INTERPEDUNCULAR NUCLEUS 203 



Each one of the other classes of afferents may have its own specific 

 type of trans-synaptic transmission, depending on the histological 

 and chemical structure and the strength and timing of volleys de- 

 livered. 



Provision is thus made for discriminative responses to a consider- 

 able range of different kinds of activity that may be in process else- 

 where in the brain — olfactory, visceral, somesthetic, and so on. The 

 number of interpeduncular neurons activated and the temporal 

 rhythm of their discharge may be determined not only by the alge- 

 braic sum of afferent impulses received but also by their timing. It is 

 believed that this sort of analysis is characteristic of the nervous 

 adjusters. 



The outflow from the interpeduncular nucleus is distributed in 

 part to remote regions and in part within the interpeduncular field, 

 the latter providing apparatus for summation and intensification of 

 the discharge. The remote effects of this outflow will depend on the 

 location, structure, and connections of the motor pools entered by 

 the efferent fibers. 



The entire interpeduncular field is a well-circumscribed motor pool, 

 in Sherrington's sense, in which every intrinsic nervous element is 

 subject to activation by a wide variety of nervous impulses coming 

 from diverse sources, each of which has a specific form of synaptic 

 junction. The different structural patterns of these synapses pre- 

 sumably involve physiological differences in the type of activation. 

 The dominant member of this complex of afferents evidently is the 

 spiral of habenulo-interpeduncular fibers; every fiber of this tract 

 may activate (or inhibit) every neuron of the nucleus, and, indeed, 

 in successive turns of the spiral it may act upon several dendrites of 

 the same neuron. There is no provision for separate localization of 

 specific function here. The nucleus inevitably acts as a whole. But 

 there is provision for summation of the effect in a large way. The 

 diffuse neuropil also may act upon these neurons. Volleys may be dis- 

 charged into it from many sources, each of which may reinforce (or 

 inhibit) the activity instigated by the spiral. 



Though temporal summation at an individual synapse is regarded 

 as practically inoperative (Fulton, '43, p. 58), there is ample provi- 

 sion in this pool for spatial summation. The glomeruli present a 

 series of difficult problems. Each of these small nodes of denser neuro- 

 pil contains tufted dendritic terminals and tufted axonal terminals, 

 and the latter come from two sources, intrinsic and extrinsic. The 



