Fig. 6.

A simple model of a nonuniform distribution of presynaptic Ca²⁺ channel subtypes accounts for our data. Presynaptic terminals are assumed to be either highP_r or low P_r and to contain only P/Q-type Ca²⁺ channels (QQ), only N-type (NN), or a mixture of the two (NQ) in the same relative proportions for both high and low P_r sites. When N-type channels are blocked by adding ω-CTx GVIA, NN-type terminals are blocked completely, QQ-type terminals are unaltered, andNQ-type terminals have theirP_r either reduced or unaffected, depending on the initial P_r. A similar argument applies to the block of P/Q-type channels by ω-Aga. For further details, see Discussion. The percentages shownabove the control terminals are the estimated relative number of terminals in each category when the model was optimized to fit our data (Table 1). The pie graphs give the percentages of functional high and low P_rterminals in each condition.