 86, 158) and, as has been de- 

 scribed in connection with other structures (e.g. 

 cortex, cerebellum, etc.), probably elements of each 

 are to be discerned. In view of rekindling of clinical 

 interest in the matter, however, it is painfully clear 

 that striopallidal mechanisms require considerable 

 reassessment. 



SENSORY INPUTS. As excitation of peripheral nerves 

 or sensory pathways in the central nervous system is 

 known to activate cephalically-oriented influences 

 mediated by the reticular formation (e.g. the arousal 

 response), it might be supposed that afiferent excita- 

 tion elicited in the brain stem would also energize 

 caudally-directed activities such as those exerted 

 upon muscle function and sensory conduction. Affirma- 

 tive evidence is extensive which indicates that reticulo- 

 spinal energies are influenced by sensory inputs, hence 

 only a few illustrative examples will be cited. Abbie 

 & Adey (i) showed that the frog with cerebral and 

 cerebellar structures removed was still able to main- 

 tain a good posture, presumably through retained 

 function of its brain stem. When dorsal roots were 

 divided, however, the postural response was elimi- 

 nated, von Euler & Soderberg (273) were able to 

 show that, while cells in the brain stem normally ex- 

 hibit a background of activity, the deaff'erented me- 

 dulla was relatively silent when tested for unit firing. 

 It has been demonstrated also that bulbospinal 

 discharges can be driven by sensory stimulation (270). 

 On the other hand, removal of sensory inputs is known 

 to deplete spinally-directed influences from the retic- 

 ular formation as division of the vestibular nerves 

 resulted in increased latency and diminished ampli- 

 tude of the monosynaptic response (96). 



Spasticity 



It is probable that all motor functions — not only 

 maintenance of normal tone but also mediation of 

 conditioned responses and voluntary motor move- 

 ment — can be explained on the basis of segmental 

 reflex activity as modified locally by inter- and 

 suprasegmental influences. Eldred et al. (71), following 

 a study of brain-stem control over gamma efferent 

 neuron activity and thus of the muscle spindle, 

 concluded, "Rough measurements . . . indicate that 



the range of bias at the command of supraspinal 

 centers is adequate to cover the whole physiological 

 range of movement." Hence, it is necessary to con- 

 sider that the flexibility necessary to perform all 

 degrees and kinds of muscle activity resides in each 

 segment of the spinal cord. Skilled and voluntary 

 actions are initiated when the responsiveness of these 

 local mechanisms is changed and this change is 

 elicited by highly integrated facilitatory and inhibi- 

 tory influences arising in cerebral structures. 



It is probably inappropriate to attempt a division 

 of suprasegmental influences into separate systems 

 which concern primarily tonic or postural phenomena 

 and those which modify phasic activity. There is no 

 doubt now that brain-stem stimulation can elicit both 

 nonreciprocal and reciprocal neuromuscular re- 

 sponses. The former clearly subserves not only tone 

 and posture but also the stabilization of joints by 

 nonreciprocal contraction of muscles during volun- 

 tary motor activity (93). The latter permits posi- 

 tioning of the trunk or limbs in keeping with the 

 requirements of voluntary movement. Implicit in 

 reciprocal responses is the fact that the myotatic 

 reflex of the agonist must be facilitated at the same 

 time that the myotatic reflex of the antagonist is in- 

 hibited. It is true that some division exists in the 

 brain stem between stimulus sites which elicit non- 

 reciprocal responses and those which induce recipro- 

 cal patterns, yet this parcellation is by no means 

 mutually exclusive. Further, inputs to these respective 

 brain-stem zones in general can no longer be classified 

 specifically as 'tonic' or 'phasic" contributors. For ex- 

 ample, the vestibule connects with both enhancing and 

 suppressing portions of the reticular formation and 

 activity elicited within the labyrinth induces change 

 in tone as well as position (97)- 



While anatomical di\ision of the brain stem into 

 ■facilitatory" and "inhibitory" zones is difHcult, clearly 

 such a division exists functionally. Also these brain- 

 stem 'centers,' while exhibiting some sustained or 

 automatic activity of their own (60, 155) are con- 

 trolled by inputs from the spinal cord, cerebellum, 

 cortex, vestibule and basal ganglia. In general, each 

 of these inputs appears capable of supplying either 

 facilitatory or inhibitory energies; doubtless one of 

 the two predominates normally in all systems, 

 although the determining or governing features re- 

 main obscure. 



It is diflicult, and possibly unnecessary, to tamper 

 extensively with classical concepts of decerebration. 

 Extensor rigidity increases as brain-stem transection 

 approaches the le\el of the vestibular nuclei and dis- 



