HARRY GRUNDFEST 139 



also seen in figure 7 and was first described by Wedensky. The summated 

 depolarization produced by repetitively evoked p.s.p.'s itself probably causes 

 the inactivation of the electrically excitable, spike generating membrane of the 

 ganglion cell. This blockade is therefore similar to that caused by 'dej)olarizing' 

 or synapse activating (100) drugs. 



g) Synaptic Transducer Membrane. Structures observed. In view of the likeli- 

 hood that transducer function resides in membrane structures of molecular 

 dimensions, it is not surprising, although disappointing, that electron micros- 

 copy has not yet revealed new, functional differentiations between pre- and 

 post-synaptic membrane or between the latter and the electrically excitable 

 spike generator components of the post-junctional cell (156, 167). In the muscle 

 end-plate and at the innervated face of electroplaques, cytological and histo- 

 chemical differentiations have long been known, but of obscure significance 

 (5 1^53) • Ii'i electron micrographs of the eel electroplaque, the differences seen 

 between the responsive, innervated caudal membrane of the cell and the an- 

 terior non-responsive membrane are minor (15). Some differentiations are 

 reported in electroplaques of Torpedo (53). 



Structure deduced from function. Functional considerations provide some 

 specifications for the molecular structure of the postsynaptic membrane. The 

 'receptor' located at the external face of the membrane (38) probably has 

 specific sites for attachment of, and action by, the transmitter. For the acetyl- 

 choline members of the family, two somewhat similar structural schemes have 

 been proposed (46, 189). In some cells of the autonomic system (e.g., cardiac 

 pacemaker cells; refs. 40, 118) the cholinergic transmitter causes inhibition 

 (i.e., tendency toward hyperpolarization) while the adrenergic acts oppositely 

 and is excitant (i.e., tending toward depolarization). Therefore the same cell 

 must be endowed with different and reciprocally-acting synaptic transducer 

 membrane. In other cells, the cholinergic is an excitant and depolarizer, while 

 the adrenergic is a depressant and hyperpolarizer. Thus a double reciprocal 

 relation appears. 



Transmitter Electrogenic response Transmitter 



I — > i) Depolarizing 2) Hyperpolarizing < — 1 



Cholinergic Adrenergic (5) 



' — > j) Hyperpolarizing 4) Depolarizing < — ' 



The cholinergically activated membrane constituent / tends to be associated 

 with the adrenergic type 2, but when the cholinergic is of type j the adrenergic 

 component is 4. These relations, therefore, suggest that some common basic 

 molecular structure of the membrane undergoes modification in one direction 

 or another to evolve all four synaptic receptors. A common relation between 

 molecular structures of cholinergic and adrenergic synapses (cf. 180) may also 



