894 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



in cats by means of electrical stimulation has demon- 

 strated separate mechanisms responsible for down- 

 ward movements of the head and anterior body 

 (fig. 9); their rostral limit is located just in front of 

 the posterior commissure according to Hess (106). 

 The effects following low-frequency stimulation show 

 synchrony with the stimulus frequency but slow 

 recovery after each stimulus; thus a progressive 

 lowering of the head and anterior body occurs until 

 the cat touches the ground with its head. A few areas 

 located very close to the mid-line, when stimulated, 

 show laehavioral depression of the animal in addition 

 to the tilting effect. 



The mechanism for downward movements is also 

 tonically active. Its destruction produces an upward 

 posture of the head with upward looking eyes. The 

 cat than stalks around and lifts its feet unusually high. 

 The defect is compensated after a few days. 



The structure coordinating these movements is 

 the nucleus precommissuralis (fig. 8) located im- 

 mediately rostrally to the posterior commissure and 

 dorsally to the aqueduct, according to Hassler (un- 

 published observations based on material in the 

 Hess collection). Its efferent pathway is the tractus 

 precommissurotegmentalis, corresponding to the 

 thalamopretectotegmental tract of Bucher and 

 Biirgi. The pathway follows the fasciculus longi- 

 tudinalis medialis within its lateral area, sends 

 collaterals to the nucleus nervi al^ducens and to the 

 reticular formation of the medulla oblongata, and 

 terminates in the area of the inferior olives. Only a 

 few collaterals go rostrally to the caudal lamella 

 medialis thalami which belongs to the hypnogenic 

 zone of Hess. 



NEURONAL MECHANISMS OF IPSIVERSIVE TURNING 



MOVEMENTS. Movements in the horizontal plane are 

 coordinated by two separate systems: one for ipsi- 

 versive and the other for contraversive turning move- 

 ments. 



In freely moving animals ipsiversive movements 

 can be induced by stimulation of the pontine and 

 mesencephalic reticular formation. The often dis- 

 cussed so-called 'tegmental reaction' (of Thiele as 

 well as of Ingram, Ranson and co-workers) is nothing 

 hut an ipsiversive movement distorted by anesthesia 

 and by the fact that the animal is restrained [Biirgi 

 (27), Hassler (89)]. This effect looks very much like 

 the compensatory vestibular movements in the 

 horizontal plane produced by Bartorelli and VVyss 

 using rotational stimuli. In contrast to rotation, 

 upward and downward moxcments, ipsiversive 



turning movements do not show synchrony with the 

 stimulus frequency, even at frequencies as low as 8 

 per sec, but are continuous. Both onset and dis- 

 appearance occur with a certain latency. The eyes 

 do not precede in the direction of the movements. 

 The corresponding anatomical structure is a 

 fiber Ijundle within the dorsolateral reticular forma- 

 tion, the \estibuloreticulothalamic tract [Hassler 

 (89)]. This nondecussating pathway consists of the 

 fasciculus tegmenti dorsolateralis and of its prolonga- 

 tion in the tegmental fascicles of Forel terminating 

 in the nucleus ventrointermedius of the thalamus 

 (see fig. 11). Many fibers end in the mesencephalic 

 reticular formation and are replaced by new fibers. 

 After removal of the cereljrum, ipsiversive turning 

 movements can also be produced by stimulation of 

 the exposed surface of the mesencephalon (Thiele) 

 which activates descending reticular pathways. The 

 sustained character of the movements results from 

 the transmission of the impulses through a great 

 number of reticular synapses before they reach the 

 efferent reticulospinal pathways leading to the spinal 

 cord. The highest destination of this pathway, the 

 nucleus ventrointermedius thalami, projects to the 

 region of the central gyri. Walzl & Mountcastle 

 (290), Kempinski (143) and especially Mickle & 

 Ades (191) were able to detect changes in electrical 

 activity along this pathway following electrical or 

 physiological vestibular stimulation. In cats the 

 vestibular projection to the cortex is located in the 

 anterior suprasylvian gyrus, corresponding to the 

 inferior postcentral gyrus in primates. In human 

 subjects Penfield & Rasmussen (209) stimulated the 

 area of the central g>rus; the eye movement most 

 frequently obtained was a conjugate lateral eye 

 deviation to the ipsilateral side which is understand- 

 able on the basis of the existence of the above men- 

 tioned pathway. (In man eye movements are to a 

 considerable extent independent of movements of 

 the head and trunk.) Because of certain features of 

 the nerve fiber systems involved, one of us (Hassler) 

 considers area 3a, which contains giant pyramidal 

 cells, as the central vestibulocortical projection. 



NEURONAL MECHANISMS OF CONTRAVERSIVE TURNING 



MOVEMENTS. Contraversive turning movements are 

 much more varied in their physiological characteristics 

 and anatomical substrates than those just described. 

 The adversive movements following stimulation of 

 many extrapyramidal cortical areas also belong 

 to this group. Such movements generally occur after 

 a short latency but occur continuously, even at 



