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HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY II 



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FIG. 2. Nerve cell discharges in the central 

 nucleus of the amygdala evoked by single 

 shock stimulation of the contralateral sciatic 

 nerve (&) and of the tooth pulp (Tc) and by 

 natural somatosensory stimuli, such as light 

 touch of the fur of the contralateral hind leg 

 and stretching of the gastrocnemius muscle. 

 [From Machne & Segundo (i66).] 



Tc 



Touch Ni I'^ m l m 



i t"f i' f"| ' i ' 



jUMkHw^ii <i " ji j 'i w 



'Miiiiiiiui i .;i iiiii ! I II 1 1 I 



Stretch I i|| |M \U\ L I J 



1 1 1 i I li 



m. 



|»'!'lHililii|ll Mlllhl! 



i i yi l||! lll!l!l| | |l l l ! l! t ll(i ' H 



been demonstrated electropliysiologically. .Some anat- 

 omists describe thalamoamygdaloid fibers but their 

 statements do not seem to Ije well documented (ii8, 



177. 184)- 



Physiological studies, however, show that evoked 

 potentials to all sensory modalities can be elicited in 

 the amygdala (52, 53, 90, 166, 211). For classical 

 evoked potentials the latency of these responses may 

 vary from 8 to 25 msec. (52) and for unitary dis- 

 charges may even measure anywhere from 20 to 500 

 msec. (166). This suggests mediation over long poly- 

 synaptic pathways. With the classic evoked-potential 

 technique, responses to stimulation of the vagus and 

 lingual nerves were most prominent {52, 53, 172); 

 but unitary discharges recorded with microelectrodes 

 seemed to be fired most effectively by somatic sensory 

 stimuli, especially by a very slight touch to the skin 

 (fig. 2) {166). These responses were mostly recorded 

 from the basolateral complex. Responses to auditory 

 and visual stimuli and to stimulation of the tooth 

 pulp, presumably eliciting pain, were more difficult 

 to obtain and more scattered in their distribution 

 (52, 53, 166). Impulses originating from widely 

 distant areas of the body surface, as well as impulses 

 evoked by stimuli of different sensory modalities, 

 were found to converge upon one and the same 

 amygdaloid cell. The most frequent type of conver- 

 gence was that of olfactory and somatosensory signals. 

 Thus there is no topographical representation of 

 sensory modalities or local areas of the body. The 

 specificity of the sensory message is probably lost in 

 this system, unless a specific temporal pattern of 

 unitary discharge may serve as a code assuring its 

 retention, a possibility suggested by some of the re- 

 ported findings. Units were either directly fired in 

 response to stimulus or they were activated in their 

 spontaneous discharge pattern (fig. 2); more rarely. 



they were inhibited. Sometimes the response con- 

 sisted merely of a shift in pace of the unit firing. 



In view of their extensive overlapping these sensory 

 evoked responses, including the olfactory ones, are 

 probably unrelated to specific perceptual mechanisms 

 as sucli (52). It is therefore of interest to recall at this 

 point that the secondary sensory response of Forbes 

 and Morison, most likely an unspecific sensory mes- 

 sage, seems to be transmitted to the cortex via a 

 region close to the temporal horn of the lateral 

 ventricle, in which the most significant structure is 

 the anterior pole of the amygdala (189). 



Stimulation studies in man give additional evi- 

 dence that the amygdala is involved in sensory func- 

 tions (129). Patients in whom the amygdaloid region 

 was stimulated frequently reported sensory experi- 

 ences of ill-defined quality. The sensation was often 

 referred to the body as a whole or to some part of it 

 such as the head, the abdomen or even the extremities. 

 Some animal studies also suggest that amygdaloid 

 stimulation may evoke sensations, for instance when a 

 cat upon amygdaloid stimulation fixes its gaze on a 

 particular spot of its body and then proceeds to sniff 

 and lick it intensely, or when the contralateral paws 

 are lifted from the floor as if to remove them from an 

 unpleasant contact (82, 135). 



NONSENSORY .AFFERENT CONNECTIONS TO AMYGD.-VL.^. 



A few other afferent connections to the amygdala 

 have been described. Some of them are controversial 

 from an anatomical point of view and so far unproved 

 electrophysiologically, such as those from the mid-line 

 and intralaminar nuclei of the thalamus (165, 203, 

 204, 215). 



Other subcortical structures sending afferent con- 

 nections to the amygdala are the reticular formation 

 of the brain stem (166) and the striatum (132). The 



