RELATIONSHIPS BETWEEN MORPHOLOGY AND PHYSIOLOGY OF PYRAMID-PYRAMID SINGLE AXON CONNECTIONS IN RAT NEOCORTEX IN-VITRO

1. Double intracellular recordings were made from 1163 pairs of pyrami dal neurones in layer V-VI of the rat somatomotor cortex in vitro usin g sharp electrodes filled with biocytin. Monosynaptically connected pa irs of cells were identified when an action potential in one could eli cit a constant latency excitatory postsynaptic potential (EPSP) in the other and the cells were filled with biocytin. Labelled cells were su bsequently identified histologically with avidin-horseradish peroxidas e. 2. Thirty-four pairs of cells were found to be monosynaptically con nected. Fifteen of these pairs were sufficiently stable for electrophy siological recordings and three of these were recovered sufficiently t o permit full morphological reconstruction. 3. The EPSP recorded betwe en the first pair of pyramids varied in amplitude between 0 and 3 m V (mean 1.33+/-1.06 m V) and fluctuated considerably (coefficient of var iation, 0.796). This was largely due to a high incidence of apparent f ailures of transmission. On reconstruction two boutons from the presyn aptic pyramid axon were in close apposition to the proximal portions o f basal dendrites of the postsynaptic cell. 4. In the second pair of p yramids the EPSP had a mean amplitude of 1.06 m V, and displayed a 10- 90% rise time of 2.8 ms and a width at half-amplitude of 23 ms. This E PSP did not alter significantly with changes in membrane potential at the soma. The presynaptic axon closely apposed the distal apical dendr ite of the postsynaptic cell in eight places. 5. In the third pair of pyramids, the EPSPs, recorded at a relatively depolarized membrane pot ential, were long lasting and could elicit slow dendritic spikes with long and variable latencies. These slow spikes suggested that the post synaptic recording site was dendritic and on reconstruction a possible location was identified on the apical dendrite. A total of five presy naptic boutons closely apposed three separate, proximal branches of th e postsynaptic apical dendrite. 6. These results provide the first ill ustration of a morphological basis for variations in functional proper ties of pyramid-pyramid connections in the neocortex.