EXCITATION BY HYPERPOL ARIZING POTENTIALS 337 



between the sensory receptor cells and neurons and gland cells. Some of the 

 neurosecretory cells respond to external stimuh such as changes in luminous 

 or thermal radiant energy, others to changes in various chemical compo- 

 nents, osmotic pressure and temperature of body fluids. Still others are 

 probably activated only by specific excitatory innervation. Thus, the line 

 between sense organs and neurosecretory cells appears to be tenuous. 



A discussion of the evolutionary implications of these concepts will be 

 found elsewhere (Grundfest, 1961). At this point, however, it may be appro- 

 priate to point to possible relations between secretory and neurosecretory 



Fig. 9. Postsynaptic potentials and spikes in spinal cord electromotor neuron 

 of Sternarchus albifwns. A. Activity of single neuron {lower trace) is synchronous 

 with organ discharge at about 620/sec (upper trace), b-d. Upper trace is monitor 

 for hyperpolarizing currents applied to the neuron. Current of organ discharges 

 also appear in this trace. B. Weak hyperpolarization delayed the spike some- 

 what, c. Superimposed traces show variation in response with stronger hyper- 

 polarization. D. Still stronger hyperpolarization eliminated spikes, leaving 

 p.s.p. behind as evidence that each neuronal spike arises by activation through a 

 descending bulbo-spinal tract system (Bennett and Grundfest, unpublished 



data). 



cells on the one hand and various receptors and neurons on the other. One 

 may expect to find six types of cells (Fig. 10) arranged in two general groups 

 that are differentiated by the presence or absence of an innervation. Simple 

 epithehocytes without innervation (la) might be gland cells or sensory receptor 

 elements of exteroceptive or enteroceptive function. The same cells with 

 innervation (Ila) would represent the simple epithelial gland cells. A more 

 complicated pair of cell types (lb, lib) would be secretory or neurosecretory 

 cells with direct processes which might or might not develop spikes. Other 

 neurosecretory cells might fall into the third group (Ic, lie), the ducts being 

 axons which produce spikes and secretion being confined to the terminal. 

 One neurosectory cell type which probably generates spikes is known at 

 present, that o{ Lophius (Potter and Loewenstein, 1955). 



