■;;'• 



HANDBOOK OF I'HYSII il ll(IV 



NKlkOPHYSIOLOtiY III 



ficd as being important to emotion differ from other 

 parts of the cerebral mantle. First to be considered are 

 -in ue broad distinctions between the limbic cortex 

 and neocortex. The limbic cortex, in contrast to the 

 neocortex, has a wealth of large connecting pathways 

 with the hypothalamus the importance of which in 

 emotional expression hardly needs to be restated. 

 These include the medial forebrain bundle, stria 

 terminalis, fornix and mammillothalamic tract. Re- 

 cently Nauta (49) has confirmed Ramon y Cajal's 

 findings (55) of a sizeable bundle of fibers in the 

 fornix that project to the tubercle nuclei which sit 

 astride the portal circulation of the pituitary. It has 

 also been learned that the fornix has connections with 

 the central gray of the midbrain (49), the importance 

 of which in emotional behavior is becoming increas- 

 ingly evident. 



Another distinction pertains to cytoarchitecture. 

 The limbic cortex is structurally primitive compared 

 with the neocortex. In broad terms it can be distin- 

 guished from the latter by a) the absence or poor 

 development of the supragranular layers, b) the end- 

 ing of the afferent plexus in the superficial layers, and 

 1 ) the relative paucity of cells of short axon. Ramon y 

 Cajal looked upon the presence of large numbers of 

 these cells in man's highly evolved cortex as the ana- 

 tomical expression of the delicacy of function of the 

 human brain. 



The neocortex has a topographical representation 

 11I ihc visual, auditory and tactile senses, as well as 

 the proprioceptive sense related to movement. Then' 

 appears to be but a small representation of viscero- 

 ceptive senses. The preponderant representation of 

 the highly discriminative exteroceptive senses indi- 

 cates that the functions of the neocortex are prin- 

 cipally concerned with refined adaptive adjustments 

 to the external world. On the contrary, comparative 

 neurology indicates that the development of the limbic 



cortex hinges on the sense of smell which is both .111 

 intero- and exteroceptive sense and is presumably 

 chemical in nature. I( is a sense that is appreciated 

 largely in terms of quality and intensity and is poorlv 

 adapted to giving spatial information. These char- 

 acteristics would seem to be correlated with the primi- 

 tive nature oi the limbic cortex. 



bin trophysiologii al Diffi rena 1 

 Electrophysiological^, .is one would expect of 



lii'jlib discriminative senses, rapid 011-ofl responses 



n« led in the various sensory areas of the neo- 

 cortex upon stimulation through the appropriate re- 



ceptor apparatus. In contrast, olfactory stimulation 

 elicits trains of rhythmically recurring potentials in 

 the pyriform cortex which may persist well beyond the 

 duration of the stimulation. Gustatory and noxious 

 stimulation elicits similar changes in the pyriform 

 area (42). In a somewhat parallel manner stimulation 

 by way of various intero- and exteroceptive systems 

 results in slow rhythmically recurring potentials 

 throughout a large part of the hippocampus (22). 

 These changes may outlast the contrasting low-voltage 

 random fast activity that characteristically appears 

 simultaneously in the neocortex generally. In this 

 connection, and in anticipation of future discussion, it 

 is to be pointed out that stimulation of the reticular 

 formation in the midbrain evokes comparable changes 

 (22). 



Biochemical Differences 



Recent biochemical and neuropharmacologies 



studies have indicated further distinctions between the 

 limbic and neocortex which have important implica- 

 tions in regard to chemotherapy of mental illness. 

 The 5-hydroxytryptamine content, for example, of 

 the hippocampus and the cortex neighboring the ol- 

 factory tracts is far in excess of that found in the neo- 

 cortex. Radioautographic studies employing S 35 

 labeled /-methionine have provided indirect evidence 

 that the protein metabolism of the limbic cortex 

 generally, and of the hippocampus in particular, is 

 higher than that of the neocortex (41 ). In view of 

 these findings it is of interest that the tranquilizing 

 drug reserpine elicits distinctive electroencephalo- 

 graphic changes in the hippocampus, and that these 

 are similar to those seen in ether anesthesia (41 ). 



Limbic and Prefrontal Cortt \ 



From the standpoint of emotional mechanisms it 

 would appear to be significant that the prefrontal 

 cortex evolves in relation to the medial, rather than 

 the lateral, group of thalamic nuclei. Thus its thalamic 

 connections with the dorsomedial nucleus brings it in 

 close association with the anterior and other niu lei 

 of the middle group which are related to the limbic 

 cortex. Neuronography studies have revealed also 

 that it has corticocortical connections with parts ,,| 

 the limbic cortex falling within the frontal lobe. Its 

 possible reciprocal relationship with the limbic cortex 

 through the reticular System will be discussed subse- 



quentl) . 



