CENTRAL AUTONOMIC MECHANISMS 



965 



hypotlialaniic area contributes especialh- to this 

 system. Some of these fibers descend through the 

 central gray of the aqueduct; otiiers fan out into the 

 tegmentum, d) Diffuse descending connections, which 

 extend caudalward in large part as continuations of 

 the medial forebrain bundle, proljably arise through- 

 out the hypothalamus, although the posterior and 

 lateral areas probably are the chief contributors. 

 This system of fibers is scattered in the lateral por- 

 tions of the tegmentum (104, 107), and physiological 

 experiments indicate that it is of prime importance 

 in the conduction of hypothalamic impulses toward 

 the lower autonomic centers, e) Hvpothalamohypo- 

 physial connections are made through supraoptico- 

 hypophysial fibers which arise from the main and 

 accessory groups of the supraoptic nucleus and termi- 

 nate in a rich branching plexus in the neurohypoph- 

 ysis, paraventriculohypophysial fibers which arise 

 in the paraventriculus nucleus and end in the neuro- 

 hypophysis, and tuberohypophysial fibers which arise 

 from scattered neurons in the tuberal region. It is 

 likely that some of these fibers terminate in the median 

 eminence and in the infundibular stem. It may be 

 noted here that many of these neurons give evidence 

 of neurosecretory activity. /) A diffuse projection 

 system to the cortex originates in the posterior portion 

 of the hypothalamus and in the nearby reticular 

 formation of the mesencephalon. This system, which 

 probably involves thalamic relays, is important in 

 the maintenance of the waking state (105). It will 

 be noted that included in the above efferent systems 

 are rich connections with the neopallium, with the 

 orbitofrontal cortex and with rhinencephalic struc- 

 tures. These provide an ample basis for elaborate 

 feed-back mechanisms and reverberating circuits (64). 

 It is readih' apparent that this small area is dis- 

 tinguished by the richness of its fiber connections. 

 It is quite likely that the influence of the cerebellum 

 is also brought to bear on the hypothalamus, although 

 the pathways are not well known. The afferent and 

 efferent systems involved in the total hypothalamic- 

 reticular formation picture are probably still more 

 complex than is indicated by the above summary, 

 and the intermingling of these cells and fibers has 

 placed great obstacles in the way of experimental 

 exploration. 



Fuiulional Co)isideralions of Hxpolhalamus 



Although the hypothalamic region was a relatively 

 large part of the brain in primitive vertebrates, 

 evolutionary development of the higher mammals 



has almost completely enveloped it with folds of 

 forebrain. It is not surprising then that the physio- 

 logical significance of this remote area escaped analy- 

 sis until recently. Interest in endocrine physiology 

 directed attention to the hypophysis, and the spectacu- 

 lar experiments of Gushing and others first ob.scured 

 and later overemphasized the nearby hypothalamus, 

 for there was at one time considerable confusion in 

 regard to the respective functional relationships of 

 these structures. We now know that nervous control 

 of the pituitary, so far as it exists, is mediated through 

 the hypothalamus and that the latter participates in 

 certain functions which were once thought to be ex- 

 clusively hypophysial. 



There is danger in thinking of the hypothalamus 

 as a distinct and isolated structural entity. We have 

 seen that it has direct and indirect nervous connec- 

 tions with many other parts of the brain, including 

 the cerebral cortex, and from the functional view- 

 point it probaialy should not be considered as separate 

 from other diencephalic and lower brain-stem mecha- 

 nisms. We have already seen how the hierarchy of 

 autonomic mechanisms is functionally interrelated 

 in the spinal cord and lower brain stem. The so-called 

 autonomic phenomena with which the hypothalamus 

 seems especially concerned are also related to other 

 parts of the forebrain, as well as the mid- and hind- 

 brain, and hypothalamic functions are probably 

 largely integrative. The hypothalamus, then, fits 

 into large schemes, some of which are not exclusively 

 autonomic and which involve certain patterns of 

 behavior, including sleep-waking phenomena and 

 some aspects of emotional activity. 



Analysis of the participation of the hypothalamus 

 in autonomic activities began with the work of 

 Karplus and Kreidl who, as early as igog, studied the 

 effect of stimulating the wall of the third ventricle in 

 animals. In recent years various modifications of 

 this approach, as well as the use of experimental 

 lesions, have been reapplied many times in carnivores 

 and primates, including man, by such investigators 

 as Hess, Ranson and many others. To summarize 

 briefly, readily elicitable and observable responses 

 to electrical stimulation include: elevations of arterial 

 pressure, cardiac acceleration, dilatation of the pupils, 

 sweating, piloerection, hyperglycemia and cessation 

 of gastrointestinal movement. The responses obtained 

 are not exclusively sympathetic, however, for with 

 different forms of electrical stimuli (68, 69), and 

 especially from the more anterior regions, para- 

 sympathetic phenomena may be produced. These 

 include bladder contraction, increased gastrointestinal 



