CHAPTER XCIX 



THE CENTRAL CONTROL OF POSTURAL REACTIONS; THE 



CEREBELLUM 



The prime essential for muscular tone and its plastic reactions is the 

 integrity of its own proprioceptive reflex arc. Certain centers in the 

 brain exert a modifying influence upon the degree of tone which this 

 arc maintains, but if the afferent fibers from the muscles are damaged, 

 these centers cannot replace them in their effect on the motor neuron 

 of the proprioceptive reflex arc. Thus the exaggerated tone of the ex- 

 tensor muscles which is produced by the action of the centers in the 

 brain of animals from which the cerebrum is removed disappears at once 

 if the afferent roots are cut. 



Since the proprioceptive reflex arc is necessary for the production 

 of tone in muscles, conditions of diminished tone or flaccidity appear 

 in disease affecting either the afferent fibers from the muscle or its 

 motor neurons. If the former alone is damaged tone will be dimin- 

 ished or lost without paralysis of the muscle. This is an unusual con- 

 dition since the afferent and efferent paths are only separated during 

 their passage into the cord through the spinal nerve roots. It occurs, 

 however, in locomotor ataxia, in which the primary lesion lies in the 

 posterior spinal ganglia, and in the ganglia of the cranial nerves. If the 

 motor neuron alone is affected the loss of tone will be accompanied by 

 paralysis of the muscles, that is a flaccid paralysis will result. This is 

 an important principle in determining where the lesion which gives rise 

 to paralysis is situated, since paralysis produced by lesions of the higher 

 motor centers and tracts is rarely accompanied by permanent flaccidity. 

 A typical flaccid paralysis occurs in anterior poliomyelitis in which 

 the lesion is situated in the ventral horn of the gray matter of the cord, 

 involving the cell bodies of the motor neurons. 



The Influence of the Brain on the Local Tonic Reflex 



When the proprioceptive reflex arc is isolated from the brain by com- 

 plete section of the spinal cord, a condition called spinal shock inter- 

 venes for a period during which reflexes may not be elicitable and the 

 muscles become atonic. After a period of time, which is longer the 

 higher the animal in the evolutionary scale, the reflexes return and tone 

 is regained, but not quite in normal degree. This condition holds true 



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