The Medial Septum: Node of the Ascending Brainstem Hippocampal Synchronizing Pathways

Abstract

The study of electroencephalographic (EEG) (field) activities indigenous to the hippocampal formation (HPC) has proven to be a fruitful approach, based on the assumption that they reflect the neural processing going on in these structures (Bland 1986). To date, as a result of the research efforts of many laboratories, we have a better appreciation of the range of field activities, the concurrent cellular activity, and the behavioral conditions with which they are associated. The study of HPC theta field activity has now been placed within the more general context of oscillation and synchrony in the central nervous system (Bland and Colom 1993). Although the nomenclature varied somewhat, earlier studies identified three major field activities: (1) theta—a sinusoidal-like rhythmical activity (also called RSA), up to 2mV in amplitude and a frequency range of 3-12 Hz in rodents, depending on the recording conditions; (2) LIA (large amplitude irregular activity)—an irregular activity with a broad-band frequency ranging from 0.5 to 25Hz; (3) beta—fast waves occuring in a frequency range from 20 to 70Hz (Stumpf 1965; Vanderwolf et al. 1975; Leung, Lopes Da Silva, and Wadman 1982; Leung 1992). The large amplitude aperiodic spikes occurring during LIA have been subsequently well characterized as sharp waves (SPWs), large amplitude (1-3mV) transient field potentials with a duration of 40-100ms (Buzsaki 1986), along with a 200Hz oscillation associated with the SPWs (see Chrobak and Buzsaki 1998; and Chrobak, Chapter 4, this volume). This chapter will focus on the HPC field activities of theta and LIA. Limbic cortex represents multiple synchronizing systems. Subsets of cells in these structures exhibit membrane potential oscillations as a result of intrinsic properties of membrane currents. These cells also receive inputs from other cells in the same structure, as well as from cells extrinsic to the structure. A major source of extrinsic inputs are the ascending brainstem HPC synchronizing pathways originating in the rostral pontine region, ascending and synapsing with midline caudal diencephalic nuclei, which in turn send projections to the medial septal region (see Figure 6.1). The medial septal region functions as the node in the ascending pathways, sending both cholinergic and GABA-ergic projections to the HPC (Vertes and Kocsis 1997; Bland and Oddie 1998; Chapter 2, current volume). A complete understanding of the functional significance of “theta band” oscillation and synchrony in limbic cortex will require an understanding of how intrinsic and extrinsic properties interact.