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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



tern of disturbance we have described results from 

 interference with any of these structures, ahhough re- 

 lease of segmental stretch reflexes in the relevant 

 muscles requires damage to the precentral gyrus or 

 pyramidal tract. Similarly the coarse reaction called 

 proprioceptive placing (2, 4, 104) is directly compa- 

 rable to the 'true grasp reflex' and emerges as a sub- 

 cortical coordination that eventually becomes very 

 facile in long term survival after removal of the pre- 

 and postcentral gyri (26). It is possible that a 

 large area of moving contact is the adequate stimulus, 

 rather than displacement (proprioception). 



Extrapyramidal Motor Responses 



Instincti\e tactile grasping and placing reactions 

 are released by mesial frontal lesions. We (22, 24, 26) 

 have found that whereas the natural reaction to a 

 light tactile stimulus is a delicate balance between a 

 slight movement of withdrawal or a tentative palpat- 

 ing movement, a lesion of any part of the cingulate 

 cortex, supplementary motor area of Woolsey or area 

 8 of Brodmann, disturbs this balance in favor of exag- 

 geration of grasping. A lesion including all this strip 

 is necessary to produce the most prolonged release of 

 grasping and placing. This release is associated with 

 disappearance of the contrary withdrawal or 'tactile 

 avoiding reaction.' Ablation of the parietal cortex or 

 of the pre- and postcentral cortex results in release of 

 such avoiding reactions. The cortical pattern of such 

 withdrawal responses appears to correspond to that 

 of the extrapyramidal 'inhibitory' effects found by 

 Tower (gg) by cortical stimulation of limbic and re- 

 lated cortex after bilateral pyramid section. A 

 detailed consideration of the cortical mechanisms 

 of tactile and visual avoiding has been presented 

 elsewhere (24, 26). It is necessary here only to point 

 out that this other type of cortical control of move- 

 ment (withdrawal) is balanced against the instinctive 

 exploratory reactions, so that damage to either re- 

 sults in enhancement of the other. We term this 

 effect 'transcortical release' (22, 23). The tactile 

 avoiding responses are independent of the pyramidal 

 system and are thus true extrapyramidal motor 

 effects. They may be extremely delicate and adroit 

 in the complete absence of both parietal lobes, 

 intermittent contact with only a few hairs then 

 sufficing to elicit elaborate avoiding of a pursuing 

 stimulus. When the 'set' of cortical reactions is al- 

 tered in this way, it is as if the unpleasant attributes 

 of every stimulus are emphasized at the expense of 

 pleasant or interesting features. The reactions to 



pain are correspondingly increased in extent and 

 duration. Similarly independent extrapyramidal 

 visual avoiding reactions, also independent of the 

 Rolandic-pyramidal system, can be identified in the 

 monkey deprived of the pre- and postcentral cortex 

 (23, 26) and occasionally in man (24). 



The cortical areas active in grasping and avoiding 

 responses are represented in the ventrolateral and 

 anterior thalamic nuclei, respectively. Those con- 

 cerned with vision are in the lateral and medial 

 pulvinar (26). Direct lesions of these structures at 

 thalamic level, or their severe degeneration as a 

 result of very extensive ablation of the corresponding 

 cortical areas, results in the more intense persistence 

 of the attitudes of grasping or a\oiding that are 

 called dystonia. Dystonia is essentially an abnormal 

 persistent attitude such that any attempt of the 

 examiner to displace the limb to another attitude 

 meets increasing resistance. The response to stimulus 

 is altered as in cortical types of grasping or avoiding, 

 Ijut the attitude is maintained for long periods with- 

 out further stimulus (24). 



It will be noted that whereas the spinal reflexes 

 present elaborate organization of the defense (flex- 

 ion) reflex and at times fleeting glimpses of proprio- 

 ceptive responses in the flexors, the known postural 

 reactions of the brain stem and cerebellum make 

 little use of flexor responses. Inhibition of extensor 

 responses is more widely found in the reticular forma- 

 tion, for example, where the presumed positive as- 

 pect of behavior has not yet been determined. More 

 highly organized withdrawal behavior appears at 

 the highest level as the avoiding responses of the cor- 

 tex, there related to the phylogenetically more primi- 

 tive cortical areas. The response to pain, for example, 

 has thus a diffuse cortical representation of this 

 type and, in association with it, negativistic reactions, 

 such as mutism and other akinetic states the physio- 

 logical mechanism of which is poorly understood. 



The function of the basal ganglia has not been 

 mentioned for the simple reason that it is defined 

 only in terms of symptoms and not of positive reac- 

 tions. The symptoms are in terms of conflicts between 

 the various reactions that we have discussed above. 

 Tonic grasping reactions with long after-discharge 

 compete with tonic avoiding reactions in the condi- 

 tion we recognize as athetosis (21, 26), resulting from 

 lesions of the lenticular nucleus in man. A profusion 

 of cortical automatisms, exploratory and avoiding, 

 often incompatilile and therefore leading to gro- 

 tesque movement, is characteristic of Huntington's 

 chorea, due to degeneration of the caudate nucleus. 



