Introduction

Rhythmic behaviour is characteristic for the nervous system at different hierarchical levels. Periodic temporal patterns can be generated both by endogenous pacemaker neurons and by multicellular neural networks. At single-cell level it was demonstrated, both experimentally and theoretically, that periodic membrane potentials could bifurcate to more complex oscillatory behaviour (ultimately identified by chaos) in response to drug treatment (Holden, Winlow & Haydon, 1982; Chay, 1984). Even the alteration of periodically synchronized oscillation and chaotic behaviour has been found in periodically forced oscillators of squid giant axons (Aihara, Matsumoto & Ichikawa, 1985).

The appearance of quasi-periodicity and chaos has been associated with abnormal neural phenomena not only at single neural level but as well at macroscopic scale connecting chaotic EEG dynamics to epileptic seizure (Babloyantz, Salazar & Nicholis, 1985). At intermediate level, chaotic behaviour was found in a model of the central dopaminergic neuronal system, and was associated with schizophrenics (King, Barchas & Huberman, 1984).

‘Normal’ and ‘abnormal’ dynamic behaviour, also at intermediate, namely synaptic level, has recently been investigated (Érdi & Barna, 1986; Éedi & Barna, 1987). Preliminary numerical calculations suggested that the regular periodic operation of synaptic level rhythmic generator of cholinergic system requires a fine-tuned neurochemical control system. Even mild impairment of the metabolism might imply ‘abnormal’ dynamic synaptic activity.

Memory disorders associated with Alzheimer's disease, partially due to disturbance of the control system of acetylcholine (ACh) synthesis, can be accompanied by change of dynamic patterns of firing frequency (Wurtman, Hefti & Melamed, 1981).