164 INVERTEBRATE PHYSIOLOGY 



from observations on vertebrates, has been summarized elsewhere (Welsh, 

 1955). 



One may picture a typical neuron, whether vertebrate or invertebrate, 

 as an elongated cell whose cell body is its main synthetic center for the 

 production of a specific transmitter agent or neurohormone. Lipid-coated 

 packets of the neurohormone flow with the axoplasm to axonal endings, 

 where they form a reserve. On the arrival of a nerve impulse, resulting 

 in an increase in membrane permeability, a certain number of vesicles (or 

 their contents) are released. The neurohormone may act at close range 

 to excite an adjacent neuron or efifector cell, or it may be carried in the 

 circulation to regulate a more lengthy process, such as the activation of 

 insect thoracic glands, which in turn produce a molting hormone. 



No longer is it possible to encompass all neurons under the headings 

 "cholinergic" and "adrenergic." Instead we must be prepared to accept 

 a terminology that will recognize a variety of chemical transmitters or 

 neurohormones. 



The Chemical Nature and Identification of Neurohormones 



In the vertebrates, acetylcholine, adrenaline, and nor-adrenaline have 

 been isolated from the nervous system and chemically identified. There is 

 adequate physiological evidence that these three substances act as chemical 

 transmitters. From the vertebrate posterior pituitary gland the neuro- 

 secretory substances, oxytocin and vasopressin, have been isolated and 

 identified as polypeptides, and their detailed structure is known. 



All too often in the invertebrates the only procedure used in attempt- 

 ing to identify a neurohormone has been to compare the physiological 

 effects of a nerve extract or of nerve stimulation with the effects pro- 

 duced by the application of a series of known candidate compounds. One 

 has often been forced to follow such a procedure, because of the very 

 small amounts of tissue available for chemical study. By use of extraction 

 and bioassay, with other common pharmacological procedures, acetylcho- 

 line has been reported to be present in representatives of most of the major 

 phyla of animals (see Prosser, 1946). Such methods do not always give 

 the true chemical identity of a substance and should, where possible, be 

 supplemented by other means of identification. This appears especially 

 desirable in connection with the identification of acetylcholine, since there 

 are other members of this class of compounds known to occur in the in- 

 vertebrates (Erspamer and Benati, 1953; Whittaker and Michaelson, 

 1954; Augustinsson and Grahn, 1954). 



Fortunately, by means of basically simple chromatographic and electro- 

 phoretic procedures, it is now possible to identify many naturally occurring 

 organic compounds even though they are available in very small amounts. 



