560 



HANDBf)OK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



fk:. 9. Responses of various cortical areas to vestibular 

 stimulation, recorded from points ;, 2 and 5 indicated on the 

 drawing of the brain. A:i, A:j and A:j from the utricle, B:i, 

 B-j and B:^ from the superior ampulla; C.i, C.jj and C.j from 

 the lateral ampulla before local strychninization. A.j, A.\f and 

 A:6 from the utricle; B:2, B:^ and B:6 from the superior 

 ampulla; C:2, C:^ and C':6 from the lateral ampulla after 

 strychninization. Time in lo msec, intervals. [From .^ndersson 

 & Gernandt (2).] 



Descending Tracts 



Through these connections vestibular impulses are 

 conveyed to the primary motoneurons of the spinal 

 cord. As far as origin and course are concerned the 

 vestibulospinal trac t seems to be the less complicated 

 of the descending pathways. This tract, which origi- 

 nates, at least for the most part, from the large ' motor' 

 type cells of the lateral vest ibular nucleus , descends 

 ventrally during its course in the medulla into the 

 aTUerior_Jmii£iilus of the s^ jne side of the cord. As 

 stated, numerous anatomical connections exist between 

 the v estibu lar nuclei and the reticular format ion, and 

 the fibers which constitute the reticulospinal tract 

 have been traced from here into the lateral andven- 

 tral parts of the spinal cord (82, 83). Descending fibers 

 forming the nigdial longilmjj nal fasciculus , being both 

 homolateral and contralateral, are derived from the 

 d escendin g_ntcd ial and lateral nuclei. Those fibers on 

 the contralateral side all terminate in the cervical 

 region, while those on the homolateral side may con- 

 tinue throughout the cord. 



X'estibular and proprioceptive systems are both 

 known to be active in posture and locomotion; 

 streams of impulses arising from receptors in each of 

 these systems must converge to influence the activity 

 of the final common path. ."Ml reflexes which aim at 

 preserving the normal posture of the body are col- 

 lectively called 'postural reflexes' (26, 74, 75, 89); 

 these are considered in Clhapter XLI by Eldred in 

 this work. The hyperextension of the extremities of a 

 decerebrate animal can be modified by passively 

 changing the position of the head. The compensatory 

 movements of all four legs are elicited by the stimula- 

 tion of the otolith organ and the proprioceptors of the 

 neck muscles. These ' tonic l abyrinthin e' and ' tonic 

 neck reflex es' operate in the s ame directi on and con- 

 sequently sum algebr aicall y when both arc elicited. 

 The tonic labyrinthine reflexes can be studied .sepa- 

 rately after excluding the tonic neck reflexes by section 

 of the upper cervical dorsal roots or by fixing the head 

 so as to prevent any movements of it in relation to the 

 body. It is then possible to move the animal about in 

 different positions and thus ascertain the effects of the 

 labyrinths upon the distribution of tone. For example, 

 placing the animal on its back with the angle of the 

 s nout approxima tely 45° ajj ove the hori7.ont aJ_jTlane 

 cau.ses the ext ensor tone to become m axim al; it is 

 minimal when the animal is in the prone position with 

 the angle of the mouth 45° below the horizontal axis. 

 When the head is brought into other positions by 

 rotation of the body around its transverse or longi- 

 tudinal axis, intermediate degrees of rigidity between 

 the two extremes result. These modifications in pos- 

 tural tone disappear if the labyrinths are destroyed. 

 More precise experiments have made it clear that the 

 reflexes are abolished by remosing the otoliths from 

 their maculae. 



The ability to stay in an upright position is a uni- 

 versal property of man and higher animals. Five 

 principal groups of reflexes of a somewhat similar 

 type, responsible for the righting tendency, have been 

 separated. Each one of these factors alone may bring a 

 more or less normal upright position; but when they 

 collaborate, greater precision and promptness in 

 righting results. These responses can be studied in 

 decorticate animals in which their reflex nature is 

 quite apparent. One of them, which is dependent on 

 the hib\rinth, will be descrilied briefly. In an animal 

 blindfolded but with the labyrinths still intact the 

 head tends to assume the natural horizontal position 

 irrespective of the position in space of the remainder 

 of the body. The reflexes causing righting of the head, 

 initiated from the otolith organ, are called the 



