12 HARRY GRUNDFEST 



the complex functioning of the brain, but in experimental situations the 

 tested effects may be small. For example, profound modification of axo- 

 dendritic electrocortical activity affects the electrical excitability of the 

 corticospinal neurons little or not at all. In contrast, stimulation of the 

 reticular formation which probably activates axosomatic as well as axo- 

 dendritic inhibitory synapses, causes marked depression of the electrical 

 excitability of corticospinal neurons, as well as changes in electrocortical 

 potentials. 



Electrical or "ephaptic" connections between cells occur more frequently 

 than has hitherto been recognized. The primary electrotonic effects are 

 excitatory, since the spike of an active cell would cause a relatively large 

 electrotonic depolarization in other cells. However, hyperpolarizing inter- 

 actions are theoretically conceivable. Electrical interactions without dis- 

 tinctive anatomical relations also occur and are designated as "field effects". 

 Excitatory and inhibitory effects of this type have been demonstrated between 

 axons in peripheral nerves and in the spinal cord. The dorsal root reflex is 

 probably due to excitation of intraspinal axon terminals by massive nuclear 

 activities excited synaptically by influx in the same or other dorsal root 

 fibers. An inhibitory effect, probably of similar origin, by ephaptic field 

 interactions, has been described in Mauthner cells of goldfish. 



INACTIVATOR TRANSMITTERS 



It may be appropriate to note at this point that transmitter agents need 

 not be activators of excitatory or inhibitory synapses, but might have quite 

 the opposite effect. Thus, synapse inactivators might be released by some 

 system and depending on the site of this synaptic action they might cause 

 "inhibition" or "excitation". Furthermore, a number of pharmacological 

 agents are known which in addition to being synaptic drugs also affect 

 electrically excitable activity. Thus, a transmitter agent that depolarizes 

 excitatory synapses might also inactivate the conductile membrane of some 

 presynaptic fibers or of the postsynaptic cell. 



CONCLUSION 



Nearly all of the foregoing examples have been experimentally established. 

 The others are at least likely possibilities which deserve serious consideration 

 in view of the vast complexities of the membranes of living cells. Unfortun- 

 ately, phenomenological schematizations of "excitatory" and "inhibitory" 

 actions are far too abundant. Especially within the nervous system, with its 

 large variety of anatomically, physiologically and/or pharmacologically dis- 

 tinct species of cell components, extreme complexity of action may be 

 expected and should be sought for analytically rather than by phenomeno- 

 logical verbalizations. 



