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HANDBOOK OF PH'^SIOLOGV 



NEUROPH^■SIOLOGV II 



and motor output. Polysensory projections have been 

 described in tlie association cortex of cat brain (cf. 

 75), and the thalamic relays for these responses 

 have been identified (51). Among others, the nucleus 

 centrum medianum has been found to relay sensory 

 information to regions in the anterior and posterior 

 suprasylvian gyrus (17). This nucleus, a part of the 

 so-called 'diffuse projection system' of the thalamus, 

 also receives projections from many cortical areas 

 and is the site of interaction of cortical descending 

 influences and ascending somatosensory volleys (cf. 

 16). Furthermore, stimulation of this nucleus, in 

 addition to producing widespread changes in elec- 

 trical activity of the corte.x, also produces changes in 

 motor functions which take place through subcortical 

 mechanisms (cf Chapter LIII by Jasper in this work). 

 This example of the centromedian nucleus is cited 

 here merely to illustrate a particular aspect of the 

 complex and involved corticosubcortical interrelation- 

 ships which might play a very important role in 

 sen.sorimotor integration. This conclusion is sug- 

 gested also by the observation of reticular units 

 which were found to be influenced by stimulation of 

 the cerebellum and motor cortex, and by sensory 

 inputs of different modalities (441, 442), as discussed 

 in Chapters LII by French and XXXV by Jung & 

 Hassler in this work. 



Certain Curticoiubcortnal Inter) elatwns 

 in Motor Mechanisms 



It has been suggested that the diffuse projection 

 nuclei of the thalamus may play a role in the initi- 

 ation of movements, in the processes of attention, or 

 in both (220). The observation of motor disability 

 and transient lethargy after thalamic lesions in the 

 region of these nuclei (391) is not inconsistent with 

 this hypothesis. 



A greater body of information is available con- 

 cerning the reciprocal relationships between the 

 cortex and reticular formation which is discussed in 

 Chapter LII by French in this work. While many 

 questions concerning the mechanisms of cortical 

 'activation' by reticular action remain unsolved (cf 

 381), it is, however, sure that the reticular formation 

 can influence intracortical mechanisms by altering 

 the probability of firing of cortical neurons, including 

 those involved in sensorimotor activities (351, 473)- 

 Discharges recorded from single fibers in the pyram- 

 idal tract are altered as a result of cortical 'activa- 

 tion' by reticular action, concomitantly with the 

 changes occurring in the cortical electrical activity 



(473). Evoked potentials in sensory projection areas 

 are also modified as a result of arousal, following 

 both natural sensory stimuli and reticular stimu- 

 lation (cf. 59). 



More work is needed before these actions can he 

 defined in precise terms, but the overall picture of 

 the electrical cortical activity in this condition of 

 arousal is the one which can be expected in the 

 course of integrative processes. This would seem to 

 imply mainly a desychronized pattern of unitary 

 activity necessitated by a temporal and spatial dis- 

 persion in which excitatory and inhibitory processes 

 act through graded and algebraically summating 

 actions (cf. 56). It would thus appear that reticular 

 actions may affect in varying degrees sensorimotor 

 cortical integration. The cortical efflux, in turn, is 

 directed to a great extent to subcortical structures, 

 including the reticular formation (cf. 156). 



It has been suggested that subcortical facilitation 

 of motor activity, as studied by means of prebulbar 

 reticular stimulation on the monosynaptic reflex, is 

 controlled at once, when initiated, by an inhibitory 

 process which is cortical in origin since it can be 

 abolished temporarily by cooling the cortical surface 

 and permanently in the chronic decorticated cat. 

 Hugelin & Bonvallet (213, 215, 216) describe these 

 inhibitory cortical actions as originating from most of 

 the cortex and mediated by 'extrapyramidal' fibers 

 which, at the level of the pes pedunculi, enter the 

 lateral hypothalamic area and reach the posterior 

 diencephalic tegmentum. This cortical inhibitory 

 action is tonic in character but increases slowly when 

 subcortical facilitation is initiated. This produces, 

 concomitantly with the action at the spinal level, a 

 cortical 'activation.' 



The quoted results resemble those obtained i>y 

 Tower (42 1 ) who demonstrated that the stimulation 

 of various cortical points, after section of the cortico- 

 spinal tract, abolishes a background of muscular 

 hypertonus and stops movements. Tower describes 

 regional differences in the capacity of the cortex to 

 produce these effects. 



While many difficulties attach to specific interpre- 

 tation of this (214) or other corticosubcortical inter- 

 relationships previously considered, their existence 

 emphasizes the interdependence of ganglionic masses 

 within the central nervous system when their ac- 

 tivities are considered in relation to a behavioral act. 

 In this respect the importance of the o\crlap between 

 pyramidal and extrapyramidal functions in in- 

 tegrative processes necessary for a successful motor 



