THE EXTRAPYRAMIDAL MOTOR SYSTEM 



883 



transitory turning movement of tiie body to the 

 opposite side since this supra\estibular system is 

 tonically active during wakefuhiess [Hassler (89)]. 

 Bipolar stimulation of this nucleus during stereotaxic 

 operations in patients with extrapyramidal motor 

 disorders is followed by a horizontal conjugate ocular 

 deviation to the side of stimulation. In man coagula- 

 tion of parts of the nucleus produces a paresis of 

 ocular movements with nystagmus to the damaged 

 side which, however, disappears after i or 2 weeks. 

 These observations seem to indicate that the nucleus 

 vcntrointermedius plays the same functional role in 

 man as in cats where it represents a part of the mecha- 

 nism responsible for ipsiversive turning movements. 



b) Nucleus venlrocaudalis thalami (I'PL and \'PM). A 

 remark appears appropriate at this point concerning 

 the motor effects following stimulation of the medial 

 lemniscus and the spinothalamic tract or of their 

 terminal thalamic nuclei, the nuclei venlrocaudalis 

 externus (V.c.e.), internus (V.c.i.) and parvocellularis 

 (V.c.pc) (VPL, VPM and VPI, according to Walker's 

 terminology). In unanesthetized, freely moving cats 

 low-frequency stimulation produces twitching of the 

 contralateral face and of the contralateral foreleg — 

 rarely of the hind leg — which is at first synchronous 

 with the stimulus but very shortly shows definite 

 summative effects (such as closing of the eyelids or 

 lifting of the bent foreleg). After the end of the stimu- 

 lation some, but not all, of the cats may shake the 

 extremity involved or may lick themselves as if an 

 unpleasant sensation had been experienced in the 

 twitching area. 



During stereotaxic operations for the relief of 

 chronic intractable pain, the sensory relay nuclei of 

 the thalamus were stimulated in conscious patients. 

 Synchronous muscular twitching accompanied by a 

 twitching or electrifying sensation in the topically 

 corresponding area of the body was evoked with a 

 stimulus frequency up to 8 per sec. When stimulation 

 of the parvocellular and ventral parts of the nucleus 

 (V.c.pc or VPI) at a rate over 20 per sec. produces 

 localized pain (in amputees it may even evoke 

 phantom pains), the ipsilateral motor symptoms are 

 enhanced and extreme pain distortions appear. 

 When stimulation does not produce painful sensa- 

 tions, these distortions apparently have to be con- 

 sidered as reflex motor activities resulting from 

 extero- or proprioceptive impulses aroused artificially 

 by electrical stimulation of central sen.sory systems. 

 These effects are not extrapyramidal motor phe- 

 nomena in the narrow sense. 



c) Nucleus uentro-oralis posterior. Stimulation of this 



nucleus and of the terminal dentatothalamic fibers 

 of the brachium conjunctivum with the Hess method 

 causes movements of the contralateral forelegs and of 

 the contralateral muscles of the face. The foreleg is 

 lifted or adduced and stretched forward. This adduct- 

 ing and forward stretching effect is also seen after 

 stimulation of the brachium conjunctivum in the 

 area between its decussation in the mesencephalon 

 and the cerebellum where the effect is only homo- 

 lateral. These motor effects obviously are due to 

 stimulation of afferent fibers going to area 4. Accord- 

 ing to Hess, coagulation of small parts of this nucleus 

 has no effect. Larger lesions (2 to 3 mm in diameter) 

 cause functional disabilities such as diminished use 

 of the limb, lack of postural control and ataxia. 



In man this nucleus (V.o.p.) has been stimulated 

 by Hassler, Riechert and Mundinger during stereo- 

 taxic operations, however, only in patients suffering 

 from myoclonic or parkinsonian disorders. When the 

 myoclonic movements or the resting tremor disappear 

 during light anesthesia, bipolar low-frequency stimu- 

 lation can cause them to reappear in the contralateral 

 limbs after a short latency. The frequency of the 

 hyperkinesia differs in this case in the arms and in the 

 legs. If the resting tremor persists during the opera- 

 tion, it is possible to change its frequency by using the 

 same type of stimulation and to block it in regular 

 intervals. Single shocks cause an enhanced flexion 

 or extension of the forearm with compensatory inter- 

 vals, depending upon the phase of the concomitant 

 tremor. Therapeutic destruction of this nucleus to a 

 considerable extent reduces or completely suppresses 

 both resting tremor and myoclonic movements. 

 Larger lesions can produce a contralateral ataxia 

 which is later compensated. Diminished use of the 

 extremities is also seen, but no pareses and no increase 

 of tone. Lesions extending to more medial areas pro- 

 duce a paresis of mimetic movements of the contra- 

 lateral facial muscles with completely intact voluntary 

 motor activity. Disorders of sensibility never occur 

 following lesions in the ventro-oral thalamic nuclei. 

 The effects of clinically occurring focalized lesions 

 in this nucleus in patients without pre\'ious extra- 

 pyramidal motor disorders include a diminished use 

 of the extremities without paralysis, contralateral 

 ataxia, and contralateral mimetic paralysis of the 

 facial muscle as \on Leyden and Nothnagel had 

 already assumed. Interruption of the dentatothalamic 

 fibers of the brachium conjuncti\um terminating in 

 this nucleus produce the same effect. This is in con- 

 trast to the results of the monkey experiments of 

 Clarrea & Mettler (35) and Carpenter (30). 



