INVERTEBRATE PHYSIOLOGY 



Input from 

 inhibitor axon 



^ fcvWon process 

 Excitability / \ov 



(smoothed) 



r-^ inhibitory process i 



Output in 



motor axon —^-^ ^ ' ' ' ' 



Time in 



seconds 



Fig. 2. Diagram of the response of a single unit of the cardiac ganglion of a 

 lobster to stimulation of the inhibitory axon. Each vertical line represents an im- 

 pulse. Maintained activity in the inhibitor produces, first, deep depression, followed by 

 partial escape or adaptation. Termination of inhibitor activity produces a postinhibi- 

 tory excitation after a brief latency or inhibitory after-effect. The observed input 

 and output are related by an algebraically summing series of processes, including 

 many of those listed in the text. Just two of these, which are moreover not directly 

 measured but inferred, are shown here. (After Maynard.) 



effects. These may be positive or negative or both in sequence, of various 

 relative durations and magnitudes. To say the same thing in more familiar 

 terms, there may be an after-discharge following cessation of presynaptic 

 excitatory bombardment or an after-inhibition following the end of in- 

 hibitory influx, and there may be, with or without this positive after-effect, 

 a rebound effect — postexcitatory depression or postinhibitory excitation. 

 When phases of opposite sign succeed each other, they may be affected 

 independently by various factors, as though manifesting separate under- 

 lying processes. 



Recently we have distinguished another property which is of importance 

 in permitting repetitive firing in response to a single presynaptic impulse 

 in the cardiac ganglion of lobsters (Hagiwara and Bullock, 1955). This 

 may be described as a sajcty factor of much less than one so that the post- 

 synaptic impulse, once initiated in the axon, cannot invade the cell body 

 (antidromically). This protects the latter from the possibility of loss of 

 any partial depolarization it may have built up. Its significance depends on 

 the asymmetrical relation between cell body and axon — the slow synaptic 

 potential of the soma can spread electronically into the axon with less 



