The histological organizarion of the pacemaker for the hippocampal theta rhythm in the rabbit.

Abstract The neurons of the upper part of the rabbit's septum discharge in bursts synchronous with the hippocampal theta rhythms, acting as its pacemaker. A detailed Golgi study of neurons and their connections was under taken with the obejct of understanding the possible neuronal mechanisms by which the septal neurons might transform the variations in degree of reticular excitation into rhythmic bursts of carying (theta) frequencies. Two fundatmentally different types of cell can be distinguished in the dorsal nucleus of the diagonal band (BD): (1) large and medium sized cells the axons of which leave the nucleus in the doral direction, and (2) small cells somewhat resembling Golgi type II neurons. Initial axon collaterals of the type (1) cells can regularly be seen and their termination on the type (2) cells is assumed. The profuse arborizations of the short type (2) cell axons can be traced easily to both cell bodies and proximal dendrites of the type (1). Various kinds of afferent axons and their terminal arborizations have been identified in the (BD) nucleus. In spite of differences in detailed arborization patterns most of these axons have the common feature of establishing contracts with various kinds of cell in relatively widespread regions of the nucleus. A considerable degree of divergence and of convergence appears to be secured by their connections with the neurons of the nucleus. On the analogy with the neuron linkage in many other regions of the CNS the small cells resembling Golgi type II neurons might be interpreted as an inhbitory feedback circuit established within the nucleus. Such feedback loops are generally assumed to produce rhythmic bursts on continuous input both in physiological observations and in model experiments.