1294 



HANDBOOK OF PHYSIOLOGY 



NEUROPH\SIOLnGY II 



moveinenl induced by stimulation of the motor cortex 

 did not occur until the animal was anesthetized. 



Both facilitatory and inhibitory influences appear 

 to be expressed from active cortical loci; it is even 

 likely that excitation and suppression both can ema- 

 nate from the same locus under dififerent conditions, 

 von Baumgarten el al. (270), for example, reported 

 that single shocks applied to the motor cortex were 

 followed at times by augmentation and at other times 

 by inhibition of responses in a single unit in the re- 

 ticular formation, depending doubtless upon the 

 local state of both stimulating and receiving cells. In 

 this regard, Livingston & Fulton (161) have called 

 attention to the fact that effects of cortical excitation 

 vary considerably according to stimulus character- 

 istics and temporal levels of excitability. Eliasson was 

 able to elicit both excitatory and inhiljitory responses 

 in gastric motility upon stimulation of the cingulate 

 gyrus (72). In the unanesthetized alert animal, 

 Segundo et al. (244) found that a state of immobility 

 was initiated by stimulation of modulating cortical 

 zones with threshold voltages while excessive activity 

 followed supraliminal stimuli. 



Considerable question has been raised concerning 

 the validity of considering the active cortical loci 

 enumerated as 'suppressor' zones (i8g, 190, 249). 

 While it is probable that many of these objections are 

 valid, it seems clear that loci to which the term 'sup- 

 pressor' was originally assigned contribute impor- 

 tantly to modulation of motor mechanisms as well as 

 to other caudally and cephalically directed influences 

 mediated by the reticular formation. 



It has been proposed that part, at least, of the 

 cortically-originating influence over motor functions 

 requires cortical re-entry after brain-stem transport 

 in order to exert its control by way of pyramidal or 

 extrapyramidal routes (46, iig, 210). Moreover, it 

 has been suggested that facilitation of motor cortical 

 response by brain-stem stimulation occurs within the 

 motor cortex itself (200). Supporting this concept are 

 the observations that motor abnormalities, such as 

 tremor, are improved by destruction of re-entrant 

 channels to the cortex from basal ganglia {57, 188) 

 and from the cerebellum (108). Yet movements elic- 

 ited from stimulating the pyramidal tract after 

 extirpation of the cortex are still facilitated by brain- 

 stem excitation (169). Reticulospinal influences ap- 

 pear to exhibit some measure of independence from 

 the cortex, therefore, although there can be no doubt 

 that properly functioning cortical structures are 

 necessary for normal control of tonic and phasic 

 muscular activitv. 



CEREBELLUM. In 1 896 Sherrinsjton (246) showed 

 that faradization of the anterior lobe of the cerebellum 

 inhibited the extensor rigidity of animals with all 

 structures above the mesencephalon destroyed. Later, 

 Bremer (32) confirmed this observation and noted in 

 addition that excision of the cortex in this region re- 

 sulted in a strong increase in decerebrate rigidity. 

 Evidence is abundant now which indicates that such 

 cerebellar influences as these o\er motor functions 

 are mediated principally through reticulospinal con- 

 nections (196, 253). 



It is now evident that both facilitatory and in- 

 hibitory influences can be elicited by stimulation of 

 the cereljellum (252, 254, 255). In fact Moruzzi sug- 

 gests that the nature of the influence induced is a 

 function of cerebellar discharge rate and that "every 

 Purkinje cell of the anterior vermis may be inhibitory 

 or facilitating according to the frequency of its dis- 

 charges" (195). Both facilitatory and inhibitory re- 

 sponses can be elicited by excitation of the same 

 cerebellar site with difTerent stimulus frequencies. 

 Nulsen et al. (206) suggested that anterior vermal 

 stimulation at faster pulse rates elicited facilitatory 

 influences, while Snider et al. (255) found reverse re- 

 lationships. Doubtless a species difference is important 

 as the cat and monkey reacted difTerently. 



The most recent proposal concerning the func- 

 tional relationship between cereljellum and reticular 

 formation is that of Chambers & Sprague (51). They 

 suggest that this relationship is oriented longitudinally 

 in both structures. The medial cerebellar zone con- 

 sists of the vermal cortex which acts through the 

 fastigial nucleus upon the central reticular formation 

 and is concerned with postural tonus equilibrium and 

 locomotion of the entire Ijody. The intermediate zone 

 arises in the paravermal cortex and acts upon the 

 lateral reticular formation via the nucleus interpositus 

 and superior peduncle. This zone is considered to 

 function in more discrete control of the use of ipsi- 

 lateral limbs. 



A feature of the cerebellar-reticular interrelation- 

 ship which bears comment is the apparent functional 

 organization which is displayed by stimulus sites in 

 the cerebellar cortex. Nulsen et al. (206) showed that 

 a high degree of specificity could be demonstrated by 

 cerebellar stimulation; discrete activation of specific 

 folia only would augment or inhibit single mu.scles 

 thrown into movement by cortical stimulation. The 

 organization of the motor propriocepti\e homunculus 

 on the cerebellum seemed to coincide quite closely 

 with the sensory homunculus described by Snider and 

 Stowcll (252, 256). By contrast, no such organization 



