558 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



to compensate for an actual sudden change of position 

 in space (112, 115) The direction and character of 

 the movement depend upon which of the labyrinthine 

 sense organs are most strongly stimulated. The semi- 

 circular canals can become sensitive to acoustic 

 stimulation when they are artificialh' exposed to it, as, 

 for instance, after a fenestration operation (12, 45, 

 112, 114). This does not mean, however, that sound 

 perception is in the natural range of functions of the 

 semicircular canals. 



L.^BYRINTHINE P.ATHW.^YS .AND REFLE.XES 



The neural connections of the vestibular organ 

 consist of numerous chains of neurons, reciprocally 

 linked in many ways and ha\ing their synapses in 

 various anatomical nuclei. All the chains work in 

 intimate collal^oration and the final pattern of reflex 

 responses is attributable largeK' to the highly complex 

 integrating activity of the center (62). The labyrin- 

 thine function is automatic, carried out in a reflex 

 fashion, in other words, mostly below the level of 

 consciousness. The brain centers through which the 

 labyrinths elicit the various appropriate muscular 

 reactions of the head, body, limbs and eyes — the 

 righting, the postural and the ocular reflexes — repre- 

 sent an intricate mechanism. The nervous connections 

 of the vestibular apparatus with the brain are, as yet, 

 imperfectly known. 



The impulses generated in response to stimulation 

 of the peripheral receptors pass for the most part to 

 Peiter's nucleus (the l ateral \estibu lar nuclejis), 

 Schwalbe's nucleus (the niedial nuclejj.s!). Be chterew 's 

 nucleus (the sup frior nucleu s^ and Roller 's nucleus 

 (the descen ding oi- sp ina l \ estibu laJL nuclettsy In 

 these nuclei originate, in turn, the ascending and 

 descending tracts. For all practical purposes these 

 four nuclei can be treated as a single functioning 

 entity. Some axons pass directly to the c erebellu m 

 (2, 25). 



The vestibular nuclei on each side of the medulla 

 are connected with each other. This connection may 

 be either direct (19, 31, 47, 90) or indirect by way of 

 the reticular formation. Ramon y Cajal (90) describes 

 a compact bundle of fibers within the vestibular 

 nerve which passes directly from the vestibular 

 (Scarp a's) ganglion across to the opposite side of the 

 bulb without synaptic relay in the ipsilateral vestib- 

 ular nuclei. Because these fibers spread out diffusely 

 after crossing the mid-line, he was uncertain whether 



they terminate in the contralateral vestibular nuclei 

 or within the contralateral bulbar reticular formation. 



Ascending Fibers 



Fibers arising from the medial and s uperior vestib - 

 ular nuclei form the medial longitudinal fasciculus , 

 the fibers of which end in the nuclei of the oc ulomotor 

 nerves of the same and opposite sides. The tract is 

 phylogenetically one of the every early ones to 

 appear. It is present in cyclostomes and is known to 

 be an important reflex pathway in fish. Its position 

 and connections are very constant throughout the 

 vertebral series. 



NYST.AGMUS. Thc position of the eyes is very markedly 

 influenced by stimulation set up in the labyrinth. 

 This is of obvious importance since, as the body 

 moves, compensation must be made b\' the eye 

 muscles in order that the gaze may remain fixed on 

 any object. In birds and reptiles most of the compensa- 

 tion is made by the neck muscles, and a head nystag- 

 mus appears during and after angular stimulation 

 (12). As the body turns the eyes swing slowly in the 

 opposite direction so as to maintain their fixation. 

 Having turned as far as possible, they swing quickly 

 back in the opposite direction to fix a new object which 

 in turn they follow by a slow deviation. The slow move- 

 ment in one direction is known as the slow component 

 of the nystagmus, and the quick movement in the 

 opposite direction is known as the quick component. 

 The reflex latency of the slow component is 50 to 80 

 msec. (21). The magnitude of the quick and slow 

 components is the same and by convention the direc- 

 tion of the nystagmus is designated as that of its quick 

 component. Thus, when the quick component of a 

 nystagmus is observed to be in the direction of the 

 subject's right, it is called a nystagmus to the right. 

 The movement of the eyes in nystagmus is in either 

 the horizontal, frontal or sagittal plane. These differ- 

 ent directions of the nystagmus can be easily demon- 

 strated in man by rotating him with eyes closed in a 

 revolving chair when different pairs of canals are 

 brought into their maximal po.sition (120). For 

 example, to stimulate the horizontal canals maximally 

 the head should be inclined forward about 30°. Dur- 

 ing rotation the quick component will be in ihe 

 direction of rotation. When the rotation is stopped a 

 postrotatory nystagmus will i)e ob.served; its quick 

 component is in the direction opposite to that of the 

 rotatorv movement. This is due to thc retardation of 



