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HANDBOOK (JF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



effects are elicited by stimulating the motor cortex, 

 the anterior lobe of the cerebellum, the habenular 

 complex and the head of the caudate nucleus. Inhibi- 

 tion of spindle activity is readily elicited by stimula- 

 tion of the medial part of the bulbar reticular forma- 

 tion — the brain-stem inhibitory region of Magoun 

 — and by excitation of the anterior lobe of the cere- 

 bellum (17; 28, p. 103; 30). Eldred has painstakingly 

 extended the exploration and anahsis of these remote 

 central spindle afferent control mechanisms (16). 



Granit & Kaada showed that gamma efferent 

 activity is facilitated by reticular stimulation at 

 strengths considerably below those which will elicit a 

 discharge of the large skeletal-muscle (alpha) moto- 

 neurons. Hence, motor facilitation by brain-stem 

 mechanisms appears to take place first through an 

 activation of the gamma efferents controlling sensory 

 input from the muscle spindles, and then bv both the 

 direct descending influences which act upon the large 

 motoneurons and the continuing indirect influence of 

 brain-stem control over muscle-spindle afferent dis- 

 charges which act back upon the same motor units. 

 As in other sensory control systems, the gamma effer- 

 ents appear to be normally under a tonic inhibitory 

 influence from above. 



In each example, the frog tactile receptor, the 

 muscle stretch receptor in Crustacea and the mam- 

 malian muscle-spindle afferent, there is evidence for 

 efferent neuronal systems which exercise an important 

 controlling effect upon the initiation of afferent nerve 

 impulses. In the case of the muscle spindle, at least, 

 the efferent fibers are in turn under the control of 

 certain remote central mechanisms. The principle of 

 central control of afferent activity is equally applicable 

 to the special senses. 



CONTROL OF ACTIVIT\- IN SPECIAL SENSE AFFERENTS 



Auditory Nerve Activity 



For many years a compact bLuidlc of libers traveling 

 with the eighth cranial nerve pair was considered to 

 be afferent (65, vol. I, figs. 319, 324). In a series of 

 critical anatomical studies, Rasniussen proved that 

 these are really efferent fibers. They arise in the 

 vicinity of the superior olive and terminate within the 

 contralateral cochlea (67, 68). Rasmussen's efferent 

 fibers appear to make contact with the afferent audi- 

 tory fibers as these pass from the hair cells to the 

 spiral ganglion. Some of the efferents may pass 

 directly to the inner hair cells but this point is un- 



LEFT 0-C 

 CUT 



/t^tN""*^ 



LEFT STAPEDIUS 

 CUT 



FIG. 2. Suppression of auditory nerve response by olivococh- 

 lear and stapedius mechanisms. A. Control auditory nerve 

 responses to click applied to each ear, right recording above 

 left. B. Suppression of both left and right responses with shocks 

 at 100 per sec. delivered to the decussation of the olivocochlear 

 bundle in the floor of the fourth ventricle. This high frequency 

 of stimulation tetanizes the stapedius muscle so as to eliminate 

 interference from that source (see E below). C. Following 

 transection of the left olivocochlear bundle, the suppression 

 shown in B occurs only on the right. D. .Another control re- 

 sponse showing that lesion made between B and C has not 

 interfered with auditory nerve response from either ear. E. 

 Single shocks to same medullary location 1 3 msec, prior to test 

 clicks suppress the eighth nerve responses from either ear 

 (stapedius effect). F. Following cutting of the tendon of left 

 stapedius muscle the suppression shown in E is seen only on 

 the right. [From Galambos (26).] 



settled. Galambos has recently shown, as illustrated 

 in figure 2, that stimulation of the medulla in the 

 region of the superior olive, and along the course of 

 the olivocochlear bundle, will cause a suppression of 

 auditory nerve responses elicited by standard click 

 stimulation (26). Such suppression does not occur 

 following division of the olivocochlear bundle at a 

 point peripheral to the locus of stimulation. The 



