552 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 4. Schematic three-dimensional diagram of one-half of 

 an ampullar crista. [From Wersall (121).] 



horizontal position, with the otoliths King on the 

 hair cells. The saccular maculae are situated ob- 

 liquely, forming an angle of about 30° with the verti- 

 cal plane. Thus, when the head is in the erect posi- 

 tion, the otoliths of the saccule are placed laterally on 

 the hair cells, embedded in the substance that covers 

 them. 



INNERVATION OF SENSORY CELLS 



Impulses from the peripheral receptors to the 

 stations in the medulla are conducted by the \estibu- 

 lar branch of the eighth nerve. These fibers make up 

 inore than half of the nerve and number about 19,000. 

 Most of them are large myelinated fillers (10 to 15 m) 

 but there are also medium and fine fibers (i to 2 /x) 

 (28, 59, 86, 90, 91). In addition, a large number of 

 unmyelinated fibers with diameters between 0.3 and 

 I M have been described (121). Nerve fibers of vary- 

 ing diameter, as pointed out by Ramon y Cajal C90) 

 and Lorcnte de No (59), have a characteristic distri- 

 bution in each crista. Thus, the large fibers innervate 

 the central region, those of medium size are distrib- 

 uted to the lateral regions, while the fine fibers go to 

 the basal region. 



Electronmicroscopic examination of the innerva- 

 tion of the sensory epithelium in the guinea pig by 

 Wersall (121} revealed two different types of nerve 

 endings. The bottle-shaped hair cells have a nerve 

 calyx enclosing the greater part of the cell, while the 

 nerve branches form loops around the basal part of 

 the cylindrical hair cells or terminate like btid-shaped 

 nerve endings (fig. 3). Stimulation of the former type 

 will give rise to impulses conducted mainly in one 

 nerve ending, the nerve calyx, and only in one nerve 

 fiber. The cylindrical hair cell, however, is usually in- 

 nervated by branches from several dififerent fibers; 

 and several hair cells, often at relatively great distance 

 from each other, are innervated by the same fiber. 

 The difference, in principle, between the innervating 

 characteristics of the two types of sensory cells may 

 be of physiological importance but no investigations 

 have, as yet, been able to re\eal clear!)' the significance 

 of the postulated different functions in the sensory 

 epitheliinu of the cristae anipullares. 



Recently PetrofT (84) has published results of ex- 

 periments with sectioning of the eighth nerve that 

 might point to the existence of thin efferent fibers in 

 the vestibular nerve. Such recurrent or feed-back 

 connections of the auditory system have previously 

 been described. Thus Rasmussen (92, 93) has found 

 an efferent cochlear bundle that forms a plexus at 

 the margin of the osseous spiral lamina around the 

 afferent fibers. Galambos (35) was able to surpress 

 the expected inflow of auditory nerve activity to 

 normal acoustic stimuli by electrical stimulation of 

 these efferent fibers. The function of the vestibular 

 efferent fibers has not yet been studied. 



The vestibular nerve has six main branches of 

 origin : one each from the posterior, superior and 

 lateral ampullae, and the utricle, and two from the 

 saccule. Galambos & Davis (36) have found In histo- 

 logical methods that the auditory nerve in the interval 

 from the internal meatus to the medulla contains 

 nerve cell bodies which probably belong to the coch- 

 lear nucleus and are therefore second-order neurons 

 in the auditory tract. There are reasons to believe 

 that corresponding second-order neuron cell bodies 

 can be found in the vestibular portion of the eighth 

 nerve (38). 



MODE OF ACTION OF VESTIBULAR APPARATUS 



Though there must be an intimate coordination 

 between activities of receptors situated in the ampullae 

 of the semicircular canals and those in the vestibular 



