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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



lar formation of the pons and the vestibular nuclei, 

 others pass through the dentate nucleus to the upper 

 levels of the thalamus and motor cortex. From there 

 cerebrospinal fibers carry impulses coming from these 

 circuits downwards. Thus the pyramidal tract may 

 function as one of the efferent pathways of the "extra- 

 pyramidal' system (fig. 12). Most of the coordinative 

 integration of the extrapyramidal system at supra- 

 spinal levels is carried out in cerebelioponto-olivary 

 centers, including the short neuron systems of the 

 reticular formation. 



Thus the complex motor regulatory influences of 

 the basal ganglia are exerted through the cerebellum 

 and the motor cortex. The main descending path in 

 the brain stem for these cerebellar circuits is the cen- 

 tral tegmental tract from the red nucleus to the 

 reticular formation and inferior olive. The result of 

 this integration in short and long chains of neurons, 

 coordinated with vestibular and cortical impulses and 

 with cervical proprioceptors, is transmitted to the 

 anterior horns. In parallel with the descending retic- 

 ulospinal tracts, the interstitiospinal and vestibulo- 

 spinal pathways influence the motor horn cells mainly 

 for the direction-specific movements of head and body. 

 The rubrospinal tract (from the magnocellular red 

 nucleus) has no significance in the human. The eflTer- 

 ent pathways of the parvicellular red nucleus are re- 

 laved through the central tegmental tract in the 

 reticular formation [VVeisschedel (291)] or coor- 

 dinated with cerebellar circuits through the inferior 

 olive. 



At the spinal level the efferent extrapyramidal im- 

 pulses carrying downwards the integrated and simpli- 

 fied result of cerebral coordination cooperate with 

 the propriospinal interneuron systems, fed by extero- 

 and proprioceptive aflferents from the periphery. Im- 

 pulses in the long cerebrospinal tracts end mostly on 

 spinal interneurons. Direct endings on alpha moto- 

 neurons, if existent, are of little significance, but end- 

 ings on gamma motoneurons may be more important. 

 This spinal sensorimotor coordination cannot work 

 properly without the external loops of the gamma 

 motoneurons, regulating proprioceptive afferent flow 

 from the muscle spindles. By this convergence of 

 neuronal circuits the highest motor centers are able to 

 control motor readiness and to command motor per- 

 formances at the lowest level of peripheral motor 

 effectors. 



The relation of higher extrapyramidal centers to 

 spinal reflexes has not yet been sufficiently investi- 

 gated. Control of the gamma motoneuron system 

 and muscle spindles over two separate pathways from 



the reticular formation seems to ije a proijable function 

 of lower extrapyramidal structures. 



The role of the extrapyramidal system in posture 

 and locomotion is clarified i^y application of Hess' 

 dynamic interpretation in terms of his concept of 

 'teleocinetic" and 'ereismatic' motility. Normal pos- 

 ture is the result of a dynamic equilibrium of central 

 antagonistic forces continuously active in the waking 

 state and controlled by afferent impulses. On the 

 lower motor mechanisms, mainly dependent upon 

 labyrinthine and cervical propriocepti\e influences, 

 is superimposed a mesodiencephalic coordinating 

 apparatus for body posture showing an elaborate 

 direction-specific differentiation, closely integrated 

 with the cerebral cortex and operating in the three 

 dimensions of space. The physiological mechanism of 

 'ereismatic' supporting motility seems to be primarily 

 proprioceptive but is regulated in anticipation of 

 action via the gamma system and its extrapyramidal 

 control. However, we are still ignorant about the 

 mechanisms of motor anticipation in the higher 

 extrapyramidal and cortical centers. 



The relation of extrapyramidal functions to in- 

 stinctive behavior is revealed by the findings of com- 

 parative physiology and some reactions of decorticate 

 mammals. In birds having a highly developed 

 striatum and little cerebral cortex, the upper and 

 lower striatum can regulate their behavior and they 

 can even exhibit some learning without the cerebral 

 cortex. Complex instinctive actions as mating and 

 nesting require the hyperstriatum but not the cortex. 

 Simpler components of their feeding, drinking, fight- 

 ing and courting ljeha\'ior are possible without the 

 cerebral cortex and hyperstriatum but only if the 

 ectostriatum and mesostriatum are intact. 



Relations of the extrapyramidal system to the 

 thalamoreticular regulating system are suggested 

 by phylogenetic, anatomical, electrophysiological and 

 clinical evidence. Apparently extrapyramidal centers 

 contain the motor mechanisms of attentive behavior 

 and of the postural accompaniments of wakefulness in 

 close coordination with the reticular formation of the 

 brain stem. Ph\logenetically the extrapyramidal cen- 

 ters seem to have been differentiated from the central 

 core of the brain stem regulating motor behavior. 

 The reticular formation is primarily a motor coor- 

 dination apparatus. The psychological aspects of at- 

 tention and consciousness are only secondary differ- 

 entiations from this basic regulation of behavior. 



In spite of the extensive literature on the extra- 

 pyramidal motor system we must confess that we know 

 surprisingly little about the essential phvsiological 



