CRITERIA FOR DISTINGUISHING BETWEEN MONOSYNAPTIC AND POLYSYNAPTIC TRANSMISSION

The methods considered apply specifically to the classical type of synaptic connection, i.e., the rapid release of transmitter at specialized appositions of neurons. Many of the tests are applicable to electrical synaptic transmission, though the presence of an intercalated neuron coupled electrically to the presynaptic (and postsynaptic) neuron could be difficult to detect. The use of radioactive markers has greatly facilitated the study of neuron pathways by EM. The labeled substance can be applied extracellularly or by direct intracellular injection, and since there is no apparent damage to ultrastructure, it is possible to identify synaptic regions. Care must be taken to discern any transfer of marker to postsynaptic neurons. To determine whether a recorded postsynaptic potential is produced by a monosynaptic connection ideally requires the anatomical identification of a synapse between the neurons involved. This is at present extremely difficult for central neurons, and may not be possible in nervous systems which show little synaptic specialization.