1040 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY H 



activity. Knowles (183) defines neurosecretory fibers 

 as those which end bhndly, which have special 

 synapses and which do not excite other neurons or 

 innervate organs in the common sense of the term. It 

 is clear that this definition may already be too re- 

 stricted in the light of present investigations (40, 194, 

 •253). It has not been determined with certainty 

 whether neurons active in the transport of neuro- 

 secretory material are able also to stimulate other 

 nerve cells. The presence of neurofibrillae in such 

 cells is questioned seriously, largely liecause of techni- 

 cal difliculties in their demonstration (130). How- 

 ever, it is a fact that in vertebrates as well as in in- 

 vertebrates these neurons are capable of transmitting 

 electrical impulses (49, 245). The term encephalo- 

 hydrocrime has been coined by Roussy & Mosinger 

 (267) to indicate the delivery of neurosecretory ma- 

 terial into the ventricles, a concept of some possible 

 significance which has received little attention out- 

 side of the French school of workers 



NEUROSECRETORY CELLS AS SPECIALIZED NEURONS 



Secretorv activity of neurons represents a further 

 specialization of these cells. Like other neurons, cells 

 active in neurosecretion exhibit all the usual morpho- 

 logical evidence of intensive protein synthesis 

 (Caspersson, Hyden) such as enlargement of nucleoli, 

 enlargement and excentric position of nuclei, evidence 

 of nuclear secretion, reconstruction of plasma ribo- 

 nucleic acids, accumulation of phosphata.se enzymes 

 and alteration in amount and distribution of Nissl 

 material as in the axon reaction (56). The streaming of 

 cytoplasm from the region of the perikaryon to the 

 periphery of the nerve cell (335) indicates not only a 

 continuous protein synthesis, but also reflects the 

 movement of material along the axon, as has been 

 observed in living nerve fibers (126, 167). This phe- 

 nomenon has been demonstrated also with the aid of 

 radioactive materials in both in vivo and in vitro nerve 

 preparations (66). The observation that living nerve 

 fibers can incorporate and move fluid droplets (126) 

 should be an indication that the finding of vacuoles 

 in ganglion cells is not necessarily an indication of 

 neurosecretion (326). The ontogenetic maturation of 

 the neurosecretory system (263) lends itself easily to 

 correlation with similar development and change in 

 the enzyme systems of nervous tissue observed by 

 earlier investigators (334). Finally, there are signs of 

 a phylogenetic specialization of neurosecretory ele- 

 ments (152, 236, 264). 



NEUROSECRETION IN VERTEBRATES 



Hypolhalamic-Pituilary System Demonstrable 

 with Chromhematoxvlin 



The most thoroughly investigated field in neuro- 

 secretion is that part of the hypothalamic-pituitary 

 system demonstrable througli staining with chrom- 

 hematoxylin or paraldehyde fuchsin. 



MORPHOLOGY. In lower forms, the nuclei concerned 

 in this part of the hypothalamic-pituitary system be- 

 long to the preoptic nucleus which difTerentiates into 

 two parts and in reptiles, birds and mammals becomes 

 the supraoptic and paraventricular nuclei. The 

 special position of these nuclei (fig. lA, B) is indicated 

 in the case of the preoptic nucleus by the relationship 

 of its dorsorostral part to the third \entricle and its 

 caudovcntral part to the ventral surface of the brain. 

 Similarly, in higher forms the paraventricular nu- 

 cleus lies close to the third \entricle and the supraoptic 

 nucleus can be seen on the ventral surface. 



In mammals the neurons constituting these groups 

 may occasionally be disseminated in the form of 

 accessory nuclei (136, 192, 237). Characteristic cells 

 of these nuclear groups may be found distributed also 

 along the pathway of their axons as far as the posterior 

 lobe (29, 144, 196). The axons of cells in the para- 

 ventricular nucleus may associate closely with those 

 of the supraoptic nucleus or pursue a separate path- 

 way to the posterior lobe (192). In the dog, the total 

 number of neurons is approximately 90,000. The 

 processes of these neurons end in large measure in the 

 posterior lobe (neiirosecretorische Balm) (29, 241). They 

 constitute accordingly the supraopticohypophyseal 

 tract of Greving (137, 138) which, with some species 

 variation, contains both myelinated and nonmye- 

 linated fibers. In lower forms, dendrites of these cells 

 may reach the ventricular wall (152, 214, 236, 266, 

 317). In all vertebrates, axons of such fibers end par- 

 tially in the hypothalamus, in the i^egion of the median 

 eminence and in the pituitary stalk as well as in the 

 ependyma of the infundibular part of the ventricle 

 (fig. iB). In earlier work, the common characteristics 

 used to identify these nuclei were the limitation of 

 the Nissl material to the periphery of the cell (variable 

 according to species), the very rich capillary bed and 

 the presence of colloid in the neuron cell body and 

 processes. Currently, greater emphasis is placed upon 

 the cytoplasmic granules and droplets made \'isible 

 by a number of stain techniques (131) but which are 

 demonstrated to ha\c special characteristics by the 



