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Department of Biophysics, University College London. Gower Street, London WC1E 6BT

1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na⁺ (i.e. in normal medium, Ca²⁺-free medium and Cl^--free medium). M.e.j.c.s were also examined in isotonic Ca²⁺ where the inward post-synaptic current was carried mainly by Ca²⁺.

2. In normal medium, mean m.e.j.c. amplitude = 2.34±0.05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) was _m.e.j.c. = 2.62±0.11 msec (V_m = -80 mV, T = 22 °C). Decay-time was not markedly changed in Ca²⁺-free or Cl^--free medium. _m.e.j.c. approaches the life-time of glutamate activated junctional channels.

3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.

4. _m.e.j.c. decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182.±24.8 mV change in V_m.

5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca²⁺-free and Cl^--free solutions.

6. The kinetics of junctional channels are altered in isotonic Ca²⁺. M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j.c. = 0.74±0.03 nA. The decay time becomes markedly briefer, _m.e.j.c. = 1.01±0.08 msec, indicating a reduction in mean channel life-time (V_m = -80 mV, T = 22 °C).

7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca²⁺, thus producing a prolongation in mean _m.e.j.c.. This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of _m.e.j.c. of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca²⁺. The equilibrium potential for transmitter action may be slightly more positive than normal.

8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.