Underlying mechanisms of facilitation and depression of transmitter release. The role of calcium in synaptic transmission.

The internal functioning of a mathematical model describing synaptic transmission and accounting for both facilitation and depression is analysed. Simulation studies reproduce all salient characteristics of both phenomena during repetitive nerve stimulation. Specifically, the analysis reveals that 1. facilitation and depression may be explained in terms of the dynamics of a common process, 2. calcium ions exert a similar influence on this process during the development of both phenomena and 3. the accumulation of an active calcium residue inside the nerve terminal is not absolutely necessary to account for facilitation. Rather, it is proposed that such a residue constitutes an important factor exerting an influence on the development and control of longer term phenomena such as augmentation and potentiation, the two components of post-tetanic potentiation.