Spino-olivary termination on spines in cat medial accessory olive

Three functional regions of the inferior olive, the caudal medial accessory olive (cMAO) and the caudal and rostral dorsal accessory olive (DAO) receive input from the spinal cord. The present study determined how spinal inputs to cMAO interact with olivary neurons. These inputs were labeled by injections in cat lumbosacral of wheat germ agglutinin conjugated to horseradish peroxidase. The tracer was visualized with tetramethylbenzidine. The morphology of the labeled spino-olivary terminals and the relationship between these terminals and postsynaptic elements were determined. Spino-olivary terminals in cMAO displayed the morphological characteristics classically associated with excitatory synapses. Almost three quarters synapsed on spines, most of which contacted other spines, forming spine clusters. The majority of postsynaptic spines also received convergent input from apparently excitatory, nonlumbosacral afferents. This postsynaptic organization provides several possible benefits for the putative role of cMAO in the control of posture. An earlier study demonstrated that in DAO, almost three quarters of lumbosacral, spino-olivary terminals synapse on dendrites (Molinari: Neuroscience 27:425-435, 1988). Thus, lumbosacral afferents appear to differ fundamentally in the way in which they interact with neurons in cMAO and DAO. These results suggest that the way information is processed may be as important in determining the functional differences between olivary regions as what information is received.