VESTIBULAR MECHANISMS 



561 



'laby rinthine righting reflexes '. The responding 

 muscles are those of the neck. The tonic labyrinthine 

 and righting reflexes are static ones and are not to be 

 confused with the vestibular reflexes which are pro- 

 voked by movements in space and initiated from the 

 semicircular canals. 



Since the classical investigations of Magnus and 

 Sherrington upon the brain-stem influences on spinal 

 motor activity were published, some more recent 

 papers concerning the maintenance and control of 

 static and phasic postural activites have appeared. 

 Magoun and coworkers (76) have studied the role of 

 the brain-stem reticular formation with respect to 

 inhibition and facilitation of spinal motor activity. 

 The importance of the vestibular nuclei as an excita- 

 tory mechanism for the cord has also, in the light of 

 recent experiments, been reinvestigated (43, 44, 97, 

 105, 118). 



It has been shown that the brain-stem reticular 

 formation receives impulses relayed from somatic and 

 auditory sensory structures. It is of interest from this 

 point of view to be able to add the vestibular organ 

 to the rest. By recording the impulse activity generated 

 in response to adequate vestibular stimulation from 

 isolated units in the reticular formation, it has been 

 demonstrated that the formation is connected with 

 both the homolateral and the contralateral vestibular 

 nuclei (42). Thus the reticular formation forms an 

 internuncial relay constituting a fundamental element 

 of the reflex arc. The bilateral distribution of impulses 

 from both labyrinths are changed in the relay into 

 excitatory and inhibitory impul.ses which influence 

 the motoneurons, i.e. the vestibular responses are 

 organized for reciprocal action on flexors and exten- 

 sors even when initiated from the reticular level C43). 



Impulses conducted in the vestibulospinal tract 

 will encounter fewer synapses on their wav from 

 periphery to periphery than those in the reticulo- 

 spinal tract bv way of the reticular formation. It is 

 therefore possible to record a two-peak response to a 

 single vestibular shock stimulus from a whole ventral 

 root because the impulse volleys are transmitted along 

 separate paths having different nuclear delays (40). 

 The descending impulses occurring in response to 

 vestibular stimulation will influence the activity of 

 both alpha and gamma fibers. The small gamma 

 eflferents, however, are activated at a lower strength 

 of stimulation than are the alpha fibers (3). 



In studying the effect of the proprioceptive im- 

 pulses upon the efferent discharge elicited by vestib- 

 ular stimulation and recorded from a ventral root, it 

 became obvious how strong and dominating this 



FIG. 10. Effect of foot joint stimulation on vestibular root 

 response. In A is sliown a control response recorded from 

 ventral root L7. In B the response is augmented by manipula- 

 tion of the tarsometatarsal joints of the ipsilateral hind foot. 

 Time scale in msec. [From Gernandt el at. (40).] 



proprioceptive control can be. The vestibular re- 

 sponse, however strong it may be, will be inhibited 

 by a muscular contraction (3). One kind of peripheral 

 stimulation found to facilitate the vestibular re- 

 sponse arises from manipulation of the joints of the 

 foot ipsilateral to the recording site (fig. 10). Rein- 

 forcement of the vestibular response by afferent 

 discharges arising from the foot joints will contribute 

 to the increased stability and strength of the corre- 

 sponding limb during standing, walking and jumping 



C40). 



The effects of vestibular stimulation upon strych- 

 nine autorh\thmic convulsive activity of the spinal 

 cord has been studied in decerebrated cats. Inhibition 

 of strychnine tetanus was obtained at all levels of the 

 cord by tilting the head or the whole animal to the 

 side, backward or forward. The inhibitory effect was 

 characterized by a progressive decrease in frequency 

 of the tetanic waves until a complete, but always 

 reversible, inhibition occurred (41). 



EFFECTS OF LABVRINTHECTOMV 



As mentioned above, a distinction is made between 

 two different functions of the vestibular apparatus. 

 One is concerned with recording the position of the 

 head in space, the other with reacting to any change in 

 the rate of movements. The former function is 

 mediated by the otolith organs, the latter by the 

 ampullary cristae of the semicircular canals. The 

 observation of equilibrium disturbances resulting 



