Abstract
1. Decreases in the frequency and increases in the regularity of the bursts of type I–II cells of the medial septal nucleus-diagonal band nucleus (MS-DB) occur to various extents during surgical (transection of the medial forebrain bundle) and pharmacological (Nembutal) shut-offs of ascending afferentations; however, phase-frequency drives of theta cycles to a low-frequency stimulus differ substantially under these conditions. 2. Nembutal sharply restricts the upper boundary for overall (up to 8 Hz) and precise (up to 5 Hz) driving of the background theta cycles to a stimulus. When basal undercutting of the septum is done, the upper boundary of overall (10 Hz) and precise (8 Hz) driving differ insignificantly from the corresponding control parameters (12 and 9 Hz). 3. Nembutal significantly increases the duration of the initial pause in cells with driving “by pause” (from 55.1±2.7 to 95.2±6.7 msec). This does not occur with basal undercutting. 4. Under both conditions, there is significant improvement in comparison with normal driving to low frequencies of stimulation near the background frequency of the theta cycles. 5. MS-DB cells with weak rhythmic modulation (type III) can react to stimulation with two types of responses: the driving of background bursts and the following of single spikes. These responses differ with respect to latent periods, frequency optima, and pharmacological sensitivities.
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