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FIG. 7. Activity recorded from a single unit at the level of the lower hippocampal pyramids 

 gain higher for a than for b, c or d. Records a and h show the effect of a flashing light. ( A smal 

 second unit's activity is detectable in a which also shows regular bursts on stimulation, bu t is no 

 precisely in phase with the large one.) Record r is of spontaneous activity; d, ot continuous visua 

 excitation. The unit was not affected by other modalities of sensory stimulation. [From Green 

 & Machne (49).] 



campus itselt Ijut from some other region in which 

 seizure activity is induced. 



Field Potentials and Unit Activity 



Renshaw et al. (92) in an early study of subcortical 

 unit activity investigated the hippocampus and drew 

 attention to the advantages of its single lamina of cells. 

 While the neocortex has six recognized laminae in 

 which axons, dendrites and cell bodies are almost in- 

 extricably intermingled, the pyramidal cell layer of 

 the hippocampus is arranged with the somata tightly 

 packed together, with the apical dendrites passing 

 centripetally and the axons and basal dendrites 

 passing at first centrifugally. The axons then turn in 

 the alveus at right angles forming a thin surface layer. 

 The hippocampus may be approached by removing 

 the cortex and corpus callosum above the lateral 

 ventricles in animals which possess a dorsal hippo- 

 campus, such as the cat. The blood supply enters 

 the hippocampus from the septal arteries, a series of 

 branches which emerge from an arterial arcade formed 

 by the anterior and posterior choroidal arteries. This 

 group of septal vessels penetrates between the gyrus 

 dentatus and the hippocainpus and supplies Amnion's 

 horn from this cleft which, however, is incompletely 

 filled with pia and is crossed by part of the temporo- 



ammonic tract system of fibers. The result is that 

 when the hippocampus is exposed from above, it is 

 approached \'ia the ependyma rather than the pia. 

 Its blood supply entering from below is not 

 endangered. Under these circumstances, Green & 

 Adey (45) were able to isolate the hippocampus by 

 sectioning the fornix and the psalterium, and by re- 

 moving the entorhinal cortex and amygdala. A single 

 hippocampus isolated in this way is silent during acute 

 experiments lasting .several hours. A single shock 

 applied to the hippocampus may be followed by an 

 evoked potential in almost any other part of the struc- 

 ture. A single shock to the fornix under these circum- 

 stances also activates the hippocampus. If this re- 

 sponse is recorded with active electrodes on the surface 

 of the hippocampus, it is found not to be altered by 

 making a circumscribed cut around the point from 

 which the record is taken (45, 1 12), providing this cut 

 extends half a millimeter or so, and no more, beneath 

 the surface. A deep cut, parallel to the fimbria, 

 abolishes or greatly modifies the response. It was con- 

 cluded that the response recorded with a latency of 4 

 to 5 msec, was postsynaptic, for it was greatly de- 

 pressed by anoxemia and by pentobarbital, and it was 

 potentiated by strychnine or by repetitive stimulation 

 and showed, in addition, tetanic and posttetanic po- 

 tentiation. It was also concluded that the afferents 



