Types of neurons and synaptic relations in the lateral superior olive of the cat: normal structure and experimental observations.

Light and electron microscope studies on normal and experimental material in the lateral superior olive (LSO) of cat revealed the presence of three types of neurons: (i) fusiform cells characterized by the large number of terminals articulating with them and projecting to the nuclei of the lateral lemniscus (NLL) and central nucleus of the inferior colliculus (CNIC) (ii) marginal cells embedded in the neuropil of the fibrous capsule and sharing input and output characteristics with the fusiform neurons (iii) multipolar cells with spinous dendrites, local axonal spread and synaptic relation restricted to few afferents only. Four distinct types of axon terminals were distinguished on the basis of their size, vesicle content, membrane attachments, postsynaptic relations and origin. (i) Large terminals with asymmetrical membrane contacts and round vesicles (AR) were of ipsilateral cochlear nucleus origin and articulated predominantly with fusiform and marginal projection cells. (ii) The majority of small AR type terminals arose from the spherical cells of the ipsilateral nucleus of the trapezoid body (NTB). Their number was larger on the fusiform and marginal cells. (iii) Medium sized terminals of symmetrical membrane apposition and pleomorphic vesicles originated from the ipsilateral NLL and CNIC. Their quantitative distribution indicated preference for the multipolar cells. (iv) Profiles with symmetrical membrane contacts and flattened vesicles (SF) type were considered to be terminals of the spinous multipolar interneurons, thus local in origin.