CHAPTER XX 



EFFERENT PATHS AND REFLEX ARCS 



THE motor apparatus is a complex mechanism into which the pyramidal 

 system enters as a single factor. The primary motor neurons of the brain stem 

 and spinal cord are also under the influence of other motor centers than those 

 found in the cerebral cortex. They receive impulses from the corpora quadri- 

 gemina through the tectospinal tract, from the lateral vestibular nucleus by way 

 of the vestibulospinal tract, from the large motor cells of the reticular formation 

 through the reticulospinal path, from the cerebellum, and probably also from 

 the corpus striatum by way of the red nucleus and the rubrospinal fasciculus. 

 Perhaps, also, impulses descend from the thalamus or subthalamus by way of a 

 thalamospinal tract. 



We must not think of the individual parts of this complex mechanism as 

 functioning separately, since each of these motor centers contributes its share 

 to the control of the primary motor neuron, upon which as the "final common 

 path" all these efferent pathways converge. Only by keeping this fact con- 

 stantly in mind can the motor functions be properly understood. The same 

 idea has been well stated by Walshe (1919): 'Tn stimulation experiments on the 

 motor cortex we see a complex motor mechanism at work under the influence 

 of an abnormally induced, crude form of hyperactivity of the predominant partner 

 in this mechanism. Conversely, after destructive lesions, we observe it at work 

 liberated from the control of this predominant partner and deprived of its actual 

 cooperation." 



On the other hand, the grave motor disturbances resulting from lesions in 

 the basal ganglia and especially the corpus striatum with little or no involvement 

 of the corticospinal tracts (paralysis agitans, Auer and McCough, 1916; bilateral 

 athetosis, Cecile Vogt, 1911; and progressive lenticular degeneration, Wilson, 

 1912-14) have recently called attention to the importance of the corpus striatum 

 and the extrapyramidal motor path (see p. 324). In these diseases voluntary 

 movements are impeded by tremor, rigidity, and athetosis; and in all probability 

 these disturbances arise because the pyramidal system is deprived of the co- 

 operation of one of the subordinate "partners" in the motor combine. 

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