Electron microscope study of sources in the cat primary auditory cortex (AI) projecting to the medial geniculate body

Abstract

An electron microscope study of retrogradely labeled pyramidal neurons in layer VI of the primary auditory cortex (AI) after injecting horseradish peroxidase (HP) into the medial geniculate body was carried out in cats. Not less than 57.8±1.9% on average of the perimeter of perikaryon profiles of corticogeniculate neurons labeled with HP were found to be covered with astroglia processes. Between three and eight synapses occupying an average of 10.8±1.0% of the perimeter length were found on the perikaryon profiles of these neurons. Nearly all synapses (a total of 98.7%) at the soma of corticogeniculate neurons had symmetrical active zones, being made up of axonal terminals with flattened synaptic vesicles. Anterogradely HP-labeled axonal terminals of geniculocortical fibers were also found in the neuropil of layer VI in area AI, in addition to retrogradely labeled neurons. They contained large round synaptic vesicles and formed asymmetrical synapses. The potential role of axosomatic synapses in the shaping of corticogeniculate neuronal activity is discussed.