VESTIBULAR MECHANISMS 



555 



lopctal direction, the stimulus being ainpullopetal 

 inertia movement of endolymph. With an increase of 

 stimulus strength a clear recruitment of sensory units 

 can be demonstrated. The maximum frequency is 

 evidently related to the acceleration, but o\\ ing to the 

 deceleration which follows it is impossible to say how 

 rapidlv the receptors would become adapted to the 

 stimulus. For the study of adaptive behavior, a con- 

 stant angular acceleration would have to be applied 

 for a protracted period of time. Some results suggest 

 that the recepto rs a dapt slowly (i, 94) but Hallpike 

 & Hood (51) and Lowenstein (64) came to the con- 

 clusion that the end organs show considerable adapta- 

 tion under conditions of sustained cupular deflection. 



An ampullofugal deviation of the cupula of the 

 horizontal canal inhibits the spontaneous impulse 

 activity. This demonstrates that a s ingle recepto r can 

 signal rotation in either direction instead of one direc- 

 tion only. In the vertical canals the discharge of 

 impulses is increased by angular displacements in 

 which the ampulla is leading__a nd an ampullofuga l 

 deviation of the cupula is elicited. An ampullopetal 

 deviation will cause an inhibition. On cessation (or 

 deceleration) of the angular stimulation, changes 

 which are the reverse of the initial ones occur. If the 

 speed of rotation is maintained at a constant level, 

 the impulse frequency falls ofT until it has reached 

 the spontaneous rate. 



Adrian (i) was the first to use a higher mammal, 

 the cat, for recording the discharge following varying 

 stimulation of the labyrinth. The activity was recorded 

 from the vestibular nuclei. Generally speaking, the 

 results obtained have not shown any marked difTer- 

 ence between the vestibular apparatus of the cat (i, 

 38) or rabbit (27) and that of the frog or the fish. 

 There are gravity receptors to signal the posture and 

 linear acceleration of the head, and rotation receptors 

 to signal the turning movements (fig. 6). Some differ- 

 ences are found, however, but they are probably due 

 to recording from second-order neurons (38). Units 

 associated with the receptors of the horizontal semi- 

 circular canal showed an increase in impulse fre- 

 quency in response to rotation toward the side of 

 recording, while rotation in the opposite direction 

 inhibited the activity. Sudden arrest of the rotatory 

 movement resulted in a reduction in impulse discharge 

 rate after ipsilateral and an increased discharge after 

 contralateral acceleration. This type of response is 

 interpretable on the basis of a mechanical tension- 

 release theory for the hair cells, excitation being; the 

 result of stress, inhibition of release. In addition to 

 this usual type of response, there were units which 



showed an increased discharge in response to rotation 

 in both directions (i, 27, 38). Both the ampullopetal 

 and ampullofugal flow of endolymph had an excita- 

 tory effect. A mechanical tension-release theory 

 would seem to be still more natural for these units 

 than for units of the previous type (51). The hair cells 

 may be assumed to be pulled upon by the movement 

 of endolyinph and cupula in both directions. This 

 type of response appears in about 12 per cent of units. 

 An inhibitory effect of rotation in both directions has 

 been noted also during recording of the electrical 

 activity from second-order neurons. This inhibition 

 can hardly be regarded as due to a peripheral mech- 

 anism, a fact suggesting a difference in function be- 

 tween higher mammals and simpler organisms. An 

 inhibition in both directions of rotation should, how- 

 ever, not provide greater difficulties to a tension- 

 release theory than inhibition in one direction only. 

 In both cases we have to account for the nature of 

 the release by internal forces of tension for which so 

 far there is no evidence. Once impelled, by the 

 mechanical theory, to add unidirectional tensile 

 forces inside the receptive organ to account for these 

 findings, we might as well assume the existence of 

 structures pulling upon the hair cells in such a well- 

 balanced fashion that release follows when the cupula 

 swings either way. Alternatively, the mechanical 

 theory should be given up altogether in favor of the 

 assumption that the impulses recorded are from cell 

 bodies of second-order neurons, and that the pull on 

 certain hair cells sets up inhibition at the first synapse, 

 in the manner of the well-known retinal inhibition. 

 This alternative seems to be the more probable. 

 Another assumption is that these neurons may have 



FIG. 6. Diagram to illustrate average time course of impulse 

 discharge from a semicircular canal showing after-discharge and 

 silent periods when acceleration and deceleration are separated 

 by an intervzJ of steady rotation. [From Adrian (l)] 



