AMYGDALA 



1401 



complex may be transmitted over the stria terminalis 

 as well Ijut with latencies so long as to indicate that 

 they are synaptically relayed in the corticomedial 

 nuclei. This would presuppose the existence of an 

 intraamygdaloid association system with short axons, 

 a postulate supported by anatomical findings of 

 various workers (62, 63, 118, 132, 154, Crosby, per- 

 sonal communication). Direct subcortical connec- 

 tions from the basolateral complex seem to be identi- 

 cal with the ventral amygdalosubcortical pathways 

 described by the anatomists (see p. 1400). 



Direct connections from the amygdala to the 

 mesencephalic tegmentum may also exist in addition 

 to those relaved over polysynaptic hypothalamic re- 

 lays of the 'secondary projection field' since some 

 shorter latencv responses ranging between 10 and 15 

 msec, reappeared behind the posterior hypothalamus 

 where the latencies measured between 15 and 25 

 msec, or even more. It is therefore possible that a 

 direct amygdalomesencephalic pathway, such as is 

 known to be present in birds (43, 55, 107, 112) and 

 reptiles (137), may also exist in mainmals where so 

 far only scanty evidence for its existence could be 

 derived from some physiological observations (250) 

 and from some histological findings in marsupials 

 (162). 



The cortical projection field of the amygdala is 

 rather restricted. Short-latency responses were re- 

 corded from the piriform, anterior temporal and 

 insular cortex. An important but polysynaptic path- 

 way relays from the amygdala to the hippocampus. 

 It involves several synapses, presumably in the piri- 

 form cortex, as suggested by the long latencies of the 

 hippocampal responses to amygdaloid stimulation 

 which often exceeded 20 msec. The hippocampal re- 

 sponses mediated over this pathway inay become very 

 large with repetitive stimulation, as will be further 

 described below, suggesting that this connection, al- 

 though indirect, may be quite important functionally, 

 a view also expressed by Crosby (personal communi- 

 cation) on the basis of anatomical studies. 



This analysis of the neuronal organization of the 

 amygdaloid projection systems reveals a very intimate 

 relationship of the amygdala with highly integrative 

 subcortical structures (fig. 3). This relationship is not 

 a simple one; divergent paths from the amygdala 

 converge again upon common neuronal pools, as 

 evidenced by the existence of direct and indirect 

 amygdaloseptal, amygdalohypothalamic and amyg- 

 dalomesencephalic pathways. Some of the indirect 

 pathways relay impulses through short intraamyg- 

 daloid connections. Thus impulses from the baso- 



lateral complex may ije conducted to the stria ter- 

 minalis via the corticomedial nuclei and, finally, both 

 tiie direct and indirect systems converge upon the 

 same septohypothalamic zone. In addition there is 

 still a third possible route with polysynaptic relays via 

 the piriform cortex to the hippocampus which in its 

 turn discharges into the same general septohypo- 

 thalamic zone by way of the fornix. This arrangement, 

 together with the polysynaptic organization of the 

 subcortical system upon which these pathways pro- 

 ject, creates favorable conditions for a very flexible 

 play of excitatory processes. 



This flexibility is clearly revealed when repetitive 

 electrical stimulation of the amygdala is applied to 

 the study of its projection system (97). With relatively 

 low-frequency (10 to 50 cps) repetitive stimulation 

 the following changes of excitability were observed. 



a) Recruitment, i.e. a gradual increase in the 

 amplitude of the evoked response during repetitive 

 stimulation (fig. 4), was ordinarily seen but occasion- 

 ally the opposite phenomenon, a gradual decrease of 

 the amplitude of the response, for which the term 

 obliteration was used, appeared. 



b) Potentiation, i.e. a state of residual facilitation 

 outlasting the period of 'tetanic' repetitive stimula- 

 tion to which it is due, manifests itself by an increased 

 amplitude of 'posttetanic' single shock responses as 

 compared to "pretetanic' single shock responses (fig. 



4)- 



c) Latency changes may be observed, either de- 

 creases or increases occurring without any constant 

 correlation with the type of amplitude change. 



These three types of changes were, as a rule, more 

 prominent the longer the latency of the response; in 

 other words, increase in the number of synaptic pas- 

 sages seems to amplify the excitatory changes condi- 

 tioned by repetitive stimulation. Recruitment and 

 latency changes were absent in the 'primary projec- 

 tion field' and potentiation here was minimal.'' On 

 the other hand, such changes were rather prominent 

 in the 'secondary projection field' and were most 

 conspicuous in the hippocampus where the longest 

 latency responses were found. The short latency neo- 

 cortical responses in the 'temporoinsular' cortex 

 showed none of these changes. 



^ There is evidence however that a certain degree of po- 

 tentiation already occurs at the actual site of stimulation in the 

 amygdala. This contention is based on observations that the 

 utilization time shortens after repetitive amygdaloid stimula- 

 tion. Thus pre\iously ineffective short pulses may be rendered 

 effective in the posttetanic phase following a sequence of 

 repetitive stimuli of long pulse duration (97). 



