338 



HARRY GRUNDFEST 



Cells of type lie in which secretory activity is concentrated at the terminus 

 of the axon is also a neuron, while cell type Ic is a primary receptor neuron. 

 Combinations of la and Ic; lb and Ic; or of all three cell types would then 

 represent secondary and tertiary receptor systems. Some neurosecretory 

 cells of types Ic and/or lie have both synaptic vesicles and neurosecretory 

 granules, the two differing markedly in size. This may indicate existence of a 

 double mechanism, the nerve impulses releasing "transmitter agent" from 

 the vesicles, and the substance then exciting the secretory activity of an 

 effector membrane (DeRobertis, 1961). 



lann 



specifically sensitive input membrane 

 Innervated chemosensitive synaptic membrane 

 Secretory output membrane 



Fig. 10. Diagrammatic representation of six possible types of related secretory, 

 neurosecretory, receptor and neuron cells. Group I: Cells which receive excitation 

 from external or internal specific stimuli. Group II: Cells that are excited by 

 innervation, (a) Simple columnar cells which might be glands, neurosecretory 

 cells, or receptor cells, (b) Neurosecretory or receptor cells, (c) Neurosecretory, 

 receptor, and correlational neurons. Further explanation in text. 



The conductile properties of the nervous system are essential for all but 

 the smallest of the co-ordinated Metazoa. Accordingly, electrical excitabiUty 

 and spikes are already found in the nervous system of coelenterates (Horridge, 

 1954) and have been studied with intracellular recording in earthworm 

 (Kao and Grundfest, 1957). Both coelenterates and worms exhibit complex 

 central integrative patterns. Perhaps, the Porifora may still yield examples 

 of primitive electrically inexcitable receptor-effector cells (Grundfest, 1959b). 

 However, it seems rather more likely that closer study of various types of 

 gland and neuroserectory cells might be more rewarding in establishing 

 evolutionary relations. 



REFERENCES 



Adrian, E. A. (1932) The Mechanisms of Nervous Action: Electrical Studies of the Neurone. 



University of Pennsylvania Press. 

 Alvarez-Buylla, R. and Ramirez de Arellano, J. (1953) Local responses in Pacinian 



corpuscles. Am. J. Physiol. Ill : 237-244. 



