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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



interruption from the contralateral cochlear nuclei, 

 many more from the ipsilateral superior olivary 

 nucleus and a few (proiaably) from the nucleus of the 

 lateral lemniscus, ipsi- and contralateral. The input 

 to this nucleus, therefore, represents a degree of diver- 

 sity and temporal dispersion still greater than that of 

 the superior olive and cochlear nuclei. This is demon- 

 strated by the relatively greater protraction of re- 

 sponse to brief stimuli than is seen in the more caudally 

 located stations of the pathway (5, 90). 



The inferior colliculus is one of the most highly 

 organized and largest nuclei of the brain stem. In- 

 deed, its position, size, pattern of organization and 

 multiplicity of afferent and efferent connections would 

 make it seem more logical to consider it, together with 

 the superior colliculus, suprasegmental rather than a 

 brain-stem nucleus in the usual meaning. At any rate, 

 the organization and fiber connections are such that, 

 despite the fact that the inferior colliculus must func- 

 tion to some extent as a relay in the ascending audi- 

 torv pathway, its significance can by no means be 

 limited to its relay function. This will be discussed 

 further in a different context in a later section of this 

 chapter. 



Inferior Qiiadrigermnal Braclnum 



The colliculus, in addition to efferent pathways to 

 superior colliculus and pons (77), has, as its principal 

 route of discharge, the brachium of the inferior collic- 

 ulus (or inferior quadrigeminal brachium). This 

 tract is composed predominantly of fibers arising in 

 both ipsilateral and contralateral colliculi, those from 

 the latter passing through the commissure of the in- 

 ferior colliculus (ill). In addition, there is present 

 in the tract the group of lemniscal fibers, noted in the 

 preceding section, that bypasses the colliculus. The 

 entire brachium passes rostrally and .somewhat later- 

 ally to terminate in the medial geniculate body. 



Medial Geniculate Body 



The medial geniculate body is the thalamic nucleus 

 of the auditory pathway. It is described as having a 

 pars principalis compo.sed of small closely-packed 

 cells arranged in a laterodorsally curving band, and 

 a pars magnocellularis, lying medioventral to the 

 pars principalis and composed of large cells (10, 23). 

 There is some doubt that the magnocellular part 

 should be considered a part of the true auditory 

 thalamic relay, although the terminology which 



makes it a part of the medial geniculate has been 

 generally accepted for many years. 



The pars principalis appears to be fairly homogene- 

 ous with respect to cell size and distribution, except 

 for a slightly decreasing gradient of density from lat- 

 eral to medial aspects (85). Thus, there is none of the 

 conspicuous organizational complexity of the brain- 

 stem acoustic nuclei. The principal input to the nu- 

 cleus consists of the terminations of the inferior 

 quadrigeminal brachium. Other than these, the only 

 fibers reported as afFerents to the nucleus are recurrent 

 projections from the cortical projection area (62). 

 Aside from a small number of fillers which are dis- 

 tributed rather diffusely to other parts of the thalamus 

 (i) and a few which retrace the lower projection path- 

 way (i), the main efferent outflow from the medial 

 geniculate is the acoustic radiation. These fibers 

 proceed bv way of the posterior limb (sublenticular 

 portion) of the internal capsule to part of the superior 

 face of the superior temporal gyrus and adjacent 

 insular and parietal opercular cortex in primates and 

 the corresponding cortex in carnivors which lack a 

 true temporal lobe. Discussion of the projection areas 

 forms the subject matter of a later section of this 

 chapter. 



As noted above, the medial geniculate, pars princi- 

 palis, shows little or no histologically demonstrable 

 organization; however, there is other evidence indicat- 

 ing that there is, nevertheless, at least a spatial type 

 of organization. This is inseparable from evidence of 

 similar organization in other parts of the auditory 

 system and a separate section will be devoted to spa- 

 tial and tonotopic aspects of the projection pathway. 



Auditory Connections with Cerebellum 



.Snider & .Stowell (95) in 1944 reported the hitherto 

 unknown fact that auditory stimuli (clicks) could 

 regularly evoke responses from the cortex of the 

 cerebellar \ermis in cats. In subsequent experiments 

 these findings were confirmed and the additional dis- 

 covery made that stimulation by light flashes also 

 elicits response in the same cerebellar area. The 

 responses to auditory stimulation occur with latency 

 so brief as to imply a fairly uncomplicated projection 

 from the periphery. At the time of the original ob- 

 servations, no such cochleocerebellar path was known. 

 Since then Niemer & Cheng (68) have deduced the 

 existence of a pathway by which the ventral part ot 

 the dorsal cochlear nucleus sends fibers to termina- 

 tions in the cerebellar \ermis. Their evidence consists 



