AMYGDALA 



'403 



lying these changes were recently investigated more 

 thoroughly with microelectrode techniques (98). Re- 

 cruitment and potentiation occurring in response to 

 amygdaloid stimulation were studied in the hippo- 

 campus. It was found that the excitatory changes 

 underlving recruitment and potentiation take place 

 in the layer of the long apical dendrites of the hippo- 

 campal pyramidal cells since the maximum negative 

 wave or 'sink' of the response was localized in the 

 dendritic layer. The slow dendritic response to single 

 shock often was not associated with propagated dis- 

 charge from the pyramidal cells. A progressive 

 i)uild-up of the dendritic excitatory state was re- 

 c|uired to enable the pyramidal cell to fire off action 

 potentials. This was signaled by the appearance of imit 

 spikes riding on the crest of a negative deflection 

 within the trough of the positive wave which was the 

 electrical sign of the evoked dendritic response when 

 recorded from within the cell layer (fig. 4.I to D). 



Since these excitatory changes are most prominent 

 with polysynaptic responses, it is proiiable that this 

 modulating dendritic mechansim may operate at 

 successive synaptic junctions, thus amplifying the 

 eflfects with each synaptic passage. Some of these 

 synaptic way stations inay actually be present within 

 the intraamygdaloid association system since it was 

 observed that the stria terminalis response may be- 

 come quite complex under the influence of repetitive 

 amygdaloid stimulation. It is thus apparent that the 

 rate of amygdaloid firing is capable of regulating the 

 mode of transmission of the responses in a very large 

 degree. Certain synaptic barriers are not transgressed 

 under certain conditions, but under others facilitatory 

 processes may build up to open these synaptic barriers. 



The mechanisms underlying the changes in latency 

 still remain unclear. Decrease in latency may be due 

 in some cases to shortening of synaptic delays due to 

 the facilitatory effect of dendritic depolarization. In 

 other cases latency changes may be due to differential 

 facilitatory effects within vicarious routes involving a 

 greater or smaller number of synaptic stations. If the 

 excitatory changes induced by repetitive stimulation 

 favor conduction over the shorter pathways, the 

 latency will decrease; if, on the contrary, facilitation 

 occurs predominantly in more involved pathways, 

 then the latency will increase and yet the response 

 will recruit. 



Commissural Connections 



Fibers of the anterior commissure connect the 

 amygdala with its fellow on the contralateral side. 



The.se fibers enter the commissure directly, except for 

 those forining the commissural component of the stria 

 terminalis which run with this bundle first until it 

 crosses the anterior commissure (26, 66, 109, 122, 

 130, 132, 233). The amygdaloid fibers taking a direct 

 course into the anterior commissure were for a long 

 time believed to originate from the basolateral 

 amygdala (118, 122, 130, 157, 256), but more re- 

 cently Brodal (33) has demonstrated that they orig- 

 inate in the corticomedial complex. No detailed 

 electrophysiological studies on these commissural 

 connections have so far been reported. 



ELECTRICAL ACTIVITY OF ."WIY'GDALA 



The spontaneous amygdaloid electrogram in cats 

 shows irregular 4 to 6 cps rhythms of 100 to 200 /iv 

 amplitude (103). Often rapid spindles at 16 to 26 cps 

 are seen which tend to be synchronous with the 

 respiratory rhythm, are usually arrested by blocking 

 of the nasal airway and are enhanced by olfactory 

 (103, 172) or reticular (59) stimulation. This activa- 

 tion may therefore represent the arousal pattern of 

 theamygdala (see footnote on p. 1397). Other studies 

 (20) however revealed no change in the amygdaloid 

 electrogram upon reticular stimulation. Some ob- 

 servers (158) recorded spontaneous spike discharges 

 from the amygdala, but it seems that this must be 

 interpreted as an injury discharge (102). 



STIMULATION STUDIES 



Excitabilily 



Electrical stimulation of tiie amygdala and the 

 neighboring cortex shows that this region is electrically 

 highly excitable. Stimulation responses as well as 

 seizure discharges are elicited at a threshold which is 

 generally lower than that in other parts of the brain, 

 except for the hippocampus (93, 133, 160, 175, 190). 



Electrocorticographic Responses 



Electrical stimulation of the amygdala is apt to 

 produce diflTu.se flattening of the cortical electrogram 

 together with an increase in background frequency 

 and asynchrony (20, 59-61, 133). Simultaneously, 

 barbiturate spindle bursts, slow waves and epilepti- 

 form spikes, including those evoked by a weak 

 strychnine solution may be suppressed. These 



