[The ultrastructural characteristics of the pyramidal callosal neurons of layer III in the primary auditory area (A1) of the cat cortex].

An electron microscope study of retrogradely labelled pyramidal neurons in layer III of the primary auditory cortex (AI) after HRP injections into the contralateral AI has been carried out in cats. From 4 to 10 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 20.0% of the somatic surface of these neurons. All of the revealed synapses on the somata of callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. Average length of these synaptic contacts was 1.6 microns. In layer III anterogradely labelled terminals of callosal fibres were also revealed. The majority of them contained large round synaptic vesicles and formed asymmetric contacts on spines. Three labelled axon terminals with small elongated vesicles were found to form symmetric axo-somatic synapses on callosal neurons of layer III.