THE EXTRAPYRAMIDAL MOTOR SYSTEM 



891 



ter, longit. bundle 



FIG. 8. Diagram of nuclei and tracts involved in posture and responsible for direction-specific 

 motor effects of mesodiencephalic stimulation. Rotation movements are regulated by the interstitial 



nucleus {Ist) and its fiber systems ( ). Raising movements are regulated by the praestitial nucleus 



{P.sl) and its fiber systems ( ). This tonically active nucleus sends short fibers to the nucleus 



ruber magnocellularis (/?u) from which arises the rubrospinal tract («/. spi.). Lowering movements 

 are regulated by the praecommissural nucleus {Pr. co) and its fiber systems (+-(- + + -|-). The 

 efferent fibers constitute the praecommissurotegmental tract. All descending fiber tracts of the di- 

 rection-specific systems send collaterals to the nuclei of the ocular muscles (n. ///, IV and Vl) and 

 the reticular formation. 



the body or even the whole body in such a way that 

 the cat rolls around its longitudinal axis. In unre- 

 strained animals the position of the eyes is normal in 

 relation to the head. However, if the head rotation is 

 impossible, an intermittent rotation of both eyes in 

 the same direction appears which is easy to recognize 

 becau.se of the slit-shaped pupils. In encephale isole 

 cats, where head rotations are impossible, stimulation 

 of the same central area produces only conjugate 

 rotatory eye movements [Hyde & Eliasson (124, 



125)]. 



Coagulation of an area producing rotatory move- 

 ments is followed by a inirror-image defect: the head 

 of the cat is continuously rotated to the opposite 

 side, while the eyes are kept in normal position in 

 relation to the head (fig. 10). Electrical stimulation 

 of the coagulated area has no effect. These mirror- 



image disorders usually last over a period of several 

 days and then disappear slowly. However, they also 

 can last several months. 



The mirror-image defect is attributable to the 

 fact that under normal conditions the inechanisms 

 for rotatory movements in both hemispheres are in 

 tonic activity during wakefulness and that there is a 

 balanced relationship between both sides which 

 assures the head of being kept in the normal position. 

 Following destruction of one mechanism, the other 

 side becomes predominant and produces rotation 

 of the head toward its side. 



The mechanism responsible for rotation around 

 the longitudinal a.xis lies in the nucleus interstitialis 

 of Cajal with its descending and ascending con- 

 nections [Hassler & Hess (91)]. Its large nerve cells 

 give rise to the particularly large interstitiospinal 



