THE EXTRAPYRAMIDAL MOTOR SYSTEM 



907 



tive lesions. However, tegmental mesencephalic 

 lesions are still among the main condition for decere- 

 bration. The motor functions of the midbrain struc- 

 tures have appeared in a new light since recent 

 studies of the supraspinal control of the gamma moto- 

 neuron system. We must distinguish between two 

 types of decerebrate rigidity reacting differently to 

 posterior root section and to chlorpromazine. a) The 

 classical Sherrington decerebration following inter- 

 collicular transection shows a very marked hyper- 

 activity of gamma efferents which can be diminished 

 by chlorpromazine [Henatsch & Ingvar (95)]. b) The 

 decerebration rigidity following lesions of the anterior 

 cerebellum or produced by the anemic method of 

 Pollock and Davis shows no hyperactivity of the 

 gamma system but rather direct activation of the 

 alpha motoneurons [Granit (69)] and is less sensitive 

 to dorsal root section and chlorpromazine. An 

 anatomical correlation of these two types with differ- 

 ent brain-stem structures and their functions cannot 

 yet be made. Granit & Holmgren (70) have postu- 

 lated two different pathways for activating gamma 

 neurons, one a slow and polysynaptic mechanism 

 through several spinal segments, the other a fast 

 mechanism mediated through the lateral columns 

 probably by reticulospinal fibers. 



Slaiilf Rcac/ioii 



The startle reaction after unexpected sensory 

 stimuli (usually acoustic) seems to be mainly an 

 extrapyramidal motor response. It has been studied 

 extensively by Strauss (250) who distinguished be- 

 tween primary and secondary startle reactions by 

 kinematographic analysis of the phenomena {^usam- 

 menschrecken) following a pistol shot. Strauss believes 

 the primary reaction to be an acousticomotor reflex 

 in the lower brain stem, involving the red nucleus. 

 The secondary reactions are partly emotional or 

 voluntary movements and may involve the cerebral 

 cortex and are less constant. Increase in the primary 

 reaction was found in spastic limijs and in some aki- 

 netic patients without rigidity. Diminished primary 

 reactions were found in parkinsonian syndromes 

 showing rigidity and tremor and in chorea and 

 athetosis. Startle reactions may be absent during 

 sleep. As startle reactions and Moro's reflex are also 

 obser\ed in human anencephali, it seems highly 

 probable that they are mediated by the lower extra- 

 pyramidal centers with the reticular formation. 



Electromyographic studies of the startle reactions 

 by Duensing (46) distinguished a short latency startle 



(Schreckreflex) and a long latency startle {Schreckreak- 

 lion), involving differejit muscles. Duensing believed 

 that both startle reactions use the same efferent bulbo- 

 mesencephalic and spinal pathways, but that the 

 long latency reactions also involve the thalamus or 

 the striatum. 



Tremor 



Phylogenetic parallels between extrapyramidal 

 movements and motor performances of animals have 

 been discussed since Foerster (57) compared athetosis 

 with the climbing movements of monkeys. The simi- 

 larities of tremor with the fin movements of some fishes 

 (fig. 15) were believed by Jung (130) to indicate the 

 existence of coordinating mechanisms in the spinal 

 cord similar to those described in spinal fishes by von 

 Hoist (278). Therefore, tremor was interpreted as an 

 archaic simple form of rhythmic antagonistic move- 

 ments, released by disturbances of higher mech- 

 anisms. The physiological tremor of shivering investi- 

 gated electromyographically was found similar in 

 man [Denny-Brown d al. (43)] and animals [Burton 

 & Bronk (28)]. The latter found, as did Jung (130), 

 that most motor units discharge only once in one 

 tremor beat. 



Parkinsonian tremor, the rhythm of which may be 

 different in different limbs, shows little dependence 

 upon afferent control and seems to be mainly an 

 autogenous rhythm arising in the interneuronal sys- 

 tem of the central nervous system [Jung (130)]. 

 Central tremor rhythms independent of reflex control 

 were first postulated by Wachholder & Altenburger 

 (282). Altenburger (6) showed that tremor persisted 

 in deafferented limbs after posterior root section, 

 although Strughold (250a) and Hoff'mann (119) 

 had found some slowing of clonus rhythms after 

 loading the muscles and therefore considered propri- 

 oceptive modification possiijle. This was recently con- 

 firmed in tremor by Halliday & Redfearn (76). These 

 observations, however, seem not to be conclusive for 

 the 'servoloop' theory of tremor [Halliday & Red- 

 fearn (75)] and do not prove their contention that 

 integritv of the reflex arc is essential for tremor 

 rhythms. Proprioceptive tendon reflexes elicited 

 during parkinsonian tremor show a rhythmic facilita- 

 tion or inhibition depending upon the alternating 

 reciprocal innervation of the reflexly excited muscles 

 [Jung (130)]. In contrast to many older theories 

 considering cortical and subcortical mechanisms to 

 ije the source of tremor rhythms, Jung (130) localized 

 the central substrate of tremor in the interneuronal 



